10 Types of Microphones

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

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. They are still used in mining and chemical manufacturing because higher line voltages might cause explosions.

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. This type is best placed close to a vocalist or instrument and doesn't typically pick up sound from more than a foot away.

The modern dynamic mic is what most people probably picture when they think of a microphone, with a slender tubular body and a round recording head on top. They're an extremely common sight at live music shows and karaoke because they bring a balance of reliability, portability and sound quality.

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. They are also useful in documentary and news production, as tiny "lapel" mics which can be discreetly placed on an interview subject's clothing [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.

These mics fell out of fashion after the early radio days, and were usurped by dynamic and condenser models, because the chintzy ribbon inside made them exceedingly fragile. One of these can easily require repair after one unfortunate bump from a technician. Modern sound studios will still occasionally use ribbon mics when they're looking to record a track with an authentic "vintage" sound.

A laser microphone works by capturing vibrations off 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. This type isn't suited for general sound recording, like music, but is great for espionage, as the laser can secretly track sound over extreme distances.

A condenser microphone is essentially a capacitor, with one plate of the capacitor moving in response to sound waves. The movement changes the voltage 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. Many modern consumer-grade condenser mics can also get their power from a USB connection to your PC.

Condenser mics are often used in recoding studios. There are two varieties of condenser microphone: large diaphragm, and small diaphragm. Large diaphragm devices are popular for vocals and instruments with a lot of bass or mid-range sound. While small diaphragm mics are more compact, and pick up higher frequency sounds such as string instruments or cymbals.

The microelectromechanical microphone (MEMS for short) is an evolution of the electret design, and is beginning to replace it in some cell phones and headsets. The MEMS mic can be made even smaller than electret, at just a few millimeters wide. Within that tiny space is a microchip containing the mechanical sound diaphragm, a capacitor to transfer sound gathered as electrical current, an amplifier to boost that current's signal, and a digital converter to turn it into audio data which can be used by smartphones and computers.

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. These mics were very cheap to produce, and so found use in budget-friendly applications throughout the 20th century. Their sound quality left a lot to be desired, however, causing them to be replaced by modern condenser and dynamic microphones. Nowadays, crystal microphones are used mainly for monitoring and for automotive transmitters and sensors.

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.