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All Speakers are Microphones
Strange as it may seem, a loudspeaker can be re-tasked as a microphone. Just about any speaker can be converted to be used as a microphone. It seems an odd statement at first sight, because speakers and microphones look quite different, and it seems to defy everyday perception that you could conceivably use a speaker designed for outputting sound into something that can pick up sound. But, there it is, it's true!
How it works
What happens is that in a speaker there's a magnet and coil, and a pair of wires connecting it up to something. What is supposed to happen when the device is used for outputting sound that electricity is converted to sound. Electrical output from an amplifier is put into the speaker and turned into sound. However, the process is reversible, so it's possible for the magnet and coil in a speaker to pick up sound in the air and turn it into electrical signals in the wires.
Scope of this idea
Almost anything which is fundamentally a speaker can be retasked this way. Hi-fi speakers in imitation wood cases, speakers scavenged from televisions, huge bass-bins for use at rock concerts, etc. Note that if a device has an integral amplifier like some of the things plugged into computers, it's not "a speaker", so the same logic doesn't apply. To check this, if it's powered rather than driven, it's not "a speaker" in this sense.
How to show this is true
If you take a speaker, for example from a hi-fi music centre, and connect the two wires into an input market "mic" on some device such as a tape recorder, computer, amplifier, etc, then the speaker will act as a microphone and will pick up sound. You can check this with record and playback, or on Skype, etc.
How good a microphone is it?
The average loudspeaker when converted into use as a microphone is not going to be the quality of a Shure SM58 or AKG 880D, but curiously even an ordinary hi-fi speaker is better at being a microphone than some cheap microphones are! The bass-response is considerably better! So, if you've got a computer and you need a microphone, you might be better to recycle a secondhand 1970s music centre than to get a cheap mic!
To go into the technical details, the actual frequency-response of a reasonable speaker is actually better than a cheap microphone, but the impedance is a bit different (being typically 8 ohms), and if you are wondering about feedback-rejection, let's not even think about it.
As with many a clever notion, speakers as microphones as an idea has many applications for good or evil. At the GOOD end of the range, there is the fact that if you need a moderately-good microphone but don't want to pay good money for it, you can recycle an old speaker and liberate yourself from microphonelessness typical of having just taken delivery of a computer of the type designed for selling rather than using. Speakers as microphones in the do-it-yourself recording studio are useable. However, singing on stage into a speaker has a few snags, so this is why it's a technique seldom used!
Another application of the speaker-microphone duality is in the making of intercoms. Because the speaker at each end is also a microphone, the uses are switched over and over and over and out. It's a practical piece of electronics.
Burglars considering breaking into a house that has any hi-fi speakers in it may have to consider the fact that the speakers may be listening! If connected up to the right kind of monitoring system, speakers just standing silently in a house can hear burglars breaking in and can alert security.
The flipside of this is that it's possible to use speakers for bugging. Being a paranoid myself I am not in favour of this, but I consider that it's important to check speakers to see if they are being used for bugging. Remember that there is no little "bug" involved - it's the speaker itself that is the listening device. If you are a paranoid too, remember it's where the wires go that's important!
The "All speakers are microphones" idea is no excuse to avoid buying a high-quality microphone. If you have vocal ambitions, you should buy the best microphone you can get, and you should test various mics to get what's best for your voice! Also, reverb/delay is required. Quality microphones and studio equipment is available from Sound Control and other Music Equipment Suppliers
Would-be do-it-yourself electronics enthusiasts wishing to test out the theory of speaker=mic may experiment with reasonable safety but remember that a mic needs some kind of amplification, and you can only really use a speaker=mic where you'd use a mic! Mic-IN is more sensitive than Line-in.
A speaker can be tested to see if it's not dead by the very brief connection of a 1.5 volt battery across it to see if it crackles.
Another point of good sense is that whilst it's quite clever to use a speaker as a microphone, it's not clever at all to try to use a microphone as a speaker. They do not sound good, and do not survive well. For the same sorts of reasons as this: You might be able to push a car, but if a car pushed you it would not be good.
Other pages about this sort of thing at this site include Connecting Old HI-FI Speakers to a Computer, Computer Speakers Not What They Should Be, Decibels Explained, Truths Not Generally Realised, Shareware Inventions, Sound Equipment Shops, and a variety of other odd things which are listed in the Site Index which you may want to bookmark and come back to later!
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All Speakers are Microphones: Strange as it may seem, a loudspeaker can be re-tasked as a microphone. Just about any speaker can be converted to be used as a microphone. It seems an odd statement at first sight, because speakers and microphones look quite different, and it seems to defy everyday perception that you could conceivably use a speaker designed for outputting sound into something that can pick up sound. But, there it is, it's true! For more about this see www.zyra.org.uk/sp-mic.htm