"Richard Dobson" <richarddobson@blueyonder.co.uk> wrote in message 
news:nI3Ld.2659$8B3.1098@text.news.blueyonder.co.uk...
> Fred Marshall wrote:
>
> ..
>> The standard for SPL is in micropascals (Pa=N/m^2) and 0dB is *different* 
>> if you are working in water vs. in air.  120Db spl in air is at the 
>> threshold of pain or injury.  0dB is usually the standard or reference 
>> and at the threshold of hearing.
>>
>> 0dB == 0.0002 ubar = 1E+5 upa/ubar  * 0.0002= 20upa in air
>> and
>> 0dB == 1upa in water
>>
>> Fred
>>
>>
>>
>  But note that music studio engineers use 0dBFS to indicate digital peak 
> (i.e. maximum amplitude, +-32767 in a 16bit system), with all lesser 
> levels accordingly indicated as a negative dB value. For floating point 
> samples (in both soundfile formats such as WAVE and AIFF-C, and streaming 
> plugin formats such as DX and VST), 0dBFS is defined to be 1.0. In mixing 
> desks faders are marked from 0dB down to -inf, with typically some room 
> above for gain, up to +24dB or so. This will often translate to nominal 
> 0dB being calibrated as, for example, -18dBFS (there are other 
> conventions), allowing engineers to use a digital desk and feel they have 
> the same headroom "at the top" that they enjoyed in the analogue-tape 
> studio (using the famous "fader-creep" technique).
>
> With this measuring-downwards system, the same dB level will translate 
> trasnparently to any sample size, whether 8bit, 24 or 32bit int, 32bit 
> float and even 64bit float, which some soundfile formats can support. 
> There have even been 20bit formats.

Ah.  Very interesting and a useful thing to know for DSP!
Of course, it is independent of actual SPL and only applies to received, 
converted, levels.

Fred

Fred Marshall wrote:

..
> The standard for SPL is in micropascals (Pa=N/m^2) and 0dB is *different* if 
> you are working in water vs. in air.  120Db spl in air is at the threshold 
> of pain or injury.  0dB is usually the standard or reference and at the 
> threshold of hearing.
> 
> 0dB == 0.0002 ubar = 1E+5 upa/ubar  * 0.0002= 20upa in air
> and
> 0dB == 1upa in water
> 
> Fred
> 
> 
> 
  But note that music studio engineers use 0dBFS to indicate digital peak (i.e. 
maximum amplitude, +-32767 in a 16bit system), with all lesser levels 
accordingly indicated as a negative dB value. For floating point samples (in 
both soundfile formats such as WAVE and AIFF-C, and streaming plugin formats 
such as DX and VST), 0dBFS is defined to be 1.0. In mixing desks faders are 
marked from 0dB down to -inf, with typically some room above for gain, up to 
+24dB or so. This will often translate to nominal 0dB being calibrated as, for 
example, -18dBFS (there are other conventions), allowing engineers to use a 
digital desk and feel they have the same headroom "at the top" that they enjoyed 
in the analogue-tape studio (using the famous "fader-creep" technique).

With this measuring-downwards system, the same dB level will translate 
trasnparently to any sample size, whether 8bit, 24 or 32bit int, 32bit float and 
even 64bit float, which some soundfile formats can support. There have even been 
20bit formats.

Richard Dobson

"Clay S. Turner" <Physics@Bellsouth.net> wrote in message 
news:6oSKd.18$t67.8@bignews5.bellsouth.net...
> Hello Fred and others,
>
> I recall the standard for 120 dB spl is 0.946 watts/meter^2.  So dB spl is 
> simply a power density. Apparently this standard assumes a standard size 
> for the eardrum, etc. But at least this is a way to calibrate a sound 
> meter.
>
>
> Clay

The standard for SPL is in micropascals (Pa=N/m^2) and 0dB is *different* if 
you are working in water vs. in air.  120Db spl in air is at the threshold 
of pain or injury.  0dB is usually the standard or reference and at the 
threshold of hearing.

0dB == 0.0002 ubar = 1E+5 upa/ubar  * 0.0002= 20upa in air
and
0dB == 1upa in water

Fred

Hello Fred and others,

I recall the standard for 120 dB spl is 0.946 watts/meter^2.  So dB spl is 
simply a power density. Apparently this standard assumes a standard size for 
the eardrum, etc. But at least this is a way to calibrate a sound meter.


