OT: S/N-Ratio measurment - help

[Magnus Danielson]

From: Martin Czech <martin.czech at intermetall.de>
Subject: RE: OT: S/N-Ratio measurment - help
Date: Fri, 11 Feb 2000 09:34:06 +0100 (MET)

> 
> :::The A-curve may be rubbish, but it is established rubbish, and it is important
> :::for us to compare like measurements. I am reminded of the shady practice of
> :::spec'ing peak power on amps to make them sound much more powerful then they
> :::were. If you compare rms power, the amps sound similar. 
> :::
> :::The National Semiconductor LM3915 Spec sheet shows a filter for a VU response. I
> :::think that is an A-weighted response, but I am not sure.
> 
> Of course, for a psycho acoustical meaningful measurement some kind
> of "inverse" ear filter will be needed, for example in the case of
> determination how loud some industrial noise will be perceived.
> 
> I just wanted to mention that the experts seem to disagree what the
> right procedure will be, it is really not surprising, because if you
> look at the compared loudness curves test persons give, you'll see that
> for low levels it's extremely "bath tub", whereas it flattens for more
> volume. So the proposals speak of different filters for different volume
> levels or even for variable filters etc. etc.

But originally was this actually done!

If you look into ANSI Z24.3-1944 you will find that 3 curves are defined.
These curves are called A, B and C. These are to be used at diffrent sound
levels as follows:

24 to 55 dB	A curve
55 to 85 dB	B curve
85 db and up	C curve

The curves set the compensation filtering needed prior to the detector.
The C curve is totally flat. The B curve has a sligth slope in the bas but
is essentially flat from 400 Hz and up to about 5 kHz where it starts to fall
of again. The A curve is much more aggressive in suppressing the bas and has
0 gain at about 700 Hz and actually give a boost of up to 3 db until it starts
to fall of and at 6 kHz you have 0 gain again.

To the best of my knowledge have this ANSI standard been withdrawn and is now
replaced by some other IEC or ISO standard.

One must recall that these filters provide a mere approximation, but a
standardized approximation.

Cheers,
Magnus