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post #1 of 23 Old 11-17-2016, 07:21 PM - Thread Starter
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Modal Analysis

I'm working toward a bass treatment plan for my under-construction basement home theater. Notice I say working toward a plan - that is not just to say that I don't have a plan at present, but also that I don't expect to have a plan until I have learned about what my room is actually doing. That's what I've come here for.

The theater is heavily built as a room within a room (more or less). The walls and ceiling are all fully independent from the other structures of the house and sheathed with 1/2" OSB and 2 layers of 5/8" drywall, all coated in Green Glue sound absorbing compound. The floor is poured concrete, as are the foundation walls which surround the screen end of the to a height of about half the wall (though the walls do not touch the concrete). It's about 22 feet long, 12 feet wide, and almost 9 feet tall. There is a 14" tall riser in the rear 40% (approx) of the room, not prepared for bass trapping (mostly filled with sand), and a 14" tall stage at the screen end of the room (completely filled with sand). There are soffits running down both sides of the room at the ceiling enclosing air ducts - nearly 1 foot tall by 1.5 feet wide. Here's an image of the front of the room as it stands now. You can see a lot more details by wading through the build thread in my forum signature.



Do I have a plan at all? Well, yes. Here it is:
  1. four subwoofers. 3 cubic foot sealed boxes with Dayton Audio UM 15-22 drivers (1 each). Each sub gets it's own channel of (2) EP4000 amps. Locations are pretty well locked into the corners.
  2. I don't have EQ and processing picked out, but I expect to need something - potentially something from MiniDSP.
  3. Passive treatment begins with a perforated helmholtz trap. 25 inches deep, 96 inches wide, and 40 inches tall. It doubles as a speaker stand for LCR. I have not yet finalized optimal perforation pattern or insulation sizing and spacing, but I expect strong absorption from about 70-80 Hz up to 180-200 Hz. The trap takes up 2/3 the width of the screen wall behind an acoustically transparent screen - almost 25% of the total area of the front wall of the room. This is not in place yet.
  4. LCR speakers are DIYSoundgroup Cheap Thrills, which may not be the strongest in low end output, but are still by most any reasonable standards very capable - sealed design with 15" pro woofers. F3 is near 80Hz.

So what's the question?

The first question I have is what are the axial modal resonances? What are their frequencies and bandwidths? I think I have enough data to answer that question for the length of the theater (not the vertical or lateral modes, only the longitudinal (? is that the right word? Axial length?).

I used a Behringer ECM8000 mic and external USB sound card with REW to collect data from a series of sweeps. All the settings here are default for REW: I didn't adjust sweep duration or windowing or anything like that. No smoothing has been applied. SPL is not calibrated, and no mic or soundcard calibration file was loaded. For these measurements, one sub was driven - no mains. The sub is in the front left corner of the room. The room is mostly empty - just speakers and a few boxes laying around - no furniture, no carpet, no exposed insulation. The mic placement is the only parameter changing from one sweep to the next, and the mic placement only changes in one direction: it moves steadily from the left rear corner toward the front left corner, about 9-10 inches per sweep. The plots presented here represent movement of the mic just beyond 50% of the length of the room, always the same distance from the ceiling (not always form the floor - I extended the mic stand when I moved the stand from the riser to the floor), and always the same distance from the wall - about 5 inches. The overall highest output was the first measurement, nearest the corner. The output get steadily lower as I proceeded away from the corner.



Of particular interest to me while getting started is the set of mode markers I added via the EQ functions of REW. I set them based on what I was reading from the plots, not based on the physical dimensions of the room. The green marker at 52 Hz represents the first axial mode in the width (left-right) dimension. Since the output was very consistent in that range and peaked at 52 Hz, I adjusted the width setting in REW until that predicted resonance coincided with the peak - 52Hz. In the same way, I set the red marker at 57Hz for the secondary axial (length) mode. When the mic was in the corner, output at that frequency was a maximum. When the mic was near the center of the room, I was in or near a null at that frequency.

