### MSP and Decibel Query

I have a number box which is the result of a calculation that works out the perceived intensity of a sound source in dB at a changeable distance if the emitted source was 100 dB.

For example, if the sound source was 100 dB at 1 meter away, at 40.23 meters away, the perceived intensity in dB would be 73.93 dB.

Basically, I have got a cycle~ 440 object and I want the perceived intensity in dB to be represented on the gain~ slider, but the gain slider has a range from 0-157. How can I get the dB from the calculation to be represented on the gain~ slider. I understand that the calculation is in dBSPL and I have looked into atodb and dbtoa.

Any ideas would be greatly appreciated,

Matt

db inside digital audio and in real world are used in different manners. In digital audio 0dB is defined as the maximum possible amplitude – which equals an amplitude of 1. Therefore dB values in digital audio are usually negative. I.e. -6dB equals a amplitude of 0.5, -12db would result in 0.25 and so on.

In the ‘real’ world 0dB is defined as the lowest perceivable sound pressure – therefore the dB values are positive.

What remains the same is that if you raise a sound-pressure by 6dB you double the signal.

So the only thing you can represent inside your patch is the relative difference of the signal. In your example you have a drop in sound pressure of 26.07dB (100dB – 73.93dB).

[dbtoa] will calculate for you that this difference equals an amplitude (inside the digital audio processing) of 0.05.

To scale your signal feed [dbtoa] with the difference in dB of your ‘real world’ values (in your example it would be -26.07 as it is a drop in sound pressure) and multiply your signal from the [cycle~] with the resulting value using [+~].

If it comes to the [gain~] object you have to notice that it applies a logarithmic scale internally and that is uses what is called headroom. (See the help patch). At a value of 128 is multiplies the in incoming signal by 1 (in other words the signal is unchanged). A drop of 10 steps at the [gain~] equals a drop of 6dB or half of the amplitude. So at 118 the [gain~] will multiply the incoming signal by 0.5 (== -6dB), at 108 with 0.25 (== -12dB) and so on.

The rest is just a very little math….

Jan

"and multiply your signal from the [cycle~] with the resulting value using [+~]."

–> i meant [*~] of course… ;)

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