From music recording to podcasting, microphones play an essential role in many of our lives. Each recording situation requires a different kind of microphone and one of the best ways to get started is to know some of the essential mechanisms and terms in sound recording.
Microphones are such a staple of the modern landscape that it can easily escape us that we don’t know that much about how they do their magic. And even once you get your head around how they capture sound, terms like polar pattern, frequency response, and sensitivity can send you spinning again.
So what do you really need to know before buying a microphone? Keep reading to ground yourself.
What Are Sound Waves?
When you pluck a guitar string the vibration of the string causes a disturbance in the surrounding air particles. The movement of this disturbance through the air is known as a sound wave.
As the sound wave travels through the air, it will collide with particles in other objects like walls, and each collision causes the sound wave to lose energy. That is why the closer you are to the origin of the sound, the louder it seems to you.
A sound wave can be represented on a graph where the amplitude is the distance between the undisturbed position and the maximum disturbance. In audio applications the amplitude of a wave usually refers to the loudness. A wavelength is the distance between a point on one wave, and the same point on the next.
A low frequency means that the waves are distantly separated, whereas a high frequency means that the waves are closer together. The frequency is equivalent to the pitch of the sound. Thus, a high frequency wave is a high pitched while a low frequency wave is low pitched.
How Microphones Work
Microphones capture the energy from the sound waves and, in a couple of different ways, are able to convert the sound into electrical energy. There are two main methods of doing this, either by using a condenser or a dynamic microphone.
A condenser microphone uses a capacitor to capture the sound. The wave enters the microphone and causes the front plate of the capacitor to vibrate, moving it closer to the back plate, which creates a change in capacitance. In order to register the change in capacitance, condenser microphones require phantom power of +48 volts across the capacitor.
A dynamic microphone uses electromagnetic induction to transduce (convert) the sound into electrical energy. Dynamic microphones have a diaphragm with an induction coil attached, which is placed in the magnetic field of a permanent magnet. When the sound waves hit the diaphragm, they cause it to vibrate. This causes the coil to move within the magnetic field, creating a varying current and transducing the sound into electrical energy.
Ribbon microphones also operate using electromagnetic induction. Although instead of using a moving coil, the ribbon is directly placed inside the magnetic field. This allows for better performance at capturing high-frequency detail.
Since a microphone sits in 3D space, the physical microphone is able to receive sounds from all directions. However, that may not be the effect that you are after. For example, if you want to mic the singer of a live band, you would only want to receive sounds from the singer and not the audience.
Different microphones have been created for different recording situations, and each have characteristics that allow them to only receive sound from particular directions. Polar patterns describe the direction that the microphone can pick up sound from.
An omnidirectional microphone can pick up sounds equally from all directions around it. While it wouldn’t be suitable for the aforementioned live event, it’s the ideal choice for recording an orchestra or choir. It is also commonly used for live studio performances, where the goal is to recreate the sound of the room.
A good example of this type of microphone is the Rode Reporter. Although it is primarily targeted at voice recording in interviews, thanks largely to its tailored frequency response, it is a well-regarded omnidirectional microphone.
Bi-Directional (Figure 8)
A bi-directional microphone is equally sensitive to sound from the front and rear. However, it has very high rejection of sound hitting the microphone from the side. Although they aren’t frequently used in many studio or live recording setups, they are especially useful for creating a Blumlein pair (a method of creating stereo recordings).
There aren’t many companies that make pure Figure 8 microphones. However, German audio equipment manufacturer Neumann offers one in the U87. Although the price is relatively high, it justifies the price by allowing you to switch between Figure 8, Cardioid, and Omnidirectional polar patterns.
Cardioid microphones get their name from the shape of their sensitivity pattern, which is roughly heart-shaped. They are commonly used to pick up vocals or speech as they only hear sounds directly in front. As they are unable to pick up sounds from the side and rear, this prevents feedback loops.
One of the most popular cardioid microphones is the Shure SM58. The SM58 has been in production since 1966 — an indicator of its quality — and, along with the SM57, is one of the world’s best-selling microphones.
Frequency Range and Response
You may have heard of dog whistles that produce sound at a frequency outside of the range of human hearing. A microphone also has a frequency range that it can “hear”. On a microphone specification sheet, this is listed as a range of numbers.
However, as is also similar with humans, some microphone designs are able to record some frequencies better than others. The microphone’s response to its frequency range will be shown in a frequency response graph. This gives you some very useful information when choosing microphones for different applications. For example, a microphone that performs better at lower frequencies may lend itself to bass or drum recordings.
The sensitivity of a microphone is a measure of how much electrical output it produces for a given sound pressure input. This makes it a useful way to compare different microphones. Most manufacturers test with a sound pressure level of 1 Pascal, which equates to 94 dB. So a microphone with a higher output voltage is said to be more sensitive than a microphone with a lower output.
Maximum Sound Pressure Level (SPL)
Another measure of a microphone’s performance is the maximum Sound Pressure Level (max SPL). This is usually expressed in decibels (dBs) and is the maximum volume that the microphone can handle without distortion.
This is typically only given for condenser microphones as dynamic microphones are unlikely to create distortion. It is theoretically possible for the coil to vibrate enough to hit the microphone’s physical housing and distort, but this is not known to happen in practice.
Time to Rock the Mic
Sound recording has at least one thing in common with most technology: lots of terminology. Understanding the basic workings of a microphone, the different types, and microphone classifications will help you make the right choice for your project. With the fundamentals of microphones in your grasp, you should be well on your way to making awesome sounding recordings.
How many of these terms did you already know? Do you think we missed something? What microphone will you choose? Tell us about your projects in the comments below!