# Measuring and understanding THD and THD+N for real world tracks



## jmcomp124 (Feb 27, 2007)

I want to measure and understand THD and THD+N for of an electrical component and it's various modes. For example, I want to measure the THD and THD+N of the subwoofer preamp output on a real world bass intense track like Finding Nemo, Darla tapping the fish tank. The attached picture shows what the spectrum looks like. I use RTA 1/12 Octave mode, 16 Averages. What do the THD numbers 30.8 and THD+N 87.1 mean? In this case, the subwoofer output of the preamp is in the right input of the UCA 202 external card. The output of the UCA is left open. I see the same type of high numbers if I use the output of a CD player DAC playing a bass track an I use an in-line 12dB attenuator to avoid clippin and keep the levels correct the VU meters. If I use an internally generated sine wave and loopback cable, then the THD value is within 1% and and THD+N is a higher value. I want to measure THD and THD+N values relative to pre-amp volume levels and when EQ is enabled and so forth. Help me understand how to interpret this and those values like 87% are actually sound card distortions? How do I factor those out? 
Thanks,
-Jai


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## laser188139 (Sep 19, 2009)

Looking at the helpfile under Graph Panel -> Spectrum, the section Distortion Measurements describes how it calculates THD. It assumes the input was intended to be a sine wave, determines its frequency by looking at the strongest signal in the spectrum, then evaluates everything else as being an unintended harmonic of the primary signal. So I don't think it can do what you are trying to do, determine the amount of distortion from some real world content, by accepting the real world content directly on some other channel and comparing the input and the output from the receiver. 

On the spectrum display, one can use the Generator button at the top to generate a wave at the desired test frequency. The resulting spectrum will show you the distortion introduced by the system. 

Bill


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## JohnM (Apr 11, 2006)

As Bill says, the THD and THD+N measures are only valid for a single tone input signal.


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## glaufman (Nov 25, 2007)

Absolutely true. But I just came up with a question John... how would REW evaluate say two sine waves playing at the same time, but harmonically unrelated to each other?


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## JohnM (Apr 11, 2006)

glaufman said:


> How would REW evaluate say two sine waves playing at the same time, but harmonically unrelated to each other?


It wouldn't. REW is not set up for intermodulation measurements, only THD and THD+N of single tones.


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## glaufman (Nov 25, 2007)

I understand, I'm just curious what kind of output to expect... I assume the plot would show the second tone, but would REW then ignore that and calculate only what it sees at it's predicted harmonics of the lower fundamental?


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## jmcomp124 (Feb 27, 2007)

.


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## jmcomp124 (Feb 27, 2007)

JohnM said:


> It wouldn't. REW is not set up for intermodulation measurements, only THD and THD+N of single tones.


Would the THD and THD+N numbers be valid with an externally generated sine wave, say from a blu-ray player to a receiver/pre-amp and then looping back the preamp "subwoofer" output into the sound card input? Assume that receiver crossover for mains is set to 100Hz and I use a 30 or 40hz sine wave into the CD input of the receiver and I enable bass management by using Stereo listening mode for a 2.1 type of setup. Will the measurement be valid if I enable a room correction feature like Audyssey. It is possible that the Audyssey filters may change the sine wave. What are the implications in such a case? If there are implications that would introduce errors, are there ways to factor the error out? 
Thanks, 
-Jai


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## glaufman (Nov 25, 2007)

Ignoring Audessey for the moment, yes that should be valid, but you have to take care not to overdrive or clip your soundcard input... this really is true regardless of how the signal is generated...
Audessey: although it will indeed alter the sine wave, I would certainly hope it wouldn't do anything that could be picked up as distortion. But truthfully, I don't know for sure.


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## jmcomp124 (Feb 27, 2007)

glaufman said:


> Ignoring Audessey for the moment, yes that should be valid, but you have to take care not to overdrive or clip your soundcard input... this really is true regardless of how the signal is generated...
> Audessey: although it will indeed alter the sine wave, I would certainly hope it wouldn't do anything that could be picked up as distortion. But truthfully, I don't know for sure.


In RTA mode once the spectrum captures an input signal like the one I just posted, how do I deduce the maximum input voltage (Vrms Max) as seen at the input of the sound card for the signal that was just sampled?


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## glaufman (Nov 25, 2007)

All I can think of is to feed in a signal of known magnitude and compare the dBFS readings. Why do you need to know this?


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## laser188139 (Sep 19, 2009)

Jai, if you want to know the voltage of the signal from the preamp's sub output, the simplest answer would be to attach a high impedance voltmeter across the sub output terminals at the preamp/rcvr in parallel with the line going to the soundcard. 

Bill


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## jmcomp124 (Feb 27, 2007)

glaufman said:


> All I can think of is to feed in a signal of known magnitude and compare the dBFS readings. Why do you need to know this?


This is very useful information for adjusting amp sensitivity and subwoofer trim levels.


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> Jai, if you want to know the voltage of the signal from the preamp's sub output, the simplest answer would be to attach a high impedance voltmeter across the sub output terminals at the preamp/rcvr in parallel with the line going to the soundcard.
> 
> Bill


I use a cheap Rshack voltmeter now and I don't know how accurate it is. A sound card with a high resolution sampling would be a good alternative if the software can latch on the Max input value observed. For example, if you are playing a very bass intense track like War of the worlds or 9, I want to know at reference level what is the peak voltage it puts through at the subwoofer output.
I know that the sound card ideally would not want to see more than 1 volt. WOTW pod emerges for example registers as high as 8 to 9 volts :yikes: at reference levels and subwoofer trim at 0 with Audyssey enabled. I have pairs of 6dB and 12dB in-line attenuators to bring the signal down to 1 volt. If I know the max voltage registered, I can factor out the attenuators.
Does this make sense?
:scratch:


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## Ricci (May 23, 2007)

Not to push another product but ARTA can do some nice distortion measurements. Sweeps and the like. 

