# Reduced level when measuring drivers in parallel?



## foxfire3 (Apr 7, 2011)

I'm using REW v.5.0 to measure a woofer, tweeter and both in parallel.
The woofer is a Tang Band 6" unit, the tweeter is Fountek 3.0 and all are mounted in a box.
The amplifier is a $100 Sherwood 2 channel receiver from Radio Shack.

When I measure the woofer and tweeter in parallel(using a 30uF cap. in series with the ribbon), the level is lower than measuring the woofer and tweeter by themselves.
I'm not changing any volume controls or moving the mic.

Is this normal, or am I doing something wrong? :scratch:

I've included a photo:
Red is the summed response...gold is the woofer by itself and purple is the tweeter by itself.


----------



## robbo266317 (Sep 22, 2008)

You should use a proper crossover to get proper measurements, even a simple LC will suffice.

Here is a simple crossover calculator:- http://www.ajdesigner.com/crossover/crossoverfirst.php
Plug in the impedance of the drivers and the desired crossover frequency. 

In your case it's probably going to be around 2kHz 
Assuming they are 8Ω drivers this gives C1 = 9.9 microfarads (round up to ten) and L1 = 0.64 millihenries 

Cheers,
Bill.


----------



## foxfire3 (Apr 7, 2011)

Bill, I'm not trying to come up with a crossover right now.

I'm measuring the drivers' responses and phase so I can import them into crossover design software later.

My question is really why the paralleled response is lower in level than the individual responses?


----------



## robbo266317 (Sep 22, 2008)

foxfire3 said:


> Bill, I'm not trying to come up with a crossover right now.
> 
> I'm measuring the drivers' responses and phase so I can import them into crossover design software later.
> 
> My question is really why the paralleled response is lower in level than the individual responses?


Ok, to get frequency and phase response you only need the individual drivers responses which should be referenced to the system you are using to measure it. It's only after you get these figures that it enables you to design the crossover and then you can get the full system frequency vs phase response.

In effect your parallel response is lower because the load you are applying to the amp with a simple 30 microfarad cap is not good. Assuming the drivers are 8Ω then you are effectively crossing over around 700 to 800 Hz.

For accurate crossover design software you really need to obtain the Theile/Small parameters and model your drivers and then choose a suitable crossover point.

Without the Thiele/Small parameters you either need to measure the individual drivers response and design the system from there, or you need to measure the combined response with a "best guess" crossover in place.

Unfortunately there are no shortcuts that will give meaningful results as you have demonstrated.

Can you tell me the driver models and manufacturer so I have a better idea as to how they will interact?

Cheers,
Bill.

PS A 10 microfarad cap and 0.64 millihenry inductor should only set you back a couple of dollars to do some preliminary measurements.


----------



## foxfire3 (Apr 7, 2011)

Bill, thank you for your replies.

The cost of the inductor and capacitor really isn't an issue.
I probably have one of each lying around in one of my parts boxes.

The woofer is a Tang Band W6-789E and the tweeter is the Fountek Neo3.

I measured the same drivers using HOLM in the photo below.
The black line is the woofer, blue is the tweeter and red is the summed response.
As you can tell their levels are pretty close, unlike what I'm getting with REW above.

I'm not here trying to make HOLM look great and REW look bad...NOT at all.
I really like REW, but I'm trying to figure out what is causing the difference in summed response level in REW.


----------



## AvantGuy (Oct 21, 2011)

But there are still a couple dips in the middle, I think. Could this possibly be a result of phase cancellation? Unless you're certain the drivers are wired in phase, you might take a minute or two to retest with one of the drivers reversed from its current polarity.

--Bob


----------



## foxfire3 (Apr 7, 2011)

AvantGuy said:


> But there are still a couple dips in the middle, I think. Could this possibly be a result of phase cancellation? Unless you're certain the drivers are wired in phase, you might take a minute or two to retest with one of the drivers reversed from its current polarity.
> 
> --Bob


Hi Bob,

You are correct...there is quite a bit of phase cancellation where the tweeter's response overlaps the woofer's.
If I reverse the polarity on the tweeter, most of the cancellation goes away.

Still I'm wondering why REW's summed response is almost 26dB lower than the individual responses?

I also tested these same drivers using the free version of ARTA.
Again, the summed response was very close in level to the individual responses.


