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G8MNY > TECH 12.03.04 19:58l 68 Lines 3094 Bytes #999 (0) @ WW
BID : 47950_GB7CIP
Read: DL7NDF DB0FHN GUEST OE7FMI
Subj: Sound Levels, dBA dBB & dBC
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Sent: 040312/1118Z @:GB7CIP.#32.GBR.EU #:47950 [Caterham] $:47950_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
(updated Mar 04)
The human ear has a very wide frequency & level range. From 0dB that
approximates to the threshold of hearing & represents an ear drum movement in
the order of the width of an atom, to energy levels > million million times
stronger, or > million times that movement. This is achieved by an AGC action
in the ear that uses a muscles to dampen the movement of the acoustic path to
the cochlea. The minimum level change that can be detected is about 2dB over
this range.
Too much sound for short time results in ringing in the ear, some damage
usually has happened if you hear this. Long periods of loud sound actually
breaks off the frequency sensing hairs in the cochlea!
Frequency resolution is about 2% at most frequencies, eg 20Hz @ 1KHz. Frequency
range & sensitivity changes with age, from 20Hz-20KHz when young, to -20dB @
4KHz when old with some 20-40dB reduction in overall sensitivity as well.
Here are some typical sound levels taken from a 1963 DAWE Instruments Meter
handbook. Some sounds may be louder now! (eg noisy Jets, louder HGVs etc)
EXAMPLE dB Wtg NOISE Pressure mPa
Jet Aircraft @ 500ft 120 dBC Deafening 20,000
Boiler Making Factory 110 dBC Deafening 6,300
Pneumatic Drill 110 dBC Deafening 2,000
Motor Horn 105 dBC Deafening 600
Inside Old Tube Train 95 dBC Very Loud 200
Busy Street 93 dBC Very Loud
Workshop 88 dBC Very Loud
Small car @ 24ft 83 dBB Very Loud
Noisy Office 78 dBB Loud
Inside small Car 73 dBB Loud
Large Shop 68 dBB Loud
Radio Set @ Full Volume 65 dBB Loud 68
Normal Conversation 58 dBB Moderate
Urban House 53 dBA Moderate
Quiet Office 48 dBA Moderate
Rural House 43 dBA Moderate
Public Library 38 dBA Faint
Quiet Conversation 33 dBA Faint
Rustle of Paper 28 dBA Faint
Whisper 23 dBA Faint
Quiet Church 18 dBA Very Faint
Sill Night in Country 13 dBA Very Faint
Sound Proof Room 5 dBA Very Faint 0.6
Threshold of Sound 0 dBA Extremely Faint 0.2
dBC is a frequency response weighting & not dBc which is dBs with respect to an
RF carrier. The weighting changes with level to reflect the human ear response.
Weighting dBA is for sound levels up to 55dB it has a response peaking at 2KHz
with heavy LF cut of -50dB @ 20Hz & -10dB @ 20KHz
Weighting dBB is for sound levels between 55dB & 85dB, it is flatter than dBA
with only -24dB @ 20Hz LF roll off & -10dB @ 20KHz
And dBC Weighting is level above 85dB, it is quite flat 30Hz-10KHz, and only
-10dB roll off at 10Hz & 20KHz.
These tonal level changes are the principle behind Loudness controls on modern
audio domestic systems, eg boost low frequency Bass & top end Treble, as it
will sound much louder to the ear.
/QSL
73 De John, G8MNY @ GB7CIP
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