[John Casler]

This question is directed to Ethan, but anyone feel free to answer.

I have a rather good "practical" or intuitive sense about how bass traps work, but wonder what the reality is??

I am looking for a simple (non-mathematical or acoustically jargonesque) explanation.

Now since they are generally more effective in the corners/intersecting boundaries, I see that they are designed to (as their name implies) "stop/absorb/trap" the higher power bass waves in the areas where they tend to be directed and collect.

Now if we accept that the bass wave travels through space and is limited/directed at its "outmost" to travel along the rooms boundaries to an intersection, then it seems logical that when this energy "meets or combines" from a second boundary, we will have increased pressurization.

My assumption is that bass traps are "rigid" enough to slightly impede and absorb some of the "direct wave" (that which comes directly through space without encounterig any surface that directs it) and that which does pass through, finally hits the wall and pressurizes (and moves back out into the room) is then reduced in energy, and as it travels back out of the intersecting boundaries then again encounters a barrier to its re-entering the room with enough power or energy to damage the "pure" "direct" waves coming from the drivers.

Now given that bass is "not" only along the walls (it is only contained by them) and it is, in fact, pressurizing the whole room aperiodically according to frequency, the flat room boundary/surfaces that are at a higher angle of incidence (90 degrees being the highest) also need these traps to "meet" the bass wave that comes from the speaker, passes through the listener, and then travels to the back wall to be actively "reflected" right back toward the speaker again.

My take is that these "direct attack" traps on the back wall:

1) Again absorb some of the bass energy before it "hits" the back wall
2) and what does get through has been reduced in power
4) and then they "inhibit/trap" the return wave which has to "pass back through them on its way back into the room toward its original destination.

It would seem then that this too would reduce the negative affects that this "anti-wave" would have in damaging the "direct wave", which we want to hear.

Is this close?

and if so I have a few more questions.

[youngho]

I don't have a copy of F Alton Everest's Master Handbook of Acoustics handy, but my recollection was that bass traps can come in a few different varieties, like Helmholtz resonators and membrane absorbers.

As I recall, the membrane absorber uses a relative thin rigid surface (like, say, plyood) that is designed to vibrate at certain frequencies or a range of frequencies. This vibration of the membrane results in some loss of energy as kinetic energy is converted to heat (think about what happens when you bend a spoon back and forth), and the membrane can be dampened further by the presence of absorbant material such as fiberglass behind the membrane. These seem to be more effective when there is an air cavity present behind the membrane, though I forget why that is. The membrane absorber is placed at boundaries or intersections of boundraries, as these are areas where standing waves due to axial modes are of maximum amplitude, in order to reduce standing wave formation by reducing the back-and-forth movement of air at these locations.

Helmholtz resonators, if I'm not mistaken, are essentially cavities or bodies of air with an aperture that are typically placed within room boundaries with dampening material placed in the resonator. However, this part was a bit beyond me.

Essentially, both methods seem to rely on the presence of absorbant material for conversion of the longitudinal sound waves into kinetic energy, or heat. This conversion or absorption would apply to both the original wave and its reflection.

However, it's been a while, so I may be mistaken in some of the above.

Young-Ho

[ctviggen]

No, you're right.  I'm reading that book now, although I've only skimmed some of the resonator sections.  He discusses several resonators, one of which has a tube that's adjusted in length to resonate at a certain frequency.  I'm thinking of building one of these for a huge peak I have at 28Hz, although I've not yet looked into the mathematics.

[Ethan Winer]

John,

> I am looking for a simple (non-mathematical or acoustically jargonesque) explanation. <

You combined a few different issues together in your questions. One question is "how does bass trapping solve bass problems?" Another is "where do you put them for the most effect?" Yet another is "How do they absorb sound?" I could spend all day doing nothing but writing a detailed reply, so forgive the brevity.

All acoustic problems are caused by reflections. At low frequencies these reflections create peaks and nulls in the response at various frequencies in different parts of the room. They also foster resonances that cause some frequencies to linger after the original sound has stopped.

Bass traps absorb the reflected sound, and so reduce both the peaks and nulls, and they also absorb the resonance.

There are a few types of bass traps, and I'm most familiar with porous absorbers such as fiberglass and foam, and also wood panel traps which are pressure absorbers. These two trap types work on opposite principles.