----------



## foxfire3 (Apr 7, 2011)

Wow...129 views and no one knows why the lower level in the paralleled response?

Perhaps I'm the only one who measures drivers in parallel to establish their acoustic offsets.

I was hoping JohnM might know what's causing this.


----------



## jtalden (Mar 12, 2009)

I have no good guess as to what went wrong here and there is little info provided. 
The HOLMImpulse measurements appear correct. The REW measurements are not. To find out where the problem is, we probably need to start with a proper calibration (it's doubtful that your noise floor was at 100 dB for instance. 

Start by showing us the soundcard cal and mic cal on you charts and get a proper adjustment of the REW SPL Meter. Calibration is probably not the major issue here, but if that isn't correct is harder to get to the real problem. 

Some questions:
Did you repeat the series of measurements and replicate these same results?
Are you using multiple sweeps? if so, use just one and see if that has an impact. It should not.
Is your soundcard and REW set to the same sample rate?
The mic was in the same position and none of the mic or speaker levels were changed?


----------



## JohnM (Apr 11, 2006)

foxfire3 said:


> Is this normal, or am I doing something wrong?


Strange result indeed. Could you attach the mdat file with the three measurements?


----------



## foxfire3 (Apr 7, 2011)

JohnM said:


> Strange result indeed. Could you attach the mdat file with the three measurements?


Sure, here it is:

I did use mic. and soundcard calibration files when measuring.

Also, I didn't move the mic. or change the soundcard or amplifier's volume.

Thank you...


----------



## EarlK (Jan 1, 2010)

*Normalization of Audio Files *

> I looked at your mdat file & here are a few comments ;

> Most users are not going to get credible results ( especially in the observation of relative levels ) when* mixing raw mdat files together with imported WAVE files *( such as you have done here ) due to that pesky feature ( which is under your control btw , called *"file normalization"* ) .

> According to your posted mdat file, you've imported 2 Wave files ( the separate woofer & tweeter responses ) that were either normalized by REW ( as part of the export process ) or normalized by HolmImpluse ( as part of it's export process ). You then used them with a non-normalized 3rd file ( the combined response ) .

Read this ( taken ) from  *here ! * 



foxfire3_Nov26_2011 said:


> Hi everyone,
> 
> I just started trying to learn how to measure drivers with v.5.00. ,,,,,,snip,,,,,,,,
> 
> ...


> If this was the process that you went through and then decided to use ( imported , "Normalized" files from either REW or HolmImpluse ) as the basis for your observations within the title of this thread / then your observations are fundamentally flawed . 

> I recommend that you start over & make 3 new REW files ( following stringent control of all levels & mic positions , etc, etc, etc, with no export/import tricks) to get some real comparisons .

> Something to note, your old Sherwood receiver may not put out the same amout of watts/level at different operating impedances . This will also somewhat skew your observed results ( by a few db )

<> :sn:

PS ; Most users ( that I know ) export from HolmImpluse typically using its' "normalize to 1" feature &/or are already capturing & displaying data "Normalized" to "0" . 

( ie ; it seems typical HolmImpulse usage encourages users to work with it in a highly normalized fashion which is quite dangerous when mixing files with a program that really encourages getting actual SPL levels into the testing regime ) .


----------



## foxfire3 (Apr 7, 2011)

EarlK said:


> *Normalization of Audio Files *
> 
> > I looked at your mdat file & here are a few comments ;
> 
> ...


Sorry there Earl...I could have gotten my wires(and in this case files) mixed up on that particular mdat file.
If they were wav files, then I was attempting to export them with minimum phase as you pointed out.

My Sherwood receiver isn't old. It's only 3 months old.

I just repeated the test with a different woofer and the same Fountek tweeter using the same procedure as before.

When checking the levels before measuring, REW stated the levels were "ok" for the woofer and tweeter when tested by themselves.
When I wired the 2 drivers in parallel and checked the level again, it stated that the level was "too low".
I know changing the level at that point would certainly be a "no-no", so I tested both paralleled at that level anyway.
Here are the results without any wav files involved.
I've been using a 30uF cap. in series with the ribbon tweeter for protection.
This looks better to me, but I'm curious what you guys think.
I'm thinking I may have to test at a even higher level to get the paralleled response to be "ok" in level.

