Helmholtz resonators and frequency heterogeneity in the HT - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 222 Old 07-10-2003, 05:36 AM - Thread Starter
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I'm sure this will be no great revelation to many of you guys/gals, but I found this interesting and wanted to share what I've learned. :)

I've been researching, over the last couple of days, ways of dealing with frequency peaks and nulls within a HT, particularly the low frequencies produced by subwoofers. Some have advocated precise seat placement:

http://www.hometheatermag.com/printarchives.cgi?137

and

http://www.hometheatermag.com/printarchives.cgi?140:6

In these articles they talk about calculating harmonics, peaks, nulls, and seating position based on a rectangular room with perfectly reflecting surfaces. They then say place the seat so that your head is in not in a null. However, if you move even just a little bit, your head could be in a null. I've noticed this many times before. So keep your perfect posture throughout a 2 hour movie! :rolleyes: But as Dennis has pointed out soooo many times with regards to SGHT's advocated spreadsheet, the surfaces are not perfectly reflective, and the addition of stages, soffits, risers, screens, etc will make peak and null location prediction exceedingly difficult. Besides, I like recliners!

Another route is speaker placement. It is not practical to place a full range speaker in every possible null position, there are so many, and the nulls and peaks are so close together above a certain frequency that it is irrelevant. However, subwoofer placement is somewhat practical as described in:

http://www.harman.com/wp/pdf/multsubs.pdf

and seems to effectively "smooth out" the low frequency peaks and nulls across a seating area. They found the best placements were symetrical, with either 2 or 4 subs. The 2 sub arrangement amounted to one placed at the 1/2 point on the front and back walls. The four sub arrangement was (1) one in each corner, (2) one at the 1/2 point on each wall, or (3) one at each of what appears to be the 1/4 and 3/4 points of the front and back walls. The two sub arrangement puts them at the null positions for the first resonance freqency in one direction. In the 4 sub arrangements, arrangement (2) puts them in the null positions for the 1st resonance frequency in two directions, while arrangement (3) puts them in the null positions for the second resonance freqency in two directions. I don't really understand how arrangement (1) is effective. So, I could buy either 2 or 4 subwoofers and place them in various positions with the likelihood one of them would be centered on the front wall below my screen. I'm not sure I would like that or having to buy four additional subs. However, I built my own subs, and building 2 more might not be too bad.

Then I ran across a thread where Dennis mentioned how columns, stages, and risers could be used as helmoltz resonators. :confused: So I decided to research them and found these:

http://www.avsforum.com/avs-vb/showt...oltz+resonator

and

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

After reading them, and if I correctly understood them and what Dennis was intimating, these replace and function as well as subs in places within the room. The amazing thing was they could be as simple as well sealed cardboard boxes with a port. The only trick it seemed, was making sure they were tuned to the correct frequency, which is what the second link is about. I believe I could now build a helmoltz resonator for the appropriate resonance frequency. The question is, appropriate placement given all the room angles, and will they really function as well as a subwoofer to deal with in-room frequency heterogeneity? I'm not saying they replace the function of a sub, but used in addition to the subs.
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post #2 of 222 Old 07-10-2003, 08:29 AM
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Buph,

None of those are effective ways to solve low frequency room problems. As you observed, speaker placement can help one location, but not others. Even then it does nothing to reduce low frequency reverb time that muds up the sound. Likewise for Helmholtz traps. They are very effective at one frequency, but do nothing for all the other problem frequencies.

In truth, all rooms need absorption at all low frequencies, not just those that correspond to the room's dimensions. The real cause of peaks and dips that vary around the room is acoustic interference. This is created when sound waves reflect off the walls, floor, and ceiling and collide with each other and with the direct sound from the loudspeakers.

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post #3 of 222 Old 07-10-2003, 09:09 AM
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As can be found in the work of Toole and Olive, general bass broadband absorption is significantly frowned up and is highly counterproductive. Equally problematic with general purpose absorption of this nature is you simply end up chasing your tail. Furthermore intermodulation distortion artifacts will occur at any time (aka at all times) unless you have an anechoic chamber and at all frequencies. On the other hand, that's exactly the way it would occur at a live performance! Understand, the playback environment you're attempting to duplicate is that of a larger room (theater, concert hall, etc.).

