does acoustic foam only absorb high frequency sound? - Page 1
Forum section: electronics / general

author: engineheat
author_rank: Frequent Contributor
timestamp: October 16, 2020, 03:24:30 pm
content: I have a sound source enclosed in a thick plexiglass chamber with a microphone. I used a chamber to isolate ambient noise. This caused quite a bit of echo. I found that the echo caused the signal amplitude picked up by the mic to rise dramatically. My first question is:1. Does echo merely changes the amplitude of the sound (as picked up by the mic) or can it also alter the frequency? In another word, say the sound source is a pure 1000 Hz tune, would the presence of echo merely cause the amplitude to rise inside the chamber (as detected by the mic) or can it also introduce changes in frequency or other attributes of the sound? It's not obvious to me so far.In an attempt to prevent echo, I placed acoustic foam inside the chamber. While this cut down on the echo, as indicated by the amplitudes of the signal that the mic picked up, I noticed that the sound signal looked different. Specifically, I noticed the low frequency component of the sound got more pronounced.My hypothesis is that the foam was able to absorb most of the high frequency echo but not the low frequency, so the low frequency component of the sound is amplified in the chamber due to the echo. When I looked at the sound wave plotted against time, I see a noticeable low frequency sinusoidal component introduced, and this was also shown on the subsequent FFT plot.Is that what's happening? If so, what can I do to absorb the low frequency component?Thanks

author: duckduck
author_rank: Frequent Contributor
timestamp: October 16, 2020, 03:52:00 pm
content: Google "diaphragmatic absorbers"

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 16, 2020, 04:16:58 pm
content: You have dived in a very deep pool.1. Simple absorbers cannot introduce new frequencies. But any non-linear behavior in the absorber could potentially cause mixing and introduction of new frequencies. So your solution might work at low volume, but as intensity rises things go south.2. All mechanical systems will move under the force of sound wave pressure changes. Thus they will have resonant frequencies. If stiff enough the resonances will be above your frequency range of interest. But absorbers must move to absorb energy, thus tend not to be stiff, potentially lowering resonances to audio ranges. The desirable low Q of absorbers broadens the resonance peaks allowing them to be excited by a broad range of frequencies. The tuning of all of these effects to get good results is as much art as science.3. Echo is usually dissipated by complex surface shape (the pyramids seen in anechoic chambers). The shapes need to be of dimension comparable or larger than the wavelength to be effective, so in general the echo will remain strongest at lower frequencies. Chamber size should be large with respect to frequencies of interest. This is a problem for most home projects. Maybe a field trip to a remote quiet location would be the easy solution.I have personally only worked these problems as a student and observer, but the real pros at this are rare and highly valued.One of the better solutions for sound damping is problematic these days. Uncovered lead wool. Used to be widely available for packing cast iron plumbing joints but is now relatively expensive, and many people are uncomfortable using lead at all.

author: Cerebus
author_rank: Super Contributor
timestamp: October 16, 2020, 04:57:25 pm
content: Also note that the container itself can have a huge effect, both from the build up of standing waves between parallel surfaces and from resonance of the whole volume (see Helmholtz resonances for the latter). It's no accident that recording studios often have sloping ceilings and internal walls that are not parallel to each other (often with resonant absorbers hidden behind internal walls in the spaces created between them and ordinary rectilinear external walls to deal with the inevitable 'left over' nodal resonances).

author: eti
author_rank: Super Contributor
timestamp: October 16, 2020, 11:40:23 pm
content: Save the thread some half-guesses and wasted speculation, I'll send you to someone who's doing lots of research and who knows his stuff:

author: GlennSprigg
author_rank: Super Contributor
timestamp: October 17, 2020, 10:56:53 am
content: Long ago, I used to wonder why "Bose" systems/speakers where purportedly so 'great', even though tiny!!I couldn't/can't see what the specific small speakers engineering was having on their products... Beyond me...

author: ConKbot
author_rank: Super Contributor
timestamp: October 18, 2020, 08:09:42 am
content: Short answer is the low frequency performance of an anechoic chamber is driven by the height of the feature doing the absorbing. (Pyramids/ridges). Look at the wavelength of low frequencies, vs the height of the cheap/common absorbing foams, and you'll see a large disparity.Long answer: it's really complicated, with other effects all having their turn causing a headache.

author: engineheat
author_rank: Frequent Contributor
timestamp: October 18, 2020, 03:06:20 pm
content: Thanks.maybe I'm better off removing the foam and just use the rigid plexiglass box. It does create more echo and dramatically increases the amplitude of the sound as recorded by the microphone, but unlike the foam it shouldn't treat different frequencies differently (right?) In another word, if the sound source outputs a 1000Hz wave at x magnitude and a 4000 Hz wave at y magnitude, due to the echo, the microphone should pick up the 1000Hz sound at A*x magnitude and the 4000 Hz and A*y magnitude theoretically right?For my application, I need to isolate outside noise. If the echo makes the sound louder (as described above) that's acceptable, but I can't have other attributes changed.

author: bdunham7
author_rank: Super Contributor
timestamp: October 18, 2020, 03:15:35 pm
content: Quote from: engineheat on October 18, 2020, 03:06:20 pmNo, you will have resonance modes with peaks and nulls that will cause wild swings in the amplitude response in relation to frequency. And all of that will vary greatly with the positioning of your sound source and microphone and with the dimensions of the box. What is your sound source and what are you doing?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 18, 2020, 05:16:23 pm
content: If you are only concerned about external sound you put your absorber on the outside of your box, where you may have more room. You will still have echo and other effects inside the box, but might be able to deal with that analytically, as long as you don't exceed the dynamic range of your instrumentation. eti's video suggests some good options for materials and configurations. Depending on where the external sound is sourced you might just want to cover lab walls with anechoic material. Not decorative, but it would make your lab a more pleasant place to work.The other thing eti's video confirms is that this isn't a simple problem. A person who is well informed in the field still has to do a substantial amount of head scratching and experimentation to develop an acceptable solution. You will undoubtedly need the same.

author: eti
author_rank: Super Contributor
timestamp: October 18, 2020, 05:30:16 pm
content: Quote from: engineheat on October 16, 2020, 03:24:30 pmWatch the video in my previous post, save yourself hours or days of debate and speculation here. The guy in the video has done vast amounts of exceptionally pedantic testing with acoustic enclosures and speakers.

author: fourfathom
author_rank: Super Contributor
timestamp: October 18, 2020, 05:35:38 pm
content: Look at "Bass Traps". These are designed to absorb lower frequencies and are often placed in the corners of a room. There are foam traps, and also some made with various types of fiber -- some people use rolls of fiberglass insulation. I got some pre-made bass trap panels that used a high-density fiber filling, and they did make a difference in my studio.

author: pwlps
author_rank: Frequent Contributor
timestamp: October 18, 2020, 06:43:22 pm
content: Quote from: fourfathom on October 18, 2020, 05:35:38 pmThere are also active bass traps:https://www.psiaudio.swiss/avaa-c20-active-bass-trap/As I understand these use a sort of speaker driven in quadrature with respect to the sound pressure (at least this is how I understand "active velocity") to make its acoustic impedance real and with the gain adjusted to match the acoustic impedance of the air. Actually they say the acoustic impedance is even lower than that but it's not clear for me why they are doing so.

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2020, 03:22:06 pm
content: Quote from: engineheat on October 16, 2020, 03:24:30 pmThe echos alter amplitude and phase through constructive and destructive interference but do not alter frequency.QuoteThe acoustical foam dampened high frequencies more than low frequencies.