Thank you...


----------



## jtalden (Mar 12, 2009)

You may want reread EarlK's comments regarding normalization. These traces are again normalized and thus the relative levels have been lost. 

That said, it also appears that the "summed response" had a loss of timing between the 2 sweeps (or possibly a signal dropout). The SPL level looks to be depressed below about 1 kHz. You may want to change the setting to a single sweep, make sure the soundcard and REW are both set to the same sample rate and check that the buffer size is still at 32K (the default I think). With those checks completed run the measurements again and do not export/import the data. That way we can see the original SPL levels.

You were correct to not adjust the level during the set of 3 measurements, but the mic gain can be adjusted up a little before the next set is taken if you want to avoid the low level warning. You should reset the REW SPL meter if you do this. I don't believe that this is a factor in this issue however. The results should track well over a broad range of levels and if the warning didn't show up on all 3 measurements there should be no problem leaving it as is.

By the way, running the two drivers together with the cap on the TW is not a factor in this issue. You will get similar results to the HOLMImpulse results once the setup is correct.


----------



## foxfire3 (Apr 7, 2011)

Ok, I read Earl's Wikipedia link to audio normalization and I really don't understand what it is implying or just how it applies here.

I also have no idea what you mean by "the traces are normalized and thus the relative levels have been lost".

If you would like to suggest what I may be doing wrong that's causing me to "normalize traces" and "lose levels", then I would be more than glad to try it.

I know you guys are trying to help me, but I'm a novice and a lot of times just don't understand some of the technical terms here.

As for the soundcard I'm using, it's a Sound Blaster SB1095 USB soundcard.
Specs. state 24-bit/96KHz.
Under "preferences", I have sample rate set at 44.1KHz and buffer is at 64K(the default setting).
If the sampling rate is 96KHz for this card, I don't see an option for 96KHz under soundcard "sample rate".
If the buffer and sample rate I'm using are wrong, please correct me and I'll change it before attempting any more measurements.

I will also try one single sweep instead of 2 sweeps.

Also, in the above file nothing was imported or exported. They were the original SPL measurements.

Thank you...


----------



## jtalden (Mar 12, 2009)

foxfire3 said:


> > > I also have no idea what you mean by "the traces are normalized and thus the relative levels have been lost".
> > >
> > > If you would like to suggest what I may be doing wrong that's causing me to "normalize traces" and "lose levels", then I would be more than glad to try it.
> >
> > ...


----------



## JohnM (Apr 11, 2006)

The relative levels of the measurements in SB-Fountek-summed.mdat look more like what one would expect, so all seems OK there. A couple of questions on that: 
1 - when you view those graphs, what SPL figures are you seeing at around 2.5kHz, where the traces are all at about the same level? I'm seeing around 141dB so I'm not sure whether there is a bug in the way the calibration offset is being applied for measurements loaded on systems other than the one they were generated on. 
2 - If you bring up the measurement info window for one of those measurements (click the info button in the main toolbar) what figure do you see listed for the "Data Offset"?

On the levels warning, you can ignore that. The levels check for full range measurements uses a test signal that is centred around 1kHz. Your summed measurement has a lower level in that range, likely due to the phase offset between the tweeter and woofer, so the level check comes out a bit low. You do not need to go through a levels check every time you measure, just keep an eye on the headroom figure to make sure the highest level in the measurement is not overly low.


----------



## foxfire3 (Apr 7, 2011)

JohnM said:


> The relative levels of the measurements in SB-Fountek-summed.mdat look more like what one would expect, so all seems OK there. A couple of questions on that:
> 1 - when you view those graphs, what SPL figures are you seeing at around 2.5kHz, where the traces are all at about the same level? I'm seeing around 141dB so I'm not sure whether there is a bug in the way the calibration offset is being applied for measurements loaded on systems other than the one they were generated on.
> 2 - If you bring up the measurement info window for one of those measurements (click the info button in the main toolbar) what figure do you see listed for the "Data Offset"?
> 
> On the levels warning, you can ignore that. The levels check for full range measurements uses a test signal that is centred around 1kHz. Your summed measurement has a lower level in that range, likely due to the phase offset between the tweeter and woofer, so the level check comes out a bit low. You do not need to go through a levels check every time you measure, just keep an eye on the headroom figure to make sure the highest level in the measurement is not overly low.