The things that trouble me are two: First, while it looks to me like I have set those two markers at the correct locations, based on performance in that range, outside that range I see very little further agreement between the modal markers and the actual peaks and nulls. Notice that if the secondary axial length mode is centered at 57Hz as I have placed it, the primary must have been around 28-29 Hz, but my measured peak is lower than that - about 26Hz. Second, and not plainly visible in this, is that the physical dimensions associated with the markers I have placed on this plot are smaller than my actual physical dimensions. I expected them to err in the other direction. I have come to expect (via reading, not via practice) that rooms actually resonate at frequencies lower than their dimensions would predict. Is this not true? Do online calculators like the one built into REW make assumptions about room contruction that my room violates? Have I misread the plots and set the markers wrong?

I'm in this for the long haul to fully understand what my room is doing in the hope of learning some things, like how effective can one bass trap be, and how much can I gain moving from one sub to several - how does phase effect that, and so on.

Please dive in with whatever you see, and I will keep posting as I collect and analyse more data. I expect over the next week or so to add data that should illuminate behavior in other directions - width and height. Once I have developed an understanding of the raw performance of the room in all three dimensions, I'll start adding other variables, like second, third and fourth subs, then the helmholtz trap, and eventually EQ. Once I am satisfied with the low end, I will move on to mains integration (absent persuasive advice to the contrary), and eventually to midrange and high frequency assessment and treatment - probably in other threads.

Thanks for any thoughts you have about this.

Fred
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post #2 of 23 Old 01-11-2017, 11:42 PM
 
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Fred you didn't seem to get a good reply, I can give a shot when I am not on my mobile. Did you run one of the modal charts to compare to your measurements?

Do you measure only subs? Or speakers too? Sometimes speakers show SBIR and other problems above 80hz.
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post #3 of 23 Old 01-12-2017, 06:43 AM
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Google "room mode calculator" to find various online calculators. I have a program but it is in Mathcad (or Matlab, I forget) and somewhat user-hostile. Here is one: http://www.bobgolds.com/Mode/RoomModes.htm There is another on the RealTraps site: http://realtraps.com/modecalc.htm

To optimize multiple sub placement one of our own members, @andyc56 , has developed an excellent program for that. See the thread in the subwoofer forum: Optimizing subwoofers and integration with mains: multi sub optimizer and the downloadable program is here: http://andyc.diy-audio-engineering.org/mso/html/

HTH - Don
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post #4 of 23 Old 01-12-2017, 07:27 AM
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Originally Posted by DonH50 View Post
Google "room mode calculator" to find various online calculators. I have a program but it is in Mathcad (or Matlab, I forget) and somewhat user-hostile. Here is one: http://www.bobgolds.com/Mode/RoomModes.htm There is another on the RealTraps site: http://realtraps.com/modecalc.htm

To optimize multiple sub placement one of our own members, @AnD cy56, has developed an excellent program for that. See the thread in the subwoofer forum: Optimizing subwoofers and integration with mains: multi sub optimizer and the downloadable program is here: http://andyc.diy-audio-engineering.org/mso/html/

HTH - Don

Don, Thanks very much. I still have bass issues in this room and still not completely happy. As soon as I figure out what I'm looking at, this may well help me.

I've got the L & R placed precisely where I want for soundstage, imaging, depth, height, clarity, focus and all those metrics, but still not there with the bass. Bass management is not available for me so still running the L & R full range so of course that can cause conflicts with the two stereo subs. I have no way to really 'fix' anything other than correct placement. Dipoles don't make things any easier.

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post #5 of 23 Old 01-12-2017, 08:39 AM
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No problem, Scott, hope it helps. As you know I have dealt with dipoles for decades as well and they can be a trial. My current subs have continuous phase control and a single-band PEQ that were invaluable in dialing them in, and I still had to wait until putting another pair of subs in the room before really taming a fundamental room mode. Bad rooms are, well, bad!

Anytime you move the mic an inch and the sound changes by a mile you are likely dealing with room modes, SBIR, or other comb filter effects that can be challenging to fix. The rewards are great, however.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #6 of 23 Old 01-12-2017, 08:58 AM
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Originally Posted by HopefulFred View Post
The first question I have is what are the axial modal resonances?