I don't think anything is going to measure the distortion of a complex normal signal like a movie soundtrack. How would the measurement system know exactly what is original content and what is added content (distortion/noise) if it isn't generating the signal itself? 

REW won't auto calculate the THD for multiple sine waves, but you could capture the signal and manually calculate it yourself. It might not be perfect but you could get an idea of the THD levels.


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## jmcomp124 (Feb 27, 2007)

Ricci said:


> I don't think anything is going to measure the distortion of a complex normal signal like a movie soundtrack. How would the measurement system know exactly what is original content and what is added content (distortion/noise) if it isn't generating the signal itself?


If a feature was enabled wherein a movie soundtrack can be played from within REW then it would have a reference to compare to. 
Alternatively a two pass option where the first pass is for sampling a baseline and the subsequent passes compares to that baseline maybe a way to do this. I don't understand the nuances of THD and THD+N measurements, so this maybe a naive statement on my part. Just thinking out loud onder:


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## bjs (Jun 12, 2008)

jmcomp124 said:


> This is very useful information for adjusting amp sensitivity and subwoofer trim levels.


Ultimately you want the subwoofer acoustic level to match your mains. So turn your amp to maximum sensitivity and adjust the receiver subwoofer trims for the right level. If it is too loud, turn the receiver subwoofer trim down (amp sensitivity too if needed). If too quiet, turn the receiver subwoofer trim up. Measuring voltage levels shouldn't be necessary.


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## jmcomp124 (Feb 27, 2007)

bjs said:


> Ultimately you want the subwoofer acoustic level to match your mains. So turn your amp to maximum sensitivity and adjust the receiver subwoofer trims for the right level. If it is too loud, turn the receiver subwoofer trim down (amp sensitivity too if needed). If too quiet, turn the receiver subwoofer trim up. Measuring voltage levels shouldn't be necessary.


It depends on your usage. If you are like me and you at times push your system to the very limits, then it helps to know what the maximum line level voltage from the subwoofer pre-amp/receiver output is. I use Audyssey to calibrate my system to correct for both magnitude and phase. Audyssey for example, set the subwoofer trim level at -4 in my case (this is a common case BTW). It also does an excellent job in level matching the mains. So ultimately you do have your subwoofer level matched to your mains. So what is the issue? Let's say your amp sensitivity has been set to 1.4v (33.8dB). My CE4000s supports 0.775v, 1.4v and 3.6v sensitivity settings. If you turn your amp to maximum sensitivity, that is 0.775v. It requires only 0.775v to drive the amp to full power (3600 watts for 4 ohm bridged on a 240v circuit). If the voltage level of the subwoofer output > 0.775, you begin to clip. In my particular case, with amp sensitivity at 1.4v during Audyssey calibration, it set the trim at -4 (As indicated earlier). Guess what a voltmeter reads at reference level for a bass intense scene (Example WOTW pod emerges), about 4volts!!! Yes, a huge spike over the clipping limits. Depending on what you have downstream, different systems would handle this differently. Some may cause ugly distortions that appear to be awesomely loud for some. In other cases it may bottom out the sub...etc. You get the idea?
You need to have a worst case scenario in mind to calibrate this. In my case, the worst case is WOTW pod emerges at reference level. If my amp sensitivity is at 1.4v, I never want to cross this voltage level even at times when I push the system to an insane level like reference level on such type of material. 
Once you limit the subwoofer output to 1.4v (or 1.3 to be safer or lower) using an in-line attenuator if needed or simply using the subwoofer trim adjustment, then you play with the gain to level match. Hope this explains why I am even thinking of this.


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## glaufman (Nov 25, 2007)

If you have to use a real scene, you'll need a meter that reads in true rms... most cost effective way to do this is with the trusty old d'arsenval movement (read: analog meter with a moving needle)... but those won't have max functions...
At the other end of the spectrum, pardon the pun, get a scope that'll calculate from a complex waveform...
Somewhere in the middle is a cheap USB scope. I say cheap more for the quality than the price...
But you say you've done voltmeter measurements, so...
Measure the voltage from the mains pre-out playing the internal test tones from your AVR... add 40dB to that. Add to that any higher than 75dB you want to listen. No movie should ever drive the sub pre-output higher than that number.


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## laser188139 (Sep 19, 2009)

REW alone is not a calibrated measuring tool. By itself, it cannot give you absolute measures, it computes these relative to a reference you have already established. For voltage measures, I can see something similar to how one calibrates the sub preout for sweeps working. You could use REW to generate a sine wave signal, measure this with your voltmeter at the preout while calibrating REW to use this as its 75dB level. On your dynamic content, you can then use the RTA spectrum and the SPL meter to measure its dB level relative to 75dB, and calculate the voltage. 

Of course, the REW SPL meter is not an instantaneous measure. I've only used it at Slow and Very Slow; for your purpose you may want to use Fast. 

As to your general problem, I think if the sub bottoms out at reference levels, that will occur regardless of where the gain is in the chain, in the preamp/receiver or in the sub's amplifier. If the sub amplifier is multistage with limits in the first stages, I can see your scenario that a high level from the preamp/receiver could cause clipping on content that the sub would otherwise handle. 

Bill


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## bjs (Jun 12, 2008)

jmc: If the sub/mains acoustic level is matched properly then the fact the amp clips at reference is a problem that can't be solved with gain structure. You need more efficient subwoofers or more power than your 3600 watts.