Porous absorbers work as the sound waves pass through them, and they absorb by converting the sound energy to heat. The path of sound through a porous absorber is impeded by the pores and tunnels. Since sound has to pass through to be absorbed, a porous absorber works best when spaced 1/4 wavelength away from a room boundary for a given frequency. That's where the sound wave's velocity is highest.

A wood panel trap is basically a shock absorber for sound waves, and it's based on a mass-spring principle. The wood panel is the mass, and the air trapped inside the sealed cavity serves as the spring. Fiberglass is also added inside the cavity to damp the vibration so the panel doesn't continue to ring like a drum head after the sound source stops. Unlike a porous absorber, a panel trap works best when placed at the room boundary, not spaced away. A panel trap works on wave pressure, not velocity, and the pressure is highest at a room boundary.

In electrical terms a panel trap is an RLC shunt network. The panel is the inductor, the air is the capacitor, and the fiberglass is a series resistor that lowers the circuit Q.

--Ethan

[ctviggen]

Ethan,

I take you mean "all acoustic problems in a room" are caused by reflections.  I'm sure my neighbors are too happy with my sound system!

What you said implies that thickeness is important.  Have you tested the absoption if you tape two of your traps together?  If so, does this help with low frequency absoption?

[Ethan Winer]

Bob,

> in a room <

Yes.

> What you said implies that thickeness is important. <

Yup.

> Have you tested the absoption if you tape two of your traps together? <

That wouldn't work well because it puts one of the front membranes inside, sandwiched between two traps. But in theory what you suggest is reasonable. MondoTraps are thicker than MiniTraps, and that's why they absorb more below 100 Hz.

--Ethan

[John Casler]

Thanks

So it seems that Bass "TRAPS" are of a couple varieties;

1) Hemholtz type, which I gather are more "frequency specialized"
2) Foam Type LENRDS which are only helpful in the upper bass areas
3) Wood Panel Pressure absorbers for the larger power waves
4) Fiberglass based Panels that are more broad band

If that is correct then it would seem that the next most important thing is "when and where" is the best place to use the traps.

Now it would seem, that many would initially place their first traps in the four wall/wall L&R rear corners.

(what I mean by that is, that you have two surface boundaries that make up an intersection or corner.   Examples would be wall/wall, floor/wall, ceiling/wall.  Obviously "tri-corners" would be combos of 3 boundaries.)

But, while bass is directed to the corners, I am under the impression that some of the most "dangerous" bass is that which travels directly and uninterupted from the bass driver to the "back wall" so it hits the wall "straight on" (90 degrees) and is then "reflected" absolutley straight back into the room to meet direct approaching waves "head on", to provide a "specific" cancelling affect only ameliorated by their energy reduction, which was absorbed by the back wall.

So, what I am driving at here is, "PRIORITY" of trapping, if you will.

If one is to begin the trapping process, where is the best place to start.

Is the rear wall right behind the listener, more important than the rear L&R wall/wall corners?

And to be more specific, the rear wall has three primary areas that seem to need addressing.

1) Floor/wall intersection
2) Main flat wall surface at "listening position" level
3) Ceiling/wall intersection

So again, I'm looking for "priority".

Strangely enough, I would think that since most woofers and subs are located closer to the floor, and the angle of incidence to the ceiling is greater, that the Floor/wall intersection, and the Direct wall sections at listening level, would certainly be high in priority.

But how do they rank opposed to L&R rear "wall/wall" corners?  To each other?

And..... same questions about L&R rear "wall/wall" corners.  Is the section of that corner designated by the floor/wall/wall, more important than the "mid-wall" (listener level) wall/wall corner (hope this terminology is clear )

Again, what I'm going for is "priority" of bass treatment.

In mids and highs, "everyone" knows that the absolute priorities are the points of first reflection.

Well in bass, what is it?

I see that we have a huge number of choices, and I think it would be valuable to understand which ones should get priority.

I also understand that some of the prioritization may be dependant on room dimensions, and if so, why?

Whew   And again this is posted for Ethan (hope you don't mind, but it could help sell more traps ) but I appreciate thoughts from others too.  I have my own ideas, but different perspectives improve awarness.

I have more questions, but I will pause here to see what the responses are.