Sanjay
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post #7 of 23 Old 01-12-2017, 09:42 AM
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No problem, Scott, hope it helps. As you know I have dealt with dipoles for decades as well and they can be a trial. My current subs have continuous phase control and a single-band PEQ that were invaluable in dialing them in, and I still had to wait until putting another pair of subs in the room before really taming a fundamental room mode. Bad rooms are, well, bad!

Anytime you move the mic an inch and the sound changes by a mile you are likely dealing with room modes, SBIR, or other comb filter effects that can be challenging to fix. The rewards are great, however.

Thanks. This exact same system had wonderful bass at the house I just moved from two years ago. In this house all the other metrics are far improved except for down there and down there is a hilly road with pot holes and speed bumps. I had many pairs of Tympanis back in the day, then Dayton Wright XG8 mk2s then later, big ML Statements, then later yet, big full range Sound Labs, A-1s and M-1s so I know what bass can sound like, but I'm not there yet.

I did the old sub woofer crawl again the other night and managed to move the subs out another couple of inches and pick up quantity, but lost detail and speed so moar bass, but a wooly mammoth instead of tight and lean.
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Thanks. This exact same system had wonderful bass at the house I just moved from two years ago. In this house all the other metrics are far improved except for down there and down there is a hilly road with pot holes and speed bumps. I had many pairs of Tympanis back in the day, then Dayton Wright XG8 mk2s then later, big ML Statements, then later yet, big full range Sound Labs, A-1s and M-1s so I know what bass can sound like, but I'm not there yet.

I did the old sub woofer crawl again the other night and managed to move the subs out another couple of inches and pick up quantity, but lost detail and speed so moar bass, but a wooly mammoth instead of tight and lean.
I hate the wooly mammoth!
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post #9 of 23 Old 01-12-2017, 11:24 AM
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I hate the wooly mammoth!

Welcome back!
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Welcome back!
Thank you! I had a nose bleed...
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post #11 of 23 Old 01-12-2017, 11:51 AM
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Thank you! I had a nose bleed...

Must have been a mighty big nose!
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Must have been a mighty big nose!
Well, it was more a case of the finger picking it, being much bigger than the nostril...
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Well, it was more a case of the finger picking it, being much bigger than the nostril...

Ewwww. A big bloody booger!
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post #14 of 23 Old 01-12-2017, 06:10 PM - Thread Starter
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Fred you didn't seem to get a good reply, I can give a shot when I am not on my mobile. Did you run one of the modal charts to compare to your measurements?

Do you measure only subs? Or speakers too? Sometimes speakers show SBIR and other problems above 80hz.
I appreciate you bringing this thread back up - I need to get back into this process.

As I described in the first post, this is sub only - no speakers.

I have run dozens of model predictions. What I want is to understand how they are right and wrong. I'll get into that in a following post.
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Thanks for the links Don. I was unaware of Andy's work. I'll get into that soon.
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post #16 of 23 Old 01-12-2017, 06:40 PM - Thread Starter
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Sanjay's image is a useful visual reference. I have done the same thing Sanjay has, using the EQ functions built into REW. That places markers on the plots indicating the frequencies of the predicted resonances. What I want to understand is what the actual, measured resonant frequencies are. Here's my measurements (same raw measurements as above) but with revised predicted modal resonances that match Sanjay's



Notice Sanjay has predicted the primary axial length modal resonance at 26 Hz, just as is marked by REW with the red marker. Given how these plots were created, we should see clear evidence of that resonance as the measurement location changes moving down the length of the room. By my reading of the plots, the actual resonance is slightly higher in frequency than predicted - maybe 27-28Hz

Working from left to right through the plots, the second red marker (third overall marker) represents the secondary axial length mode - predicted both by the calculator Sanjay used and the REW calculator to be just above 50 Hz (double the frequency of the primary mode). Using Sanjay's image as a reference, notice that 51Hz should show a peak in response when measured in the corner (the top trace in my plots). The measurement taken at 1/4 of the room length (labelled 5 2/4 in Sanjay's diagram) should show a null. The measurement taken at the center of the length of the room should again be a peak, the same as the measurement take in the corner. Can you see evidence of that in my measurements? Where? or if not, why not?
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post #17 of 23 Old 01-12-2017, 08:54 PM
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No problem, Scott, hope it helps. As you know I have dealt with dipoles for decades as well and they can be a trial. My current subs have continuous phase control and a single-band PEQ that were invaluable in dialing them in, and I still had to wait until putting another pair of subs in the room before really taming a fundamental room mode. Bad rooms are, well, bad!