1. I'm seeing about 140 or so dB at 2.5KHz., also.
2. For Data Offset, it states 165.29 dB.

Is there a way I can get the measurements in the 80dB or so range without getting the "level is too low" message?
Seems like when I check the levels before measuring, I get a figure like -22dB or so.

I'll have to look and see if I can find where the headroom measurement is located.

Thank you...


----------



## JohnM (Apr 11, 2006)

Headroom is the big number that appears in the middle of the measurement panel (above the progress bar) while you are making a measurement.

To get levels displaying correctly you need to do the SPL Calibration. On the REW SPL meter click the "Calibrate" button, select "Use REW Speaker Cal signal" and follow the instructions.


----------



## jtalden (Mar 12, 2009)

JohnM said:


> The relative levels of the measurements in SB-Fountek-summed.mdat look more like what one would expect, so all seems OK there. A couple of questions on that:
> 1 - when you view those graphs, what SPL figures are you seeing at around 2.5kHz, where the traces are all at about the same level? I'm seeing around 141dB so I'm not sure whether there is a bug in the way the calibration offset is being applied for measurements loaded on systems other than the one they were generated on.
> 2 - If you bring up the measurement info window for one of those measurements (click the info button in the main toolbar) what figure do you see listed for the "Data Offset"?
> 
> On the levels warning, you can ignore that. The levels check for full range measurements uses a test signal that is centred around 1kHz. Your summed measurement has a lower level in that range, likely due to the phase offset between the tweeter and woofer, so the level check comes out a bit low. You do not need to go through a levels check every time you measure, just keep an eye on the headroom figure to make sure the highest level in the measurement is not overly low.



JohnM, I want to point out that the first set of measurements looks okay if the relative levels for the imported woofer and tweeter are adjusted, but in the second set of measurements the OP provided from another set of measurements (done entirely within REW) the summed response is in error for some reason. Below is a screen shot. The summed response is down 10 to 20 dB below 1kHz.

This is why I was thinking maybe there was an issue with resampling or timing of multiple sweeps.


----------



## JohnM (Apr 11, 2006)

jtalden said:


> in the second set of measurements the OP provided from another set of measurements (done entirely within REW) the summed response is in error for some reason. Below is a screen shot. The summed response is down 10 to 20 dB below 1kHz.


I don't think that is a measurement error.

Foxfire3, when you say you are using a 30uF capacitor where is it connected when you are making your measurements, in series with the tweeter only (so the woofer is always connected directly to the amp) or between amp and woofer+tweeter? The low end roll-off of the 'summed' response looks a bit like one would get with a series cap.


----------



## foxfire3 (Apr 7, 2011)

JohnM said:


> I don't think that is a measurement error.
> 
> Foxfire3, when you say you are using a 30uF capacitor where is it connected when you are making your measurements, in series with the tweeter only (so the woofer is always connected directly to the amp) or between amp and woofer+tweeter? The low end roll-off of the 'summed' response looks a bit like one would get with a series cap.


John, I'm placing the 30uF cap. in the + wire going to the tweeter only.
Like you stated, the woofer is connected straight to the amp. with no caps. or filters.

Thank you...


----------



## foxfire3 (Apr 7, 2011)

JohnM said:


> Headroom is the big number that appears in the middle of the measurement panel (above the progress bar) while you are making a measurement.
> 
> To get levels displaying correctly you need to do the SPL Calibration. On the REW SPL meter click the "Calibrate" button, select "Use REW Speaker Cal signal" and follow the instructions.


John, can you please tell me what a good headroom figure should be?

I don't have a SPL meter, so I guess I can't really calibrate things.

The 150dB levels don't bother me since I bring all levels down an equal amount in other software before importing them into crossover software.

When raising and lower levels, is it best to adjust the amp's volume, the mixer's output level, or the soundcard's volume control?

Thank you...


----------



## JohnM (Apr 11, 2006)

foxfire3 said:


> John, can you please tell me what a good headroom figure should be?


At least 6 dB, preferably not more than 20dB. REW will tell you if the levels are too far either way.



> I don't have a SPL meter, so I guess I can't really calibrate things.


