Can Somebody Explain This To Me?

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harrysand

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Apr 17, 2013
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Okay, so I have an Acoustic Research wireless audio system sitting around the house. It's a two part system, an audio transmitter that plugs into an mp3 player or something, and a wireless speaker so you can bring music around the house, yard, whatever.

Here is a link to the product on amazon: Amazon.com: Audiovox Acoustic Research AW825 Mission Style Wireless Speaker (Bronze): Electronics

I saw a sticker on the bottom of the transmitter that said 900MHz and I thought it would be fun to listen to the signal through my sdr dongle, so I did. It seems to just be an FM transmitter, and sounds great on the computer. Cool. What was strange was when I unplugged my iPhone and picked up the transmitter. I heard a "BOOM" in my computer speakers just as I touched the device. Well I played around tapping it for a bit and noticed if I speak loud enough near the transmitter I could hear my voice in the transmission!

Is this some kind of interference or is there actually a hidden microphone in this thing???

At around 12 seconds you can hear me say "testing 123" and tap a couple time before and after that. Sorry for the zip file, the forums wont let me share an mp3 or wav...
 

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n0nhp

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Grand Junction
It is a not too uncommon attribute of inexpensive transmitters, and sometimes expensive transmitters. And as may be guessed it is called "microphonic". Being an FM transmitter, any portion of the transmit path that is not tight will act as a microphone. Think of it the same way that a laser listening device works. Any surface will oscillate with the sound waves. In the transmitter it is usually something like an open air inductor or a capacitor that has long leads on it. Potting the board in silicone seal usually will stop most microphonics. I have had metal shields covering the transmitter cause microphonics due to capacitive coupling.
The early EFJohnson 800 MHz mobile radios had a shield that would act as a microphone in receive. In construction machinery the speaker had to be turned almost max and all that could be heard was a loud feedback squeal any time another radio keyed up. I used many tubes of silicone seal and just hoped I never had to see those radios again for receive problems.

Bruce
 
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