The Science of Sound Recording: How Your Microphone Works
Learn how different microphone types generate signals, why they respond differently at angles, and what it all means when using them to record sound
Up until the mid 1800s, no sound had ever been recorded. Sounds were one-time occurrences that would vanish into thin air just as quickly as they were produced. Through the ideas of curious thinkers and a snowball of brilliant inventions, a revolution that would lead us to modern day audio had begun. The audio technology we utilize (and often overlook) in our daily routine was absolutely unheard of back then, but the principal ideas remain the same: capturing and reproducing sound.
At the core of this revolution is a fairly simple device, the microphone. Microphones allow us to transfer the energy of sound waves into an electrical signal that can then be recorded, manipulated, distributed, and even broadcast right back into the air!
By design, different microphone types yield different sounding results and are thus suited for different applications. We’ll take a deeper look at the methods in which microphones capture sound, their directionality (polar patterns), and common problems that arise when using them.
Before we dive into the inner workings of a microphone, it’s helpful to understand the basics of how sound works, and how we interact with it.
The vibration of an object causes its surrounding air molecules to vibrate. This creates a trajectory of changing air pressure that travels until it inevitably loses energy and comes to a halt. When these changes in air pressure arrive at our eardrums, they’re then converted by our hearing system and our brain into what we perceive as sound.
A microphone is a type of transducer – a device that converts energy from one form to another (in this case, the kinetic energy or air movement to an electrical current). Amongst other factors, microphone quality, and often price, is determined by how accurately the electrical signal generated by this process represents the physical sound that arrived at the microphone.
Aside from objective fidelity, many recording engineers are also interested in the sonic characteristics of the microphone, and how ear-pleasing the resulting signal is.
Known for their simple, yet sturdy design, dynamic microphones are a common asset in any engineer’s mic collection. They work through electromagnetism, a concept of physics that involves an electric conductor moving in a magnetic field to generate an electric signal.
In dynamic microphones, the diaphragm (a component that responds to vibrations in the air, just like the eardrum does) is attached to a coil of wire, which surrounds a strong magnet. As sound nudges the diaphragm back and forth, the coil’s interaction with the magnet generates a corresponding electric signal.
Although the physics of this process may seem complex, the parts that make it possible are quite simple, durable, and much less sensitive to high sound pressure than other microphone types. This makes dynamic microphones an extremely rugged, reliable option for live sound and recording loud sources (drums, guitar amps, explosions, etc.).
Condenser microphones (less commonly referred to as capacitor mics) are an equally popular microphone type. In this type of circuit, a diaphragm sits near a fixed metal plate. The microphone is connected to a battery or external power source, resulting in an electrical charge between the two.
As sound waves strike the diaphragm, it vibrates closer and further away from the static backplate, changing the current between these two components and creating a flow of signal. This output signal is essentially a proportional representation of the change in the diaphragm’s default position. It typically lacks a strong enough current to travel to the next piece of equipment in the audio chain, so additional components are used to amplify it before leaving the microphone.
Phantom Power in Condenser Microphones
Historically, condenser mics relied on external power supply units and special cables to operate, however, it is now common to see this power supplied directly from the microphone input on a mixer, console, or audio interface (labeled 48V or Phantom Power) over a standard XLR cable.
It is worth noting that you will certainly still run into condenser microphones with external power supplies – this is to accommodate enough power for an amplifying component known as a “tube.” Tubes have been largely replaced by solid-state electronics in modern microphone technology, though they still have a specialty interest for their “warm” sound and unique characteristics under strain.
The Advantages of Condenser Microphones
Since there is no coil attached to the diaphragm in a condenser microphone, it is lighter and can vibrate more freely than in a dynamic microphone. This gives condenser microphones the ability to more precisely represent sound waves with great detail and nuance. For this reason, they are often the preferred choice for high fidelity studio recording.
As a fair warning, there are plenty of cheaply constructed condenser microphones out there that produce unpleasant results, so do your research before making any decisions!
A ribbon microphone is a type of dynamic microphone that uses different components to accomplish the same electromagnetic process. Instead of a diaphragm and coil, an extremely thin sheet of aluminum, or aluminum alloy, is suspended between a magnet and does both jobs!
While condenser and dynamic microphones react to the fluctuation of surrounding air pressure, ribbon microphones respond to the velocity of air particles, and are sometimes referred to as velocity microphones for this reason. The ribbon element handles various frequencies differently, and usually results in a darker, smoother, and natural sounding result that is highly praised in the audio community.
It is considered best practice to protect the often-delicate ribbon from blasts of air pressure or phantom power, though plenty of modern ribbon designs offer a sturdy alternative to their fragile older counterparts.