You don't have to have a meter, just generate a signal at a comfortable level and tell REW it is 75dB.



> When raising and lower levels, is it best to adjust the amp's volume, the mixer's output level, or the soundcard's volume control?


Doesn't much matter, but probably best to ensure the soundcard and mixer output levels are about 3dB below their max output to steer clear of any possible distortion issues at max level then just use the amp volume, but make sure the output level from each stage is comfortably within the input range of the next.


----------



## foxfire3 (Apr 7, 2011)

JohnM said:


> At least 6 dB, preferably not more than 20dB. REW will tell you if the levels are too far either way.
> 
> You don't have to have a meter, just generate a signal at a comfortable level and tell REW it is 75dB.
> 
> Doesn't much matter, but probably best to ensure the soundcard and mixer output levels are about 3dB below their max output to steer clear of any possible distortion issues at max level then just use the amp volume, but make sure the output level from each stage is comfortably within the input range of the next.


John, I'm not sure just how to tell REW that the level is 75dB.

Also, I've been trying to set the levels under the preferences tab and I'm a little confused.
I finally got a -18dB input level after turning the amp's volume up pretty high.
I included a photo of what I have.
When I click on the dB SPL 83 box, while the loud pink noise is playing, I get a very high reading in the 140dB range.
Other than getting the input level at -18dB, I'm not sure what else to check or calibrate here.

Thank you...


----------



## jtalden (Mar 12, 2009)

foxfire3 said:


> John, I'm not sure just how to tell REW that the level is 75dB.
> 
> Also, I've been trying to set the levels under the preferences tab and I'm a little confused.
> I finally got a -18dB input level after turning the amp's volume up pretty high.
> ...


I'm not JohnM, but I’ll try to help again.

I started to walk through calibration steps in more detail for you, but found myself recreating the REW help screens (only poorly) so I dumped it and wrote this instead. I trust you have carefully studied the REW help screens. Specifically the sections titled "Checking Levels" and "Calibrating the SPL Level"? Help is right in the REW menu bar and it’s also repeated on the Forum.

Regarding levels, it looks like you are okay on the output levels so just adjust the AVR to provide a reasonable measuring noise level in the room. If the mic input level is then too low (headroom too high) turn up the Mic gain on the soundcard.

Regarding REW SPL Meter Calibration, the part you may have missed is that since you don't have a separate handheld SPL meter you just tell the REW meter that the noise level you selected for testing is 75 dB.
To do that you open the REW Meter, select "calibrate", then select "REW speaker cal signal" you should then hear the reasonable noise level that you selected. Then just enter "75" into the dialog box and press "Finish". REW will then know that level as "75". It couldn't be easier. There are pictures of the dialogs boxes in the help screens.

As I read this I guess it's clear that I don't really understand what you are missing here as most if not all of this has been mentioned before, but maybe this new wording will help.

Good Luck!


----------



## foxfire3 (Apr 7, 2011)

Apparently there's something I'm missing here.

There's too many things I have hear to adjust(avr volume, mixer gain, soundcard volume control) and not sure which to adjust to do what.
I/ve been leaving the mixer's gain in the middle and adjusting the avr volume and soundcard volume controls.

If I turn up the volume on the AVR to a point that is comfortable(say 75dB), then I get the message that level is too low to measure with.

If I turn up the avr's volume until I get -18dB input level like shown above, the output level through the speaker is quite loud.

I now understand the part about setting the SPL meter to 75dB...thank you.


----------



## jtalden (Mar 12, 2009)

Did you try to turn up the mic gain on the SB soundcard as I suggested?


----------



## foxfire3 (Apr 7, 2011)

jtalden said:


> Did you try to turn up the mic gain on the SB soundcard as I suggested?


Yes, when I turn the volume up on the soundcard the pink noise through the speaker gets louder and if I increase it enough I get the message "level is ok" for measuring.

I have no idea where some of these controls should be set, but I'll tell you how I have things set and maybe you John or someone else can tell me what is set too low.

The avr receiver has a digital readout for volume.
I have it set at 38.

The SB soundcard's volume is adjustable from 0 to 100.
I have it set at 70.

The Behringer mixer's gain is set at 12:00 or straight up and down.

In order to get -18dB input level shown above, I have to turn the avr's volume near 50 and increase the soundcard's to about 80.