[Ethan Winer]

John,

> So it seems that Bass "TRAPS" are of a couple varieties <

Yes. The tuned trap types absorb most at a center frequency and less above and below. The porous absorbers work over a wider range, but all fall off at very low frequencies.

> while bass is directed to the corners, I am under the impression that some of the most "dangerous" bass is that which travels directly and uninterupted from the bass driver to the "back wall" <

Both are important. I'm not a physicist but I believe the bass that finds its way to the corners is mainly modal. The bass frequencies (and all other frequencies too) that reflect off the rear wall are caused by a different phenomenon. In that case the effect is straight comb filtering, so the peak and null frequencies are directly related to the distance from that wall. For example, at 100 Hz there's a deep null 34 inches in front of the rear wall. The same thing happens at other boundaries, but as you observed, when the angle is not perpendicular that shifts the frequency/distance calculations.

> what I am driving at here is, "PRIORITY" of trapping <

Corners first, always. Not only because it helps the peaks and nulls, but that's also the best place to reduce modal ringing which is just as important as the raw LF response.

> Is the rear wall right behind the listener, more important than the rear L&R wall/wall corners? <

They're all important. That's why I have 31 traps in my living room.

The ideal room would have bass trapping covering 100 percent of all surfaces. Though you would not want all of the trapping to be absorbent at mid and high frequencies. If you could make every surface absorb 100 percent below 200 to 300 Hz I think that would be perfect.

> I also understand that some of the prioritization may be dependant on room dimensions, and if so, why? <

Only because some dimensions / ratios create worse peaks than others.

--Ethan

[John Casler]

Thanks Ethan,

So to continue on the "priority" vein.

If we have to prioritize the corners we have them divided into:

1) Front wall L&R
2) Rear wall L&R

and in the individual corners themselves we have three sections:

1) Floor/wall/wall (tri-corner)
2) Wall/ wall
3) Ceiling/wall wall (tri-corner)

My assumption would be:

Rear wall L&R would be more important than Front wall L&R???

and Floor/wall/wall would be slightly more important than Wall/wall or Ceiling/wall/wall???

Could that be right? and if not why?

I know they're "all" important, but as you say no one will be able to do the 100% bass trap/wall coverage, but it would be great to have an educated assumption as to what areas would offer the "highest percentage" result, so that audiophiles could approach the task by being most effieicnt when possible and then "grow" into more trapping, much like they now "refine" their systems by "adding" components with higher percentage improvements.

I would also have to say that the "ultimate" goal (IMO) would be to make "ALL" room boundaries for practical purposes, "acoustically transparent".

I know some disagree, but removing as much room interaction as possible will get you to the pure recording. (again IMO)

[klh]

Sorry to interject, but I have three quick questions. In my room the seated position is about 40 inches from the back wall. How (exactly) did you calculate that there is a 100 Hz null at 34"? Also, if one was to put a trap behind the seated position, it seems to me the best option would be to have broadband absorber focused at the problematic frequency. I don't think a panel trap would enhance the sound that much since even though it would work great for the problematic frequency, it would reflect the mids and highs, which is obviously undesirable. I presume a better option would be to use 4" OC705 spaced a specific distance from the wall that would make it most efficient at absorbing the problematic frequency? How do I calculate that distance? Thanks in advance.

[youngho]

KLH: The speed of sound is 565 ft/sec. 100 Hz (the definition of hertz is "per second") is 565 ft/sec divided by 100 Hz or 5.65 feet. 5.65 feet is the wavelength of 100 Hz. If you can understand that a reflection is 180 degrees out of phase with the original signal, you'll see that at a distance of 5.65/2 feet from the wall, the original signal and reflection will result in negative interference (the reflection will be of lower amplitude, so will not cancel out entirely). 5.65/2 is 2.6825 feet, multiply by 12 inches/ft, and you get 33.9 inches. You can divide 565 by room dimensions to calculate primary axial modes or resonances, but the effects of these resonances is dependent on position of speaker and listener.

Hope this helps,

Young-Ho

[klh]

Thanks for the detailed explanation.  That is exactly what I was looking for. As for ways to ameliorate the problem, any suggestions?

[klh]

Ethan (or others),

I believe you stated for a broadband absorber to be centered on a given frequency, it should be placed a distance one fourth of it's corresponding wavelength from the wall. If my recollection is correct, then, based on Young-Ho's mathamatical explanation, a good way to dissipated the problematic frequency would be to place an absorber (e.g. 4" OC705 or your MondoTraps) half way between the listener and the rear wall? It would be pretty easy to make such an absorber on stands (or use your MondoTraps) and store them against the wall but pull them out whenever intense listening is done.

