Thread: Awkward shaped room

> My problem is the large cupboard with 2 alcoves either side of it. <

If it's behind you, that's less damaging than if it's beside you or in front on the sides.

> I'm worried that the reflections from the front of the cupboard will be too strong and that bass will gather in the alcoves either side. <

Bass gathers most in the corners, so that's where you should plan to put bass traps. As many as you can, in as many corners as you can. Including the wall-ceiling corners if possible.

--Ethan

Originally Posted by z60611

But I'm wondering about your phrasing of

Originally Posted by Ethan

Bass gathers most in corners.

Originally Posted by Dennis Erskine

from http://archive.avsforum.com/avs-vb/s...16#post2421916

Modes do not build up in the corners! They don't today, they didn't yesterday and, unless the laws of physics are changing, they won't tommorrow either.

Room modes are created by standing waves (NOT acoustic interference*). Standing waves are always at peak pressure at the room boundary (wall, floor, ceiling). At the seating position, the standing wave, or mode, can be either a null or a peak.

Z,

You don't mean this isn't it?
That's the endless discussion again.

I find Erskine's explenation also partly very questionable.
Room modes are not created by standing waves. The words are often used interchangeable but litterally: Standing waves establish by exciting room modes. Room modes are not created by standing waves.

RPG followed the reasoning of Dennis with their first corner resonant membrane absorber.
You remember yourself the quote you gave from the Cox & d'Antonio book were they stated that velocity systems didn't work in corners. It's still in the book.
Yet RPG herself introduced corner absorbers now based on velocity, which must be around the same time that the book was published in which they wrote it didn't work.

And in fact standing waves, while no energy source DO accumulate energy by storing it in the same manner you can store your energy in a swing by pushing a child sitting on that swing.

As such while this build-up is maybe a bit a popular approach visualizing they all combine somehow in the corner, I find Dennis' explenation meant to be scientifically more exact much more sinning.
If standing waves are not related to acoustic interference, than I wonder how they can excist ....
Standing waves are the result of opposing waves, thus more than 1 wave, otherwise it should be a traveling wave. Once you have more than 1 wave they interfere. Depending on specific boundary conditions this interference can cause standing waves or other interference patterns.

To be honest I rather like a vague layman's approach which gives the feel or a picture than a wrong pseudo scientific approach which is entered to set the physical record straight.

If one put this red sentence as answer in an exam, one can be sure one is sacked, or this asterix must refer to some special text you didn't enter here.

If you express accumulating/storing energy as build-up than this word isn't that bad.
That's linguistics for experts.

You store energy until the level that the added energy balances with the disipated (absorbed) energy.

Is this the same ???:

You build up energy until the level that the added energy balances with the disipated (absorbed) energy.

I leave this for language experts.

This build-up in the corners in function of individual standing waves is wrong, when compared relative to other peaks of the same standing wave free in space.
Corners only guarantee that they all are in their highest pressure zone there. Hence you can call it a build-up when you take a broad bandwidth (summation of lots of frequencies), not when you see individual resonant frequencies.
If you take the theoretical standing wave with little of no damping than all peaks are equal, hence the ones in the corner aren't higher than the ones free in the room.

The build up against walls and in trihedral corners relate to stastistical random noise where you have a theoretical Q factor of 1 free in space, 2 against a wall, a Q of 4 in a Dihedral corner and a Q of 8 in a trihedral corner.
Translated in dB this gives + 0 dB [10log(1)] free in space, + 3 dB [10log(2)] against a wall, +6 dB [10log(4)] in a dihedral corner and + 9 dB [10log[8]] in a trihedral corner.

And those values 1,2,4,8 represent the denominator of the fraction of the sphere over which the energy is distributed.
free in space = all directions = 1/1 sphere
against a wall is 1/2 sphere
in a dihedral corner is 1/4 sphere
in a trihedral corner is 1/8 sphere
Hence the local pressure inverse increases with the diminishing surface over which the total energy is distributed.

Originally Posted by z60611

The first sentence seems similar to the last sentence.

:D Next time I will study my text better. See the last as a free bis song.

Originally Posted by z60611

But for example I haven't decided if by 'broad bandwidth' you mean an octave band with one to a few dozen modal frequencies in it, eor 20hz to 20000hz.

Doesn't matter.
Either you see resonances as discrete frequencies, or you see them combined over a broader bandwidth were the peaks will be random distributed throughout the room, rather than combining (summed pressure level of multiple frequencies) as they do in the corners.

look here:
http://www.isvr.soton.ac.uk/SPCG/Tut...ding-rooms.htm
This are standing waves. The pressure levels in ALL antinodes are equal. The ones in the corners aren't higher than the other ones.

Hence for individual standing waves the expression: "Bass build-up in the corners" is senseless.
A standing wave in a rectangular room is just a nice static repetitive pattern, whether axial, tangential or oblique.

I hear this bass build-up in corners more as a harmless: Hey guys if you do something then do it in the corners, I don't hear that as a physical description.

This 4pi loading etc.
I always wonder why electro guys like to make things incomprehensible.
It's simple: your pressure is defined by the power divided over a certain surface. Increase that surface and the pressure lowers inverse linear and vice versa.
By going to the core of things everybody understands what it's all about rather than remembering formulas they have no idea they originate from a simple surface calculation they are all familiar with.

There is a related picture in my NoiseDecay-RoomRadius MS Word document at Studiotips showing this phenomenon.
http://forum.studiotips.com/viewtopic.php?t=1414