To determine which frequencies are modal in a room you really need to take measurements once the room is finished with the furniture (particularily the seating) installed. Using a real time analysizer (certainly something more than an SPL meter), place your microphone at the size wall. Any peak is a mode. Repeat at the rear (or front wall)...again, any peak is a mode and repeat again at the floor or ceiling. If you place your microphone at the corner, just above the floor, you'll see all modal frequencies (a peak at the boundary will be a mode without regard to whether the mode is manifest at the seating location as a peak or null.) Armed with that information, you can now calculate what is needing for your slotted or perforated resonator.

With general purpose broadband absorption, you're absorbing all frequencies ... those that need absorption, those that are at a perfect level already, and those that need to be boosted. You're using fiberglass to create heat from mechanical energy. To then solve that problem you add subs and amps which brings you back to where you started. Next step, more amps and subs.

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post #4 of 222 Old 07-10-2003, 10:27 AM - Thread Starter
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Dennis, I understand what you are saying with regards to actually measuring the modal frequencies. However, it is almost impractical to assess the real world situation each time a decision needs to be made.
When you design a theater for somebody, do you attempt to model the future HT taking into consideration all the contributions from stages, columns, riser, carpet, chairs, etc? Or do you defer until you have the information from a real time analyzer?
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post #5 of 222 Old 07-10-2003, 11:45 AM
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I design the room taking into account all best available information, past experience and various modelling processes. What should follow in the final build/install stage are those items necessary to fine tune since no prediction method can be either 100% reliable nor 100% accurate. Further, various prediction models tend to be less and less accurate as room size decreases (particularily to residential sized rooms).

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post #6 of 222 Old 07-10-2003, 12:29 PM
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Dennis,

> With general purpose broadband absorption, you're absorbing all frequencies ... those that need absorption, those that are at a perfect level already <

Of course I'm talking about the range of low frequencies, not all frequencies.

But I'm really surprised to hear you say that "general bass broadband absorption is significantly frowned upon" because that simply is not true in any of the professional recording and mastering studios I know of or have dealt with. In fact, broadband bass trapping is the only way to achieve a flat response throughout the entire low end in a "normal" sized room.

The real issue is acoustic interference, as explained above, which causes severe peaks and dips in the low frequency response. In larger rooms low frequency reverb is an additional problem that requires the same absorption solution.

If you have any relevant reference links to share I'd love to see them. Thanks.

--Ethan
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post #7 of 222 Old 07-10-2003, 06:22 PM
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Quote:
But I'm really surprised to hear you say that "general bass broadband absorption is significantly frowned upon" because that simply is not true in any of the professional recording and mastering studios I know of or have dealt with.
I have been involved with recording and recording/mastering studios as well...as far back as the first direct to digital recording. The key Ethan is that the requirements for a recording studio are entirely different than a playback environment. Significantly different. Among the better ways to screw up a playback environment is to build it like a recording studio. Indeed, the acoustics should be variable...the reverberation required for Segovia was different than Yma Sumac or the Cleveland Winds.

Quote:
The real issue is acoustic interference, as explained above, which causes severe peaks and dips in the low frequency response.
Not entirely the case. Modes will create peaks and dips all by themselves. Modes are a unique form of reverberation (reverberation but with separate and unique characteristics). Modal interaction is defined as the interaction between two different modes or two differental modal frequencies. Modal interaction is one reason predicted results do not map to real results until you measure the modes as I've described previously. Once that is resolved, you're now in a position to determine if you have remaining undersireable artifacts...noting that all rooms, at all frequencies have such interactions (including the best concert halls on the planet) and that is, in part, what contributes to their richness. You do not solve such frequency specific problems by throwing out the baby with the bath water. Modes are a reverberation problem in both large and small rooms albeit less so as the room becomes larger. Bob Stuart has published some excellent work in this area.

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post #8 of 222 Old 07-11-2003, 07:26 AM
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Quote:
Originally posted by Dennis Erskine
To determine which frequencies are modal in a room you really need to take measurements once the room is finished with the furniture (particularily the seating) installed. Using a real time analysizer (certainly something more than an SPL meter), place your microphone at the size wall. Any peak is a mode. Repeat at the rear (or front wall)...again, any peak is a mode and repeat again at the floor or ceiling. If you place your microphone at the corner, just above the floor, you'll see all modal frequencies (a peak at the boundary will be a mode without regard to whether the mode is manifest at the seating location as a peak or null.) Armed with that information, you can now calculate what is needing for your slotted or perforated resonator.
Hi Dennis,

In determining which frequencies to build your slotted or perforated resonators, why wouldn't you take your real time analyzer readings at the seating locations rather than the room boundaries?

