Input sound from microphone into dB indicator

[rolfk]

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@Bob_Schor wrote:

Speaking in terms of sound, the decibel scale gives relative loudness on a logarithmic scale.  0 dB is around the softest sounds humans can hear.  As the sound pressure level grows by a factor of 10, dB increases by 20.  Generally sounds >85 dB are sufficiently loud to damage hearing over time (shorter times for louder sounds).

If you want to express your sound in dB, you need to define what 0 dB is.  You then measure your sound, divide it by the 0 dB level, take the log, multiply by 20, and there's your answer.  Note that 0 dB cannot be zero (otherwise you'd be dividing by zero!).

Bob Schor

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Actually, strictly speaking 0dB only means that it is the same as the reference value, but without further specifying what the reference value is, it means nothing!

That is why you have dBm, dBV and many others, which is a further specification of the reference value.

dBm means in reference to 1mW power, but you normally measure voltage so you also need to know the impedance on which this voltage was measured. (Common values are on 600 Ohm for telephony or 50 Ohm for radio frequency).

1mV on 600 Ohm results in a voltage of 0.775V. If you specifically want to reference to this voltage independent of the actual load impedance, it is usually written as dBu.

dBV usually means 1V independent of the impedance.

If you mean any of these dBs, you obviously need to have a calibrated voltage measurement in order to be able to make a meaningful dB calculation.

Another possible dB scale would be the sound pressure (SP) or sound pressure level (SPL). Here the reference for 0dB is often 2 * 10^-5 Pa. This is often the scale used when wanting to display the sound pressure, but in addition to a calibrated electrical measurement path you also need to have a calibrated microphone too, to be able to calculate any meaningful dB value.