# Thread: noise floor to ceiling.

1. ## noise floor to ceiling.

BOw. HEre is a question I have,

How does one take into account a low-frequency diffusion system in reguards to the modal response of a room?

Say the deep bass diffusion is on the rear wall, do you avarage the distance btween the front wall and the various well depths? This new dimention would in turn affect the width and ceiling dimentions. Does a low frequency diffusor shift the focus away from a standard golden modal formula? Bow bow, happy tuesday everyone

J.
A.
M!

2. Wow, how big of a diffuser are we talking about here?

3. ## that big

I was planning on about 5 feet deep, up to 23 feet wide.
BASIC calcs propose it would function down to 54 Hz??
I'm wonder what course should be taken to balance such
a beast inside the rest of the room. I may be try to
tame something WAY to big for myself too, but diffusion is so sensual spacially. J.A.M!

4. ## Re: noise floor to ceiling.

Pewep,

> How does one take into account a low-frequency diffusion system in reguards to the modal response of a room? <

I think you're looking at the wrong solution. Maybe you could build a diffusor that works down to very low frequencies, but what's the point? It seems to me that all rooms need absorption at low frequencies far more than they need diffusion.

What are you actually trying to accomplish?

--Ethan

5. Yeah, you're calculation of the low frequency limit seems accurate, but i'm with Ethan - this seems strange. I've never heard of any studio, auditorium or anything having diffusers of this size.

6. One investigation on LF diffusion was presented by James Angus at AES back in 1999. (There have been other studies.) Dr. Angus' paper: The Effect of Diffusers on Frequency Dependent Room Mode Decay (AES Preprint #5060) even had some scale models of LF diffusion and how well it worked (or didn't work - you decide! :D ).

As for Pewep's inquiry, no one's asked him how large the room is in which he'd like to build this uber-diffusor?

7. Everest shows pictures of some studios in Europe with extraordinarily large and deep diffusors.

JAM, you raise a good valid question, and Jeff, ever on the ball, that study sounds very interesting. I haven't ante'd up the \$10 to read it yet, but I'd love to know the results. One would think that a properly designed 5-foot deep diffusor could work wonders for smoothing out a room's modal response. In theory, the result of such an approach would be a much more lively room with so many more nulls and peaks that no null or peak would be dominant, and the entire response of the room would be more even. Then the room wouldn't require so much absorption.

Jeff, any more clues you can give about the conclusion of the paper?

JAM, please provide your proposed room dimensions. And exactly what kind of diffusor did you have in mind to build? How deep the wells, what pattern, how tall, how thick the dividers, what material?

Lee

8. Hey acoustics mods, this thread should really be renamed to something like 'bass diffusion' so people can search for it.

9. Jazz,

I think I can shed some light. The proposed technique is to vary room shape according to a diffusion sequence such that the number of modes is increased. In theory, this increase in modal density provides for a more diffuse low frequency sound field. (The "more nulls and peaks" thing you mentioned.)

To test the theory, Dr. Angus performed electromagnetic measurements in a room before and after certain shaped surfaces were added. (Very novel approach, I thought.) The conclusions Dr. Angus drew from his studies were as follows:

Originally Posted by Dr. James Angus, AES Preprint 5060, Section 7
We have discussed the behaviour of rooms at low frequencies. In particular we have examined the definition of a mode, modal decay, the asymptotic density of modes in different shaped rooms, and the criterion required for a diffuse field. In particular we have shown that at low frequencies the shape of the room does affect the modal density and shape. However the total increase in modes is small, although the eveness and the amplitude variation seems to be improved.

We have also presented an analysis of the effect of diffusers on room mode decay which demonstrates that they are effective in both improving the decay and increasing the effective modal bandwidth. This should have the effect of reducing the spectral variation and the diffuse field in a room. Measurements on an electromagnetic scale model confirm these hypotheses and also indicate an improvement in the mode density. In fact the experiments show a more significant increase, it is possible that this is due to the input energy being coupled into more of the modes. If this is the case this would further improve the diffusion performance of the room.

These results augur well for the development of techniques to improve the low frequency behaviour of rooms.

10. Jeff, thanks.

Lee

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