# Repositioned sub and started to re-EQ



## Malice (May 1, 2006)

Well, having lived with the sub in one position for over a year and having got continual gripes from 'er indoors about the "boominess" (even though I had used the BFD to flatten the response), I decided to move the sub to the front next to the TV and inside the front speakers. I thought it may integrate better and also means that the wife is not sat right next to it when watching a film!!

I shan't bore you with the many sweeps and graphs of different combinations of crossover, PEQ on the amp and phase settings on the sub. But suffice it to say I was nearly disappearing up my own **** with all the data sets. What I will say however is that I could not believe that for a different XO on the amp, the optimum phase on the sub had to change. On a 60Hz XO to get the best overall response of mains and sub together around the XO point the phase on the sub had to be 0 deg. (I swept for 0, 45, 90, 135 and 180 and compared the output). When I switched to an 80 Hz XO, the optimum phase on the sub was found to be 180 deg. That was a surprise as I assumed the amp would have been consistent in its output across all XO.

Anyway, to the point.

I wanted to sweep the mains and sub with the BFD in the loop but with no filters active, so that if there was a delay/phase shift through the BFD I could "optimise" the phase from the outset.

As far as I am concerned, this is the best I achieved when having the amp set to "Normal" PEQ (for the upper frequency EQ), a manual check of levels using the SPL meter and the Denon test tones, but leaving the distances as automatically calculated by the amp on auto set up.


(See attached JPG)

This is the response WITH NO EQ AT ALL!!! Note I get in room response of 15Hz at the target levle of 75db!

Question is, should I try to tame the peak at 25Hz? Is it too much?

Thanks

Bob

EDIT I've added the SUB ONLY graph and MAINS ONLY graph.


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## brucek (Apr 11, 2006)

> Question is, should I try to tame the peak at 25Hz? Is it too much?


Yes........................

brucek


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## Malice (May 1, 2006)

Well, I've created two BFD presets:

(1) Flat (ish)
(2) House curve with 20 Hz + 6db and 50 Hz at 0 db

I'll live with both for a while and see what's what!

But I am really pleased with the end result as these are the final combined responses of mains and sub.


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## cyberbri (Apr 27, 2006)

Maybe cut 25Hz 10~12dB...


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## Malice (May 1, 2006)

I have to assume it is an anomoly in the calibration file and sound calibration because I cannot believe my system has this response....

:scared:


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## cyberbri (Apr 27, 2006)

Malice said:


> I have to assume it is an anomoly in the calibration file and sound calibration because I cannot believe my system has this response....
> 
> :scared:


Yeah, don't look at anything below 10Hz or so.


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## DrWho (Sep 27, 2006)

Are your mains set to large or small?


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## brucek (Apr 11, 2006)

> I have to assume it is an anomoly in the calibration file and sound calibration because I cannot believe my system has this response


No, that's a result of the meter cal file stopping at 10Hz and then the C-weight carrying on down very low. When that curve reaches the noise, it compensates just as a standard calibration file does to any signal, by applying the inverse gain to it.

As said, ignore anything below 10Hz.........

brucek


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## Malice (May 1, 2006)

DrWho said:


> Are your mains set to large or small?


Small. If they were large the MAINS ONLY SWEEP would look like very similar to the SUB down to just below 30Hz. My MAINS go low with room gain.

The top line is the MAINS with no processing (i.e. LARGE) and then 40, 60, 80, 100 XO's.


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## Malice (May 1, 2006)

I think what is now clear to me is that I have accidentally stumbled upon a reasonable position for my mains and sub. The MAINS in their current position get the life sucked out of them at about 68Hz, and although there is a similar dip in the sub at the same frequency, I presume its position / phase setting has minimised this to an extent where the mains fall off has been all but compensated by the Sub. More or less.

I good few evenings' work I'd say!


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## DrWho (Sep 27, 2006)

Something isn't right if you've got over 15dB of gain at 25Hz...that's on the order of what you would get in a car - I doubt your room is that small. In fact, your speakers are rated at -6dB at 30Hz...so call it -9dB at 25Hz and you're looking at over 24dB. This is not normal at all.