Can you tell from any of these settings what may be set wrong?

Thank you...


----------



## foxfire3 (Apr 7, 2011)

Well, I must have done something right because my levels are more within reason now.

Instead of being in the 140dB range, they look more like 90dB now.

When I check the level right before measuring, it states -21.3dB, level is ok.

Here's what I have:


----------



## jtalden (Mar 12, 2009)

If it is not too loud when measureing then you are okay as is.

If it is still too loud: 
You probably have not yet found the volume for the microphone input. It is probably adjusted in the SB application software someplace. You may want to check your manual or get help at the SB site or maybe someone else here has a SB soundcard and can help. The reason I don't think you found the mic volume yet is that it will not increase the noise level if it is turned up. It will only show a higher input level for the same noise level.


----------



## foxfire3 (Apr 7, 2011)

jtalden said:


> If it is not too loud when measureing then you are okay as is.
> 
> If it is still too loud:
> You probably have not yet found the volume for the microphone input. It is probably adjusted in the SB application software someplace. You may want to check your manual or get help at the SB site or maybe someone else here has a SB soundcard and can help. The reason I don't think you found the mic volume yet is that it will not increase the noise level if it is turned up. It will only show a higher input level for the same noise level.


I think this may be what you're talking about.

If I increase it, the "check levels" before measuring seems to go up a couple of dB's without touching the soundcard volume or the avr's volume.


----------



## foxfire3 (Apr 7, 2011)

Ok, I just measured a woofer.

The level turned out to be around 88-90dB when I measured it as you can see in the red response.

So I exported that file as "Impulse response as wav" file, along with minimum phase, just as John instructed me to do early on.

When I imported that same wav file, the level has mysteriously increased about 20dB's as seen in the green response. :scratch:


----------



## EarlK (Jan 1, 2010)

> When I imported that same wav file, the level has mysteriously increased about 20dB's as seen in the green respon.



> Hmmm, I handed you the answer to that :scratch: in post 12 ( obviously spoon-feeding doesn't work ).

> As an experiment , do the export/import thing again ( of that same file ) , just leave off ticking the "normalize" box .

> Then hopefully you'll start to understand where a couple of your perplexing assumptions stem from .

:sn:


----------



## jtalden (Mar 12, 2009)

foxfire3 said:


> I think this may be what you're talking about.
> 
> If I increase it, the "check levels" before measuring seems to go up a couple of dB's without touching the soundcard volume or the avr's volume.


Great you found it. 
If you haven't made the final adjustments yet:
Set the AVR volume to the noise level you want. Adjust the mic volume to get to get the correct input level (headroom). Recalibrate the REW SPL meter to show that level as something around 75 dB and calibration is done. 
[It looks fine as is, but you didn't say if the noise level is where you want it and what the recording level or headroom was. The REW SPL meter is calibrated at about 82 dB and that is close enough to to the 75 dB recommended setting to leave it as is).]

Now that calibration is done it is time to read this sticky thread on the REW forum. 
Please Read: Posting a Graph

It's always something else.


----------



## foxfire3 (Apr 7, 2011)

EarlK said:


> > Hmmm, I handed you the answer to that :scratch: in post 12 ( obviously spoon-feeding doesn't work ).
> 
> > As an experiment , do the export/import thing again ( of that same file ) , just leave off ticking the "normalize" box .
> 
> ...


Thank you Earl...unchecking the "normalize' box recreated the same level.

Thanks again...


----------



## foxfire3 (Apr 7, 2011)

jtalden said:


> Great you found it.
> If you haven't made the final adjustments yet:
> Set the AVR volume to the noise level you want. Adjust the mic volume to get to get the correct input level (headroom). Recalibrate the REW SPL meter to show that level as something around 75 dB and calibration is done.
> [It looks fine as is, but you didn't say if the noise level is where you want it and what the recording level or headroom was. The REW SPL meter is calibrated at about 82 dB and that is close enough to to the 75 dB recommended setting to leave it as is).]
> ...


I will read "Posting a Graph".

I like learning new things, even if it is the correct way to post a graph.

I would have never thought turning the mic. volume up would increase the input level.

During measuring, headroom stays green and goes up steadily to about 9.6.

I remember John said it should stay green and not go to red.

Thank you...


----------