By the way, I really enjoyed your articles in this month's Home Theater Builder magazine, especially the one on columns which touches on the perforated resonator design. In the article you refer the reader to the Internet to obtain the appropriate formulas. I found this web page, LOW - MID ABSORBERS. Here's an excerpt with regard to a formula:

Quote:
Another form of helmholtz resonator is created using perforated plywood - i.e. plywood with hundreds of holes in it. We call it pegboard in Oz, you see it in hardware stores holding up tools etc. If you place a panel of this over an air cavity like in a panel absorber not only do the little holes act like bottle necks the whole panel acts as a low frequency panel absorber!

The formula for calculating the helmholtz resonant frequency is:

f = 2160 x sqrt ( r / (( d x D ) + ( r + w )))

Where:

f = resonant frequency in Hertz (Hz)
r = slot width.
w = slat width.
d = effective depth of slot. (1.2 x the actual thickness of the slat)
D = depth of box.
Would this be the appropriate formula to use for both a slotted or perforated resonator?

Thanks.

Larry
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post #9 of 222 Old 07-11-2003, 08:07 AM
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Dennis,

Thanks, this is all interesting stuff.

> Indeed, the acoustics should be variable <

You mean in a recording studio room, not a home theater, right? I probably wasn't clear enough because what I really meant was a control room, where recording engineers make mixing and mastering decisions. Not where the performance is recorded and where being able to vary reverb time can be useful. But I'm still not following you here:

> Modes will create peaks and dips all by themselves. <

Modes are resonances, and so will give a longer reverb time and also an increase in volume. I don't see how a mode could create a dip in the response. Either way, only absorption can tame the excess reverb and resonances.

The real issue in most home-sized listening rooms is the skewed frequency response caused by acoustic interference. This is due to reflections off the walls, floor, and ceiling combining with each other and with the original sound coming from the loudspeakers.

If you know a way to make a room have a flat response throughout the low end and in all seating positions without broadband bass trapping, please let me know!

--Ethan
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post #10 of 222 Old 07-11-2003, 08:24 AM
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Quote:
Originally posted by formerly HTbuph
After reading them, and if I correctly understood them and what Dennis was intimating, these replace and function as well as subs in places within the room. The amazing thing was they could be as simple as well sealed cardboard boxes with a port. The only trick it seemed, was making sure they were tuned to the correct frequency, which is what the second link is about. I believe I could now build a helmoltz resonator for the appropriate resonance frequency. The question is, appropriate placement given all the room angles, and will they really function as well as a subwoofer to deal with in-room frequency heterogeneity? I'm not saying they replace the function of a sub, but used in addition to the subs.
Hi formerly;

This is a great thread that you started!

In response to your question regarding resonator placement, my guess is that in most dedicated home theaters we won't have a lot of latitude in the placement. What I mean is that aesthetics and function dictate the locations of stages, risers and columns. Most enthusiasts would want to hide the resonators in these structures rather than scattering cardboard boxes embedded with toilet paper rolls around the room. ;) Sorry I just couldn't resist.

However, its possible that placement might not be all that critical in the first place. If we've got resonators hidden in these structures and tuned to reduce problematic frequencies, would it really hurt anything where in the room the absorption was occurring so long as it was sufficient at the listening position to correct the problem?

Larry
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post #11 of 222 Old 07-11-2003, 10:03 AM - Thread Starter
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Larry,

Just think what you could do with all those left over toilet paper cylinders....and think about the possibilities of using paper towel cylinders! ;) I wasn't advocating using cardboard boxes, only pointing out that they didn't appear to be difficult to build as long as you know the appropriate box and port dimensions.

I haven't read Dennis' article in HTB magazine. I'll have to look for it. Is it on-line anywhere?

I don't know that it is necessary to have the resonator in the middle of the room, but rather on the boundaries of the rectangular space where the problem mode exists. That is why they can be disguised as columns. I guess if I wanted one in the middle of the room somewhere, I'd use one of the spaces between the 2X8s, 2X6s, or whatever lumber used in the riser. Of course the space would need to be sealed except for the port, and then put a little grill over the port opening.

