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post #1 of 33 Old 07-21-2013, 10:44 AM - Thread Starter
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What's up, guys! My name is Chris, this is actually my first post on this site, but I ran across this conversation and can't help but jump in...
I started reading this question by devgcl regarding if EQ was necessary in a two-channel playback system...

I've been doing live sound professionally for a little while now, and recently decided that I needed to step up my home listening experience. The past year has seen the acquisition of a pair of B&W 685s and a Hafler TA1600. I run direct out of my PC (line level) into the amp, straight to the speakers. It's not the absolute cleanest hi-fi signal chain, but hey, I have student debt, car payments and beer to pay for and FLAC isn't that bad.

So I'm curious to hear opinions on EQ applicaiton and how you guys go about it. I struck out this weekend to figure this out myself, borrowing some gear from work and twisting some knobs, here's what I did --

Using Rational Acoustics SMAART v7, a Sound Devices USBPre and Earthworks M23 measurement mic, I measured each of my 685s at 3'. I measured them at full-range, as well as woofer only and tweeter only to see what the passive crossover was up to... (I'm using last three digits of the S/N to keep track of which speaker is which...)

'Speaker 249' -


'Speaker 250' -


Basically, all of the data is the result of comparing 'Reference' input to the 'Test' mic input. SMAART is sending Pink Noise out of the USBPre, which is a two input/two output interface. One output is literally plugged straight into one of the inputs, this way you create a 'Reference' of what Pink Noise 'should sound like,' and then the second input is the M23 measurement mic.

The top graph shows impulse response. In large rooms you can see wall reflections and stuff here, for now we just want to be sure the mic and the reference loop are in time with each other.

The second graph is Phase Response, however it should be noted all traces have been 'aligned' for comparison to each other (normally Phase Response graphs look much different...).

The third graph shows Frequency Response and Coherence. Frequency Response is pretty obvious, it's the difference between the Reference Pink Noise and what the Test mic is hearing (amp+speakers+room acoustics). The line at the top is Coherence, which is basically data quality. I've set this so the lines disappear if the Coherence (read - Data Quality) is less than 60%.

You can see a color legend on the right to figure out which trace is which. Sorry for the funky naming, the computer I was working on wasn't allow me to type t's and a few other letters for some reason?

This is in my apartment, so speaker placement is less than ideal and there's a fair bit of backround noise (A/C, fridge, neighbor playing Call of Duty...). Nevertheless, we can see things aren't very flat. This is 1/24th octave smoothing, which is a much higher resolution than we're used to seeing (most spec sheets are 1/6 or 1/3 octave smoothing). In real life, things are never going to be a beautiful curving flat line, so we're looking for large frequency bands that seem out of place.

Both boxes are dealing with a crazy boost above 10 KHz, which makes sense, these do sound crispy sometimes. One more-so than the other, manufacturer tolerance? Acoustics? (Sorry, B&W if I'm I'm putting the magnifying glass on...). Things also seem week around 2.5 KHz, in the crossover region, only to hype back up 600 Hz. And there's this iinnssaannee rumble at 90 Hz or so -- really the box, not the fridge or something else in the room, that boost is coming from the system (most likely the speakers). It sounds like the Shuttle taking off if you isolate it...

So I used (on loan from work, so I gotta take it back... mad.gif) a Meyer CP-10 (http://www.meyersound.com/pdf/products/electronics/cp-10_ds.pdf). This thing is sweet, watch the phase traces below and tell me if you see it making things all kinds of whack - it doesn't. A crappy EQ you can hear it working, it's that nasty hollow hiss that you hear (even when cutting), this thing, you can make it a super tight Q and cut 12dB and never hear anything except a certain frequency disappear - it's nuts.

I did the 'Live Sound Guy' thing and flattened the speakers out -- eek.gif

Speaker 249 -


Speaker 250 -


Some of the lines make it look like they are overall much less quieter than the others, this is because I wasn't correcting for EQ differences as much as I should have - things you learn after your done...

Ultimately, I high-passed them at around 120 Hz to get rid of that nasty low crap (you can't really expect these speakers to reproduce lower than they are meant to anyway...) I also shot and re-EQ'd based on the listening location, not a fixed 3' away. There's a lot more I need to work on to get this system 'optimized,' both on the electronic level and room treatment level, and more measuring and experimenting well be done - but coming from the Audiophile prospective, do you guys correct for room and speaker sound or stay true to the most direct, cleanest audio path possible???

