[GreenKeys] Re: teletype machine noise levels

[Ray Robinson]

Hi Gill,

Sound, its all relative!

I have scanned a chart of levels and it is at:
http://www.shlrc.mq.edu.au/~robinson/Information/SPL.jpg
It doesn't have TTYs on it, but perhaps they come in at 80db SPL.
and another table here:
http://home.new.rr.com/trumpetb/audio/dBexamp.html

How loud is it?
Depends upon how close are you to the source, and what frequency it is.
I've forgotten, but I think you hold the SPL meter 1 meter away
and to the side of the noise source.
That is, if measuring a vehicle noise output, you don't shove it up the tail pipe.

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Some definitions:

SPL (Sound Pressure Level) is used to measure sound (equivalent to voltage ie volts)
The ratio between the actual SOUND PRESSURE and a fixed reference pressure. 
This reference pressure is the THRESHOLD OF HEARING (defined as 0db SPL)
which has been internationally agreed upon as having the value .0002 dynes/cm2. 

SI (Sound Intensity) is the sound energy transmitted per unit time through a unit area, 
being a measure of the magnitude of a sound (equivalent to power ie watts). 
The unit of measurement is the erg per second per square centimeter, 
or the watt per square meter. 

Which means, instantaneous level is SPL, and dosage over time is SI.

The THRESHOLD OF HEARING lies at 10-12  watts/m2
The THRESHOLD OF PAIN is about 1 watt/m2.

A normal SPL meter measures sound with a flat frequency response,
but is has several weighting curves that can be switched in.
The weighting "A" approximates the normal human ear.
With this switched in, the SPL meter now measures SOUND LEVEL.
Just to complicate things,
the curve changes depending on how loud the sound is.
Thats why you have a loudness tap on the volume control in your stereo,
it increases the Low frequencies at low volumes.

There is more techo info here:
http://www2.sfu.ca/sonic-studio/handbook/Sound_Pressure_Level.html
and I've quoted some of the stuff above from it.

All this stuff applies to the real world.
When sound gets coverted to voltage
and starts running around inside an amplifier,
things are different, VUs, dbM, and other rules apply.

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I work in this area and have to deal with people talking this stuff,
and I have to pretend to understand it.
Sorry to get techo.

Regards
Ray vk2ilv