What kind of mic are you using? Is everything calibrated correctly and all that? What's the size of your room?


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## Malice (May 1, 2006)

Hi Mike,

I'm using an old style (square) Ratshack SPL meter with the appropriate calibration file. The REW and PC calibrate fine with internally generated tones (both speaker pink noise and sub pink noise) and this validated OK when using the Denon's test tones (once the Denon had auto EQ'd @ 75db using the Denon Mic) Everything seems to align with respect to calibration and levels.

Room is not entirely rectangular!


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## DrWho (Sep 27, 2006)

Room gain for your room shouldn't start any sooner than 30Hz, so there is no acoustical explanation for the insane boost you're seeing. And then it would be an engineers dream come true if your speakers were exhibiting this natural behavior - but I highly doubt any marketing firm would allow underated specs for hte frequency response...

Assuming your entire process is correct, then the only explanation is that the calibration file for your meter isn't accurate - basically your meter is exhibiting wierd behavior.

What do your ears tell you?


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## Malice (May 1, 2006)

Mike,

Before I knew about calibration files for the ratshack meter and before I knew about REW, I used Sonnies test tones and burned a disk and manually plotted a response in Excel. This was when the SPL meter was new out of the box. I had been aiming for a target level of 80db!

As far as my ears are concerned, "untamed" feels too boomy at the lower levels. Tamed and the sweep levels feel about even, but perhaps a little flat at the lower end.

EDIT, just as a note, I remember when I did the manual 1hz test tones, the sub was significantly louder to my ears over the 20 - 35 range, and borne out by the SPL readings.


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## JohnM (Apr 11, 2006)

That's not room gain at the bottom end, it is the first length modal resonance at around 26Hz and the first width mode at or just over 30Hz. Can easily get modal gain of 15dB or more at those frequencies.


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## brucek (Apr 11, 2006)

> it is the first length modal resonance




















brucek


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## Malice (May 1, 2006)

Me being a complete half wit with the finer technical nuances in this thought room gain and modal gain were one and the same thing.

JohnM, your use of the word "resonance" perfectly describes what I appear to hear near the frequencies that Brucek has kindly chipped in with; with single test tones that I did over a year ago, the room seemed to take on a life of its own and nearly took off! People in the other part of the house were complaining about the loudness of it. TBH when I first set out to manually measure and EQ the sub, I tried to achieve a target level of 85db, but as I approached (with hindsight) the modal frequencies, particularly the one about 26Hz, the SPL meter was registering 115dB. No wonder there were complaints!


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## DrWho (Sep 27, 2006)

Serves me right for rushing numbers trying to catch the bus - there's quite a difference between 11 and 21 feet.... :dumbcrazy: 

I would like to take a look at the energy time curves...if it's really entirely the result of a standing wave, shouldn't you see a series of spikes with 38ms intervals? And then wouldn't it take at least 32 equal magnitude reflections to achieve 15dB of gain? 

I ask because I don't know and am skeptical of the possibility for 15dB gain on a standing wave...especially considering that a good portion of the energy will be transmitted through the walls/windows. Where is the energy coming from?

On a side note, if you reduce the response of the subwoofer to flatten out the peak, then aren't you also reducing the sub in compensation for the response of the mains? If so, then aren't you compromising the LFE channel that is only being sent to the sub? If you set the LFE to Sub + Mains, does it still engage the highpass filter to the mains?


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## Malice (May 1, 2006)

> I would like to take a look at the energy time curves...if it's really entirely the result of a standing wave, shouldn't you see a series of spikes with 38ms intervals? And then wouldn't it take at least 32 equal magnitude reflections to achieve 15dB of gain?


I've attached the mdat for the sub on its own with the 80 XO.
NOTE: In order to upload the file I had to zip it and then rename it to mdat as at 750.2 Kb it exceeded the upload limit of 750kb! So "save as", rename to a "zip" extension, unzip and there you have my raw data. Make of it what you will coz I certainly I am not on your wavelength (sic) at all with the level of detail you are talking about! This is obviously an area of expertise for you!

If you want the mdat of the mains on their own with no XO in play, let me know as well!