My personal take is this - The 865s, like most speakers in the world, are 'hyped' the way their manufacturer likes. These guys defiantly have a low and high boost, and take out some mids (my ears tell me that more than the software). I'm not sure I like this 'better' or not, but only taking time to listen to a lot of music both ways will tell (currently Daft Punk's Random Access Memories, just got done with Pink Floyd's Momentary Lapse of Reason). It's certainly very different than it was before...

Now, I'm not about to put on a pair of headphones. I like the impact a pair of speakers in a room provides, and (for me) it puts me in the room with the music more than headphones. So to some extent, I'm probably going to like it when 100 Hz and below are up three or six dB (I *really* need to decide on a sub...) and when the highs cut a little more to stay on top. But if I'm trying to reproduce the music as the engineer wanted me to hear it, then the system has to be tuned for flat response, and this requires (in order of importance) proper speaker placement, room treatment and probably some (subtractive) equalization.

So what do you guys think? To EQ or not to EQ?

-Chris

-Chris
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post #2 of 33 Old 07-21-2013, 11:26 AM
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So what do you guys think? To EQ or not to EQ?

EQ is useful when you want to change the sound of your speakers, or change the sound of a recording. It's only minimally useful for compensating for poor room acoustics. This article explains why in detail:

Audyssey Report

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post #3 of 33 Old 07-21-2013, 11:27 AM
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EQ or not?

Indoor, it's theoretically prudent to only EQ the minimum phase energy. Therefore, one would EQ from the transition frequency region and below.

The reflected energy above the transition, and it's extremely complex characteristics regarding the blending of the direct energy and the reflected energy, just makes EQ'ing anything above a few hundred hertz a difficult proposition. Fortunately, we hear much more discriminately than our mics can capture. So relying on measurements to EQ above the transition, isn't necessarily the right thing to pursue.

Good luck and welcome

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post #4 of 33 Old 07-21-2013, 01:10 PM
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Some additional thoughts on room EQ:
http://www.dirac.se/media/12044/on_room_correction.pdf

Markus

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post #5 of 33 Old 07-21-2013, 05:29 PM
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My approach to EQ is to use as little as possible as the 'icing on the cake' kind of thing.

I first try and optimise speakers and subs and listening position placement as best I can to get a smooth frequency response. (and at the same time speaker distances from walls and listening position etc for good soundstaging and imaging)

Then I start applying room treatments to address additional problematic areas.

Then any EQ on the lower frequencies, say below 200hz, if needed.

So even before I think about EQ, I have speaker and listening position placements, sub placements and integration with crossover points, crossover slopes, phase and distance adjustments. I can experiment with port plugs in any combination with the speakers or subs. Experiment with leaving any particular doors open into adjoining rooms. And also experiment with broadband absorption near corners or large sections of baer wall. All can hopefully get things pretty close.

I will then experiment a little with EQ and only run with it only if it makes my system sound better to me, regardless what the frequency response measurement looks like.
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post #6 of 33 Old 07-22-2013, 09:16 AM
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Indoor, it's theoretically prudent to only EQ the minimum phase energy. Therefore, one would EQ from the transition frequency region and below.

It might seem that way, but even at very low frequencies EQ will make some places worse rather than better. Besides the Figure 3 graphs in my article linked above, look at Figure 1 in this article:

A common-sense explanation of audiophile beliefs

Even at very low frequencies the response can change dramatically over small distances. I know people use "minimum phase" when describing room behavior, but it doesn't really apply because rooms are more complex than a simple electronic filter.

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post #7 of 33 Old 07-22-2013, 09:39 AM
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Even at very low frequencies the response can change dramatically over small distances. I know people use "minimum phase" when describing room behavior, but it doesn't really apply because rooms are more complex than a simple electronic filter.

Well, there are regions where the response at low frequencies is minimum phase. Here's an article by John Mulcahy (author of REW) which describes how to visualize (non-) minimum phase behavior:
http://www.hometheatershack.com/roomeq/wizardhelpv5/help_en-GB/html/minimumphase.html

EQ can and will fix frequency and time domain errors at the same time if the response is minimum phase. The problem is that the transfer function within a room varies from location to location. The equalization that worked at one location might not work at another location. What really needs to be done before running any equalization is to remove these point-to-point differences so the whole area will benefit from equalization and not just a single point in space.