Ribbon mics are bidirectional, giving them a figure-8 polar pattern. Sound arriving at the sides of the ribbon won’t cause it to move at all, resulting in a response only at the front and rear of the ribbon. This polar pattern provides a strategic option in recording, as you can place sound sources that you’d like to avoid capturing on the left or right axis of the microphone. It can also lend itself useful for capturing a sound, while equally capturing the way that sound interacts with the room/space behind the mic.
Boundary mics (also known as PZM mics), are small condensers that are usually mounted to a plate and placed directly on a surface, such as a wall or floor. This is advantageous when aiming to avoid phase issues caused by interference between sound reflections and the direct sound. In the recording studio, these mics are often blended with the signal from other microphones, or used for sonic experimentation.
Contact microphones use specific materials that respond to mechanical vibrations as opposed to air vibrations. These mics are almost only concerned with sound that travels through structures or objects. They are also referred to as piezo microphones because the process in which this electric charge accumulates is known as piezoelectricity.
Contact mics are often confused with boundary mics as they are both typically placed directly on a surface, though the process in which they generate a signal is (and sounds) completely different. They generally don’t offer a full sounding result on their own, but can be used in conjunction with other microphones to add depth and body to the recording.
Lavalier (lav) or lapel microphones are small sized condenser microphones attached to a clip worn by a speaker or performer. Their hands-free design and wireless capabilities make them an excellent choice for theatre, tv, or public speaking.
Shotgun mics, technically known as interference tube microphones, are highly directional microphones that were designed to capture a specific sound source, from a distance, while ignoring most unwanted noise in the surrounding environment. These are great for field recording, or any type of dialogue in which a standard mic or lavalier is not the optimal choice.
The components responsible for producing a signal are housed in what’s called a capsule. Capsules vary in sizes, although they have mostly been standardized to two categories in condenser mics; large and small diaphragm (roughly 1-inch and ½-inch, respectively). This measurement refers to the size of the diaphragm inside of the capsule, however, some manufacturers’ measurements refer to the entire capsule.
From a technical standpoint, it’s good to know that as the size of the diaphragm increases, so does its weight, meaning that its low-frequency response will be better, and its high-frequency response will become worse.
However, a larger surface area also means that more acoustic energy is captured, producing a stronger electric signal with a lower signal-to-noise ratio.
In the recording community, large-diaphragm condensers are considered the go-to choice when tracking vocals and other solo instruments. They are known for their gorgeous representation of sound, especially in the low frequencies. Small-diaphragm condensers are praised for their neutrality and transient response. They can work wonders when aiming to achieve natural, uncolored results and are often used on stringed instruments and percussion for these reasons.
You may have noticed a small symbol near the capsule of a microphone before. These symbols indicate the direction in which a microphone best picks up sound from, or its ‘polar pattern’. Understanding polar patterns will help you strategically pick a microphone and its placement in various recording scenarios.
There are two fundamental polar patterns; omnidirectional and figure-8. Omnidirectional (or simply ‘omni’) microphones pick up sound from all directions equally. This can be visualized as a sphere around the capsule of the mic. Many lavalier models utilize this polar pattern as it will capture the subject’s voice, regardless of the direction their head is facing. As we learned with ribbon mics, the figure-8 (bidirectional) polar pattern responds to sounds from the front and back, while disregarding sounds coming from the sides.
It was eventually discovered that by combining the omni and figure-8 polar patterns, more directional polar patterns could be achieved. Most notably, the cardioid polar pattern and its variations; supercardioid and hypercardioid. The below diagram demonstrates this by laying the figure-8 pattern on the omni pattern. You’ll notice that the positive half of the figure-8 doubles with the omni pattern, the negative half cancels out, while the sides remain unchanged.
Supercardioid and hypercardioid increase the blend of the figure-8 pattern, resulting in an even narrower directionality in the front, at the expense of the rear cancelation cardioid provides us. Be sure to pay attention to off-axis sounds with these patterns, as they will almost definitely suffer from a dampening of high frequencies.
The cardioid pattern is incredibly useful when aiming to capture a sound source with as little bleed from its surroundings as possible. It is the foundation of live sound feedback prevention techniques, and a recording studio staple.
Understanding the basic functionality of microphones is a great first step to improving your audio skills. When it comes down to making equipment decisions, it’s always helpful to start by carefully listening to the sound source. What tonal characteristics does it have? How loud is it? Is it producing sharp transients? How does the space around it sound? Are there other sounds happening at the same time (for instance; a noise source, a live monitor wedge, or another instrument in a multi-tracking situation)?
Once you’ve answered these questions, you can start narrowing down your options from a technical standpoint, and make a selection that compliments the sound source’s characteristics. Utilize the information you’ve learned on polar patterns to get strategic with your placement, and creative with your results. Remember, some of the most incredible recordings originated from experimentation—Don’t be afraid to get creative and try new combinations.