> I ask because I don't know and am skeptical of the possibility for 15dB gain on a standing wave...especially considering that a good portion of the energy will be transmitted through the walls/windows. Where is the energy coming from?


Is there any way there is an unusual contribution from the fact that I have a bay area with walls at 45 deg and an entrance at a 45 deg angle as well?




> On a side note, if you reduce the response of the subwoofer to flatten out the peak, then aren't you also reducing the sub in compensation for the response of the mains? If so, then aren't you compromising the LFE channel that is only being sent to the sub? If you set the LFE to Sub + Mains, does it still engage the highpass filter to the mains?


In order to get the option to have LFE + Mains I have to set the speaker config to set the MAINS as "Large" in which case this bypasses the crossover filter and would make EQ'ing too difficult or impossible perhaps! As it is setting all the speakers to SMALL will route MAINS below the XO and the LFE to the sub.

If you look at the first post you will see that the curve for the combined MAINS and SUB is almost identical to the sub on its own up to about 65 Hz, so I assume if I tame the sub to a flatish response then any LFE will also be tamed, but not to the extent of it being non-existent.


Bob


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## DrWho (Sep 27, 2006)

Hey Bob,

I downloaded your file, converted to .zip and then unpacked and tried to load the measurement in REW and I get an error saying that it's not compatible with REW4. I wonder if something got messed up during the whole process?

Maybe try sending it via email? mebentz2 -at- gmail.com
(better yet, you could send me your impulse response - which you can obtain via the export option).

Btw, I'm by no means an expert - I know just enough to get myself in trouble 

I'm sure John is right, but I'm just not seeing it...I've always been told that the rule of thumb was 6dB for standing waves? onder:


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## Chrisbee (Apr 20, 2006)

The two slanting windows behind the listening position seem to focus optically slightly in front of the hot seat. 

One could well imagine that long soundwaves would also be focussed (or concentrated) just there. 

I can't save any images in *** Paint.NET or I'd post a picture to show what I mean.


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## Malice (May 1, 2006)

I know what you mean Chris, no need to stretch your Painting skillz for me!

Whether or not I have made any difference to this potential issue is not clear, but behind the seating position I have some very heavy "velvet" curtains that go all around the bay with about 400mm of the bay wall showing top and bottom.

I doubt they will be much of a barrier for the LF though!

MIKE

I've emailed the MDAT. The WAV export didn't appear to work for me and the text export was bigger than the MDAT!

FWIW, I just downloaded the file to another directory, renamed the extension to zip, unzipped the mdat and it loaded fine into REW with no issues for me. :scratch: 

Just to whet your appetite, this is the Energy time graph

JOHNM: I could not get the Y axis to scale above +20. It appears to be a max?


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## MACCA350 (Apr 25, 2006)

Malice said:


> I could not get the Y axis to scale above +20. It appears to be a max?


Select the 'Plot Response Normalized' option down the bottom, this will fix it.

cheers


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## Malice (May 1, 2006)

I had tried that MACCA350, but as it puts the highest peak at 0 you have to do mental maths to calculate how many dbs any peak is over the target response. Having relative dbs below the highest peak could be useful in other situations, but I think it would help me better to understand the mechanics at play and how and by how much I appear to be getting up to nearly 20db boost with room modes at certain frequencies.

Unless I am misreading the normalised graph?


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## MACCA350 (Apr 25, 2006)

Malice said:


> I had tried that MACCA350, but as it puts the highest peak at 0 you have to do mental maths to calculate how many dbs any peak is over the target response. Having relative dbs below the highest peak could be useful in other situations, but I think it would help me better to understand the mechanics at play and how and by how much I appear to be getting up to nearly 20db boost with room modes at certain frequencies.
> 
> Unless I am misreading the normalised graph?


As I understand it, the Energy-Time graph helps you determine the Reverberation Time Decay RT60(time required for a sound in a room to decay by 60 dB) which in the average home theater is normally between 300-600ms. To do this you will need to normalize the peak to 0db so you can see where the -60db point is easily.

cheers


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## DrWho (Sep 27, 2006)

An ETC is showing you the energy at the microphone as a function of time. When the frequency sweep is recorded, some math is done to pretend that every frequency was played at the same time. If you listen to the impulse.wav file, you'll notice that it sounds like a click....not like the frequency sweep. If you listen really closely, you'll notice that it sounds like the "reverb" in your room.