But still, determining the right frequency(s) and position...?
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post #12 of 222 Old 07-11-2003, 10:56 AM
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Quote:
Originally posted by formerly HTbuph
I wasn't advocating using cardboard boxes,...
I know.:D

Quote:
I haven't read Dennis' article in HTB magazine. I'll have to look for it. Is it on-line anywhere?
No, the magazine is by subscription. You may be able to buy it directly from their web site, or by calling them.

Quote:
I don't know that it is necessary to have the resonator in the middle of the room, but rather on the boundaries of the rectangular space where the problem mode exists.
This sort of gets back to my question to Dennis regarding where the RTA measurements should be taken. To me the only relevant problem modes are those affecting the listening positions which, for the most part, should be toward the center of the room away from those boundaries.

Quote:
But still, determining the right frequency(s) and position...?
Despite the difficulty it may entail, I'm inclined to agree with Dennis that the only meaningful way to accurately determine the right frequencies is to measure them with an RTA. I've dabbled with some acoustic software in my home theater to plot the frequency response of the room at the listening positions. As you pointed out, those simplistic spreadsheets didn't accurately predict the peaks.

As for the positioning the resonators, hopefully Dennis will weigh in here, but I think its more important to get properly tuned resonators somewhere in the room to absorb the peaks, and less important where their precise locations are.

Larry
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post #13 of 222 Old 07-11-2003, 11:53 AM
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Larry,

> I think its more important to get properly tuned resonators somewhere in the room to absorb the peaks, and less important where their precise locations are. <

The best place to install any absorption is where the reflections occur. Low frequencies build up the most in the room corners, so that's always the best place to put bass traps.

--Ethan
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post #14 of 222 Old 07-11-2003, 02:10 PM
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Modes do not build up in the corners! They don't today, they didn't yesterday and, unless the laws of physics are changing, they won't tommorrow either.

Room modes are created by standing waves (NOT acoustic interference*). Standing waves are always at peak pressure at the room boundary (wall, floor, ceiling). At the seating position, the standing wave, or mode, can be either a null or a peak. If you have a peak, a null, or something in between at a seating locating, that mode will be at its peak at the boundary. A microphone at the seating location can identify those frequencies which are at a peak or null at the microphone location....but a mode that happens to be in between its peak and null phase will not show up. Thus, placing the microphone at the boundary, all modal frequencies will appear as peaks and can be identified (you get to identify which of those you care to muck with).

Because the mode that is causing the problem at the seating position is at its high pressure point at the boundary, that is the ideal location for a Helmholtz or similar resonator. (If you have no, or low pressure, the resonator ain't going to work.) The reason you measure at a wall is to confirm that the mode is as a result of that boundary. I.E., a resonator on the side wall will not be helpful if the mode you're dealing with is one created as a result of the front/back walls. So...to address width modes, the resonator is on the side walls, and so forth.

Modes do not "pile up", "build up", or "accummulate" in corners**. Corners are not "bass magnets" or magic repositories for bass frequencies. Corners, as some would want to you to believe, do not generate energy (only the speaker does that). The corner is the intersection of *all* the boundaries creating the modes. Therefore, you can measure all modes (without respect to their being width, length or height originating) with the microphone in the corner. By the same token, since the corner is the intersection of all the boundaries, you can place your resonator(s) in the corners and deal with all modal frequencies in those (four) locations.

*"Acoustic interference", if this references to the interaction between frequencies, (a) does not go away with absorption; and, (b) exists at any time you have either two or more frequencies and/or two or more sources of sound in the same physical space (be that inside, outside, big rooms, concert halls, or cardboard boxes). You can, however, get rid of "acoustic interference"...just turn off the system or send the musicians on a break.

**Remember the 'bean bags' people would put in the corners between the two walls and the ceiling? Well, they don't (and did not) do squat for bass response. That location is a right tricorner and it has a negative impact on high frequency room response.

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post #15 of 222 Old 07-11-2003, 05:42 PM
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Quote:
Originally posted by Dennis Erskine

The reason you measure at a wall is to confirm that the mode is as a result of that boundary. I.E., a resonator on the side wall will not be helpful if the mode you're dealing with is one created as a result of the front/back walls. So...to address width modes, the resonator is on the side walls, and so forth.
Hi Dennis,

Thanks for the response. I now undertstand why measuring modes at the boundaries assists us in confirming the modal frequencies effecting the listening position.

