# Microphone and Pre-amplifier question

Discussion in 'Electronic Basics' started by MRW, May 24, 2007.

1. ### MRWGuest

Good day!

I'm looking at the following products:

Electret Condenser Microphone
http://www.knowlesacoustics.com/images/specs/MB6022APC-1-A-2004-06-01.pdf

MAX 4469 Mic-Preamp
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2469

I was hoping you can help me answer some questions that I have.

On the MB6022APC datasheet, they show a 33pF & 10pF capacitors in
parallel. What is the meaning of this? Why is it not just a single
43pF capacitor?

I also checked Knowles Acoustic design guide, and they mentioned that
their microphones are either 33pF, 10pF, or 33&10 pF. I don't know
what this means.

Also for the MB6022APC mic, the datasheet shows a sensitivity of -42dB
(min) at 1kHz with 1PA pressure (0dB = 1V/Pa). Does this translate to
a voltage output of .007943V at 1kHz with 1PA pressure for this
microphone?

If that's the case, my calculations show the following:

At 20dB SPL, microphone voltage out is 1.5886 microvolts. At 50 dB
SPL, microphone voltage out is 50 microvolts.

Maximum workable gain for MAX4469 is 30V/V with a bandwidth of 20kHz.

Using these values, the output of the MAX4469 pre-amp at 50dB SPL is
1.5mV and 47 microvolt at 20dB SPL.

If I got these numbers correctly, then are 1.5mV and 47 microvolt
usually workable levels in terms of audio? Would I need another
amplifier stage following the MAX4469 pre-amplifier to boost the
voltage levels even higher?

Thanks!

3. ### BobGGuest

I think the caps select an attenuation on the output of the capacitive
transducer.. I have some Shure SM81s that inset a 10pf cap in series
for 10dB attenuation. This is BEFORE the mic preamp, so it effectively
attenuates the mic output... useful when recording live artillery fire
demonstrations, for example.

4. ### MRWGuest

I have this chip ISD1110S (Windbond Voice Recorder) from an old
project. I want to utilize it into some useless, novelty gadget. I had
an idea of creating a small recording device that picks up, from about
30ft away, any sound that surpasses a certain sound pressure level. I
was thinking maybe 20 dB SPL.

The device will be used to prank my friends while, say, they are using
the toilet. The device would emit a loud shriek and then record my
friends' reaction on the voice recorder. If I put it at least 30ft
away, then I can mount it on top of the ceiling so they won't be able
to get it down right away to destroy it.

5. ### EeyoreGuest

You're going to have more than a little trouble doing that with a cheap electret
capsule for sure ! Chances are that the capsule itself has an equivalent noise
of > 20dB SPL.

I'm not clear why you want to use the Maxim part either. Was it simply a part
you came across by chance ?

LOL !

Graham

6. ### MRWGuest

Yeah, I figured I'd use some "ready-made" pre-amp IC. That was one of
the things that showed up while searching for samples on Maxim's site.

What kind of microphone and pre-amp do you suggest?

The mic data sheet says that it has a minimum SNR of 60dB at f=1kHz
with 1 Pa sound pressure.

7. ### EeyoreGuest

You could use any old (not too noisy) op-amp of your choice. The electret capsule
already has the jfet buffer in it.

Your biggest problem at 30 feet is going to be localising the sound source. For that
you need high directional sensitivity and the way to do that seems to be the 'shotgun
microphone' that does clever acoustic things to achieve the result. I wonder if you
could build your own ?

http://en.wikipedia.org/wiki/Image:Shotgun_microphone.jpg

Well... 0dB SPL is 20 uPa which makes 1Pa = 94dB SPL.

So the mic's equivalent noise is ~ 34dB SPL. Not too surprising. Typical of the genre
I expect.

Graham

8. ### MRWGuest

Shucks. I thought this project was as simple as using an electret mic.
I guess not. I haven't looked into shotgun microphones, yet. However,
when my dad was shopping one for his camera, it was worth about \$80
and looked pretty bulky.

I wonder what kind of mic my bluetooth headset uses. Every time I use
it in an area with a higher background noise level, the people on the
other line complain that they can hear the background.

Hmm.. so this mic is not suitable for any sounds below 34 dB SPL. I
guess there's so much about microphones that I didn't take into
account .... and need to learn. I guess 34 dB SPL is workable for my
needs because the SPL for a quiet room is supposed to be around 40 dB.
I guess as long as the device triggers with the sound of water
flowing then that should be good enough.

9. ### EeyoreGuest

I'm sure it'll be similar to the other one. You might be surprised how loud 'background
noise' can be.

I nearly forgot the parabolic reflector. Somewhat large but has the advantage that it also
amplifies the SPL at the mic.
http://www.bio.umass.edu/biology/kunkel/gjk/microphone.htm
http://www.canford.co.uk/blueprintonline/blueprintaug2003/bigears.aspx

Graham

10. ### MRWGuest

Parabolic reflectors, huh. Cool. I'll read up on it. I don't know much
about mics. I thought it was as simple as hooking them up into an
opamp and so on.

11. ### Rich GriseGuest

Of course - here's just one:
http://members.shaw.ca/roma/twenty-eight.html

Cheers!
Rich

12. ### Rich GriseGuest

Admit it - you just want sound for your toilet-cam, right? ;-)

Cheers!
Rich

13. ### phaetonGuest

Doesn't everyone have quadraphonic toilet sounds?

14. ### MRWGuest

Haha. Not my cup of tea. Really, I think it'd be interesting to come
up with some device like this because it's fun and that other kids
will want to learn how to build it.

15. ### EeyoreGuest

There are still kids who want to learn how to build things ???? !!!!

Slightly not fair since I know my nephew does. When he gives me the rail
currents for the various voltages I'll be posting about it in s.e.d

Graham

16. ### MRWGuest

That's the problem. The interest is not there. So I figured if we show
them something fun, then they'll be interested in learning some more.
In my local IEEE student branch, we do an expo show for elementary
kids every semester. But the kids are really not interested in just
seeing a basic inverting configuration op-amp circuit laid out on a
breadboard and its output connected to the oscilloscope.

That's also the main reason why I try to learn more about electronics
by asking a bunch of dumb questions because eventually, I'll be able
to understand it better and in turn explain it better.

17. ### EeyoreGuest

Sounds good.
There's no such thing as a dumb question (well as long as you did *some*
homework). If only more ppl asked things instead of making often erroneous
assumptions, usually because they're scared to ask !

Graham

18. ### Eric R SnowGuest

A parabolic reflector will limit the frequency response depending on
the size. Since the wavelengths of the sounds we can hear are pretty
large using a small diameter reflector will filter out low frequency
sounds. I wonder what frequency range most farts are in? Maybe you can
use a small parabolic dish to find out how tight your friends are.
ERS

19. ### EeyoreGuest

For long distance work this tends to be a beneficial side effect. Much unwanted background noise is LF.

Graham