VO-BB - 20 YEARS OLD!

[D Voice]

Can someone please explain the 38% rule?

I have seen it mentioned, but can't find more details.

TIA

[asnively]

Do you mean the 80/20 rule?
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[Deirdre]

He's referring to This Thread Where Mike Sommer asks:

[Diane Maggipinto]

as long as it's not 110%
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[Deirdre]

I know— I hate that.
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[Mike Sommer]

Oops, I thought I had covered this before -Sorry.

38% is often the optimal listing and recording position in a room.

If one is sitting at this position in a room, sound is less likely to be effected by bass Modes and Nulls. Taking a rooms size, one can mathematically predict modal resonance spacing. On average sitting 38% off the wall, of a rooms length, sets one in a neutral area. This means that sound is neither effected by a rooms modal resonance or a null. This is very important when recording or mixing. If you are in a position where where a bass mode lands, sound at that frequency will be colored, or will not be heard in a null.

This is why if you ever mixed something, and it sounds great in the studio, but in the car or another system it sounds to boomy (the result of a null), or to thin (the result of a sitting in the middle of a frequency).

This image illustrates what I'm talking about:
Taking the rooms length of 15-feet, setting back 38% puts one at about 4'-6" away from the wall.


The smaller the room the closer the frequencies come together, and the higher the room resonance frequencies being. Every room has its own frequency. Like a slide whistle, the longer the tube, the lower the frequency, the shorter the tube the higher the frequency.

The same thing with a Violin or a Stand Up Bass. A violin resonates at a higher frequency than a standup bass. And the same thing goes for our rooms. Our rooms are nothing more than a box full of air. When the air is vibrated in that box, the dimensions of that box determines the frequency at which the sound resonates. If the box resonates at Middle C, then all the cords off Middle C will be enhances. Think of it as a Guitar. Even though you pluck one string of the guitar, the strings next to it vibrate also. That's why a guitar is tuned the way it is. This way when all the strings are fingered correctly in a cord, they all work together as a total instrument. (This is very simplistic but it's the general idea)

A rooms resonate frequency is crated by it's dimensions. The time that it takes sound to travel in a room, to make a full cycle, is it's frequency. The rooms measured length is only a half cycle. To make a full cycle, sound needs to travel the length and back again-- that is a full cycle.

To calculate for a room dimension 10 feet long: divided 1130 (the speed of sound in feet per second) by twice the rooms length 10 feet: 10 X 2 = 20.

1130 /(10 x 2) = 56.5 Hz

For a room 15 feet long : 1130 / (15x2) = 15.6 Hz

To figure out all the other frequencies affected but the room resonance you just need to do some simple math. To find the next frequency affected by the rooms resonance, simply add the rooms resonance frequency to itself.

For a 10-foot room the room's First mode frequency is 56.5 Hz.
So, 56.5 + 56.5 = 113.0 is the second mode frequency.
113.0 + 56.5 = 169.0 is the third mode frequency, and so on...

All subsequent modes are just multiples of that - just keep adding the rooms resonance frequency to the last sum, until you reach 400 Hz. This will be your rooms Resonate Bass Frequencies. We are only worried about bass because this is what effects our sound the most. It's what makes us sound muddy and unclear.

The effected frequencies of a room:
10-feet long are-
56.5 Hz (Room resonance frequency)
113.0 Hz
169.5 Hz
226.5 Hz
339.0 Hz

16.5-feet long are-
34.2 Hz (Room resonance frequency)
68.4 Hz
102.6 Hz
136.8 Hz
171.0 Hz
205.2 Hz
239.4 Hz
273.6 Hz
307.8 Hz

21.5-feet long are-
26.3 Hz (Room resonance frequency)
52.6 Hz
78.2 Hz
105.2 Hz
131.5 Hz
157.8 Hz
184.1 Hz
210.4 Hz
263.7 Hz
289.3 Hz
315.6 Hz

As you can see by these numbers, the longer the room the lower the frequencies start. And the longer the room is, the more evenly spaced these frequencies are.

(We usually figure a rooms dimension to 400 Hz. These are the frequencies that can be colored, and create the most problems)

Ask questions of you don't understand.
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Acoustics are counter-intuitive. If one thing is certain about acoustics, it is that if anything seems obvious it is probably wrong.

[asnively]

</brain_explodes>
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[Philip Banks]

Ok. I admit. I laughed. 28% of me was nodding in approval but you couldn't hear it because I gated everything below -12db.

[ballenberg]

Crap, I've spent my entire career recording at 39.678 percent.

Time for new career.

Philip and I are now going into the curry industry. And we'll corner 38 percent of the market in both Scotland and New Jersey.

[Philip Banks]

"How would you like me to position my Vindaloo?" sounds like a sordid euphamism but it will be part of our Curry strategy.

[ballenberg]

Yes, it will indeed. And I do believe we have twisted this poor thread mercilessly.

Kindly position the shrimp vindaloo at 42.389 and the chicken at the aforementioned 38 minus pi times the distance of your kitchen to the wee huttie.

I must now rest.

Before Mike takes a contract on my life.

[Pam]

I usually just set my dial to 11 and that covers it.
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Now what did I come in here for?

[JBarrett]

What I find interesting is that if you take the answer to life, the universe, and everything (42) and add it to Mike's 38, you get the 80 part of the 80/20 rule. So where does the 20 come from?
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[Philip Banks]

Isn't your 20 where you are now as opposed to where you are from, good buddy..c'mon?

[D Voice]

how about for recording? with the back of the microphone to a corner?