Anytime you move the mic an inch and the sound changes by a mile you are likely dealing with room modes, SBIR, or other comb filter effects that can be challenging to fix. The rewards are great, however.

"My current subs have continuous phase control"

Now that's an idea!!! I only have 0, 90 and 180. I played around a little tonight and an inch makes a difference. It's the speed versus the panel that always drives me nuts.
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post #18 of 23 Old 01-13-2017, 04:49 AM
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@Scotth3886 : I use Rythmik. In addition to a nice set of controls they are servo, which means less ringing and deeper extension than some other subs, and better integration with "fast" planar speakers. The phase control helps align the wavefront between sub and mains with less (not "no"!) sensitivity to placement. It allows me to place the subs where they sound best in the room and still align the subs to the mains.

Small movements affect how the sub interacts with the mains and the room. Lot of variables, lot of time tweaking, to get that final sound that's so nice with 'stats (and Maggies, natch!)

I noticed in your earlier post that we have listened to a lot of the same speakers, though I think I am a bit younger (57) and did not own most of them, but the list made your "wooly mammoth" comment all the more apropos.

As for driving me nuts, well, yeah, but it's a short drive.

Enjoy! - Don

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post #19 of 23 Old 01-13-2017, 04:57 AM
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Originally Posted by HopefulFred View Post
Sanjay's image is a useful visual reference. I have done the same thing Sanjay has, using the EQ functions built into REW. That places markers on the plots indicating the frequencies of the predicted resonances. What I want to understand is what the actual, measured resonant frequencies are. Here's my measurements (same raw measurements as above) but with revised predicted modal resonances that match Sanjay's

<pix elided>

Notice Sanjay has predicted the primary axial length modal resonance at 26 Hz, just as is marked by REW with the red marker. Given how these plots were created, we should see clear evidence of that resonance as the measurement location changes moving down the length of the room. By my reading of the plots, the actual resonance is slightly higher in frequency than predicted - maybe 27-28Hz

Working from left to right through the plots, the second red marker (third overall marker) represents the secondary axial length mode - predicted both by the calculator Sanjay used and the REW calculator to be just above 50 Hz (double the frequency of the primary mode). Using Sanjay's image as a reference, notice that 51Hz should show a peak in response when measured in the corner (the top trace in my plots). The measurement taken at 1/4 of the room length (labelled 5 2/4 in Sanjay's diagram) should show a null. The measurement taken at the center of the length of the room should again be a peak, the same as the measurement take in the corner. Can you see evidence of that in my measurements? Where? or if not, why not?
I don't have time to wade through this now but note the calculators assume a uniform room with perfectly reflective (infinitely rigid) surfaces. Any deviation from that will change the actual nodal parameters, including depth, width, and frequency. Any openings can create a resonance point, anything in the room like furniture or tables can change the frequency, my room has one angled wall and a dropped ceiling duct path on one side that complicates the analysis, normal walls flex a little so the nulls will not be as deep and they may change in frequency a bit, etc. Etc. Etc.! To do a really thorough analysis is a job for a flow model simulator like COMSOL with a bunch of material parameters to enter into the model. There are also some specialized acoustics programs for room analysis that are pretty slick but pretty pricey -- I have used a few in years past but do not currently own one. I Use Mathcad or Matlab to model but it's really just to get an idea, then measure.

