Absolute Noise Level

[Sahara]

Hello,

I am working for one sound level meter project but have a doubt as term ABSOLUTE ??

Does anyone help me to understand what is Absolute sound level meter or absolute noise level meter? What is a value in dB for absolute noise?

any technical document note/application note/ppt will help me to understand if any one can share

 

[chuckey]

I would guess that the absolute audio noise floor is set by the brownian air molecules motion. i.e. the random motion of the air molecules due to thermal agitation. It definitely exists, though I am not sure whether, as a theoretical figure it offers us any use. i.e., suppose the level is -200 dB, what then?, we can't hear it and any real world measurements would be so high compared to this figure that it can be ignored.
Good luck.
Frank

 

[jpanhalt]

I believe the term "absolute" sound level probably refers to the actual pressure of the sound in pascals at a particular distance (http://www.nchearingloss.org/spl.htm?fromncshhh).
Wikipedia also discusses sound pressure (http://en.wikipedia.org/wiki/Sound_pressure ), but you may find this article and table a bit more useful (http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm ).



John

 

[Dan Mills]

In this context the term absolute probably means that the instrument measures level relative to a defined level, typically 20 uPa is considered 0dB in spl measurements, rather then simply giving a relative indication.

Note that the normal reference level used does vary, in underwater work for example you see 1 uPa being used as the reference most of the time.

HTH.

Regards, Dan.

 

[Sahara]

Thanks for the reply to everyone, but it is still not clear to me about Absolute Noise level.

First of all, the question arries to me is that Does it any such meter avaiable readly in market which talks about Absolute Noise meter? Second, if we measure noise level through conventional sound level meter avaiable in market, what values should be considered Absolute?

Somewhere, I read that SPL is a ratio for absolute to reference, but as we dont know Absolute, we are not be able to get SPL, right??

 

[Dan Mills]

0dB SPL = 20uPa, that is your reference level.

So make a measurement, figure out how many dB it is above or below 20uPa and that is your absolute measurement ref 20uPa.

Now there are various filters that may need to be applied before doing the conversion to dB, depends on the required 'weighting' and you should probably limit the bandwidth into the log amp to something sane (and specified), but the absolute measurement itself is simply dB relative to 20uPa.

 

[FvM]

if we measure noise level through conventional sound level meter avaiable in market, what values should be considered Absolute?

You can safely assume that a "conventional sound level meter" measures absolute sound level, ususally A- or C-weighted (unit "dBA" or "dBC"), using the said 20 µPa reference. That's simply because 0 dBA is considered as the standard human hearing threshold.