Determining decibel level of sound clip

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CAPLAB
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Determining decibel level of sound clip

Post by CAPLAB » Mon Apr 12, 2010 3:51 pm

Is there a way to determine the actual decibel level of an audio clip when it's imported into the program? When I go to Analyze, Plot Spectrum it sets the audio clip at 0dB for reference I'm guessing. Or does anyone know any websites that provide the decibel levels of audio clips already?

kozikowski
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Re: Determining decibel level of sound clip

Post by kozikowski » Mon Apr 12, 2010 4:06 pm

You stepped into a squishy problem. Audacity plays fast and loose with sound levels. The bouncing light sound meters only meter the top half of the waveforms and the blue waveforms themselves are not tracked in the negative direction with any accuracy.

People like to post isolated questions. What's the real job? You didn't wake up one morning and say, "I think today I'll measure the dB level of some sounds..."

Koz

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Re: Determining decibel level of sound clip

Post by CAPLAB » Mon Apr 12, 2010 4:21 pm

Hmm, well that's not terribly convenient.... do you know any free audio programs that might be more useful to me in this aspect?
As far as the job is concerned, I simply need an audio file that I can manipulate to several decibel levels to be used for research purposes, ei. 60dB, 80 dB, etc. But I can't seem to find any clips which straight out define the starting decibel level. Unfortunate.

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Re: Determining decibel level of sound clip

Post by whomper » Mon Apr 12, 2010 4:53 pm

so start with any file and then amplify it to 0db
now you know that you have 0dBFS file to start with

now that you have one that is the max that you can do without clipping you can start lowering it ("amplify" it down) to get other relative output levels

how loud it actually plays will depend on your other equipment
settings. plus loudspeaker efficiency. etc. you probably need an external SPL meter to set an absolute loudness. but then the others should be roughly aligned like you want depending on the linearity of your other equipment.

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Re: Determining decibel level of sound clip

Post by kozikowski » Mon Apr 12, 2010 5:31 pm

<<<so start with any file and then amplify it to 0db
now you know that you have 0dBFS file to start with >>>

...on the one highest peak in the performance. Not the average loudness or any of the other ways to measure level.

<<<the starting decibel level.>>>

Of what? The answers change a great deal depending on the sound file. If it's straight, pure tones, then Whomper's solution will work. If it's just about anything else, it's fogged in complexity.

You wouldn't be the first researcher trying to nail down all the variables to perform a project. Did the sound start out inside the computer? If it's an external sound, then you're going to run into all the variables of your sound card or other interface device.

Then there's the technical specification. 16-bit sound only has about a 90dB range between maximum peak loudness and the channel floor -- and that's before you try to squeeze the test through analog electronics which tend to be far worse.

If you start out life with a "normal" audio file of someone speaking and reduce it 60dB, then there will be almost no sound left if you try to play it through a sound system whose noise floor is -60dB.

There are some absolutes. If you start with an arbitrary correct sound file and Effect > Amplify it -10dB, then the result will be 10dB lower than whatever it was you started with. That's easy as long as you don't hit the limits at either end. If you start with a -80 tone and reduce it 20dB, you will get flat-line in 16-bit sound.

You see we're working in a vacuum until you tell us a lot more about the "show."

Koz

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Re: Determining decibel level of sound clip

Post by whomper » Mon Apr 12, 2010 7:17 pm

i inferred he had sine tones at different freqs and different volumes.

BUT for any given file, my solution *does* work to set his relative levels point by point in time for a given file. Now if his files vary that is his problem to deal with.

What he really needs an SPL meter with A&C weighting to measure the ACTUAL sound levels he gets from his amplifier (at whatever setting) and his actual speakers at whatever efficiency. Those will be the determining factors. But my approach will get him close enough for govt work wrt relative loudness of files burned to a cd.

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Re: Determining decibel level of sound clip

Post by kozikowski » Mon Apr 12, 2010 8:30 pm

<<<What he really needs an SPL meter with A&C weighting to measure the ACTUAL sound levels he gets from his amplifier >>

He didn't say anything about Sound Pressure Level or an amplifier. He didn't say anything about tones either.

<<<BUT for any given file, my solution *does* work to set his relative levels point by point in time for a given file.>>>

Only one point per file. Amplify stops changing when the first single sound peak in the whole performance strikes zero.

Single peak values are a notoriously unstable way to measure levels. Until the poster tells us more, we're solving your problems, not his.

Koz

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Re: Determining decibel level of sound clip

Post by whomper » Tue Apr 13, 2010 1:01 am

??????
if select the whole track and then amplify you get more than one point
amplified and it does not stop at a zero crossing.

dont knwo about the new version of amplify but the one in 1.3.9 appearently will let you amplify the whole thing into clipping if you want.

he strongly implied the external SPL imho.

dont know what problem you are solving.
i dont have a problem here. i was trying to fix his based on the little info he gave us. now maybe he has some truly irrational and illogical use in mind -- i was trying to match some real world use.

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Re: Determining decibel level of sound clip

Post by steve » Tue Apr 13, 2010 1:57 am

CAPLAB wrote:I simply need an audio file that I can manipulate to several decibel levels to be used for research purposes, ei. 60dB, 80 dB, etc. But I can't seem to find any clips which straight out define the starting decibel level.
Decibels (dB) are a relative measure, not an absolute measure. It is a measure of magnitude relative to a reference level.
In the case of digital audio, the reference level is represented numerically by sample values of +/- 1.0
You can see this numerical scale in Audacity if you look at the vertical track scale in the default track view.

Signal levels are usually described with reference to the "full scale" magnitude (dBfs). If you generate a tone in Audacity with "Amplitude = 1.0", the generated tone will have a peak level of 0dBfs. Similarly, if you import or record a sound and amplify it so that the peak level reaches the top/bottom of the track (+/- 1.0), then the peak level is 0dB.

In digital audio, 0dB is usually considered to be the maximum valid signal level (though in certain special circumstances it is possible to exceed 0 dB).
All other signals are less than 0 dB and are measured as negative values - for example;
If you have a signal that has a peak amplitude of 1.0, then it has a peak level of 0 dB. (more precisely it should be 0dBfs, but commonly just called 0dB).
If you reduce the level of this signal so that it has a peak amplitude of 0.5, then it has a peak level of -6dB (minus 6 decibels full scale)

When dealing with "loudness" of sound, there are a number of different standards that are used for measuring the magnitude. These take into account the power, distance and frequency content of the sound. The general term for sound magnitude is "Sound Pressure Level" (SPL). SPL will usually be quoted in dB relative to a very quiet sound, where 0dB is the threshold of hearing (virtually silent). Sounds that are louder than this will be measured in positive dB values.

There is (obviously) no direct relationship between the dBfs level of a signal, and the SPL of that signal when played through an amplifier and speakers.
With the same signal, a small amplifier and loudspeaker will obviously produce a much lower SPL than an enormous rock PA system cranked up to 11.

For more information about the dB scale: http://en.wikipedia.org/wiki/Decibel
9/10 questions are answered in the FREQUENTLY ASKED QUESTIONS (FAQ)

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Re: Determining decibel level of sound clip

Post by kozikowski » Tue Apr 13, 2010 3:36 am

<<<i was trying to match some real world use.>>>

I vote we let the poster tell us if any of this is valuable.

Koz

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