I am taking input from an electret microphone amplified using LM358 amplifier from my PIC16F877A's ADC unit. I am getting the readings in volts from the ADC which ranges from 2.5V to 5V.
How can I convert these readings into dB?
I am taking input from an electret microphone amplified using LM358 amplifier from my PIC16F877A's ADC unit. I am getting the readings in volts from the ADC which ranges from 2.5V to 5V.
How can I convert these readings into dB?
dB SPL is a pressure measuring unit.
You can't convert a voltage to a dB SPL reading unless you know:
Your microphone has a sensitivity of -46dBV/Pa , this gives 0.005012 V RMS/ Pa
1 Pa (pascal) equals 94 dB sound pressure (SPL)
The dB equation for voltage is \$ 20 \times \log \frac {V_1}{V_o} \$
where \$ V_1 \$ is the voltage being measured, and \$ V_0 \$ is the reference level.
If we do an example calculation for the measurement of 2.5 V (assuming a unity gain for the amplifier) we get
\$ 20 \times \log \frac {2.5}{0.005012} = 53.96 \ \text{dB} \$
so the SPL will be (-46) + 53.96 = 7.95 + 94 = 101.95 dB SPL
We assumed a unity gain for the preamplifier; if the actual gain was 20dB then the SPL becomes: 101.95 - 20 = 81.95 dB SPL
If the actual gain was 10dB then the SPL becomes:
101.95 - 10 = 91.95 dB SPL
...
Coming here many years later and I am still confused by the top answer, only because I am trying to solve this exact problem.
As stated in the top answer, voltage gain is calculated as 20*log10(V1/Vref) where V1 is the Vrms of the measured voltage and Vref is the microphone sensitivity. Then, the microphone sensitive in dB is subtracted off (-46dBV/Pa) before being added to dB SPL standard (94 dB SPL), but this doesn't quite make sense to me.
If you put a standardized tone generator outputting 94 dB SPL at 1 kHz, you would expect (and I have experimentally validated) to get exactly the value of the microphone sensitivity (in your case, 0.005012).
Plugging this into the top answers formula, you would get 20*log10(.005012/.005012) = 0 dB - 46 (db rel V/Pa; microphone sensitivity) + 94 db = 48 dB, when it should be 94 dB, because we are using a standardized tone generator.
I believe the correct conversion would just be 20*log10(V1/Vref) + 94 dB SPL, where V1 is the rms voltage of the signal and Vref is the microphone sensitivity.
-46dB V/Pa is how I read it and 1 Pa is the sound pressure in newtons per sq metre. 0dB SPL is 20 micro Pascal therefore, 1 Pa is 50,000 times bigger or, in dB it is 94 dB SPL.
So, if you are measuring -46 dBV then you are measuring a SPL of 94 dB. -46 dBV is near enough 5 mV RMS so, again, if you measure 5mV RMS then the SPL is 94dB.
If you have a pre-amplifier with a gain of ten, then 50mV RMS equates to 94dB SPL and 5mV would equate to a SPL of 74 dB.
This should be enough to get you started.