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Thread ID: 106256 2010-01-02 21:53:00 The Decibel B.M. (505) PC World Chat
Post ID Timestamp Content User
844908 2010-01-02 21:53:00 Maybe one of you learned people out there could explain to me what a decibel is these days?

When I used to work with them 0dbm was 1 milliwatt across 600 Ohms.

Then a gain of 3db equated to twice the power.

So, in the case of modern Noise Meters and the like used commonly these days by Councils and Acoustic Engineers, what are they saying when they say the noise is 45db?

Or, to put it another way, it’s 45db up on what?

Also, if it were 48db would it be twice as much as 45db?
Maybe one of you learned people out there could explain to me what a decibel is these days?

When I used to work with them 0dbm was 1 milliwatt across 600 Ohms.

Then a gain of 3db equated to twice the power.

So, in the case of modern Noise Meters and the like used commonly these days by Councils and Acoustic Engineers what are they saying when they say the noise is 45db?

Or, to put it another way, it’s 45db up on what?

:thanks in advance 		B.M. (505)
844909 2010-01-02 22:37:00 It hasn't changed :) However it is a most misused parameter, I wont say unit because it has no units, it is dimensionless, the logarithm of a ratio of quantities .

Quoting from Wikipedia:

"Acoustics

Probably the most common usage of "decibels" in reference to sound loudness is dB SPL, referenced to the nominal threshold of human hearing:[11]

dB(SPL)

dB (sound pressure level) — for sound in air and other gases, relative to 20 micropascals (μPa) = 2×10−5 Pa, the quietest sound a human can hear. This is roughly the sound of a mosquito flying 3 metres away. This is often abbreviated to just "dB", which gives some the erroneous notion that "dB" is an absolute unit by itself. For sound in water and other liquids, a reference pressure of 1 μPa is used.[12]

(my emphasis, this is how it is misused every day)

en.wikipedia.org 		Terry Porritt (14)
844910 2010-01-03 07:10:00 Thanks for that Terry. :thumbs:

It comes as some relief to find that a Decibel is still what it’s always been, despite some Pirates trying to hijack it.

However, moving onto the calibration of these machines that are calibrated for noise. Just how would one calibrate them to 20 micropascals? My Barometer measures Hectopascals and I’ve never heard it make a sound. :confused: A Mosquito flying at 3 metres from you seems like a most unreliable way to calibrate a scientific instrument? :rolleyes:

All that aside, can somebody with a Logarithm book tell me how many times 25db would increase a sound? :thanks

I think we’ve established that 3db is double and 10db is 10 times so it’s a matter of how many fold 25db represents? :cool:

PS does XL have a Logarithm function? 		B.M. (505)
844911 2010-01-03 08:02:00 Perhaps this may help:-

www.thecharge.org 		Sweep (90)
844912 2010-01-03 08:05:00 The decibel is just a cheap-jack substitute for the original user-friendly Dozibell ;)
This in turn came from the nautical time keeping event of "12 bells" which in turn meant "It's very late and I'm confused". 		R2x1 (4628)
844913 2010-01-03 08:15:00 The pressure in sound pressure measurements is the root mean square of the sound wave, not a static pressure as you measure with a barometer.

The power in a sound wave is proportional to pressure squared, so your example of 25dB would be written

10log(P²/Pr²) = 25 , Pr=reference pressure=20micropascals

log(P²/Pr²) = 2.5, getting the antilog 10^2.5 gives

P²/Pr² = 316.22

So the pressure ratio would be 17.8

I seem to remember from physics about 50+ years ago something called the Weber-Fechner law, the sound perceived by the ear approximates logarithmic. Perhaps someone could look it up, I'm off to bed shortly....old age catching up.... 		Terry Porritt (14)
844914 2010-01-03 18:04:00 Heavy, Man!!

Ken :p 		kenj (9738)
844915 2010-01-03 20:36:00 Perhaps this may help:-

www.thecharge.org

Thanks for that link Sweep it further explained what I was looking for and jogged the old brain back 50 years. :D

However, whilst downloading I got hit by that Security Tool Virus so don't know if that site has been infected or if it was pure coincidence. :groan:

But I'll make that the subject of another thread. ;) 		B.M. (505)
844916 2010-01-04 02:08:00 The pressure in sound pressure measurements is the root mean square of the sound wave, not a static pressure as you measure with a barometer.

The power in a sound wave is proportional to pressure squared, so your example of 25dB would be written

10log( P²/Pr² ) = 25 , Pr=reference pressure=20micropascals

log( P²/Pr² ) = 2.5, getting the antilog 10^2.5 gives

P²/Pr² = 316.22

So the pressure ratio would be 17.8

I seem to remember from physics about 50+ years ago something called the Weber-Fechner law, the sound perceived by the ear approximates logarithmic. Perhaps someone could look it up, I'm off to bed shortly....old age catching up....

Bed by 9:15,you are indeed getting on. 		Cicero (40)
844917 2010-01-04 02:23:00 Hey youse guys! I should not have to tell you about your own ciphering rules but even I know that a decibel is 10-bells. SurferJoe46 (51)
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