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## Ricci (May 23, 2007)

bjs said:


> jmc: If the sub/mains acoustic level is matched properly then the fact the amp clips at reference is a problem that can't be solved with gain structure. You need more efficient subwoofers or more power than your 3600 watts.


Right. You seem to be assuming that your system is capable of outputting the scene in question at REF level. (BTW you do know that this DTS track you are referencing is set 4db louder than a normal DD track right, so that REF would actually be 4db past REF using that DTS track? A lot of DTS tracks are +4db and that's a whole other conversation).) Anyway...If you calibrate your system and then while trying the track at REF your subs get bottomed out or the amps are clipping it means that you simply do not have enough horsepower available to do what you want. :sad:.

I'd suggest an alternative method of simply starting at a safe level after calibrating/ eqing and playing the test scene while monitoring the subs and amplifier clip lights. Turn it up 1 notch at a time until either the subs or speakers make objectionable noises, or the subs appear about to bottom out in this case, or the clip lights flash on the amp. What ever overload indicator happens first mark the volume on the main receiver control and subtract 1 db/notch. That is your maximum safe volume level and it may be -9 or -6db from REF level or whatever it may be. ( You should be fine at +4 louder with most DD tracks and music should be safer at a much higher level as long as you keep away from amp clipping). 

That's the way I did mine and it keeps me out of trouble.


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## jmcomp124 (Feb 27, 2007)

bjs said:


> jmc: If the sub/mains acoustic level is matched properly then the fact the amp clips at reference is a problem that can't be solved with gain structure. You need more efficient subwoofers or more power than your 3600 watts.


It's funny when someone is trying to explain to me about level matching subwoofers to mains, more subwoofer or more power or watching the clip LEDs I can clearly see my point didn't come across and I didn't explain myself well, because I actually preach about this :rofl2:
These are based on my subs and I have two of those with 2 CE4000s in a mediocre room. My subs don't bottom out as long as I set the HP filter correctly and I have that magic number through extensive simulations and backed by real tests. 
Let me clarify guys, here is my goal,
1. I want the sub gain knob set at 0 attenuation and at a sensitivity say 1.4v. This guarantees me that if I send 1.4v to my amp, they push it to it's full clean potential (maybe I should say 1.2v-1.3v accounting for measurement errors and bursts). 
2. Now I want to have a sorted list of movies starting with the most potent content. The list would look something like this for Master Volume -6 ....
1. WOTW, Pod Emerges, 1.4v
2. LOTR ring fall, 1.3v 
3. FOThePhoenix crash, 1.3v
4. Nemo,Darla, 1.0v
so on....
This can be translated to WOTW -6 MV, LOTR -4MV, FOTP -4MV, Darla 0MV. As new movies come in like 9, based on prior knowledge and when folks point out which time stamp to look out/forward for, I would like to do a simple test for example using REW to get the max voltage, so I know at what volume I can comfortably watch this knowing, I won't push the subs beyond max amp power. I don't want to be blasting the daylights out of people trying to figure out when the LEDs are clipping. I want to do it in a nice an quiet setup when no one is bothered and then I crank it up at it's designed levels. 
Hope this makes sense. 
No more talk about needing more subs :boxer:
I am trying to be funny, don't take exception to the live emoticon . Hope you get my point on what I am trying to measure and why I am asking for what I am asking.


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## jmcomp124 (Feb 27, 2007)

glaufman said:


> If you have to use a real scene, you'll need a meter that reads in true rms... most cost effective way to do this is with the trusty old d'arsenval movement (read: analog meter with a moving needle)... but those won't have max functions...
> At the other end of the spectrum, pardon the pun, get a scope that'll calculate from a complex waveform...
> Somewhere in the middle is a cheap USB scope. I say cheap more for the quality than the price...
> But you say you've done voltmeter measurements, so...
> Measure the voltage from the mains pre-out playing the internal test tones from your AVR... add 40dB to that. Add to that any higher than 75dB you want to listen. No movie should ever drive the sub pre-output higher than that number.


Thanks Greg. This makes a lot of sense. I would have to factor movies recorded hot.


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> REW alone is not a calibrated measuring tool. By itself, it cannot give you absolute measures, it computes these relative to a reference you have already established. For voltage measures, I can see something similar to how one calibrates the sub preout for sweeps working. You could use REW to generate a sine wave signal, measure this with your voltmeter at the preout while calibrating REW to use this as its 75dB level. On your dynamic content, you can then use the RTA spectrum and the SPL meter to measure its dB level relative to 75dB, and calculate the voltage.
> 
> Of course, the REW SPL meter is not an instantaneous measure. I've only used it at Slow and Very Slow; for your purpose you may want to use Fast.
> 
> ...


Bill,
Thanks for your useful feedback. Sorry about this basic question, what does that "input rms 68.5dB" in my first post translate to in volts? I am just getting started with REW. Have mostly been an ETF user. Your pointer comes very close to what I am looking for. Just a few more clarifications and I should be set.
Thanks,
-Jai


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## laser188139 (Sep 19, 2009)

To give an example of what I was thinking: suppose you generate a 50Hz sine wave in REW, pass it through the receiver, measure 2.0V at the sub lineout, and calibrate this to be 75dB into REW. Now suppose you play some other content, which measures 69dB, i.e., 75db - 6dB. Given that it is 6dB less than the initial signal, that would be 10 ^ (- (6/20)), or 1/2 the voltage, and 1/2 of the 2.0V measured during calibration would be 1.0V. If, on the complex signal, REW reported an SPL of 87dB, that would be 12dB more than 75dB, and 10 ^ (12/20) would be 4 times the voltage, or in this hypothetical case, 8.0V. 