Clay




"Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in message 
news:gt-dnZXWKaXQAWfcRVn-iw@centurytel.net...
>
> "Jon Harris" <goldentully@hotmail.com> wrote in message 
> news:35ve69F4s60c0U1@individual.net...
>> "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in message
>> news:IbKdnZQLGLNhLmTcRVn-2w@centurytel.net...
>>>
>>> "Jon Harris" <goldentully@hotmail.com> wrote in message
>>> news:35t81qF4r6eq0U1@individual.net...
>>> >
>>> > A very important parameter is the distance between the sound source 
>>> > and
>>> > the
>>> > microphone.  I think 1m is a typical specification.  You also need to 
>>> > make
>>> > some
>>> > assumptions about the recording environment--consider that a drum 
>>> > played
>>> > in a
>>> > small reflective room sounds louder than the same drum outside on a 
>>> > lawn
>>> > due to
>>> > reflections.
>>>
>>> 1m is a typical *reference* distance for the sound pressure measured. 
>>> It
>>> isn't necessarily typical at all for the distance of the actual 
>>> measurement.
>>>
>>> Example:
>>>
>>> Measure sound pressure level in free space (or reasonable facsimile 
>>> thereof)
>>> at 10 meters distance from projector to receiver.
>>> Assume spherical spreading and correct the sound pressure level back to 
>>> 1m
>>> using 20log(R1/R0) so 20dB where R1 is 10m and R0 is 1m.  At least 
>>> that's
>>> how I remember it.....
>>
>> That makes sense.  I've always seen the charts that say at 1m, but if you 
>> are
>> measuring say a jet engine, it might be a little easier to back off a bit 
>> and
>> then compensate mathematically when doing the actual measurement!  :-)
>
> Right.  The charts or specifications use 1m because that's the standard 
> for comparison.  Otherwise you'd be forever trying to figure out how to 
> compare two sources / projectors.  This way, assuming the measurement and 
> the distance correction were both reasonable, you can compare two sources 
> directly.  Radiation patterns come into the equation of course....
>
> Fred
> 

To make a calibrated recording you must first use good equipment that is 
linear with both frequency and level over the ranges of concern.  Then, you 
must make sure the equipment does not have an automatic gain control that 
will attenuate loud sounds and amplify soft ones in an attempt to squeeze 
everything into the best range of the system.  Then, you must record a 
signal of known level on the recording, noting all gain settings of the 
system.  This is most commonly done with a field calibrator - a device 
placed over the microphone that produces a tone of known level.  If you make 
any changes in gain settings between the recording of the calibration tone 
and the recording of the sound of interest, then you must note these and 
make appropriate adjustments to the level at play back.
<50295@web.de> wrote in message 
news:1106848073.031008.198620@z14g2000cwz.googlegroups.com...
> Hi everyone,
>
> I suppose this is an easy question for some, but I'm trying to
> calculate in dB, the amplitude of a recorded WAV sound file. Is this
> possible? How can it be done? Are there any libraries/APIs that do
> this?
>
> Thanks,
>
> - Olumide
> 

"Jon Harris" <goldentully@hotmail.com> wrote in message 
news:35ve69F4s60c0U1@individual.net...
> "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in message
> news:IbKdnZQLGLNhLmTcRVn-2w@centurytel.net...
>>
>> "Jon Harris" <goldentully@hotmail.com> wrote in message
>> news:35t81qF4r6eq0U1@individual.net...
>> >
>> > A very important parameter is the distance between the sound source and
>> > the
>> > microphone.  I think 1m is a typical specification.  You also need to 
>> > make
>> > some
>> > assumptions about the recording environment--consider that a drum 
>> > played
>> > in a
>> > small reflective room sounds louder than the same drum outside on a 
>> > lawn
>> > due to
>> > reflections.
>>
>> 1m is a typical *reference* distance for the sound pressure measured.  It
>> isn't necessarily typical at all for the distance of the actual 
>> measurement.
>>
>> Example:
>>
>> Measure sound pressure level in free space (or reasonable facsimile 
>> thereof)
>> at 10 meters distance from projector to receiver.
>> Assume spherical spreading and correct the sound pressure level back to 
>> 1m
>> using 20log(R1/R0) so 20dB where R1 is 10m and R0 is 1m.  At least that's
>> how I remember it.....
>
> That makes sense.  I've always seen the charts that say at 1m, but if you 
> are
> measuring say a jet engine, it might be a little easier to back off a bit 
> and
> then compensate mathematically when doing the actual measurement!  :-)

Right.  The charts or specifications use 1m because that's the standard for 
comparison.  Otherwise you'd be forever trying to figure out how to compare 
two sources / projectors.  This way, assuming the measurement and the 
distance correction were both reasonable, you can compare two sources 
directly.  Radiation patterns come into the equation of course....

Fred 

"Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in message
news:IbKdnZQLGLNhLmTcRVn-2w@centurytel.net...
>
> "Jon Harris" <goldentully@hotmail.com> wrote in message
> news:35t81qF4r6eq0U1@individual.net...
> >
> > A very important parameter is the distance between the sound source and
> > the
> > microphone.  I think 1m is a typical specification.  You also need to make
> > some
> > assumptions about the recording environment--consider that a drum played
> > in a
> > small reflective room sounds louder than the same drum outside on a lawn
> > due to
> > reflections.
>
> 1m is a typical *reference* distance for the sound pressure measured.  It
> isn't necessarily typical at all for the distance of the actual measurement.
>
> Example:
>
> Measure sound pressure level in free space (or reasonable facsimile thereof)
> at 10 meters distance from projector to receiver.
> Assume spherical spreading and correct the sound pressure level back to 1m
> using 20log(R1/R0) so 20dB where R1 is 10m and R0 is 1m.  At least that's
> how I remember it.....