[youngho]

klh: if possible, it would be helpful to measure bass response in your room. I assume you actually have a relative null at 100 Hz? If so, you might find moving your seat backward or forward a good idea. This will shift the cancellation up or down, accordingly, but you may be able to compensate if there is an axial mode, for example.

Young-Ho

[Ulas]

The speed of sound is 565 ft/sec.

That may be so at -261 degrees C, but at 20 C the speed of sound is about 1127 ft/sec. (see: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe.html among others)

[youngho]

Sorry, you're right. I meant 1130 ft/sec, but you divide by twice the wavelength to calculate the modal resonances. Thanks for the correction.

Young-Ho

[Ulas]

you divide by twice the wavelength to calculate the modal resonances.

Huh? I think your initial premise is wrong and that's why you have to double the wavelength or half the speed of sound to get the right answer.

[Ethan Winer]

John,

> Rear wall L&R would be more important than Front wall L&R??? <

I used to think that if the choice is one or the other, the rear makes more sense because that's where the initial reflection occurs. But after a few people told me they got better results with (only) two traps in the front wall-wall corners, I realized:

1) Modal peaks require both surfaces in order to resonate, so front versus back doesn't really matter in that case.

2) Having broadband traps in the front of the room also tightens up the midrange "chatter" that exists around the vicinity of the loudspeakers. This is not a bass problem, but it clarifies the sound quite a bit anyway and so is welcome.

3) The corners directly behind the speakers reflect earlier the bass that emits omnidirectionally from all loudspeakers.

> Floor/wall/wall would be slightly more important than Wall/wall or Ceiling/wall/wall??? <

This is a gray area because there are so many factors: Where the speakers are in relation to all six boundaries, where you're sitting, the size and shape of the room, speaker directionality at low frequencies, and probably a few other things too.

> it would be great to have an educated assumption <

There are ways to identify the best places for bass traps, before you go to the trouble to install them. One is to play bass-heavy music and stick your head in various corners. You'll need to climb up a ladder to investigate the ceiling corners. Wherever the bass booms the most is where traps will do the most good. Better than music, but irritating to listen to, is pink noise. With pink noise you're assured of having all low frequencies sounding, versus only those in the key of the music that may or may not align with the room's problem frequencies.

Eddie Ciletti, a columnist for Mix magazine, described how he found the best places for his eight MiniTraps. However, it requires a matched pair of microphones and a dual-channel oscilloscope. Here's a link to that article:

http://mixonline.com/mag/audio_bassic_instincts/index.html

Look under the heading "The eyes have it."

--Ethan

[ctviggen]

John, what I plan on doing is measuring (with ETF) my room before traps (I'll take all the ones I have out) and then placing certain traps in certain locations and remeasuring.  Once I find the best location for two traps, I'll try the same thing with two more, and so on.  This way, I'll come up with a set of locations that actually measure the best (although I assume that putting my Mondo traps in the front corners will be better than anywhere else).  The only problem with this scenario is determining, when faced with two frequency spectrums from two different configurations of traps, which frequency spectrum is the "best" one.  That could be problematic, in certain instances.  Another problem is that I can't (for practical purposes) use every possible location, as I have three doors along one wall.  

However, I'll at least have the perception that I'm doing something "scientific."  I'm also going to use this method to tune my RM40s/subs, especially the crossover region between the two.  

If I build a Helmholtz resonator, I may use the technique where you play the tone for which the resonator is designed, then go around the room until you find the highest sound pressure level.  That's where you put the resonator.

[Ethan Winer]

KLH,

Young-Ho gave you the right answer. I wrote a litle program for the RealTraps web site that calculates frequency from distance and vice versa. It's a clever program because you can enter either and the program knows to display the other. Here's a link to that page:

www.realtraps.com/sbirlbir.htm

> for a broadband absorber to be centered on a given frequency, it should be placed a distance one fourth of it's corresponding wavelength from the wall. <

Yes, but nobody in their right mind is willing to put a trap 3 feet out from the wall. In practice, even an air gap of a few inches helps a lot.

--Ethan