I think 20dB represents a 10 fold change in voltage. See here for details. You can check this when you calibrate the levels with a sine wave, by increasing the volume control on the receiver by 6dB and verifying that the voltmeter measures a voltage twice as high. 

Bill


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## glaufman (Nov 25, 2007)

jmcomp124 said:


> Thanks Greg. This makes a lot of sense. I would have to factor movies recorded hot.


See, I don't think you do. When you're playing at "reference" let's assume you've used your AVRs test tones to achieve 75dB on each channel. This couldn't be called reference if it wasn't with a given input. That input, on the main channels is -30dBFS. That's 30dB down from FULL SCALE, or the highest signal that can be encoded. That means when playing at this level the max SPL you can get is 105dB. The sub channel can go 10 dB higher, to 115dB. If you're playing at this reference, it should not be possible for any track to drive any channel harder than that. The only danger I see is if you start increasing the volume to hear low dialogue, so you're now higher than this reference, and then go to full scale.



jmcomp124 said:


> Bill,
> Thanks for your useful feedback. Sorry about this basic question, what does that "input rms 68.5dB" in my first post translate to in volts? I am just getting started with REW. Have mostly been an ETF user. Your pointer comes very close to what I am looking for. Just a few more clarifications and I should be set.
> Thanks,
> -Jai


See, that's what Bill was saying... it completely depends on what the input voltage was when you calibrated REWs SPL.


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## jmcomp124 (Feb 27, 2007)

glaufman said:


> See, I don't think you do. When you're playing at "reference" let's assume you've used your AVRs test tones to achieve 75dB on each channel. This couldn't be called reference if it wasn't with a given input. That input, on the main channels is -30dBFS. That's 30dB down from FULL SCALE, or the highest signal that can be encoded. That means when playing at this level the max SPL you can get is 105dB. The sub channel can go 10 dB higher, to 115dB. If you're playing at this reference, it should not be possible for any track to drive any channel harder than that. The only danger I see is if you start increasing the volume to hear low dialogue, so you're now higher than this reference, and then go to full scale.
> 
> 
> 
> See, that's what Bill was saying... it completely depends on what the input voltage was when you calibrated REWs SPL.


So you are implying if your AVRs test tone for the subwoofer records 75dB at MV=0, then "no movie" played at MV=0 will exceed 115dB for the subwoofer channel?


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> To give an example of what I was thinking: suppose you generate a 50Hz sine wave in REW, pass it through the receiver, measure 2.0V at the sub lineout, and calibrate this to be 75dB into REW. Now suppose you play some other content, which measures 69dB, i.e., 75db - 6dB. Given that it is 6dB less than the initial signal, that would be 10 ^ (- (6/20)), or 1/2 the voltage, and 1/2 of the 2.0V measured during calibration would be 1.0V. If, on the complex signal, REW reported an SPL of 87dB, that would be 12dB more than 75dB, and 10 ^ (12/20) would be 4 times the voltage, or in this hypothetical case, 8.0V.
> 
> I think 20dB represents a 10 fold change in voltage. See here for details. You can check this when you calibrate the levels with a sine wave, by increasing the volume control on the receiver by 6dB and verifying that the voltmeter measures a voltage twice as high.
> 
> Bill


Again, I am not sure REW is the answer and that is what I am trying to find out guys. Here is my assumption, and please correct me if I am wrong. 
Assume that for a real world movie track, if you are able to measure the Max Voltage(rms) at the subwoofer preamp output (MV=0 and the whole movie is sampled) and you are able to bound that voltage to the amp sensitivity setting being the ceiling (1.4v), then you can prevent the amp from clipping (assume 0 attenuation in the gain).
Is this assumption correct?


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## laser188139 (Sep 19, 2009)

jmcomp124 said:


> So you are implying if your AVRs test tone for the subwoofer records 75dB at MV=0, then "no movie" played at MV=0 will exceed 115dB for the subwoofer channel?


Greg simplified slightly. If the test tone measured at 75dB, then the LFE output should never exceed 115dB at reference levels. But the 5 or 7 satellite channels contribute to the sub output below their crossovers. So the theoretical maximum for film is usually given as 121dB. The chances of all the channels having 115dB/105dB of bass content at the same time are small. 

Of course, this was calibrated at your listening position. If you are sitting more than 1m from the speakers, even more SPL is needed, as you are going to lose 6dB every time the distance doubles. 

When looking at amplifier power, of course you have to recognize that the impedance of the speaker is not really constant with respect to frequency. If the impedance of the speaker drops to 4ohms instead of 8ohms at the tuned frequency, then it will need 4 times as much amplifier power for the same SPL. Which is why most people regard the calculation of amplifier power for a given sound level as an estimated, to be tempered by empirical performance. In other words, if the sub bottoms out or the amplifier clips, don't drive it quite that hard.


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## laser188139 (Sep 19, 2009)

jmcomp124 said:


> ... Assume that for a real world movie track, if you are able to measure the Max Voltage(rms) at the subwoofer preamp output (MV=0 and the whole movie is sampled) and you are able to bound that voltage to the amp sensitivity setting being the ceiling (1.4v), then you can prevent the amp from clipping (assume 0 attenuation in the gain).
> Is this assumption correct?


Looking around on the web, I think your understanding of amplifier sensitivity is correct, that it is the voltage that will drive the amp to maximum power, for a given gain setting and output impedance. 

I think the problem with determining a priori where clipping will occur arises with the impedance of the speaker. Amplifiers are designed using assumptions about the load of the speaker, but in the real world the impedance of the speaker is not constant. So it may draw more current at some frequencies than at others, and the amplifier clips when its output current limit is reached. At least, that's what I've gathered reading about satellite speakers, and I assume the same is true of the sub woofer even over its limited range. 

