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B.M.
03-01-2010, 09:53 AM
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, itís 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, itís 45db up on what?

:thanks in advance

Terry Porritt
03-01-2010, 10:37 AM
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) ó 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)

http://en.wikipedia.org/wiki/Decibel

B.M.
03-01-2010, 07:10 PM
Thanks for that Terry. :thumbs:

It comes as some relief to find that a Decibel is still what itís 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 Iíve 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 weíve established that 3db is double and 10db is 10 times so itís a matter of how many fold 25db represents? :cool:

PS does XL have a Logarithm function?

Sweep
03-01-2010, 08:02 PM
Perhaps this may help:-

http://www.thecharge.org/decibelcalculations.pdf

R2x1
03-01-2010, 08:05 PM
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".

Terry Porritt
03-01-2010, 08:15 PM
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(P≤/Pr≤) = 25 , Pr=reference pressure=20micropascals

log(P≤/Pr≤) = 2.5, getting the antilog 10^2.5 gives

P≤/Pr≤ = 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....

kenj
04-01-2010, 06:04 AM
Heavy, Man!!

Ken :p

B.M.
04-01-2010, 08:36 AM
Perhaps this may help:-

http://www.thecharge.org/decibelcalculations.pdf

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. ;)

Cicero
04-01-2010, 02:08 PM
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(P≤/Pr≤) = 25 , Pr=reference pressure=20micropascals

log(P≤/Pr≤) = 2.5, getting the antilog 10^2.5 gives

P≤/Pr≤ = 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.

SurferJoe46
04-01-2010, 02:23 PM
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.

Terry Porritt
04-01-2010, 02:52 PM
Bed by 9:15,you are indeed getting on.

9.30pm for me these days is about bedtime...though I do get up at 6 - 6.30am.

Terry Porritt
04-01-2010, 02:54 PM
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.

I do hope your hearing has not suffered as a result :rolleyes:

Cicero
04-01-2010, 03:08 PM
9.30pm for me these days is about bedtime...though I do get up at 6 - 6.30am.

Thanks for info Ter,I am getting up to date with the ways of the aged.

decibel
04-01-2010, 07:46 PM
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.
Actually, one-tenth of a bel (and that's with one L, not two)

Cicero
04-01-2010, 07:55 PM
Thanks for info Ter,I am getting up to date with the ways of the aged.

Just about ready for the jammies.

http://www.youtube.com/watch?v=T_Y1hcHnUrg&feature=related

Terry Porritt
04-01-2010, 08:27 PM
Just about ready for the jammies.

http://www.youtube.com/watch?v=T_Y1hcHnUrg&feature=related

Your taste is getting better, Deano is not bad, but I prefer Al Bowlly....

http://www.youtube.com/watch?v=MxiC28aaHww

Now it is goodnight from me.....

Cicero
04-01-2010, 08:36 PM
Your taste is getting better, Deano is not bad, but I prefer Al Bowlly....

http://www.youtube.com/watch?v=MxiC28aaHww

Now it is goodnight from me.....

Charlie will send you off to bye byes.

http://www.youtube.com/watch?v=k5lwTlpMVRA

Sweep
04-01-2010, 09:05 PM
Your taste is getting better, Deano is not bad, but I prefer Al Bowlly....

http://www.youtube.com/watch?v=MxiC28aaHww

Now it is goodnight from me.....

And it's goodnight from him.

Cicero
04-01-2010, 09:15 PM
And it's goodnight from him.

Terr will be tucked in now with his teddy.


We know about the two Ronnie's don't we Sweep?

Sweep
04-01-2010, 09:28 PM
Terr will be tucked in now with his teddy.


We know about the two Ronnie's don't we Sweep?

Yep. Also Morecambe and Wise maybe?

And Mr Bean has a teddy but I'd prefer a female wearing one.

Cicero
04-01-2010, 09:41 PM
Yep. Also Morecambe and Wise maybe?

And Mr Bean has a teddy but I'd prefer a female wearing one.

I am an authority on the subject of who said what..............

Ronnie Corbett used to say, referring to the following week's show, ''Until then, it's goodnight from me...'' and Ronnie Barker would say ''...and it's goodnight from him''. Then both would say ''Goodnight!'' at the same time.

Wouldn't be bad if you knew what to do when meeting girl in Teddy.

KarameaDave
04-01-2010, 09:43 PM
Harry Worth was funny as well
http://en.wikipedia.org/wiki/Harry_Worth