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post #8 of 33 Old 07-22-2013, 01:18 PM
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Originally Posted by FOH View Post

Indoor, it's theoretically prudent to only EQ the minimum phase energy. Therefore, one would EQ from the transition frequency region and below.

Quote:
Originally Posted by Ethan Winer View Post

It might seem that way, but even at very low frequencies EQ will make some places worse rather than better.

Even at very low frequencies the response can change dramatically over small distances. I know people use "minimum phase" when describing room behavior, but it doesn't really apply because rooms are more complex than a simple electronic filter.


That's precisely why I stated "it's theoretically prudent". smile.gif

BTW; ... I've recently hit upon a wonderful synergy combo of placement and low passing of my three subs (quad 18 IB, (2)double 15" small sealed) , whereby no additional EQ'ing is needed and I've achieved an exceptionally smooth (yet somewhat rising LF) frequency response at the important seating area. I've been experimenting with differing seating config, so we'll se how all of this ultimately shakes out.

I appreciate the link, however I believe I've read a great deal of your content (linking to it and mentioning various pieces in forum discussions).

I'm with you on comb filtering. Even outdoor, at live performances, it's amazing how well delineated coverage interactions can clearly be heard,..even quite prominently in the LF. As you pointed out, merely moving the mic/one's head a small bit ... huge differences can be heard/measured. With poorly executed (quite often frown.gif ) sub arraying, huge differences can be heard over short distances. One live production whereby I mixed FOH at a 20,000 person outdoor amphitheater, the lobing of the coverage pattern of the subs was so sharply heard I was not happy. Unfortunately, I wasn't in a position to spec the PA hang or ground stack config of the subs. I was merely a hired hand to mix the act. Well, within the small area designated for the FOH mix position, one could move laterally maybe 12 feet and all the oomph and impact of the LF was gone... that sucks. Most shows have better consistency than that.

There's a casino with a live band center stage, about a half hour from where we live. It's essentially wide open, high ceiling,... without boundaries in any direction for hundreds of feet. Never been involved with any productions there, however the polar lobing is incredibly well delineated there. One sub and one top per side. You can walk all the way around the entire bar area, 360 degrees, and hear the problematic destructive nulling that occurs. It's quite fascinating really, and very easy to hear. They should co-locate the subs, ... this would certainly help. Or, maybe dedicate each sub for certain use (ie kick drum in one, and bass and synth for the other). Without the boundary nearby, the box to box interactions are very easy to hear.


Thanks

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post #9 of 33 Old 07-23-2013, 10:30 AM
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The problem is that the transfer function within a room varies from location to location. The equalization that worked at one location might not work at another location.

Exactly. And those two locations can be very close together. It's not like you can improve the response and ringing for all three seats on the couch, if not ten feet away along a side wall etc. EQ that improves one place can make things much worse even a few inches away.
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What really needs to be done before running any equalization is to remove these point-to-point differences so the whole area will benefit from equalization and not just a single point in space.

The way to minimize changes with distance is with bass traps. But once that's done, there's no need for EQ. Or possibly a very modest amount of cut (only) at one or two remaining very low frequency peaks.

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post #10 of 33 Old 07-23-2013, 10:41 AM
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One live production whereby I mixed FOH at a 20,000 person outdoor amphitheater ...

Huge respect for guys like you who mix live sound. It takes me weeks (or months) to get a mix I'm happy with for a single song. You guys have to do it in real time, song after song.

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post #11 of 33 Old 07-23-2013, 01:03 PM
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Exactly. And those two locations can be very close together. It's not like you can improve the response and ringing for all three seats on the couch, if not ten feet away along a side wall etc. EQ that improves one place can make things much worse even a few inches away.

Multiple subs can be used as "active" absorption which will reduce point-to-point variance. This has been shown by a couple of researchers, e.g. Todd Welti.
Another solution is to reduce influence of the room by placing subs very close to the listening seat. Here's some data on that:
http://mehlau.net/audio/dual_nearfield_sub/
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The way to minimize changes with distance is with bass traps. But once that's done, there's no need for EQ. Or possibly a very modest amount of cut (only) at one or two remaining very low frequency peaks.

--Ethan

Problem is you need "tons" of absorptive material to reduce ringing and the frequency response still might look pretty ugly.