With regard to positioning resonators, while I understand that a side wall resonator might not be effective in dealing with long wall modes, am I correct that the precise positioning of resonators along the appropriate walls is not critical to their effectiveness?

It would seem that if precise positioning were a necessity we would rarely be able to disguise helmholtz resonators inside columns because there are a number of other unrelated design factors that it determines where columns should be located.

Thanks.

Larry
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post #16 of 222 Old 07-12-2003, 04:36 AM - Thread Starter
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Is the 1st resonance within a room equal to 1/2 the wavelength? For example, the length of a HT is 22'. Using Pablo's spreadsheet, the 1st resonance is 24.7 Hz. The wavelength for that frequency is 45.7'

http://www.mcsquared.com/wavelength.htm

While not exactly 2x the room length, it is close enough that possible rounding errors would allow the calculated wavelength to have confidence bounds that may encompass the 22' length.

Also, how precise is the resonator? While it is supposed to be tuned to say 24 Hz, is that the only wavelength it absorbs?
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post #17 of 222 Old 07-12-2003, 05:30 AM
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Larry--

Precise positioning is not required...you are reducing the acoustic energy at that frequency from the room. Having said that, however, having the resonators more closely aligned with seating positions will have greater affect at the seating positions than less.

Formerly --

You will not often find the modal frequencies matching the predicted exactly. The speadsheets are forced to use assumptions which do not match reality. Because of the various physical properties of rooms and their boundaries, you'll find the frequency dimensions (I'm inventing a term here) are different than the physical dimensions of the space. I'd suspect you have a concrete wall or walls just on the opposite side of 2x4 framing. For example, if you can measure 24.7 Hz sounds outside your theater, that would be your first clue the 'acoustic dimension' of your room is different than the physical dimension at that frequency. In other words, there is a boundary outside your room which is acting as the boundary at that frequency.

A resonator will absorb frequencies to either side of the target frequency. How far to either side, and how much energy is absorbed, is a function of the resonator's Q. The design, and type of resonator will affect its Q value. Resonators are good choices to resolve nulls (since you cannot reasonably boost those frequencies). A parametric EQ, such as the QSC DSP3, is a an excellent tool for dealing with peaks at the listening position. One reason is the QSC software allows you to adjust the Q of the filter to match the Q as measured by your RTA.

http://www.qscaudio.com/products/dsp/dsp3/dsp3.htm

Your seating platform can make an excellent slotted resonator for ceiling/floor interactions and can serve as a 'bass trap' if you find you need general bass absorption (understand, the impacts of doing this before you do, however).

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Dennis,

> Room modes are created by standing waves. Acoustic interference ... does not go away with absorption <

Oy vey.

Okay, I won't argue this further here except to say that room modes are resonances whose frequencies are determined by the room's dimensions. Nodes are created by standing waves, and standing waves occur at all frequencies in all rooms.

Here's a direct link to the section of my Acoustics article that explains room modes:

www.recording.org/users/acoustics/#modecalc

Other parts of the article explain acoustic interference and how to get rid of it with low frequency absorption.

If you don't mind I'll move this to email.

--Ethan
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I don't have a HT built right now. But the dimensions of the space will be around 22' x 12' x 8 when the house is completed in about a year. I believe the ceiling will be vaulted up to around 9' in the middle. It will be a second story room, so no concrete walls around it.

I understand what you are saying about the physical vs sonic boundaries. But, assuming that the physical dimensions are the "acoustic dimesions", then the 1st resonance will be 1/2 the wavelength?
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Buph,

> the dimensions of the space will be around 22' x 12' x 8 <

Those are not ideal dimensions, but they would be if you shorten the 22 foot length down to 20 feet. If you want to see how the room modes change with various dimensions, download my ModeCalc program (only 56 KB - works in DOS and Windows) here:

www.ethanwiner.com/MODECALC.EXE

--Ethan
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But, assuming that the physical dimensions are the "acoustic dimesions", then the 1st resonance will be 1/2 the wavelength?
Yes. Resonance occurs when a wave whose wavelength is twice the distance between two boundaries is reflected back onto it's own axis. This effect is observed on multiples of the fundamental frequency. (Same principle as a pipe organ.)