That makes sense.  I've always seen the charts that say at 1m, but if you are
measuring say a jet engine, it might be a little easier to back off a bit and
then compensate mathematically when doing the actual measurement!  :-)

50295@web.de wrote:
> I suppose this is an easy question for some, but I'm trying to
> calculate in dB, the amplitude of a recorded WAV sound file. Is this
> possible? How can it be done? Are there any libraries/APIs that do
> this?

You need to know the sensitivity of the microphone in millivolts per 
pascal, and the sesitivity od the sound board in amplitude uits per volt.

Else, you need to have placed the output tone of a calibrator onto that 
or accompanying .WAV file, and the board purveyance has to be linear (NO 
AGC, clipping or compressing).

Lots of luck!

Angelo Campanella

"Jon Harris" <goldentully@hotmail.com> wrote in message 
news:35t81qF4r6eq0U1@individual.net...
> "Ronald H. Nicholson Jr." <rhn@mauve.rahul.net> wrote in message
> news:ctbl8j$feu$1@blue.rahul.net...
>> In article <8LidnVK9FtyAs2TcRVn-rg@giganews.com>,
>> Ethan Winer <ethanw at ethanwiner dot com> wrote:
>> >Olumide,
>> >
>> >> I'm trying to calculate in dB, the amplitude of a recorded WAV sound 
>> >> file.
>> >Is this possible? <
>> >
>> >I'm afraid it's not possible to determine from a recording how loud the
>> >sound was when it was recorded originally.
>>
>> Is it possible with some sort of calibration sound file, and knowledge
>> of if and/or how much the input amplification changed between files?
>
> With enough knowledge of _all_ the conditions, this would be possible.
>
>> If so, how would one go about recording a calibrated sound input?
>
> Calibrated microphone, calibrated gain, calibrated environment 
> (echo/damping and
> noise would affect the result), calibrated microphone-to-source distance,
> calibrated recorder, etc..  Difficult, but not impossible.
>
>> I seem to recall reading some childrens science book which had a table
>> of the sound dB values of whispering, normal conversation, cars, trains,
>> jet aircraft on takeoff, etc.
>
> A very important parameter is the distance between the sound source and 
> the
> microphone.  I think 1m is a typical specification.  You also need to make 
> some
> assumptions about the recording environment--consider that a drum played 
> in a
> small reflective room sounds louder than the same drum outside on a lawn 
> due to
> reflections.

1m is a typical *reference* distance for the sound pressure measured.  It 
isn't necessarily typical at all for the distance of the actual measurement.

Example:

Measure sound pressure level in free space (or reasonable facsimile thereof) 
at 10 meters distance from projector to receiver.
Assume spherical spreading and correct the sound pressure level back to 1m 
using 20log(R1/R0) so 20dB where R1 is 10m and R0 is 1m.  At least that's 
how I remember it.....

Fred 

"Ronald H. Nicholson Jr." <rhn@mauve.rahul.net> wrote in message
news:ctbl8j$feu$1@blue.rahul.net...
> In article <8LidnVK9FtyAs2TcRVn-rg@giganews.com>,
> Ethan Winer <ethanw at ethanwiner dot com> wrote:
> >Olumide,
> >
> >> I'm trying to calculate in dB, the amplitude of a recorded WAV sound file.
> >Is this possible? <
> >
> >I'm afraid it's not possible to determine from a recording how loud the
> >sound was when it was recorded originally.
>
> Is it possible with some sort of calibration sound file, and knowledge
> of if and/or how much the input amplification changed between files?

With enough knowledge of _all_ the conditions, this would be possible.

> If so, how would one go about recording a calibrated sound input?

Calibrated microphone, calibrated gain, calibrated environment (echo/damping and
noise would affect the result), calibrated microphone-to-source distance,
calibrated recorder, etc..  Difficult, but not impossible.

> I seem to recall reading some childrens science book which had a table
> of the sound dB values of whispering, normal conversation, cars, trains,
> jet aircraft on takeoff, etc.

A very important parameter is the distance between the sound source and the
microphone.  I think 1m is a typical specification.  You also need to make some
assumptions about the recording environment--consider that a drum played in a
small reflective room sounds louder than the same drum outside on a lawn due to
reflections.

> So I might be tempted to take my sound
> recorder and travel about recording conversations, trains, airport noises,
> etc., either without changing the input volume or documenting the volume
> setting for each recording, and then compare the rms amplitude of those
> files to my unknown.

You would also want to document microphone-to-source distance as explained
above.