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July 31, 2008 - 6:27am
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Bass Traps as Mid/High absorbers
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I like you Pradeep because you always ask good questions!
Yes. Even though flat in the middle of walls is not the best place for bass traps, you still get some bass absorption which is always welcome.
All frequencies gather in corners, sort of like cupping your hand over your ear, or the way a satellite dish works by focusing the sound. The concave shape of a corner does the same thing. No matter what room surface sound strikes, if there's even a small angle it will travel along the wall and eventually find its way to a corner. Bass waves tend to propagate along surfaces like that more than higher frequencies which tend to bounce at an angle like a cue ball on a pool table.
--Ethan
Thank you again Ethan for both the answer and the complement.
Oh, so that's how a satellite dish works. Thanks.
So, please send me info on your products for my dish that will improve the focusing ability of my dish.
You can buy a set of focusing devices from many high-end video dealers. These are made from fiberglass and attach to the outside edge of your sattelite dish. They are shaped like the palms of human hands. The most commonly found brand is the Model Eh.
Folks...let us try to explore the world of music and audio in this forum. I think Ethan was trying to give an anology of a concave surface directing reflected waves to its focal point. If you could give a better and clearer answer for why base frequencies tend to collect at the corners, I am sure everyone will appreciate. Or at the least you can also make your point by explaining why Ethan's explanation cannot be correct instead of being sarcastic. After all we are just discussing and trying to find answers!
First of all, lets not take ourselves to seriously. A bit of friendly sarcastic ribbing keeps things in perspective.
Second, by definition a concave surface is curved. The corner of a room is not concave. It is a trihedral.
Third, what data proves that bass frequencies gather in corners?
What does frequency gathering even mean?
High pressure, low velocity areas exist in every room based on the ratio of room dimension vs wavelength. This high pressure exists equally along the entire length of a wall at certain frequencies. The same thing happens at the opposite pair of walls and at the ceiling/floor dimension.
If all three of these dimensions are the same then the combination of their pressures along each surface will create a very high pressure in the corner at certain frequencies. But since I don't listen sitting in the corner why should I care?
As to proving or disproving Ethans posts, that is more difficult. Wavelength, room size, etc, etc, etc makes for a difficult moving target for analysis.
If its friendly, I understand.
I agree...but without being very rigorous, I think a cupping surface was referred to. And I also dont understand
how a cupping surface scenario explains why bass frequency tends to collect at the corners.
Figuring out such things was the idea of the question. I wish there is some litrature
out there. I just tried walking around the room, listening in the corners to verify this
and I could see the bass was high in the corners.
I had the same question before. But what I have learnt and
understood with time is that absorbing low frequencies eliminates standing waves thus
avoiding the peaks and valleys across the room and reduces the
reverb time so that a bass note does not linger for too long
and reduce the clarity.
I understand it is complex. Would it be easier to take a particular case and
disprove. For eg can we take a room of particular dimension and explain that the
whole theory does not apply for the case considered.
This is trivial to prove with a few low frequency test tones and a Radio Shack SPL meter. If you want hard data, below is the absorption for some of my company's products where the same products were tested out in the room versus straddling corners. Compare the solid blue and red lines (corners) with the dashed versions (out in the room). You can see that at bass frequencies the same absorbers have more affect in corners. This is because there's more bass in the corners to absorb.
--Ethan
It is interesting to consider the analogy to a woofer driving a room. Corner placement produces more bass, and one way to view this is that the room boundaries are acoustical reflectors, similar to mirrors with images.
Consider the image woofers provided by the boundaries working with the one real woofer.
There is an analogy to absorbers, we can think of the boundary providing an image absorber. This is supported by the increase in absorption, but also the absorber is acoustically larger as a result of the images since the cutoff frequency is lower than when not in the corner.
Think of the walls as mirrors, we would see multiple woofers or absorbers in those mirrors and that is what is seen by the sound.
Pete B.
Exactly, Pete.
--Ethan
My post was not meant to claim that all of you were wrong, but to point out that many of your concepts while basically correct are oversimplified and just part of a much bigger system.
Do low frequencies "gather" in corners?
Not sure what "gather" means, but yes corners tend to create a summing effect of all of the modes created by the rooms three major dimensions.
The analogy of the bass reinforcement of a speaker placed in the corner is an excellent one. As is the mirror analogy.
Except for the fact that this mirror is very non-linear over its frequency range.
As for the quote "But what I have learnt and
understood with time is that absorbing low frequencies eliminates standing waves thus
avoiding the peaks and valleys across the room and reduces the
reverb time so that a bass note does not linger for too long
and reduce the clarity."
Absorbing low frequencies does not eliminate standing waves.
It just reduces their effect.
If you want to reduce low frequencies you could just use a speaker with a limited bass response.
I'm pretty sure that is not what you want.
Instead you probably want to reduce SELECTED low frequencies. This require a carefull combination of speaker placement, listener position and room treatment.
But you all knew that already.
It does if you absorb them 100 percent! Otherwise I agree with your post.
--Ethan
Regarding waves bunching in corners- it would seem then a quarter round collumn in each corner would do a lot to break up those waves, no? Just curious.
If by "column" you mean solid material, that's not a good idea. You don't want to "break up" the waves as much as absorb them.
--Ethan
Got it- thanks for clarifying.
400hz
That may or may not be a good idea depending on your room.
A solid curved surface provides a good method for obtaining dispersion. A useful tool. If you are needing absorbtion, then not so useful.
Yes, but not at bass frequencies, which I understood was the issue. Curved surfaces are not great at higher frequencies either, at least not in smaller rooms. In a large room curves are okay. See All About Diffusion partway down the list on the RealTraps Videos page.
--Ethan