Markus

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post #12 of 33 Old 07-24-2013, 11:06 AM
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Multiple subs can be used as "active" absorption which will reduce point-to-point variance. This has been shown by a couple of researchers, e.g. Todd Welti.

Do you have a link to that, or more information? I'm aware of active bass traps, but I've never seen the response and ringing at places other than where the calibration microphone was placed.
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Problem is you need "tons" of absorptive material to reduce ringing and the frequency response still might look pretty ugly.

Yes, achieving excellent bass response and ringing does require a lot of bass trapping, though three or more high quality subwoofers probably costs even more. biggrin.gif

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post #13 of 33 Old 07-24-2013, 11:38 AM
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Huge respect for guys like you who mix live sound. It takes me weeks (or months) to get a mix I'm happy with for a single song. You guys have to do it in real time, song after song.

--Ethan


Well, much appreciated.

But you're assuming I'm adept at FOH engineering! ... wink.gif j/k ... smile.gif

In my opinion, there's never enough time for a.) getting the system the way I want it, or b.) getting a mix just the way I want it, not kidding, there's just not enough time.


These days, the two most important elements in a contemporary mix are the lead vocal and the kick-drum.
We all love a superbly executed kick-drum. However, even more important is the lead vocal. For me specifically, working live there's a very fine line between adequate presence and brightness, and excessive harshness. Fortunately with judicious parametric EQ and some transparent processing/compression, I can typically achieve just that right balance of perfectly clear well delineated vocals, without the edge that can accompany them if over-done.

This precise balance is not a trivial task, especially when involved in a live performance and all that live show entails. I attend many live shows when I'm not the one mixing, and I can very clearly hear the difference between a well engineered lead vocal, and one that has been phoned in. The proliferation of live music on TV (American Idol, The Voice, The Grammys/Oscars etc) is no different. With a high performance home system, the varying techniques used are often quite apparent. BTW, anyone catch Shirley Bassey perform Goldfinger during the Oscars,...damn, superb performance,... she brought down the house... "goldfingahhh", damn powerful at 76 years old, what a voice.

In music with vocals, the lead vocal has got to be right. All too often we all see some sound guy that's not even fully engaged, ... merely sitting there, not doing anything. Granted, these may be marginal gigs, but come on, you're doing the performers an injustice.

Back to large scale/high production value live sound, most people likely have no idea the phenomenal technology involved ... and how hard most productions strive for the best sound possible.


I was employed by the former RCA Dome, and subsequently the new Lucas Oil Stadium in Indianapolis. Well a few years ago, our first big live show was the largest stadium tour around; Kenney Chesney and several big support acts in his typical summer tour. A massive 53 semi, 29 motor coach tour (all motor coaches parked indoors smile.gif ). Anyway, mucho complaints regarding sound quality following the first show. We really went to great lengths to address this for the following year. We brought in the "experts" rolleyes.gif and spent hundreds of thousands of dollars on absorption and heavy acoustic drapery. You have no idea what a big deal this was, and how importantly the mgmt. team valued the acoustic reputation of our facility.

So, we booked them again, and we also booked the entire crew an extra day for system tuning/manipulation. This, was a big deal. Paying all the hands the extra day, also we got EV's gurus (their touring system) in addition to the usual tour tech support.

We walked the house, even the suits walked the entire bowl seating area (45k or so for end stage concert) testing articulation as differing amounts of level and delay tower coverage, etc. was employed. This was unheard of, an entire extra day, .. walking the seating sorting out delays etc, for maximum articulation, unheard of. Point being, the vast majority of concert goers, have no idea how hard the individuals involved in the production try for the best quality possible.

The show was a success, the act re-upped and committed for more ... although not involved in the mixing, I spent the entire show near the FOH position. Fortunately, the event was a great success. Relative to the previous concert, very few complaints. I've got some good pics of that show somewhere ...

These days, the big touring acts are incredible showcases of the best gear available .... the technology/materials of the loudspeakers/arraying, is significant. The effort put forth is incredible, most attendees haven't a clue.


Somewhat OT, but wth cool.gif

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post #14 of 33 Old 07-24-2013, 02:02 PM
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Do you have a link to that, or more information? I'm aware of active bass traps, but I've never seen the response and ringing at places other than where the calibration microphone was placed.