You may want to email Pablo and get a copy of his media room spreadsheet. (Pablo Roufogalis L. ... proufo@cantv.net)

Now, let's keep it simple and without inventing new terminology:

1. Standing waves occur at those frequencies as described above;
2. Their effect (peaks and nulls) are a result of the principle of superpositioning where the peaks are a result of constructive interference and the nulls as a result of destructive interference;
3. In the center of the room, odd numbered modes (1, 3, 5, etc.) will create a null and even modes will create a peak (2,4,6, etc);
4. Superpositioning will occur at any frequency, in any room and its causation is by any number of factors, resonance being but one of many;
5. Modal frequencies above the sub-woofer crossover point cannot be affected by moving the sub, adding subs, or using a virtual sub;
6. Modes that occur in non-seating locations are not relevant;
7. The more effective means of dealing with a modal peak is with the use of a parametric EQ with adjustable Q (ala the QSC DSP3);
8. Nulls cannot be reasonably solved by 'boosting' the acoustic output of the sub, they can however be resolved by means of a frequency specific resonator (a form of absorption) positioned at the high pressure area for that frequency (ie, the wall, floor or ceiling);
9. Modal spreadsheets will contain inaccuracies in the prediction of modal frequencies and cannot predict modal amplitude (which is what we really hear);
10. Broadband low frequency absorption is easy and cheap to install but largely counterproductive and will require larger, more powerful subs to overcome all the absorption...an advantage is you don't need to use math to sort it out and all you need to do to solve the downside is throw more money at the problem.
11. Solving all forms of superpositioning through the use of absorption will (a) not solve all forms of superpositioning (such as differences in the distance from the listening position to the speakers), (b) will become counterproductive since your speakers and amps will have to get bigger and you'll still have constructive/destructive interference; (c) ingnores diffusion; (d) ignores the fact that some forms of superpositioning are desireable; and, (e) you'll enjoy an excellent relationship with your fiberglass and component sales people.

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post #22 of 222 Old 07-12-2003, 01:28 PM
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Originally posted by Ethan Winer
Dennis,

> Room modes are created by standing waves. Acoustic interference ... does not go away with absorption <

Oy vey.

Okay, I won't argue this further here except to say that room modes are resonances whose frequencies are determined by the room's dimensions. Nodes are created by standing waves, and standing waves occur at all frequencies in all rooms.

Here's a direct link to the section of my Acoustics article that explains room modes:

www.recording.org/users/acoustics/#modecalc

Other parts of the article explain acoustic interference and how to get rid of it with low frequency absorption.

If you don't mind I'll move this to email.

--Ethan
Hi Ethan,

Thanks for the link and the interesting article.

It's obvious that you and Dennis certainly know a lot more about the subject of acoustics than I do. However, from reading your article it seems that we're running into semantical difficulties. Your use of the term acoustic interference may be contributing to this. To a layperson such as myself this is a very generic term. However, from reviewing your Figure 1 it seems to me that you are referring to a very specific form of acoustic interference whereby out of phase reflections off of walls reinforce or cancel the direct signal depending on the location in the room. This particular form of "acoustic interference" explains the physics causing standing waves. Therefore, in this context adding an absorber to a wall to reduce the out of phase reflections might help to mitigate peaks in the room's frequency response if the frequencies absorbed were due to problematic modal amplitudes in the listening area.

However, there are other forms of "acoustic interference" which may have no relevance to signals reflected off of walls and therefore can not be mitigated by bass traps. One form of "acoustic interference" is intermodulation distortion that Dennis mentions. It would appear to me that the mere presence of two or more signals could cause this type of acoustic interference. However, as Dennis suggests, this is not a problem because live venues, either in a room or outdoors, would be subjected to the same type of acoustic interference, i.e., its natural for us to hear this.

Just my layperson's read on the subject.

Larry
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post #23 of 222 Old 07-13-2003, 08:32 AM
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Larry,

> It's obvious that you and Dennis certainly know a lot more about the subject of acoustics than I do. <

I have a couple of emails from Dennis I'll answer later today, and hopefully we'll come to a meeting of the minds. Some of this may be due to one of us misunderstanding semantics, but these terms were all defined long ago. The concepts are not difficult to understand, and no math is required. :D

> Your use of the term acoustic interference may be contributing to this. <

Acoustic interference is the general case of sound waves colliding in the air. As I hope my Figure 1 and explanation in that article make clear, peaks and dips in the frequency response due to acoustic interference occur at all frequencies in all rooms.

