4.1 Sound Waves and Microphones

Overview of Sound Waves

Before we can start learning about audio production and microphones, let’s get a general idea of how sound waves work. Though the study of sound waves is mostly a technical matter, understanding the science behind it can help us manage it. How an audio producer adjusts the settings on microphones (method) can affect how sound waves enter the device. And ultimately, how those soundwave frequencies are manipulated within a mix can dramatically change the emotional response a piece of audio has on a listener (creative approach).

How Sound Works

Sound is the term to describe what is heard when sound waves pass through a medium to the ear. All sounds are made by vibrations of molecules through which the sound travels. For instance, when a drum or a cymbal is struck, the object vibrates. These vibrations make air molecules move. Sound waves move away from their sound source (where they came from) traveling on the air molecules. When the vibrating air molecules reach our ears, the eardrum vibrates, too. The bones of the ear vibrate in the same way that of the object that started the sound wave.

These vibrations let you hear different sounds. Even music is vibrations. Irregular vibrations are noise. People can make very complex sounds. We use them for speech.

Sound waves are longitudinal waves with two parts: compression and rarefaction. Compression is the part of the sound waves where the molecules of air are pushed (compressed) together. Rarefaction is the part of the waves where the molecules are far away from each other. Sound waves are a sequence of compression and rarefaction.

Microphones

A microphone is a form of transducer. That means it converts a sound wave into an electronic signal carried by wire. Generally, when that electric signal is sent through an amplified speaker, it is then converted back into a sound wave.  Microphones may also use a processor to convert the sound wave into signal (or code) that can be used by a computer. Understanding the technical aspects of microphones is so very important for producing good audio. It also ensures electrical safety to you and your equipment.  As you gain more experience with microphones, where you place them in relation to what is being recorded (method) will become equally important. Experimenting with both of those aspects will help you develop your own signature sounds using your favorite microphones and microphone placements (creative approach).

For many of us who started out with analog recording, microphones were the gateway into audio production. But whether you use analog or digital audio equipment, the right microphone pointed in the proper direction can help a performance stand out. There are dozens of styles of microphones and a wide variety of prices. To begin however, let’s concentrate on the two most common types of microphones: Dynamic and  Condenser.

Dynamic Microphone – In a dynamic microphone, a thin diaphragm is connected to a coil of wire, called a voice coil, which is precisely suspended over a powerful magnet.

• As the sound waves strike the diaphragm, it causes it to vibrate moving the voice coil through the magnetic field generated by the magnet generating a small bit of electricity which is sent down the output leads.
• This is the electromagnetic principle.
• ADVANTAGE: They are simply constructed and can handle loud sources without much distortion.
• DISADVANTAGE: They are weak when trying to capture soft distant sources because the diaphragm requires a lot of sound energy to move.
• DISADVANTAGE: Dynamic microphones have a heavy diaphragm along with additional weight from the coil of wire.
  • It therefore takes longer for the diaphragm to react to a sound wave causing a less accurate recording.

Condenser Microphone – Condensers use two charged plates; one fixed and one which can move acting like a diaphragm.

• There’s no coil.
• The two charged electric plates create what’s called a capacitor. As sound waves strike the electrically charged diaphragm, it moves in relation to the fixed plate changing its capacitance and generating a very small electric charge which is amplified inside the microphone.
• This is the electrostatic principle.
• ADVANTAGE: Because you’re not moving a coil, condensers are more responsive in the high frequencies.
• ADVANTAGE: Because of the lack of magnets, condenser microphones can be very small.
• Because condensers work with electrically charged plates, they require some sort of outside power.
• Some microphones have the option of an onboard battery while all condensers can utilize something called Phantom Power.

Phantom Power – +48v of energy sent down the microphone cable to a condenser microphone from the audio recording or mixing board.

• This power enables the electrically charged diaphragm to move in response to sound waves.

Directional Response – Directional response is represented by something called a polar pattern.

Polar Pattern – Polar pattern is how well the microphone “hears“ sound from different directions.

“On Axis” and “Off Axis” – On axis is directly in front of the sound source. Off axis is not directly in front of the sound source.

Omnidirectional Mics – This mic polar pattern is responsive to sound from all directions, you don’t have to be “on axis” to be picked up.

• Lavalier and lapel mics are small condenser microphones with an omnidirectional pickup pattern that can be placed on a person.
• Boundary mics are omnidirectional condenser mics. They are positioned flush with a surface that capture sound as it rolls off the flat surface. Boundary mics are used in stage production and conference tables.
• ADVANTAGE: These mics are useful for picking up sound in a general area.
• ADVANTAGE: Lavalier / lapel mics are small and can be placed just about anywhere.
• ADVANTAGE: Boundary mics do not draw attention to themselves because they lay flat on the floor or wall.
• DISADVANTAGE: They will pick up all the unwanted sound in the area.
• DISADVANTAGE: Lavalier, lapel, and boundary mics won’t have the same richness of sound as a shotgun or studio condenser mic.

Directional Mics: Cardioid Pattern – Most basic pattern.

• Heart-shaped pick up pattern.
• ADVANTAGE: Picks up what’s in front but not behind.
• ADVANTAGE: It is suited for a live performance as it picks up the sound on axis but won’t pick up what’s behind it, like crowd noise or feedback from a speaker.

Directional Mics: Hypercardioid and Supercardioid Patterns – More directional than cardioid.

• Skinnier heart-shaped pick up pattern.
• Picks up the front and sides and rejects 150 degrees to the rear.
• Shotgun mics are supercardioid.
• ADVANTAGE: Great for recording location audio while trying to filter out some of the unwanted ambient sound.
• DISADVANTAGE: Can exhibit strange phasing sound effects when used in small spaces.

Directional Mics: Figure 8 Pickup Pattern / Bi-directional – The polar pattern looks like a figure 8.

• ADVANTAGE: Useful for certain musical applications or interviews with a person on each side of the mic.

Frequency Response of Mics

Most microphones come with a manual. If not, you can find one online by searching the brand and model number of the mic. These manuals can be very helpful in three areas. First, it will show you on a chart the polar pattern of the mic. Second, it may show you what certain switches will do that are located on the microphone and what is considered the front and back of it. Third, it will feature the Frequency Response. This chart will show a line (or lines) going from 20hz all the way up to 20khz. What to pay attention to is

where this line is flattest on the chart. That area (range) is where the microphone most accurately picks up sound on the frequency spectrum. If it flattens out in low end areas of the spectrum, you can imagine that mic is best for bass sounds. If it is flat within the range of human vocals (approx. 100 to 120hz), then you can assume that particular microphone will work great on vocals. These are good places to start in your audio production development, but keep in mind that experimenting with different mics in various ways is the best way to discover what is the best mic to use for a given circumstance. If it sounds good, you’re probably doing it right!  Below is a link that explains the frequency spectrum in detail.

Attribution

Except where otherwise noted, this page is adapted from “Unit One, Part One: Sound Waves and Microphones” In Audio Production Course Manual by by Mark J. Lindquist, CC BY-SA 4.0 and “Microphone” and “Sound” from Wikipedia, CC BY-SA 4.0 . / Adjusted attribution at page level to properly match conflicting licenses, added openly licensed images, created H5P activities.