Discussion in 'Electronic Design' started by kash, Jul 10, 2003.

1. ### kashGuest

What does it mean that a microphone has a relative response of -5db at 1khz ?

Also, the above microphone is a Dynamic Vocal Microphone, but when we
look at the Electret microphones we get various specifications. What do I
need to know about Electret Microphone in order to use them ?
And Lastly, is it a good idea to connect an Electret Microphone to
a Microphone Preamplifier circuit, as part of a sound level meter design.
I can by such a circuit ready made from cpc so I was wondering if that
could be an easy way to design a db meter without too much effort.
Thanks

2. ### GregSGuest

Your talking about relative sensitivities. If you assume the mic has a flat response, then it
will output the same voltage at all audio frequencies. You also have to comput theapproximate
volume levels you will encounter. My SPL meter did not use a separate preamplifier as such. You can
simply design a one or two stage amplifier to amplify the millivolt signals to have enough drive to
push a meter.

greg

3. ### kashGuest

Your talking about relative sensitivities. If you assume the mic has a flat response, then it
Why is it -5db and not +5, where does the 5 come from ? (formula?)
Did you ever post it here, I could do we a circuit diagram if it is not overly complex ?

4. ### Paul BurridgeGuest

Because it's 5db *down* (or lower if you prefer) than the reference
level. You didn't state what the reference level was, but one assumes
it to be the peak response frequency band of the mic. The formula is
10*log(V1/V2) where V1 and V2 are the reference level and the
measurement level. Or is it 20*? I can never remember. One's power and
one's voltage and you want the voltage one. I'm pretty sure it's ten
but no doubt some clever-arse will correct me if I'm wrong.
A simple single-stage circuit would do fine for your purposes, I
imagine. Total cost to build less than \$1.

5. ### Bullwinkle JonesGuest

Paul Burridge wrote:
<snip>

I love your very polite take on the term "smart-ass".. LOL.

6. ### kashGuest

and I think it is the one with the 20, sorry for being a smart ****

7. ### Rich GriseGuest

It's 10 for power, and 20 for voltage. This is easy to remember,
because power is proportional to the square of the voltage, and
that's x2 in logarithms.

HTH!
Rich

8. ### Paul BurridgeGuest

You must specify a few things first... your mic's impedance, desired
supply voltage, load impedance and desired output level spring to mind
for starters.

9. ### kashGuest

The mic will be an electret one, they all seem to have an impedance of
1k ohm maximum. Desired supply volatage: 3.6v or less is ideal, but if
this is a problem use 9v. Use a led bar graph display (you can also use
the LM3915 if you like). Output must be logaritmic and there should be
a switch to operate the higher levels.

Thanks.