Once a single frequency has sustained long enough - a couple of cycles - to stabilize and form a static pattern, it is considered a standing wave. When this happens there will be a null in the response 1/4 wavelength away from every room boundary, regardless of how far the boundaries are from one another. This is why it takes a few cycles for standing waves to stabilize, and why standing waves can and do occur at all frequencies in all rooms. Any place in the room where a peak or null exists due to a collision of waves is called a node.

Regardless of the room's dimensions, if you play an 80 Hz sine wave and walk around the room you will find peaks and nulls in the response due to standing waves. There will always be a null 1/4 wavelength away from a wall and a peak 1/2 wavelength away. Other nulls occur at 3/4, 5/4, 7/4, etc. wavelength away, and other peaks occur at 4/4, 6/4, 8/4, etc. wavelength away. This is very easy to prove! Just play a tone and walk around the room, especially toward and away from the walls.

Now if you nudge the sine wave frequency up to 85 Hz the exact same conditions occur, except they've moved a few inches closer to the walls. Set your sine wave to 90 Hz and they've moved a little closer. The point is that regardless of the frequency, standing waves will form and stabilize in any size room, and all that changes is where the peaks and nulls occur.

In smallish rooms like you find in most homes, these peaks and dips are a huge problem. The bass is thin here, tubby over there, and nowhere in the room is it correct because the entire low end is riddled with so many peaks and valleys. Look at my Figure 2 (top) between 70 and 200 Hz. Moving the loudspeakers can affect this to some extent, but what helps a pop tune in the key of A hurts a John Williams score in the key of Eb. That is, a speaker location that's 1/4 or 1/2 wavelength from the wall at one frequency is in a different phase at a different frequency.

This is exactly why broadband low frequency absorption is the only viable solution. Absorbing the waves rather than reflecting them reduces the acoustic interference that's at the root of these problems. And because standing waves occur at all low frequencies, narrow-band traps like a Helmholtz resonator help only one small part of the problem. They can reduce the primary room resonance, but they don't affect the many mode "harmonics" nor do they reduce the peaks and dips caused by standing waves at all other frequencies. Nor do they help the overall boominess and low frequency reverb that reduce clarity and definition of notes played by bass instruments.

> One form of "acoustic interference" is intermodulation distortion ... It would appear to me that the mere presence of two or more signals could cause this type of acoustic interference. <

Not so. Intermodulation does indeed create sum and difference frequencies, but the key distinction is that IM products occur only in the presence of nonlinearity. Whenever you have regular harmonic distortion - for example, clipping - then you also have IM distortion. The two are inseparable. But IM distortion does not otherwise occur in the air in significant amounts.

I'll also point out that it's a common myth that low frequency absorption reduces the bass. While it does indeed reduce the level of the peaks, those were emphasized unnaturally and needed to be reduced! More to the point, broadband low frequency absorption reduces the depth of the nulls, which are the bigger problem, and so the overall perceived bass level in a room actually increases when absorption is applied.

--Ethan
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post #24 of 222 Old 07-13-2003, 08:47 AM - Thread Starter
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Thanks guys for your input to this thread! :)
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post #25 of 222 Old 07-13-2003, 09:17 AM - Thread Starter
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Originally posted by Dennis Erskine
10. Broadband low frequency absorption is easy and cheap to install but largely counterproductive and will require larger, more powerful subs to overcome all the absorption...an advantage is you don't need to use math to sort it out and all you need to do to solve the downside is throw more money at the problem.
So, which costs less, the EQ or the larger more powerful sub with the general low bass absorption? ;)
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post #26 of 222 Old 07-13-2003, 10:08 AM
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Ethan,

Here is a Dr. in Applied Physics that disagrees with you.

Acoustic Animations

non-resonant frequencies do not create standing waves! Only those that reflect from the wall overlapping themselves will superimpose in phase and double the magnitude of the peaks/nulls which then remain in place upon further reflection. At other frequencies the peaks/nulls travel through the room and do not get a chance to "build" because they do not reflect in phase.

This physics priniciple is observed in pipe organs and ultrasonic jewelry cleaners.

