pinhole Re: decibels

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From: pauld@exploratorium.edu
Date: Sat Oct 05 2002 - 10:36:39 PDT 

Message-Id: <200210051736.g95HacH07229@isaac.exploratorium.edu>
From: pauld@exploratorium.edu
Subject:  pinhole Re: decibels
Date: Sat, 5 Oct 2002 10:36:38 US/Pacific

Frequency is physics Pitch is psychology
Intensity is physics Loudness is psychology

Frequency is measured in hertz, cycles per second for a sound wave, the same
frequency will be perceived to have different pitches if it is played at
different intensities.

Intensity is measured in watts per square meter, it is the power carried across a
square meter by a sound wave (or the energy per second)

Loudness is measured in decibels,
loudness in decibels is related to the intensity I logarithmically
L = 10 log I/Io

Where Io is a reference level of 10^-12 watts/m^2

so a sound that carries 1 watt/m^2 has a loudness of 120 decibels.

However the ear has different responses to different frequencies.
It is most sensitive to frequencies between 1000 and 4000 hertz
 So decibel meters have two scales

dBA and dBC

dBC meters give equal weight to all frequencies
dBA scale meters reducve their sensitivity at high and low frequencies in a way
that is similar to but not identical with the human ear.

Here is a good website with more details
http://www.phys.unsw.edu.au/music/dB.html

Paul D

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