Learn how microphones function, their classifications, output configurations and how to choose the best microphone for the job
Microphones are transducers. They convert one form of energy – sound – into another form of energy electricity. Ideally, there is no loss when the sound energy is converted into electrical energy. The more efficient the process, the better the microphone.
There are three different electrical and mechanical means by which microphones do this, each with it’s own strengths and weaknesses.
Right: Cutaway of a dynamic microphone (photo courtesy of Audio-Technica)
The first type of microphone is the dynamic microphone. Dynamic microphones are pressure receivers that sense changes in air pressure. A diaphragm, which is moved by sound waves, also moves a coil of wire, which breaks magnetic flux lines to create an electrical current. In essence, a dynamic microphone is a miniature generator. The stronger the sound that hits the diaphragm, the more electricity is generated. The changes in current are what is encoded and recorded as the sound signal.
A dynamic microphone is a speaker in reverse, and many speakers are also microphones. For example, when you order a hamburger at the drive through, the same speaker is also used as microphone.
Dynamic microphones, do not require power because they, themselves, generate current. This benefit comes at a price though as they are not as sensitive as other microphone types, however they are incredibly rugged and can easily handle the rigors of production.
Left: The inside of a condenser microphone (photo courtesy of Audio-Technica)
The second type of microphone is the condenser, which used a fixed plate and a lightweight membrane. Sound eaves move the membrane, changing the capacitance of the circuit to create a changing electrical output. This electrical variance is then encoded and recorded by the recording device as the sound signal.
Condenser microphones require power to maintain the electrical field in the capacitor, but are much lighter – and smaller – then their dynamic microphone cousins. They are the microphones type of choice in shotgun, handheld, and lavalier configurations. Condensers tend to be more sensitive and are able to capture a wide frequency response, making it an ideal choice in the field.
There are two models of condenser microphone – one that requires an external power supply, and a second with a voltage impressed on the diaphragm at the time of manufacture.
Many cameras, mixers and recording devices are capable of sending up to 48 volts of DC power through an XLR cable to power a condenser microphone at the other end. This is called Phantom Power.
The third type of microphone, which is least often used, is the ribbon mic. Ribbon microphones function by creating an electric signal generated from a thin metallic ribbon suspended between the poles of a magnetic circuit. Ribbon microphones are fragile and are extremely susceptible to wind or high acoustic pressures. They are not often seen on set because of this fragility.
Sensitivity is the volume of sound a microphone can pick-up. The sound generating source will create a sound wave of a certain amplitude and frequency, and the sensitivity is a microphone’s ability to capture this.
On the surface, it can be difficult to compare the sensitivities of various microphones. The microphone type, pick-up pattern and proximity to the sound source can all impact the perception of the mic’s sensitivity.
There is always a background noise that exists in the universe caused by the Brownian motion of atoms and molecules. This background noise establishes a constant against which microphones are compared. When working on location, additional background noise is added – wind, ambience, even the sound of equipment and line noise. The signal-to-noise ratio is the ratio of the strength of the desired sound compared to the background noise. The higher the ratio, the cleaner the recording of the desired sound.
There are a number of ways to help increase the signal-to-noise ratio including:
•Reduce the ambient noise - Turning off air handling systems, stay away from freeways and turn off appliances
•Control the gain and volume – If you’re using a mixer, you can control the gain (sensitivity) of the microphone and the output volume to find the best balance between sensitivity and output.
•Move the microphone closer - As simple as it sounds, moving the mic closer to the sound source will increase the strength of the desired signal while maintaining the volume of the background noise, essentially increasing the signal-to-noise ratio.
•Use a different microphone – Choosing a microphone with a tighter pick-up pattern will pick-up desired sounds while rejecting sounds outside the pick-up patterns. Hypercardioid and shotgun microphones are ideal in high-noise environments for this reason.
Maximum Sound Pressure Level
Due to physical limitations of how much a diaphragm can move inside a microphone, there is an inherent limit as to the loudness a mic can pick-up. Called the maximum sound pressure level or SPL, when this threshold is reached, the audio will distort. If this happens, either reduce the volume of the sound source, move the microphone farther away from the sound source or place a filter between the sound source and the microphone.
Each microphone will pick-up each frequency differently, and manufactures often create a graph to demonstrate the frequency response of a microphone. It’s important to know the strengths and limitations of how each mic you use responds to each frequency so you can choose the best microphone for the job.