Types of Microphones

Sound is great. All the different sounds we hear are caused by slight pressure differences in the air around us. The great thing is that air transmits these pressure changes very well and very accurately over relatively long distances.

If you've read about how CDs work, you've learned about the first microphone. It was a metal diaphragm attached to a needle, which cut a pattern into the metal foil. The air pressure difference that occurs when someone speaks into the diaphragm moves it, which in turn moves the needle and is recorded on the slide. Later, when the needle passed over the foil again, the vibrations caught in the foil moved the membrane, reproducing the sound. The fact that this purely mechanical system works shows how much energy vibrations in the air can have.

All modern microphones attempt to achieve the same thing as the original, but do so electronically rather than mechanically. A microphone wants to record different pressure waves in the air and convert them into different electrical signals. There are several different technologies commonly used to accomplish this transformation. Take a look at the following pages to learn about different types of microphones, including the first microphone invented by Alexander Graham Bell.

1: Crystal Microphone

Certain crystals change their electrical properties as they change shape (see How Quartz Watches Work for an example of this phenomenon). By attaching a membrane to the crystal, the crystal produces a signal when sound waves hit the membrane. These microphones were so cheap to manufacture that they were used in budget-friendly applications throughout the 20th century. However, its sound quality left much to be desired, so it has been replaced by modern condenser and dynamic microphones. Today, quartz microphones are mainly used for surveillance purposes and in automotive transmitters and sensors.

As you can see, almost every conceivable technology has been used to convert sound waves into electrical signals. The most common is a diaphragm that collects sound waves and creates motion, no matter what technology is used to generate the signal.

2: Microelectromechanical Microphone

Micro-electromechanical microphones (MEMS for short) are an evolution of electret designs that are beginning to replace some mobile phones and headsets. MEMS microphones are even smaller than electrets and can be made just a few millimeters wide. This small space contains a microchip containing a mechanical sound membrane, a capacitor that transmits the collected sound as an electric current, an amplifier that amplifies the electric current signal, and a digital converter that converts it into audio data. It can be used on smartphones and computers.

3: Condensor Microphone

A condenser mic is essentially a condenser, with one plate of the condenser moving in response to sound waves. Movement changes the voltage on the capacitor, and these changes are amplified to produce a measurable signal. Condenser microphones typically require a small battery to power the condenser. Many modern consumer condenser microphones can also be powered by a USB connection to a PC.

Condenser microphones are often used in recording studios. There are two types of condenser microphones: large diaphragm and small diaphragm. Large diaphragm devices are popular for vocals and instruments that have a lot of low and midrange. Small-diaphragm microphones, on the other hand, are more compact and pick up high-frequency sounds such as strings and cymbals.

4: Laser Microphone

A laser microphone works by picking up vibrations from a flat surface, such as a window glass, and sending a signal back to a photodetector. A photodetector converts the reflected laser beam into an audio signal. When the sound hits the window glass, it bends and bends the laser beam. This can be converted into sound using a photocell. In recent years, scientists have developed a new type of laser microphone that shoots smoke into a laser beam aimed at a photocell and converts it into an audio signal. This type is not suitable for general recording such as music, but it is good for espionage as the laser can stealthily track sounds very far away.

5: Ribbon Microphone

In ribbon microphones, a thin ribbon (usually aluminum, duralumin, or nanofilm) is suspended within a magnetic field. A sound wave moves the ribbon, changing the current flowing through it. Ribbon mics are bi-directional. That is, it picks up sound from both sides of the mic.

RCA PB-31 is his one of the first ribbon microphones. Created in 1931, it changed the audio and broadcast industry by setting a new standard for clarity. Several other microphone manufacturers have produced comparable models such as the BBC-Marconi Type A and the ST&C Coles 4038. Very fragile. One of these may need a quick repair by a technician after an unfortunate crash. Modern recording studios sometimes use ribbon mics when they want to record tracks with a true "vintage" sound.

6: Electret Microphone

Electret microphones are among the most widely used microphones on the planet. Electret microphones are used in mobile phones, computers, and hands-free headsets due to their low cost and relative simplicity. An electret microphone is a type of condenser microphone, in which the external charge is replaced by an electret material. Electret materials are by definition in a permanent state of electric polarization. It's also useful for documentaries and news production, as it acts as a small 'lavalier microphone' that can be inconspicuously attached to an interviewee's clothing [Source: BeStar Acoustic Components].

7: Dynamic Microphone

Dynamic microphones make use of electromagnetic effects. When a magnet passes through a wire (or a coil of wire), it induces a current in the wire. In a dynamic microphone, when sound waves strike the diaphragm, the diaphragm moves either a magnet or a coil, and the movement creates a small electrical current. This type is best placed close to singers and instruments, and generally won't pick up sounds more than a foot away.

A modern dynamic microphone is what most people probably think of when they think of a microphone, with a slim tubular body with a round recording head on top. It's a very common sight at live music shows and karaoke due to its balance of reliability, portability, and sound quality.

8: Fiber Optic Microphone

Fiber optic systems, which use very thin glass fibers to transmit information instead of traditional metal wires, have revolutionized telecommunications in recent years, including microphone technology. So what's the big deal? Unlike traditional microphones, which are often large and transmit electrical signals, fiber optic microphones are very small and can be used in electrically sensitive environments. It can also be made metal-free, making it very useful in magnetic resonance imaging (MRI) applications and other situations where radio frequency interference is a problem [Source: Fibersound Audio].

9: Carbon Microphone

The oldest and simplest microphones use carbon dust. This is the technology that was used in the first phones and is still used in some phones today. Charcoal dust has a thin metal or plastic film on one side. When a sound wave hits the membrane, it compresses the carbon dust and changes its resistance. As current flows through the carbon, the varying resistance changes the amount of current that flows. They are still used in the mining and chemical industries as high mains voltages can cause explosions.

10: Liquid Microphone

Invented by Alexander Graham Bell and Thomas Watson, the liquid microphone was one of the first practical microphones to be developed and was the predecessor of what would later become the condenser microphone. Early liquid mics used metal cups filled with water and sulfuric acid. The diaphragm was placed over the cup with a needle on the receiving side of the diaphragm. Sound waves move the needle through the water. A small electrical current, modulated by sound vibrations, ran through the needle. The liquid mic wasn't a particularly functional device, but it's a great science experiment.