I have designed an A weighting network circuit. but i cant understand how to convert it's voltage output in to corresponding sound pressure level in dB. So How can I convert these voltage readings into dB?
Convert A weighted voltage output into sound pressure
amplifiersound
Related Solutions
DB SPL is a pressure measuring unit.
You can't convert a voltage to an DB SPL reading unless you know:
- the microphone sensitivity (or simply the analog output voltage to the input pressure ratio) which tells you essentially the voltage level it will output for a given sound pressure level
- the gain that the preamp has applied
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 V1 is the voltage being measured, and \$ V_0 \$ the reference level
If we do an example calculation for the measurement of 2.5v (assuming a unity gain for the amplifier) we get
\$ 20 \times \log \frac {2.5}{0.005012} = 53.96dB \$
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
...
The signal from the headphone socket on the android may be enough to activate a transistor or, it may not be enough without resorting to some form of voltage amplification. Given that you probably won't want to use a small battery to power the amplifier it may be sensible to consider a small voltage step-up transformer. It may not work effectively at bass frequencies but it should be able to be made small enough for operating at mid-range audio frequencies. The output would need to be rectified and smoothed and then fed to the base of a BJT via a base resistor. It may be suitable for a MOSFET too.
Another option is an energy harvesting chip such as: -
This will take the rectified and smoothed signal from a low voltage audio device and convert it to a voltage suitable for turning on a mosfet.
An LM2907 is unsuitable for turning a small dc voltage (rectified audio) into a voltage suitable for driving a BJT or mosfet - it needs a power supply to operate and it's applications are not in this field.
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Best Answer
I am assuming your output is also driving an amplifier and then some loudspeakers.
If that's the case the best way to calibrate your vu meter is put some pure tone in the amp and measure the spl with a microphone nicely placed.
You can also search for a parameter called "sensitivity", the manufacturer of your loudspeakers should be able to tell you it. Sensitivity is measured in \$\frac{\text{dB}}{\text{W}\cdot\text{m}}\$, i.e. is the spl in decibel the loudspeaker outputs for each watt of power you give to it, measured at a distance of 1m. Problem is that sensitivity depends on frequency and on the enclosing and probably other parameters.