So what you see on the impulse graph is the behavior of the driver if every note was played at once in addition to all of the reflections in the room. The Energy Time Curve is pretty much just plotting the positive area under the curve of the impulse. It's easier to see if you look at the %FS on the Y-Axis instead of dB FS.


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## DrWho (Sep 27, 2006)

The first major spike is 3ms behind - putting that 42ms spike right at the period of ~26Hz. It's not perfectly periodic (probably because your sub isn't right against the wall), but you can certainly see a lot of resonance at that frequency...and 30Hz for that matter.

Where was your mic positioned when you took the measurement? In the past I've been lazy and just laid my mic on the back of couches/chairs...until I realized that the mic would vibrate and totally skew the measurement.


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## Malice (May 1, 2006)

Hi Mike, 

Thank so much for looking at this! Has Sonnie targeted you to be a moderator yet? 

Anyway, I put the Mic on a tripod and removed the seating cushions on the settee and stood the tripod on the settee base, if you will. As the settee has nowhere else to go, I cannot see how I would get the tripod on the floor. But perhaps a heavy solid board on the settee may help? Dunno.

Now that I have got the sub "tamed" even the house curve (+6db at 20 Hz) sounds pants. I've lost the usual SVS's "thump in the chest" performance. I may need to rethinK!


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## DrWho (Sep 27, 2006)

settee? Would that be a "couch" - you'll have to pardon my American and _proper use of English _ :innocent: :bigsmile: lol

I know the use of EQ is widely used for the taming of low frequency effects, but there seems to be a lot of controversy about how much and what kinds of EQ "work". The guys over at Syn-Aud-Con are very against the use of EQ and their premise stems from looking at the behavior in the time-domain. The way I understand it, the direct sound arrives at your ears first and you're hearing what you'd hear in an anechoic chamber until the first reflections arrive (which makes sense...anechoic means no reflections). Any of the reflections that occur within the Haas window (about 20-40ms) are percieved as the same sound and affect what is percieved as the original timbre of the original sound. Anything outside that window is percieved as the semi-reverberant field of the room.

The problem with measuring anything is that you have to have a large window in order to have high resolution at low frequencies...meaning that you can't filter the difference between the direct frequency response and the reflected frequency response in the room. We can however measure our subwoofer in an anechoic environment and then calculate what effects the room is having (which can also be seen in the impulse response).

Ultimately the problem comes down to a system of psychoacoustics. I think it fair to argue that the Haas window is considerably wider at the lower frequencies. I think it's also fair to argue that our ears have some capacity to ignore the effects of the room we're listening in as well - meaning that we're more sensitive to the direct (anechoic) response of the system.

So all that to say....my opinion is that you don't want a perfectly flat frequency response. No matter how you look at it, any form of reduction with EQ is going to reduce the output of the direct signal - which is the first thing to hit your ears. Sure, after a while the reflections in the room will compound on each other and build the response back up, but by that time the note has already passed.

The best approach would be to reduce or redistribute the reflections causing the build up. Of course that is very easy to say and I know it's much harder to do in practice (especially when aesthetics are important). I think it's important to see EQ as a compromise - which follows that the ideal system has now shifted to an ideal compromised system. In other words, obtaining a flat frequency response may no longer be an ideal result in light of the compromises being made.

So in light of all this gobbly goop theory from an only slightly informed individual, allow me to propose an alternative method for determining the magnitude of your filters...

If you reduce the window of your response to 125ms in the negative and 40ms in the positive (the period of 26Hz which I'm going to assume is the limit of the Haas window). You'll notice that your output in the 26-30Hz region is about 5dB too hot. Assuming that this is representative of what your ears hear as the direct sound and assuming your ears can partially filter out the effects of the room, I would propose that the bare minimum of EQ needed would be a -5dB reduction in that region....but I would use the same center frequency and Q's as calculated by REW when you do a large window (like your original frequency response plots). Just reduce the magnitude of the filters from -15dB (or whatever it was) to around -5dB.

