John Sayers' Recording Studio Design Forum

Andre,

> To paraphrase then "By the time you move up to 4th and 5th harmonics, of the axial modes, the non-axial modes and SBIR have a much more significant effect than the room modes." Is that close? <

It seems to me that non-axial modes would diminish in significance just as quickly. But I admit I've never measured that specifically. I'm not convinced non-axial modes really account for much anyway, outside of a theoretical completely empty room having very rigid boundaries. Clearly, boundary interference is at least as important as anything else, whether you consider the distance between the speaker and the boundary or the listener and the boundary. In fact, I've argued that boundary interference is the fundamental cause of peaks and nulls in small rooms, and modes are just a subset. The following is from the Room Modes article on my company's web site:



> Is this a simplification of concepts like the Schroeder frequency? <

I don't know what Shroeder's frequency is so I can't comment on that.

--Ethan

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Andre,

I hope you don't mind my jumping in here.

By time you move up to the 4th and 5th order modes in a room, the modal density will have increased considerably. Likewise, the average modal spacing will have decreased significantly. This usually leads to an audible "evenness" in an untreated space. As the modes are damped by treatments in the room, some persnickety frequencies (often axials, sometimes tangentials, rarely obliques) could cause problems. It simply depends on what frequency range is being considered versus what frequency range is being damped and where in the room the damping is placed.

E.g., if you have treatments that damp the modes in a room down to 150 Hz, but there's a 4th order axial at 100 Hz, then it might be a much more noticeable problem after treatments.

SBIR is fully explained in myriad references. Suffice to say, at any given listening position in an otherwise empty room, you will have 6 "first order" SBIR frequencies, 6² "second" order, and so forth. The first order frequencies are usually the most troublesome. In fact, it is often only the bounces from the front and back wall that are even noticeable. And their effect in a typical listening room can usually be sufficiently damped with good broadband treatments on the front and back walls.

Finally, I am not particularly fond of the Schroeder frequency for small rooms, since Schroeder developed the formula for this frequency with large rooms in mind. A better frequency to use, IMO, is the "Davis" frequency (from Sound System Engineering, by Davis & Davis):

3*c/Lmin

where c = speed of sound in air and
Lmin = smallest room dimension.

It is worth noting that:
1. This is equivalent to the highest 6th order axial mode.
2. A comment that accompanies this frequency in the Davis text is "Do not view it as a rigid fixed frequency."

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

It is interesting to note how certain practices become formalized over time. In Acoustics and Noise Control , Rettinger calculates modes up to the fifth harmonic.

Just a quick in case takes that the wrong way. Davis and Davis helped lead the world of electronics and acoustics from one one plateau to the next one up in changing it from an art to a science.

Andre[/i]

Andre,

One of my favorite quotes:

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

Digging up...

Is this metric or imperial ?

Thanks !

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Yes. Use either consistently.

Speed of sound in m/s and Lmin in meters: or,
Speed of sound in ft/s and Lmin in feet.

Andre

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I'm going to try to find a way to fit this into my website. I have always been engaged by the golden ratio, the room ratio and the ratios presented or discussed by all you guys that have real knowledge of that.

Thanks for your time Jeff, even if it is from such a long ago post.


Brien

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Brien Holcombe
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Sound: You can't stop it, you can only try to contain it.