It is very likely that any given room could have 80Hz within Q of an axial, tangential, or oblique resonance. The whole purpose of room ratios is to find the dimensions that cause these resonances to not further reinforce each other by spreading them out.

By using bass traps all over the room - you are preventing the reflections from occuring - at the resonant frequencies as well as the non resonant frequencies. This is great for a studio console to properly hear a mix and not the room - but if reflections are not occuring entertainment listeners will consider the room to be dead. Most listeners expect to hear some reverberation when they are in the room

That is the science - it is an art how to subjectively tune those reverberation/reflections so they sound good - but this subjectivity is different for each application (movies/music/speech/recording/monitoring)
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post #27 of 222 Old 07-13-2003, 11:01 AM
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Krag,

> Here is a Dr. in Applied Physics that disagrees with you ... non-resonant frequencies do not create standing waves! <

Thanks for the link. I just sent Dr. Russell an email asking for clarification on his statement. If he replies I'll post it here immediately.

I have to assume it's a matter of semantics, or maybe the fact that the page you referenced addresses a one-dimensional room only. As I explained above, it is very easy to prove that what I'm saying is correct! All you have to do is play various low frequency sine waves and walk around the room. Regardless of frequency there will be a null 1/4 wavelength away from the walls, and a peak 1/2 wavelength away. Please try it!

Also, not to pit one Ph.D. in acoustics against another, but I discussed this exact issue in great detail a few months ago with Dr. Matthew Nobile, head of IBM's Acoustics lab in Poughkeepsie, NY. Matt confirmed that standing waves occur at all frequencies in all rooms. In fact, he made a point of saying that a lot of people don't realize this is the case!

> This is great for a studio console to properly hear a mix and not the room - but if reflections are not occuring entertainment listeners will consider the room to be dead. <

This is why I am so careful to always refer to low frequency "broadband" absorption. Yes, of course you don't want a room to be totally dead. But that's a mid/high frequency property. Pro recording studio mix rooms are never totally dead. By pro I mean a "real" studio, not the home studios you see in ads for foam treatment! :D

The goal in a pro mixing room is exactly the same as in a home theater: To provide a neutral and natural listening environment having a flat response throughout the entire range, with neither too much nor too little ambience.

--Ethan
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post #28 of 222 Old 07-13-2003, 05:03 PM
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Standing waves (modes) occur at specific frequencies in specific rooms. Not all frequencies at all rooms. Reverberation can occur at all frequencies in all rooms (if not overly absorbed or if the distance between the source and the boundary exceeds the limits of decay below audibility). Reverberation and resonance are different. If modal resonance occured at all frequencies in all rooms, your modal spreadsheet would be redundant (in the Queen's English sense of the word).

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post #29 of 222 Old 07-13-2003, 05:43 PM
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Dennis,

> Standing waves (modes) occur at specific frequencies in specific rooms. <

Mode is just another word for a natural resonance that exists in a room. A standing wave is a static interference pattern that exists in the presence of a sustained tone. Modes and standing waves are not at all the same thing. They aren't even related!

> If modal resonance occured at all frequencies in all rooms, your modal spreadsheet would be redundant <

Nobody said that modes exist at all frequencies in all rooms. I said that standing waves can exist at all frequencies in all rooms. All you have to do to prove this for yourself is play some low frequency sine waves as I described. Pick a frequency - any frequency - and you will easily find the static locations of the peaks and nulls in the room.

I am astonished that were are arguing about whether a room mode and a standing wave are the same thing. And since the sine wave test is so simple to perform, I don't know why that point is in dispute either.

--Ethan
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post #30 of 222 Old 07-13-2003, 06:19 PM
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Ethan:

Quote:
Originally posted by Ethan Winer
Dennis,

> Standing waves (modes) occur at specific frequencies in specific rooms. <

Mode is just another word for a natural resonance that exists in a room. A standing wave is a static interference pattern that exists in the presence of a sustained tone. Modes and standing waves are not at all the same thing. They aren't even related!
--Ethan
Aren't standing waves created at the resonant frequencies of the room? If you are defining mode as a natural resonance that exists in the room, then standing waves will be created when the room is subjected to frequencies for a time period t correlating to the resonant frequencies of the room. Thus, a room's modes directly correlate to the creation of standing waves.
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