But since that's the bare minimum, how bout just cut the measured difference in half? -7.5dB is really close to -5dB.

My prediction is that you won't lose any of that "SVS Punch" while also reducing partially the impact of your room.

It was once common practice in the 70's/80's to measure the response of a room with a 1/3 octave RTA and put in a 1/3 octave graphic EQ and tweak until the RTA showed a flat frequency response. It always sounded like **** and I've spent many years going through systems to undo the catastrophic effects of measuring without ears. After a while, people started using their ears and started doing half compensations instead of full compensations because it sounded more natural. With a parametric EQ, you now have the ability to match the Q of the problem frequency range and not affect other frequencies. But you still gotta trade off between the initial direct sound and the semi-reverberant field of the room.

It's free to try, so if you ever go about some of this inanity then I would love to see the measurements. I'm curious to see how the time-domain graphs change with EQ moreso than the actual frequency response, but I don't yet have a BFD to experiment with (nor will I be able to afford it on this college budget).

Man, this post turned out a lot longer than expected... :snoring:


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## Malice (May 1, 2006)

Cheers Mike. Yup Couch == Settee !!!

It is quite a PITA to drag a desktop into the lounge to remeasure, so forgive me if I just put that one on a back burner for now. I'll take your advice re the first filter as it had put in a -15db cut.

With a MIDI connection to the BFD it is sooooo easy to upload the new filters, so I may get off my butt and drag that PC in there and re-do a series of house curves for say 5 presets which will simulate a number of curves between a 12db cut and a 3db cut, i.e. have a series of house curves where the 20Hz target is 78db thru 87db.

To test my "ears" assessment of the sub's capability in its new position, I turned off all the filters and watched a movie clip I am familiar with to assess the impact/reverb. There is no doubt the "slam" was there initially, but then it was quickly (I presume milliseconds!) followed by the room resonating, i.e. a tendency to boom. As you suggest, EQ'ing in this instance may be a series of compromises to get the right balance.

My ears are definitely telling me that even with a gentle house curve of 6db the final resulting is flat.

Thanks again Mike for the detailed explanation. Most helpful


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## Malice (May 1, 2006)

> But since that's the bare minimum, how bout just cut the measured difference in half? -7.5dB is really close to -5dB.


Well, I did this the other night. I dropped the first filter as recommended by the REW, from -15db to -8db.

WOW! Awesome dude! Not only I have got the impact back but with not so much boom, but it appears to have lifted my entire soundstage!! Even my music sounds better!

Great tip Mike. "Trust your ears" is my new maxim!


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## DrWho (Sep 27, 2006)

Hey cool - I'm glad it worked out! :T 

Did you play around with the magnitude of the filter at all, or you pretty much just went with the -8dB and called it a day?


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## allredp (Feb 7, 2007)

Hey Malice,
What / how did you make your room drawing?
It looks great, but simple enough for me to do.
I'm just starting the process of calibration on my new SVS PB12+/2. The Behringer 1124P is on the way and should be here mid-next week.
Thanks to all for the info in this post as it will provide invaluable for me in coming weeks!
-- Phil


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## Malice (May 1, 2006)

DrWho said:


> Hey cool - I'm glad it worked out! :T
> 
> Did you play around with the magnitude of the filter at all, or you pretty much just went with the -8dB and called it a day?


The latter! :clap: 

Allredp: I used Coreldraw X3. But don't be drawn in by the apparent "ease" of doing it..... A few years ago we had an extension done to the house, including the lounge. The architect had done a proper layout using Autocad which I got a copy of. I imported into Corel, but put all the furniture and measurements on. Measurements are a doddle if you get the scale right as Corel will calculate the length of a dimension line automatically!! Neat!


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## allredp (Feb 7, 2007)

Malice:
Thanks for the info on the room drawing--I see what you mean about the complexity! I just tried using Google Sketch and while there are definately some easy features, I'm just dense enough not to get it to do what I really want for a simple drawing...
Good luck with all the a/v tweaking,
Phil


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