So the calculations give you a best case limit, but you might begin clipping before getting to that point. If you want to guarantee no clipping, you need to account for all the variables, or leave a conservative cushion. From your description of your amplifiers, with a 3600w limit, I would have thought that they provided so much power that for any calibration to reference level you would get no where near their limits. This exercise should be largely academic for you, right? 

Bill


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> Greg simplified slightly. If the test tone measured at 75dB, then the LFE output should never exceed 115dB at reference levels. But the 5 or 7 satellite channels contribute to the sub output below their crossovers. So the theoretical maximum for film is usually given as 121dB. The chances of all the channels having 115dB/105dB of bass content at the same time are small.
> 
> Of course, this was calibrated at your listening position. If you are sitting more than 1m from the speakers, even more SPL is needed, as you are going to lose 6dB every time the distance doubles.
> 
> When looking at amplifier power, of course you have to recognize that the impedance of the speaker is not really constant with respect to frequency. If the impedance of the speaker drops to 4ohms instead of 8ohms at the tuned frequency, then it will need 4 times as much amplifier power for the same SPL. Which is why most people regard the calculation of amplifier power for a given sound level as an estimated, to be tempered by empirical performance. In other words, if the sub bottoms out or the amplifier clips, don't drive it quite that hard.


Thanks Bill. I have noticed this with redirected bass wherein it goes louder than 115dB, in fact closer to 120dB which matches your description. You bring up a very crucial point regarding impedance. It's been at the back of my mind. A significant variation in impedance will sink all these assumptions I am making. It's been a while since I looked at my WinISD models, but now that I have the box, is there an easy way to measure impedance changes with movie tracks? I think I am asking for too much and maybe just stick to the traditional approach of using the Clip LEDs which I already have dialed in. 
My current setup has a 6dB in-line attenuator with a Rane PE-17 that is used as a subsonic filter at 17Hz. Without the attenuator, the Rane begins to clip at close to reference levels. For the best signal to noise ratio I have both input/output gain at max. This is another reason why I want to keep the Vrms at less than 2v.


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> Looking around on the web, I think your understanding of amplifier sensitivity is correct, that it is the voltage that will drive the amp to maximum power, for a given gain setting and output impedance.
> 
> I think the problem with determining a priori where clipping will occur arises with the impedance of the speaker. Amplifiers are designed using assumptions about the load of the speaker, but in the real world the impedance of the speaker is not constant. So it may draw more current at some frequencies than at others, and the amplifier clips when its output current limit is reached. At least, that's what I've gathered reading about satellite speakers, and I assume the same is true of the sub woofer even over its limited range.
> 
> ...


You are right, it is largely academic. Usually when I do such an exercise, I stumble upon other ideas. Thanks for the knowledge transfer. My previous post about the Rane also is one reason why I am thinking along these lines. 
Similar to dialogue normalization, it would be nice if movie tracks are encoded with metadata that says what the peak demand of the LFE track in that movie is. However, then it would have to account for redirected bass and things get way too complex. I am glad at least someone here is connecting with the strange ideas in my mind :T without getting irritated with my questions. 
Thanks again!


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## laser188139 (Sep 19, 2009)

You're welcome, Jai. 



jmcomp124 said:


> ... is there an easy way to measure impedance changes with movie tracks? ...


If you really want to get into measuring speaker impedance curves, my guess is your google skills are as good as mine. Fascinating as the subject is, I don't think I'm going down that path with you. 

Have fun,
Bill


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> You're welcome, Jai.
> 
> 
> 
> ...


I understand and no problem. In utopia, I would like a box that measures driver excursion in real time and modulate input voltage accounting for amplifier limits also. For now, I am not going down that path you don't want to tread either.


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> Looking around on the web, I think your understanding of amplifier sensitivity is correct, that it is the voltage that will drive the amp to maximum power, for a given gain setting and output impedance.
> 
> I think the problem with determining a priori where clipping will occur arises with the impedance of the speaker. Amplifiers are designed using assumptions about the load of the speaker, but in the real world the impedance of the speaker is not constant. So it may draw more current at some frequencies than at others, and the amplifier clips when its output current limit is reached. At least, that's what I've gathered reading about satellite speakers, and I assume the same is true of the sub woofer even over its limited range.
> 
> ...


Bill,
Pondering through this, it appears that the impedance variation may be a non-issue in my particular case because I use the LMS woofers that claim linear BL. This suggests an almost linear impedance since B will be mostly uniform with current being the larger variable but since BL is linear then impedance must be. Even if it does vary, variations may not be large so, gating the input voltage to 1.4v (amp sensitivity) using the loudest subwoofer track in the movie, should work. I will post results as I get some time to do these experiments. A friend of mine in my labs has equipment to measure peak voltage in fine granularity and he offered to contribute. I will post whatever I find, if folks here are interested?. 
Thanks,
-Jai


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## glaufman (Nov 25, 2007)

laser188139 said:


> Greg simplified slightly. If the test tone measured at 75dB, then the LFE output should never exceed 115dB at reference levels. But the 5 or 7 satellite channels contribute to the sub output below their crossovers. So the theoretical maximum for film is usually given as 121dB. The chances of all the channels having 115dB/105dB of bass content at the same time are small.


 Thanks Bill, I hadn't thought of that.


> Of course, this was calibrated at your listening position. If you are sitting more than 1m from the speakers, even more SPL is needed, as you are going to lose 6dB every time the distance doubles.


 But if the 75dB had been calibrated at the LP, the additional SPL the sub was putting out to achieve that was already accounted for?


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## laser188139 (Sep 19, 2009)

glaufman said:


> ... But if the 75dB had been calibrated at the LP, the additional SPL the sub was putting out to achieve that was already accounted for?


