Best Speaker To Use As Microphone

[abk111]

This is for a surveillance system to document harassment.
Looking to use a auto speaker as a microphone so to capture a broader range of frequencies – down to 20 Hz.
I've read I would need at least a 6 inch diameter to best capture the lower wall shaking frequencies.
I was wondering what other electronic characteristics I should look for:
Higher or lower impedance?
Higher or lower watts?
Frequency range of speaker?
Thanks

 

[cjdelphi]

Amplifying the signal then feed it into line in ? What's wrong with a normal mic? The diaphragm should pick up low frequencies as well

 

[BobK]

A 6 inch speaker would make a terrible microphone.

Why do you think you need down to 20Hz? If this is to pick up speech, which is implied by your post, 100Hz would be fine for a low end cutoff.

And a good quality microphone would be far superior to any speaker at any frequency.

Bob

 

[abk111]

Amplifying the signal then feed it into line in ? What's wrong with a normal mic? The diaphragm should pick up low frequencies as well

Was wanting to directly plug in to PC mic port. Normal voice mics seem to completely miss the lower frequencies.

 

[abk111]

A 6 inch speaker would make a terrible microphone.
Why do you think you need down to 20Hz? If this is to pick up speech, which is implied by your post, 100Hz would be fine for a low end cutoff.
And a good quality microphone would be far superior to any speaker at any frequency.
Bob

I heard that speakers can actually make great mics.
You've lived a very sheltered life if you think harassment is limited to speech.

 

[davenn]

so what specifically is making the low freq signals you want to record ?
are they from a neighbour on the other side of a wall ?

tell us more about the situation, it may help us help you with a better suggestion

Dave

 

[Gryd3]

Maybe he want to capture fart sounds...

 

[abk111]

Yes, from a neighbor, 200 to 800 ft away. Mic mounted outdoors with weather protection.
Farting from that distance would be pretty funny.

If speakers otherwise identical -- same movement of voice coil through same magnetic field -- then will a stronger mic line signal come from a higher or lower impedance (for speaker rating)?
Or do different speaker impedances mean that everything else is also different and thus some other spec I should be looking for? Would a high wattage rated speaker produce less signal?
Thanks

 

[BobK]

Normal voice mics seem to completely miss the lower frequencies.

Yes, that is why the recording industry uses speakers to make those wonderful recordings we all listen to.

Bob

 

[abk111]

Trying to get a realistic capture of lower frequencies, including infrasonics, NOT just voice frequencies. Most mics don't get down to 20Hz, and if they do there is a sharp drop off.

 

[BobK]

http://users.firstva.com/hale/Infrasound.html

Have fun.

Bob

 

[abk111]

There is nothing fun about this.
There are definitely psycho-physiological effects -- extremely detrimental ones.
But there is nothing paranormal about it -- just very wicked people.
If you choose to disregard others suffering, then that's on you.
Thanks for the link.

 

[(*steve*)]

The link seems to provide fairly rational advice on using a speaker as a low frequency microphone. Nobody has yet suggested that you're imagining things.

 

[BobK]

Whoa. That possibility had not even occurred to me.

Bob

 

[abk111]

Fine...
For someone who knows about speaker design or who has used speakers as a mic.
Trying to do a realistic capture of normal audio down to 15 Hz, or a strong 20 Hz that does not cut off at 20Hz.. Would like to also avoid line noise and bottoming out.

If speakers otherwise identical -- same movement of voice coil through same magnetic field -- then will a stronger mic line signal come from a higher or lower impedance (for speaker rating)?
Or do different speaker impedances mean that everything else is also different and thus some other spec I should be looking for? Would a high wattage rated speaker produce less signal?
Thanks

 

[(*steve*)]

One of the major issues you'll have with frequency response is the enclosure.

I'd encourage you to look up the eevblog and look for the series of video blog entries for microphone design. You'll have exactly the same issues using a speaker as a dynamic microphone element.

It is hard to say which speaker would produce the "strongest" signal. Due to the low impedance, the voltage will be very low. You might use a transformer to increase the voltage. I think a more sensitive speaker would also make a more sensitive microphne. Beware that a larger speaker will intercept more signal, so size may trump sensitivity (i.e. I wouldn't choose a very sensitive tweeter because it's just not going to be sensitive at low frequencies).

If all else was equal, I'd pick the speaker that was more sensitive. Sensitivity is usually given in dB/W at 1m

 

[BobK]

In the link I provided, the guy used an audio output transformer to up the voltage (and impedance) from a speaker. I believe the old cheap walkie-talkies that used a little 2in speaker as both mic and speaker also used that method.

Bob

 

[abk111]

One of the major issues you'll have with frequency response is the enclosure.
I'd encourage you to look up the eevblog and look for the series of video blog entries for microphone design. You'll have exactly the same issues using a speaker as a dynamic microphone element.

Thanks Steve, Here is the playlist for that EEVblog
Microphone Technology & Design

 

[abk111]

In the link I provided, the guy used an audio output transformer to up the voltage (and impedance) from a speaker. I believe the old cheap walkie-talkies that used a little 2in speaker as both mic and speaker also used that method.
Bob

Good to know if direct connection is too weak. Thanks