Look for papers by Welti, Celestinos, Hill.
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Yes, achieving excellent bass response and ringing does require a lot of bass trapping, though three or more high quality subwoofers probably costs even more. biggrin.gif

--Ethan

My point was that even after tons of passive absorption the response won't be flat and EQ is necessary anyway. Furthermore a single sub won't work for multiple seats. In that case really good results require absorption (passive and/or active) and multiple subs and EQ.

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post #15 of 33 Old 07-27-2013, 11:58 AM
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The proliferation of live music on TV (American Idol, The Voice, The Grammys/Oscars etc) is no different. With a high performance home system, the varying techniques used are often quite apparent.

Great post about live sound, and I agree about the variability of TV mixes. Before this year, the center channel on America's Got Talent was about 4 or 5 dB too soft. This year they finally got it right. Then again, on the results show two days ago the overall volume was about 15 dB too soft! Every time an ad came on it blew you out of your seat. Thankfully I have a DVR and record pretty much everything I watch in advance so I can skip the ads. biggrin.gif

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post #16 of 33 Old 07-27-2013, 12:25 PM
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My point was that even after tons of passive absorption the response won't be flat and EQ is necessary anyway. Furthermore a single sub won't work for multiple seats. In that case really good results require absorption (passive and/or active) and multiple subs and EQ.

I suppose when someone has a millionaire's budget, what you describe is a good approach. Me, I'm totally satisfied with my system with one (killer) SVS subwoofer and a lot of bass traps. I do use the one-band EQ on my sub to reduce the 40 Hz peak due to a 2nd-order length mode by about 3 dB.

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post #17 of 33 Old 07-27-2013, 01:08 PM
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Originally Posted by Ethan Winer View Post

I suppose when someone has a millionaire's budget, what you describe is a good approach. Me, I'm totally satisfied with my system with one (killer) SVS subwoofer and a lot of bass traps. I do use the one-band EQ on my sub to reduce the 40 Hz peak due to a 2nd-order length mode by about 3 dB.

--Ethan

Huh? DCX2496, a couple of capable sub driver in simple closed boxes and amps - "a millionaire's budget"??

If one doesn't have any budget then there's a pretty effective solution for 1 to 3 seats: put a single sub very close to the listening position. This will provide more than satisfying bass without the need of any absorption.

Could you post measurements of your setup across the listening position(s)?

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post #18 of 33 Old 07-28-2013, 01:26 PM
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Could you post measurements of your setup across the listening position(s)?

I'll try to get to that soon. I added new larger bass traps a year ago, and measuring again has been on my to-do list for some time!

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post #19 of 33 Old 07-29-2013, 02:35 PM
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Originally Posted by Ethan Winer View Post

I suppose when someone has a millionaire's budget, what you describe is a good approach. Me, I'm totally satisfied with my system with one (killer) SVS subwoofer and a lot of bass traps. I do use the one-band EQ on my sub to reduce the 40 Hz peak due to a 2nd-order length mode by about 3 dB.

--Ethan

Not really Ethan. One modest $1k sub is equal to basically two of your MondoTraps.

Here's my response across a row of 3 seats in my demo theater. Seat to seat variability is basically nothing. Global EQ very effective in the range the subs are playing. Above that range (say 120Hz and above) I agree with you, EQ becomes difficult because response changes a lot from place to place.


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post #20 of 33 Old 07-29-2013, 11:27 PM
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Nyal,

could you post the .mdat of the measurements above?

Markus

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post #21 of 33 Old 07-30-2013, 09:23 AM
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Markus.zip 909k .zip file

Have fun! This was from a preliminary calibration of the subwoofer array in my HT back in Jan this year. No EQ and it was also before most (all? ... I forget now) of the acoustic treatment was installed.
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File Type: zip Markus.zip (909.1 KB, 3 views)

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post #22 of 33 Old 07-30-2013, 10:33 AM
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Originally Posted by Nyal Mellor View Post

One modest $1k sub is equal to basically two of your MondoTraps.

Well, MondoTraps are $300, but point taken. When I see people talk about using multiple subs to "avoid the need for bass traps" it's often four subs at $2,000 each.
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Here's my response across a row of 3 seats in my demo theater. Seat to seat variability is basically nothing.

Very impressive. I doubt my room varies that little. How many subs are there, and how large is the room?

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post #23 of 33 Old 07-30-2013, 11:20 AM
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Not really Ethan. One modest $1k sub is equal to basically two of your MondoTraps.