You are probably right, that this does not enter into Jai's question of the maximum voltage limit where clipping would occur in the amp. I was focused on the testbed environment, where we were calibrating 75dB at REW to match a particular voltage, and confounded the two 75dBs. 

I think it can enter the question of staying within the limits of the sub's physical speaker. But there Jai has suggested real time measurement of cone excursion.


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## jmcomp124 (Feb 27, 2007)

glaufman said:


> Thanks Bill, I hadn't thought of that. But if the 75dB had been calibrated at the LP, the additional SPL the sub was putting out to achieve that was already accounted for?


I am not sure that is the case. I think the test tones are discrete to each channel. If it did contain redirected bass tones, then we would hear something in the mains during subwoofer tones due to the way crossovers are designed.


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## jmcomp124 (Feb 27, 2007)

glaufman said:


> But if the 75dB had been calibrated at the LP, the additional SPL the sub was putting out to achieve that was already accounted for?


I probably misunderstood your question. I think this question was tied to Bill's comment below...

"Of course, this was calibrated at your listening position. If you are sitting more than 1m from the speakers, even more SPL is needed, as you are going to lose 6dB every time the distance doubles"

The additional SPL the sub or any other channel would have to do, would be relative to their distance from the primary listening position. My point is that, the subwoofer test tone, does not account for the redirected bass from the mains or other channels. As a simple test, if you set your mains to full band and the center and surrounds to the lowest crossover possible and measure the SPL (C-weighted) at the listening position, you will find that it is the same as when the crossover for all main and surround channels are set to say 100Hz. This proves that the subwoofer test tone does not account for the redirected bass from movie tracks that can push it 6dB higher. So with that clarified, there are some movies recorded hot as someone (Ricci?) pointed out earlier. So you can be even 10dB off by simple extrapolation with internal test tone calibrations. If this reasoning is not correct, please let me know.
Thanks,
-Jai


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## Ricci (May 23, 2007)

jmcomp124 said:


> Bill,
> Pondering through this, it appears that the impedance variation may be a non-issue in my particular case because I use the LMS woofers that claim linear BL. This suggests an almost linear impedance since B will be mostly uniform with current being the larger variable but since BL is linear then impedance must be. Even if it does vary, variations may not be large so, gating the input voltage to 1.4v (amp sensitivity) using the loudest subwoofer track in the movie, should work. I will post results as I get some time to do these experiments. A friend of mine in my labs has equipment to measure peak voltage in fine granularity and he offered to contribute. I will post whatever I find, if folks here are interested?.
> Thanks,
> -Jai


The LMS drivers will have impedance fluctuations based on the enclosure loading they are in like any other driver. You are looking at a very complex problem that cannot be simplified too much. Your amps are not always going to clip at a certain voltage output. It can vary with freq, impedance, input ac, etc. The interaction between the amp and the load is all over the map. Your drivers will also run into trouble at some frequencies more than others. Then you get to the complex media that is being asked to be produced which may or may not be strong in the regions that are most difficult for the amp/speaker combo. 

I still think that the ghetto way to set max level, that I suggested on the first page is simplest and effective. Use the nastiest signal that you can find and push your system until you find it's limit with it. Then knock it back 1 click. There is your maximum safe output level at least until a soundtrack comes along that is even nastier than the one used for set-up at which point you'd readjust. Sure when you play normal rock type music you may be able to safely turn up another 10db but you know that nothing you put in will hurt your system if you stick to this level. That's why I'm going so far overkill on my main bass system, so I never even have to wonder. 

It seems like you want to be able to tell the absolute maximum safe level that you can run any *individual *piece of content at in a quick easy manner and it's just not as simple as looking at voltages IMHO. There are too many complex interactions going on and the signal content can vary too much. 

I guess if you really want to do the above. Run a 20 or 30 sec long, 0dbfs, 0-100hz sine sweep through your system until you get to amp clipping or the drivers make objectionable distress noises during whichever part of the sweep. (VERY DANGEROUS to your equipments health). Record the peak voltage into the amp during this. Subtract a little from it and there is your maximum safe input voltage. For example my PL9.0's clip at about 65-70hz first during a sweep like this with the cabs I'm using. There is some headroom left in other parts of the freq band, but not much left on the low end either due to driver excursion. My limitations are voltage around 65-70hz and driver excursion below 20hz. There is some unknown amount of headroom left between20-60hz and 70-100hz. 

Determining the maximum voltage out of any given piece of material at a given volume setting is another matter. You could probably do a generic CD or MP3 music setting with a loud piece of music, a DTS setting with WOTW's and a DD setting with the Hulk or whatever since each format will have completely different max output voltages at the same receiver volume. Yes there are different levels of hotness...Yes if you are calibrated to REF you can theoretically hit 121.24db at the listening position with redirected bass from 5 channels into the sub assuming that your sub is calibrated flat with the mains too...Better add a few DB if your sub is run hot. BUT...That is with a standard DD sound track. WOTW's DTS track is supposedly 4db hotter than this so you could be hitting 125.24 db if set to REF watching that movie. A lot of DTS tracks are like this...they are +4 dialnorm. REF level for these would actually be -4 from whatever your REF setting is. 

Let's say that WOTW's gives you an 8v peak. A DD encoded movie like Pulse might give you a peak below 4v. A cd could be down around 1v...


