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FLINT: Standing waves and room acoustics

The plastic is very thin and doesn stop any sound from passing through into the fiberglass. Look at it this way, if you were to hold the plastic up in front of your face between you and your speakers, you will sound hear just about everything from the speaker except maybe the absolute highest frequencies.

Sound passes through substances by moving the substance which transfers the energy hitting it on one side into sound energy coming off the other side. Typical thichness windows are not great sound blockers, especially in the mid bass range. Neither are walls. The STC measurement is what acoustic engineers use to measure a material's ability to block sound passing through. The plastic surrounding the fiberglass is likely to have zero ability to block sound at all.
 
Thanks for the writeup!

I've worked with resonant frequencies in my electronic warfare reliability days, my nuclear weapon safety job, and my last explosives job (my BS is in ME) but there's one issue I've never been able to wrap my head around, maybe you could take a stab at it (the issue).

Sound waves come off a speaker not as a flat plane but as part of a sphere, like so:
sound.gif

So, my thinking goes that the front part of the waveform hits the opposite wall first at the closest point from the speaker (ie directly across the room) and then the rest of of front part gradually moves across the wall, reflecting at slightly different times and at increasingly higher angles, like this:
wave_reflection_2_20090117112315.gif

(Google Image is the greatest thing ever)
Therefore, the front of the waveform would seem to me to appear at different places throughout the room, becoming more diffuse the longer that particular frequency is sounding. I don't quite understand how a "node" can form anywhere, even in the simplest "room" with only two opposite walls extending to infinity. :think:
 
Your analysis is 100% true for frequencies with wavelengths less than 1/16 any dimension in the room. But for bass, where the wavelengths are 1/2 to 1/4 any dimension in the room, the wavefront is far to wide to operate in the fashion you mention.
 
Flint said:
Sound passes through substances by moving the substance which transfers the energy hitting it on one side into sound energy coming off the other side.
Click to expand...
I wonder if this is similar in concept to the "membrane" that RealTraps boasts about for their bass traps. That has always confused me. I've read things like "don't glue the material to the face", "remove the foil/paper" and "don't paint the front". Then RealTraps comes along and says "We have a membrane...".
 
The super thin and flimsy plastic on covering fiberglass is absolutely nothing like the membrane on a proper mechanical bass trap. It is an acoustically transparent film which has zero affect on the sound passing through it in any way shape or form (below about 12kHz, or so).

The membrane on a proper mechanical bass trap, like the panel traps I built as defined in the "Master Handbook of Acoustics", is a heavy wooden panel which vibrates in conjunction with the acoustic energy hitting it. In the cavity behing the panel is an air gap to allow for free motion of the trap then a layer of sound absorbing material which acts as a dampener to the panels vibration. Basically, the acoustic energy in the room is transferred into mechanical energy by making the panel move (with some loss into heat), then the mechanical energy is converted into pressure (acoustical) energy inside the trap cavity where the absorbing material converts it into heat. While not the most efficient way to absorb acoustical energy, the panel trap can be extremely effective when used properly.
 
Great article Flint. It's appreciated, as always.

One thing that I feel gets "lost in translation" when dealing with bass (and especially the coverings in this case) is that the entire trap is what's being vibrated, especially at very low frequencies. Therefore, the plastic bag isn't really an issue because the sound waves are on the order of the size of the entire system (bag and it's contents).

High frequencies hitting the trap would be like raindrops hitting it; bass frequencies are like ocean waves hitting the trap.

John
 
Let's keep the same room you described above, with the issue at 56.5 Hz. Now let's assume that you are able to build a perfectly tuned trap for that frequency and are able to implement it perfectly. How would an RTA graph of the room change as a result of implementing this?

My logic behind this question:
The null is created because two waves are cancelling each other out (think infinity symbol and the microphone is at the center). The trap is going to treat both waves equally, and therefore (presumably) going to absorb both of them. The result is that now instead of no energy @ 56.5 Hz at that one location, there is no energy at 56.5 Hz anywhere in the room.

John
 
Towen is correct - if you can produce a 100% effective trap which absorbs 100% of the acoustical energy at a given frequency the result would be creating the illusion the reflective surface does not exist at that frequency (zero reflections). It would be as if there is no wall, just like being outdoors, and the sound wave would pass over the body at least once (possibly twice) and be heard once (possibly twice).

That said, if the other frequencies are still being relfected and reinforced by the walls, the room gain they experience will boost their outputs higher than the output at the bass trap frequency. Thus the frequency will still be cut in audibility when compared to the neighboring frequencies.

So, the result would still be an audible dip at the standing wave frequency, but the ringing, or sustain/resonance, at that frequency will be gone. This will tighten up the bass sound when listening to music or movies.
 
Flint said:
So, the result would still be an audible dip at the standing wave frequency, but the ringing, or sustain/resonance, at that frequency will be gone. This will tighten up the bass sound when listening to music or movies.
Click to expand...

If there will be an audible dip at that frequency, would it not make more sense to place a trap with a much broader effective range, thus reducing the boundary reinforcement of the lower/higher frequencies?

At what point does that absorption become detrimental?
 
The question is which is worse: An absense of sound at a certain frequency with excessive ringing and sustain? Or, and absense of sound at a frequency with no ringing or sustain?

I prefer the latter.

Either way the room boundaries create a problem we can address in a couple of ways. Since bass issues are typically manifest from several acoustical causes, I prefer treating ALL of the bass range rather than just one frequency. The topic at hand is just one target issue - standing waves - but we also have to consider room resonance, room gain, other ringing issues, and so on.
 
Flint, can you explain how standing waves and room nodes, react when a planer dipole speaker is used? I know the back wave cancels some of the room nodes. But would like a refresher on this aspect of room acoustics.
 
Flint said:
Your analysis is 100% true for frequencies with wavelengths less than 1/16 any dimension in the room. But for bass, where the wavelengths are 1/2 to 1/4 any dimension in the room, the wavefront is far to wide to operate in the fashion you mention.
Click to expand...
Ah. Thanks! :handgestures-thumbup:
 
To put low frequency wave lengths in perspective; If you were to "trap" a entire wave length at 20Hz, you'd need a bass trap in the neighborhood of 56', give or take a few inches depending on altitude, temperature, etc.

Rope
 
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