> I agree; the 5 dB figure may apply to urban interior noise, which
> has a lot of traffic rumble.
The noise figures which we are discussing as "excess noise" are
relative to the fundamental thermal noise of the air. This rises at
the rate of 3dB per octave so, in comparison, low frequency noise is
much greater than low frequency thermal noise. Fortunately our ears
are less sensitive to LF at low levels.
The thermal noise level from 0Hz to 1kHz is 12dB lower than the
thermal noise from 0Hz to 16kHz. I'll let someone else integrate the
noise under a A-weighting curve. :-) White noise energy is constant
over the same bandwidth, say over a range of 1kHz, but thermal noise
energy rises in proportion with frequency, hence the 3dB per octave
figure. (Why is it not 6dB per octave?)
This is why weighting is important in all noise measurements. I
haven't concentrated on the source of the 5dB figure but this is
probabably about right with A weighting.
Going back to a related subject, adding noise at -4dB to a signal at
0dB raises the level by 1dB, which is just about what we could detect
on a meter. However to the ear, adding hiss to a rumble will be very
noticeable and this is only one instance when the theory breaks down.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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