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Acoustics of corners
Acoustics of corners
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Old 22nd December 2017, 02:45 PM   #1
jlo is offline jlo  France
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Default Acoustics of corners

Some acoustics :
we know that corners are places where all low frequency modes are present with max pressure and no velocity.
For this reason, many people (and many books) say that basstraps are preferably placed in corners. This is also often said for fibrous materials (despite no velocity in corners). For sure, at opposite corner, the effect will be heard. But at listening place ?
So let's imagine a room with holes at corners (a hole gives total absorption): with or without this hole LF modes will still be present, the effect of an axial mode at listener point is due to walls and not corners and should barely be changed.
Am I missing something ?
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Old 22nd December 2017, 03:28 PM   #2
turk 182 is offline turk 182  Canada
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if the "imaginary" room has parallel walls as used in standard construction then standing modes are inevitable.
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Old 22nd December 2017, 03:33 PM   #3
jlo is offline jlo  France
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With all room shapes, there will be modes. My question is : why do some acousticians and books recommend corner placement of basstraps or absorbents ?
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Old 22nd December 2017, 04:15 PM   #4
turk 182 is offline turk 182  Canada
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my understanding is that as the intersection of two boundaries it is the place where the greatest attenuation can be achieved in the least amount of space but i'm no "acoustician".
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Old 22nd December 2017, 04:47 PM   #5
Omholt is offline Omholt  Norway
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Acoustics of corners
A corner isn't only where three surfaces meet but also where two surfaces meet.

While it's common for high pressure in the tri-corner, it's no rule and will depend on areas like the type of wall and windows. One has to measure to be sure.
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Old 22nd December 2017, 05:14 PM   #6
twinter is offline twinter  United States
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Refer to Ethan Winer for a source. He has written numerous articles on room acoustics and attenuation.

Bass Trap Myths

RealTraps - Acoustic Basics

RealTraps - Home

Last edited by twinter; 22nd December 2017 at 05:17 PM.
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Old 22nd December 2017, 05:54 PM   #7
gedlee is offline gedlee  United States
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Quote:
Originally Posted by jlo View Post
Some acoustics :
we know that corners are places where all low frequency modes are present with max pressure and no velocity.
For this reason, many people (and many books) say that basstraps are preferably placed in corners. This is also often said for fibrous materials (despite no velocity in corners). For sure, at opposite corner, the effect will be heard. But at listening place ?
So let's imagine a room with holes at corners (a hole gives total absorption): with or without this hole LF modes will still be present, the effect of an axial mode at listener point is due to walls and not corners and should barely be changed.
Am I missing something ?
A hole will have some mass, and this hole will form a Helmholtz resonator with the room. This could be a boost or a loss depending on the details. If the hole is well damped and several are used (the effect on the room will depend on the total area of the holes) of different sizes and locations then you could achieve some broadband absorption inside of the room. Of course these holes have to go somewhere and on the other side of the hole the sound level goes way up. Hence such a technique does not work in a room meant to maximize sound isolation (like mine).

I have however, used various in-room features (no leakage to the outside) to make Helmholtz resonators as LF absorbers - like a seat riser for example.

By far the best solution to LF absorption is damped and hung walls. As the walls flex they become strong absorbers. In my room there are basically no modes evident above the very lowest axial one - its along the length of the room and has the least wall area as an absorber.
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Old 22nd December 2017, 06:09 PM   #8
jlo is offline jlo  France
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Quote:
Originally Posted by twinter View Post
Refer to Ethan Winer for a source. He has written numerous articles on room acoustics and attenuation.
But this is typically the litterature I refered to in my introducing post : it says "place basstraps in corners" without any justification.


My hole example was not well understood. So I'll try another way : imagine no room, and just a front wall and a parallele rear wall (in empty space, no floor, no ceiling, no side walls). It is like having infinite corners fully absorbant (no reflection at all). Despite this, there will be axial modes between the two remaining walls, front and rear even with 100% losses of all others modes. Those axial modes can only be damped with the structure and absorption of the two walls. This simply means that the corner can have practically no inluence on the axial modes seen from a centered listening position.
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Last edited by jlo; 22nd December 2017 at 06:27 PM.
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Old 22nd December 2017, 09:13 PM   #9
Bon is offline Bon  Australia
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Paul McGowan has tried the Helmholz resonators in the corners of his custom listening room which houses the Infinity IRS iv and it didn't work.
YouTube
He does give some interesting advice on positioning that he used to deal with bass issues.
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Old 22nd December 2017, 09:56 PM   #10
gedlee is offline gedlee  United States
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Quote:
Originally Posted by jlo View Post
But this is typically the litterature I refered to in my introducing post : it says "place basstraps in corners" without any justification.
In theory a corner is the worst place to put an absorber.

Quote:
My hole example was not well understood. So I'll try another way : imagine no room, and just a front wall and a parallele rear wall (in empty space, no floor, no ceiling, no side walls). It is like having infinite corners fully absorbant (no reflection at all). Despite this, there will be axial modes between the two remaining walls, front and rear even with 100% losses of all others modes. Those axial modes can only be damped with the structure and absorption of the two walls. This simply means that the corner can have practically no inluence on the axial modes seen from a centered listening position.
There is always some leakage of modes, something that one finds in exact solutions of room acoustics, but in the simpler models it doesn't exist. So this axial mode will dissipate to the sides and top in your example. It takes perfectly reflecting side walls to keep the axial modes axial. And absorption on the side walls will dissipate an axial mode, but it takes some serious math to show this. Way beyond the simple modal model usually used.
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