Maybe in the freq domain, however as you know the LF decay characteristics are often equally as important for delineation, detail, etc.

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post #24 of 33 Old 07-30-2013, 12:56 PM
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Well, MondoTraps are $300, but point taken. When I see people talk about using multiple subs to "avoid the need for bass traps" it's often four subs at $2,000 each.
Very impressive. I doubt my room varies that little. How many subs are there, and how large is the room?

--Ethan

With a rectangular or square room and one row of seats two subs at 25%/75% of room width is all you need to get very good seat to seat consistency. Only issue with two subs and the row of seats away from the back wall is SBIR cancellation caused by the back wall. Two rows of seats or more is best dealt with by four subs for very good seat to seat consistency, though if you are clever with seat and sub placement you can get almost as good results. Four subs, two front, two back also allow SBIR mitigation by applying separate delays to the front and rear pairs.

My room has four subs and is 12x17. It scales to bigger rooms no issues.

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post #25 of 33 Old 07-30-2013, 12:57 PM
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Maybe in the freq domain, however as you know the LF decay characteristics are often equally as important for delineation, detail, etc.

At least in the subwoofer range multiple subs placed correctly will prevent some room modes from even existing in the first place because they are being driven destructively. EQ takes care of the rest since room resonances are minimum phase - correct FR, time domain follows.

Elsewhere (80Hz+) bass traps definitely required because a center speaker playing by itself is going to drive many room modes with resultant FR peaks and time domain ringing (slow SPL decay).

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post #26 of 33 Old 07-30-2013, 01:27 PM
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Originally Posted by Nyal Mellor View Post

With a rectangular or square room and one row of seats two subs at 25%/75% of room width is all you need to get very good seat to seat consistency.
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At least in the subwoofer range multiple subs placed correctly will prevent some room modes from even existing in the first place because they are being driven destructively.

To illustrate the concept:


900x900px-LL-32b9aeb4_SelectiveModeCancellation.jpeg

Sanjay
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post #27 of 33 Old 07-30-2013, 03:46 PM
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Originally Posted by Nyal Mellor View Post

At least in the subwoofer range multiple subs placed correctly will prevent some room modes from even existing in the first place because they are being driven destructively. EQ takes care of the rest since room resonances are minimum phase - correct FR, time domain follows.

Elsewhere (80Hz+) bass traps definitely required because a center speaker playing by itself is going to drive many room modes with resultant FR peaks and time domain ringing (slow SPL decay).

I'm enjoying the wonderful effects availed to us via the multi sub approach. Myself, I've experimented with selective mode cancelation, nearfield sub-woofing, and nearly every combination of placement and time alignment possible. I've tried many, many things.

Nyal, all due respect. but the problem isn't the amount of room modes, it's the lack of them actually. And someone can correct me if I'm wrong, however I believe the FR, follows the time domain,... not vice versa.

All the best

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Originally Posted by Nyal Mellor View Post

Markus.zip 909k .zip file

Have fun! This was from a preliminary calibration of the subwoofer array in my HT back in Jan this year. No EQ and it was also before most (all? ... I forget now) of the acoustic treatment was installed.

Thanks Nyal, fantastic results. The watefall looks like some absorption is present though. In general you guys in the US get good bass performance due to flimsy wall construction smile.gif
Would be interesting to see the end result in your finished theater and how you've dealt with the excess group delay peak at 75Hz.

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Originally Posted by FOH View Post

And someone can correct me if I'm wrong, however I believe the FR, follows the time domain,... not vice versa.

Please see http://www.hometheatershack.com/roomeq/wizardhelpv5/help_en-GB/html/minimumphase.html

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post #30 of 33 Old 07-31-2013, 10:02 AM
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Originally Posted by markus767 View Post

Thanks Nyal, fantastic results. The watefall looks like some absorption is present though. In general you guys in the US get good bass performance due to flimsy wall construction smile.gif
Would be interesting to see the end result in your finished theater and how you've dealt with the excess group delay peak at 75Hz.

I think the ceiling absorber was in at that point (3" of OC705 over most of the ceiling) but nothing else. The walls are actually double drywall with GG in between on hat channel on isolation clips. They do help absorb bass below 60Hz or so and the DD with GG prevents drywall ringing / resonance that you often see.

When the theater is finished finished (still tidying up some loose ends and redoing some things I am not happy with) I'll post all the measurements in a blog post over at my site.

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