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## jmcomp124 (Feb 27, 2007)

Ricci said:


> The LMS drivers will have impedance fluctuations based on the enclosure loading they are in like any other driver. You are looking at a very complex problem that cannot be simplified too much. Your amps are not always going to clip at a certain voltage output. It can vary with freq, impedance, input ac, etc. The interaction between the amp and the load is all over the map. Your drivers will also run into trouble at some frequencies more than others. Then you get to the complex media that is being asked to be produced which may or may not be strong in the regions that are most difficult for the amp/speaker combo.
> 
> I still think that the ghetto way to set max level, that I suggested on the first page is simplest and effective. Use the nastiest signal that you can find and push your system until you find it's limit with it. Then knock it back 1 click. There is your maximum safe output level at least until a soundtrack comes along that is even nastier than the one used for set-up at which point you'd readjust. Sure when you play normal rock type music you may be able to safely turn up another 10db but you know that nothing you put in will hurt your system if you stick to this level. That's why I'm going so far overkill on my main bass system, so I never even have to wonder.
> 
> ...


Ricci,
Very good points about dailnorm and movies recorded hot. In addition to that, there is the redirected bass based on what the Xovers are set at which needs to be factored in and that is why I think measuring the voltage at the subwoofer preamp output level is useful. It kind of will give you an idea of how hot (or cold) a movie is recorded accounting also for redirected bass and dialnorm. Some movies have dialnorm offsets of >5!
Again, we maybe giving this impedance issue too much weight when it probably can be treated as a non-issue. Once you gate the input voltage to a value say 1.4v for any given movie, basically what it gives you is the safe level you can play the movie at again accounting for redirected bass. For example, safe level translations would be WOTW at -6 and Pulse at -3 MV. But how would you know those values? Would you look at clip lights for all these tracks or rather simply measure the voltages using whatever peak measurement tool you can find and play that movie without being blasted with loud sound while you find the sweet spot? Once you know which timestamp is the most intense, simply forwarding to the time stamp and latching on the max value is all that is needed and you are done to find out what the MV setting should be.
Coming back to impedance. Even if the impedance fluctuates from say 3-6 ohms, what would change is your output power. The CE4000 for example is 3600 watts at 4 ohms bridged mono. A momentary dip to 3 ohm may cause the burst to go above 3600 watts momentarily, maybe it clips very briefly but still you have optimized for the averaged case of 4 ohms. 
-Jai


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## laser188139 (Sep 19, 2009)

Ricci, thank you for giving a better description of the complexity of the sub's behavior than I was doing. 



Ricci said:


> ... That is with a standard DD sound track. WOTW's DTS track is supposedly 4db hotter than this so you could be hitting 125.24 db if set to REF watching that movie. A lot of DTS tracks are like this...they are +4 dialnorm. ...


The only quibble I have with your comments is that it appears to me the DD dialnorm reduces the recorded signal to give a standard dialog level. You are right that the DTS track appears to us humans as 4dB louder, but this comes from the DD dialnorm often specifying a -4dB reduction (DD dialnorm = -27dBFS). With my Denon receiver, I tried three different audio test disks. With my master volume set to 0.0, the DVE DVD and the Audio Test DVD on avsforum render a -30dbFS test tone at 75dB at my listening position, and the AVR indicates there is no dialnorm adjustment. The AVIA DVD includes dialnorm: on its -20dBFS test signal, I measured 81dB at my listening position, and the AVR indicates an adjustment of -4dB (DD dialnorm value = -27dBFS). So, calibrating with the Denon test tones, the loudest theoretical DTS output from the sub is ~121dB with the volume at zero, and the DD track is probably softer but could be mastered just as high. 

Jai, your feeling that factors were being overlooked is correct. Although the peak signal should be ~46dB higher than the 75dB reference to which we usually calibrate, my suggestion that you could calibrate your sub preout voltage level using a 75dB sine wave left out the effect of any sub equalization. Let's consider Audyssey, with which I am familiar. The sub lineout should be flat with respect to frequency, starting well below the crossover, provided Audyssey is disabled, and the chosen frequency for calibration the voltage will have little effect. But with Audyssey, there will be boosts and dips at various points: I've measured the largest gain Audyssey applies to the sub in my Denon at 9dB. So, to measure the voltage corresponding to 75dB for comparison against the theoretical 0dbFS limit, you would need to choose a frequency at which Audyssey is applying its maximum correction. Or, you could perform your calibration measurement with Audyssey Off, and assume the peak voltage could be 55dB (46dB + 9dB) higher than this. Obviously, the same concern applies to any bass equalization system, not just Audyssey. 

Bill


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> Ricci, thank you for giving a better description of the complexity of the sub's behavior than I was doing.
> 
> 
> 
> ...


Bill,
Audyssey applies a normalization to account for boosts and dips that it does with it's filters. I have discussed this extensively with Chris and Larry in the Audyssey forum. The reason I shy away from the sine wave technique is because it is not a representation of real movie tracks. I don't want to complicate things further, but bitstreaming in the player vs bitstreaming in the receiver/preamp also has an effect on the final volume. For example, for bitstreamed sources, dial norm is applied whereas for sources decoded to PCM because of no meta-data dialnorm is not applied. Then there is dynamic EQ which is relative to master volume which also gets affected due to nuances in bitstreaming vs decoding in player. Just throwing out all the variables that made me think of the option I am considering currently which is measuring voltage output at the subwoofer preamp output. 
Thanks,
-Jai


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## laser188139 (Sep 19, 2009)

jmcomp124 said:


> Audyssey applies a normalization to account for boosts and dips that it does with it's filters. I have discussed this extensively with Chris and Larry in the Audyssey forum. ...


True. But as you seemed to be concerned with the worst case peak signal that might be sent to the amplifier, you need to take into account the peak, not the normalized average. My 9dB figure comes from my measures of the sub preout, with Audyssey On and Off, something you should do yourself if you've never looked. Thinking about this some more, I did not mention Audyssey DynEQ, which should have no impact on your question if you were actually running the receiver at 0dB, but will certainly come into the problem if the volume control is set lower because you determined that clipping would occur at that level. 



jmcomp124 said:


> ... For example, for bitstreamed sources, dial norm is applied whereas for sources decoded to PCM because of no meta-data dialnorm is not applied. ...


