Sound is an amazing thing. All of the different noises we hear are caused by minute pressure differences in the air around us. What's amazing about it is that the air transmits those pressure changes so well -- and so accurately -- over relatively long distances.
If you've read How CDs Work, you learned about the very first microphone. It was a metal diaphragm attached to a needle, and this needle scratched a pattern onto a piece of metal foil. The pressure differences in the air that occurred when someone spoke toward the diaphragm moved the diaphragm, which moved the needle, which then recorded on the foil. When the needle was later run back over the foil, the vibrations scratched on the foil would then move the diaphragm and re-create the sound. The fact that this purely mechanical system works shows how much energy the vibrations in the air can have.
All modern microphones are trying to accomplish the same thing as the original, but do it electronically rather than mechanically. A microphone wants to take varying pressure waves in the air and convert them into varying electrical signals. There are several different technologies commonly used to accomplish this conversion. Take a look at the next page to learn more about different types of mics -- including one of the first invented by Alexander Graham Bell.
Liquid microphones, invented by Alexander Graham Bell and Thomas Watson, were among the first working microphones to be developed, and they were a precursor to what would later become the condenser microphone. Early liquid microphones used a metal cup filled with water and sulfuric acid. A diaphragm was placed over the cup with a needle on the receiving side of the diaphragm. Sound waves would cause the needle to move in the water. A small electrical current ran to the needle, which was modulated by sound vibrations. The liquid microphone was never a particularly functional device, but it makes a great science experiment [source: Pemberton].
The oldest and simplest microphone uses carbon dust. This is the technology used in the first telephones and is still used in some telephones today. The carbon dust has a thin metal or plastic diaphragm on one side. As sound waves hit the diaphragm, they compress the carbon dust, which changes its resistance. By running a current through the carbon, the changing resistance changes the amount of current that flows.
Fiber Optic Microphone
Fiber-optic systems, which use super-thin strands of glass to transmit information instead of traditional metal wires, have been revolutionizing the field of telecommunications in recent years, including microphone technology. So what's the big deal? Unlike conventional mics, which are often big and send an electrical signal, fiber optic microphones can be extremely small, and they can be used in electrically sensitive environments. They can also be produced with no metal, which makes them very useful in magnetic resonance imaging (MRI) applications and other situations where radio frequency interference is an issue [source: Fibersound Audio].
A dynamic microphone takes advantage of electromagnet effects. When a magnet moves past a wire (or coil of wire), the magnet induces current to flow in the wire. In a dynamic microphone, the diaphragm moves either a magnet or a coil when sound waves hit the diaphragm, and the movement creates a small current.
Electret microphones are among the most widely used microphones on Earth. Because they're cheap and relatively simple, electret mics are used in cell phones, computers and hands-free headsets. An electret microphone is a type of condenser microphone in which the external charge is replaced with an electret material, which by definition is in a permanent state of electric polarization [source: BeStar Acoustic Components].
In a ribbon microphone, a thin ribbon -- usually aluminum, duraluminum or nanofilm -- is suspended in a magnetic field. Sound waves move the ribbon, which changes the current flowing through it. Ribbon microphones are bidirectional meaning they pick up sounds from both sides of the mic.
The RCA PB-31 was one of the first ribbon microphones. It was produced in 1931, and changed the audio and broadcasting industries because it set a new standard when it came to clarity. Several other microphone makers made comparable models, including the BBC-Marconi Type A and ST&C Coles 4038.
A laser microphone works by capturing vibrations off of a plane, like a windowpane, for example, and transmitting the signal back to a photo detector, which converts the reflected laser beam into an audio signal. When sound hits the windowpane, it bends and causes the laser beam to bend, which can be translated to sound using a photocell. In recent years, scientists have been developing a new type of laser microphone that works by streaming smoke across a laser beam that's aimed at photocell, which is then converted to an audio signal [source: Van Buskirk].
A condenser microphone is essentially a capacitor, with one plate of the capacitor moving in response to sound waves. The movement changes the capacitance of the capacitor, and these changes are amplified to create a measurable signal. Condenser microphones usually need a small battery to provide a voltage across the capacitor.
If you're looking to record sound that's located in front of and on the sides of the mic -- but not behind it -- the cardioid microphone is for you. A polar plot of the gain for cardioid is heart-shaped (hence the name), with the highest sensitivity located directly in front of the mic, and slightly less on the sides. Because of this, cardioid mics are ideal for recording live performances without capturing too much crowd noise, and many handheld microphones used to amplify vocals are cardioid mics [source: VoiceCouncil Magazine].
Certain crystals change their electrical properties as they change shape (see How Quartz Watches Work for one example of this phenomenon). By attaching a diaphragm to a crystal, the crystal will create a signal when sound waves hit the diaphragm.
As you can see, just about every technology imaginable has been harnessed to convert sound waves into electrical signals. The one thing most have in common is the diaphragm, which gathers the sound waves and creates movement in whatever technology is being used to create the signal.
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- How CDs Work
- How Telephones Work
- How Amplifiers Work
- How Speakers Work
- How Electromagnets Work
- How Capacitors Work
- 5 Miking Techniques and Tips
- 5 Home Recording Studio Essentials
- How Portable Audio Studios Work
- What's the difference between high fidelity and high definition?
- In a PA system, how does feedback work to cause that howling sound?
- BeStar Acoustic Components. "Electret Condenser Microphones." (Nov. 30, 2011) http://www.bestartech.com/base_mount.html
- Fibersound Audio. "Fibersound Fiber Optic Microphone." (Nov. 30, 2011) http://www.fibersound.com/fiberopticmicrophone.html
- Khursheed, Anjam. "FSE1202: Great Discoveries and Inventions." September, 2009. (Nov. 30, 2011) http://www.cdtl.nus.edu.sg/brief/v12n5/sec2.htm
- Pemberton, Braden. "Home Recording Gear: The Microphone." TopTenReviews. (Nov. 30, 2011) http://audio-editing-software-review.toptenreviews.com/mac-audio-editing-software/home-recording-gear-the-microphone.html
- Van Buskirk, Eliot. "Smoke and Lasers Could Disrupt Microphone Market." Wired. September 25, 2009. (Nov. 30, 2011) http://www.wired.com/epicenter/2009/09/smoke-and-lasers-could-disrupt-microphone-market/
- VoiceCouncil Magazine. "VC's Brief Guide: Mic Pick-Up Patterns." April 4, 2010. (Nov. 30, 2011) http://voicecouncil.com/vcs-brief-guide-mic-pick-up-patterns/.