HTH - Don
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post #20 of 23 Old 01-13-2017, 05:35 AM
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@Scotth3886 : I use Rythmik. In addition to a nice set of controls they are servo, which means less ringing and deeper extension than some other subs, and better integration with "fast" planar speakers. The phase control helps align the wavefront between sub and mains with less (not "no"!) sensitivity to placement. It allows me to place the subs where they sound best in the room and still align the subs to the mains.

Small movements affect how the sub interacts with the mains and the room. Lot of variables, lot of time tweaking, to get that final sound that's so nice with 'stats (and Maggies, natch!)

I noticed in your earlier post that we have listened to a lot of the same speakers, though I think I am a but younger and did not own most of them, but the list made your "wooly mammoth" comment all the more apropos.

As for driving me nuts, well, yeah, but it's a short drive.

Enjoy! - Don

"I think I am a but younger "

Everyone is younger than me!!

It can't be any more obvious from my posting that I'm about as far from an EE as you can get and getting so stinkin' old that it's getting hard to learn new ways of doing things technical. This is why I'm pretty damn good with turntables and analog and the 'old way' and have my head up my ass with regards to anything post war. I have deep experience, but very narrow. I do think I have a very experienced and keen set of ears and still have it although nothing above 14k and I've been able to always get this to work in the past.

For subs I have two of the Martin Logan W700s, which sounded great in the last house, but (very) challenging here. True, I've got about 15hz more extension using the subs, but I lose speed which has always been the nemesis using cone subs with an ESL. I was set up on the narrow wall there, but lost some of the imaging magic these are capable of, but on the wide wall here and certainly have those other soundstage metrics in abundance. Christ this hobby drive me nuts. What a disability to be born an audiophile!
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post #21 of 23 Old 01-13-2017, 12:00 PM
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One thing often overlooked in subwoofer integration with the mains is that the most important place to get phase aligned is right at the crossover frequency. You want the signal from the sub(s) and mains to all line up together at your ears. Above that frequency, the subs are dying away and matter less; below that, the mains are fading and so again it matters less and less the further away you get from the crossover frequency. You can play a tone at the crossover frequency and adjust for maximum volume at the listening position as a crude test; an SPL meter will be better, but since the room gets into play, measuring and looking at the frequency and time domain responses is the best way.

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post #22 of 23 Old 01-13-2017, 02:45 PM - Thread Starter
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Any deviation from that will change the actual nodal parameters, including depth, width, and frequency. Any openings can create a resonance point, anything in the room like furniture or tables can change the frequency,[snip] normal walls flex a little so the nulls will not be as deep and they may change in frequency a bit, etc. Etc. Etc.! To do a really thorough analysis is a job for a flow model simulator like COMSOL with a bunch of material parameters to enter into the model. There are also some specialized acoustics programs for room analysis that are pretty slick but pretty pricey -- I have used a few in years past but do not currently own one. I Use Mathcad or Matlab to model but it's really just to get an idea, then measure.
Indeed. I was expecting in fact that my slightly less than infinitely rigid walls would contribute to the modal resonances shifting to slightly lower frequencies, though I don't have a clear idea of how my doorway and soffits and stage may be influencing things. Interestingly, this is the opposite of what I think I am seeing in most of my measurements at most frequencies. I have more measurements - one set anyway - which I have not shared here yet.

One of the primary reasons I started this thread and this process was to understand what was actually happening in terms of pressure at various points in the room - mainly the listening position and at the wall boundaries where treatments are practical - so that I can use that information to help inform my choices of treatment. For example: if a peak in the response at my seat is the result of a tangential or oblique modal resonance an absorber in the middle of a wall will be much less useful.

The predictive models are useful as guides and for learning tools, but cannot exactly explain the performance of subs in any real room.
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I think I remember the equations but am not willing to put them in a post without looking them up. They are in Everst, my old grad acoustics text, and online plus there are online calculators that list the frequencies for each mode so you might see if you can find something that helps. I usually look at the null frequency(ies) to calculate a distance (vs = 1127'/s) to get me close enough to determine the path length so I can relate to a room dimension. Obviously if two or more dimensions are close it may not help, and openings add a Helmholtz resonator effect that breaks the simple equations.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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