In the papers I've read on the Dolby site, they seem quite insistent that dialnorm is applied in every Dolby decoder, regardless of whether it is in the player or the receiver. For the decoder to be licensed, there must be no option to disable dialnorm. So I expect Dolby tracks converted to PCM will still be dropped in level. Obviously this is not true of PCM or LPCM tracks on the disk itself. 

Bill


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## jmcomp124 (Feb 27, 2007)

laser188139 said:


> True. But as you seemed to be concerned with the worst case peak signal that might be sent to the amplifier, you need to take into account the peak, not the normalized average. My 9dB figure comes from my measures of the sub preout, with Audyssey On and Off, something you should do yourself if you've never looked. Thinking about this some more, I did not mention Audyssey DynEQ, which should have no impact on your question if you were actually running the receiver at 0dB, but will certainly come into the problem if the volume control is set lower because you determined that clipping would occur at that level.
> 
> 
> 
> ...


Bill,
Try a dolby TrueHD source bitstreamed vs decoded in player and measure max spl at listening position with an SPL meter. You will find that the True HD source is louder and it will match very closely to the dialnorm value. This may digress our topic of discussion but I will leave it for off-line testing and discussion. DynEQ would still be relevant to my question because the intent is not to always plat at MV=0, but find that spot in MV that gives me Vmax at subwoofer line out to match amp input sensitivity. I am still not convinced that dialnorm is what is causing the difference in levels but it is probably one of the attributes. According to my calculation it is still a dB or so off. I did some RTA measurements on this and I had to offset MV by about 5dB whereas dialnorm was 3dB, so the 2dB is unaccounted for and I don't know what is causing the difference. I always bitstream to avoid DEQ issues with Audyssey. 
Thanks,
-Jai


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## Ricci (May 23, 2007)

laser188139 said:


> Ricci, thank you for giving a better description of the complexity of the sub's behavior than I was doing.
> 
> 
> 
> ...


Thanks for the clarifications. I'm not an expert at the art of mastering movies, DD's use of Dialnorm and all. My main thrust was that there are so many formats and differences in mastering that this is a very tall order to achieve for Jai or anyone else. It should also be noted that the theoretical maximum headroom might not even be fully utilized on a given soundtrack, or other composition. 

Jai,
Are you sure that your subs will safely take the full output of your amp under all conditions for brief bursts anyway? A straight sine wave at maximum excursion above tuning? How about the Pulse hallway scene, or WOTW's highway destruction right below clipping?


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## laser188139 (Sep 19, 2009)

Ricci, after learning more about Jai's configuration in another thread, I retract my quibble. Although Dolby may intend that dialnorm always drops the level, never raises it, so that a film with a default -27dBFS dialnorm value will be dropped 4dB in the decoder, receiver manufacturers seem to be confusing the issue. Although my Denon receiver appears to be more than happy to drop the nominal film 4dB, it appears that other manufacturers have decided that -27dBFS should be the norm where the level is unchanged, so they bump Dolby tracks back up 4dB. This keeps the theoretical maximum level ~121dbFS for the typical film, just like DTS. But the side effect is that if some Dolby film track somewhere has a dialnorm value of -31dbFS, such a receiver will push the sub to ~125dbFS maximum. 

So it's not enough to do, as I did, and read the Dolby papers. The reality is biased by how the receiver and the player actually work. Which makes it difficult to extrapolating from each of our personal experiences to infer what others should see. 

Bill


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## jmcomp124 (Feb 27, 2007)

Ricci said:


> Thanks for the clarifications. I'm not an expert at the art of mastering movies, DD's use of Dialnorm and all. My main thrust was that there are so many formats and differences in mastering that this is a very tall order to achieve for Jai or anyone else. It should also be noted that the theoretical maximum headroom might not even be fully utilized on a given soundtrack, or other composition.
> 
> Jai,
> Are you sure that your subs will safely take the full output of your amp under all conditions for brief bursts anyway? A straight sine wave at maximum excursion above tuning? How about the Pulse hallway scene, or WOTW's highway destruction right below clipping?


Hi Ricci,
At least according to simulations I did, with a HP filter at 17Hz (2nd order butterworth), the driver and PRs can take a prodigious amount of power before reaching Xmax. I think it is in the order of 5000watts. It's been a while since I looked at my WinISD simulations. Also, since simulations may not be the real deal, I once video taped driver excursion with WOTW and it appeared that I still had some excursion left. Again, unless I have some way of accurately measuring driver excursion (maybe with some clever laser tricks) I cannot tell for sure. But the driver has not bottomed on me since I got the HP filters dialed in right. But next week with 3600 watts, I will approach limits very cautiously. I really wanted to try out Pulse, but horror movies are not encouraged by my wife :rolleyesno:, and I get comments like "don't bring that evil inside the house", So I have to respect it. It's amazing how much tolerance she has in accomodating my hobby that even brought a Danley DTS-20 to the family room, but as far as horror movies, there is almost 0 tolerance!
-Jai


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## bjs (Jun 12, 2008)

jmcomp124 said:


> Hi Ricci,
> ...Again, unless I have some way of accurately measuring driver excursion (maybe with some clever laser tricks) I cannot tell for sure....


One could make it complicated with laser tricks...but a small pencil placed at Xmax distance from the driver works fine. Rubber side in and let 'er rip. If the pencil hasn't moved by the end of the movie then you never reached Xmax.

Of course one could to it at some arbitrary excursion and scale the SPL from there. No need to have the volume cranked.


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