Broadcast for IT – Part 14 - Microphones

In this series of articles, we will explain broadcasting for IT engineers. Television is an illusion, there are no moving pictures and todays broadcast formats are heavily dependent on decisions engineers made in the 1930’s and 1940’s, and in this article, we investigate microphones and how they are used in television.

Converting audio sound waves to electrical signals is a fundamental requirement for any audio system and this is the role of the microphone.

Operating microphones is a highly specialized discipline and entire books have been written on them for many applications both inside and outside of television. However, there are five common designs used extensively in broadcasting; hand-held, clip-on, shotgun, boundary-effect, and contact.

Microphone designs use three main parameters to measure their performance; sensitivity, frequency response, and polar pattern.

Sensitivity refers to the size of the signal a mic can produce with a given acoustic wave input. High sensitivity mics give a better signal to noise ratio, and therefore better performance than less sensitive mics.

Frequency response governs the sensitivity at a specific frequency. In the ideal world a mic would have a flat frequency response over the entire human hearing range, that is, from 20Hz to 20KHz. However, the reality is that the response varies depending on the design of microphone and the quality of the build.

Polar pattern describes the sensitivity of a mic to its physical location. A mic may be equally sensitive to all points around it, or it may be more sensitive to sounds in front of it, depending on the application.

Dynamic Mics

The two main variants of microphone design are dynamic and condenser.

Dynamic mics use a moving-coil and operate analogously to a moving-coil loudspeaker, but in reverse. Sound waves vibrate a diaphragm situated within the mic housing, the diaphragm has a coil of fine wire attached to it which sits above a permanent magnet. As the diaphragm moves, the inductive effect generates a voltage across the coil creating a signal which varies in proportion to the frequency and amplitude of the sound around it.

Diagram 1 – Operation of the moving coil (dynamic) microphone.

Diagram 1 – Operation of the moving coil (dynamic) microphone.

Although dynamic mics require amplification using pre-amps, they do not need any electronics in them to provide an electrical signal. Combined with the moving coil assembly, they are rugged and resilient to mechanical stress.

They are resilient to high transients such as those created by drum kits and amplified electric guitars, so dynamic mics are used extensively by bands performing on stage.

Dynamic mics tend to be large and have a limited frequency response due to the inertia needed to move the diaphragm coil over the magnet.

Condenser Mics

Where smaller size, better sensitivity and frequency response is required, condenser mics are used. As well as identifying how they work, “condenser” is an old term used to describe a capacitor. Professional condenser mics deliver outstanding audio quality in a very small package and are used extensively in broadcast television and studios. A very thin metal diaphragm is stretched above a piece of flat metal thus providing the two plates needed to form a capacitor.

Sound waves vibrate the diaphragm causing the distance between the two plates to vary, thus creating a variable capacitance in sympathy with the sound around it. Applying a small voltage to the diaphragm creates a potential difference. And the capacitive effect, combined with the bias voltage, creates a voltage that varies in proportion to the sound waves applied to it.

Diagram 2 – Operation of the condenser microphone.

Diagram 2 – Operation of the condenser microphone.

The voltage created by the condenser mic is quite high, typically 80mV peak to peak, but the current is very low. Therefore, an impedance matching amplifier is needed to increase the current within the signal to sufficiently boost it and transfer it along a cable to the sound console.

Phantom Power

Fortunately, the voltage required for the bias voltage and amplifier is derived from the same source, and this is referred to as the “phantom power supply”. Some condenser mics have their own local batteries to provide the required power, especially in radio mics, but the external pre-amp or sound console will provide the phantom power feed.

The phantom power is provided along the mic cable, so no additional cables are required.

Condenser mics have a much better frequency response and sensitivity than dynamic mics and tend to be used in studios where quality of sound is of paramount importance. But they are less durable and more sensitive to weather conditions, especially moisture.

Polar Response

Pattern response describes how the sensitivity of the mic varies within its proximity. Omnidirectional mics have a uniform response around the mic, similar in shape to a football. Both dynamic and condenser mics use omnidirectional polar patterns and they have the advantage of picking up reflections and reverberation to give a natural ambience to the recording. However, they tend to pick up unwanted sounds around them.

Diagram 3 – Four diverse types of microphone polar responses, from left to right, Omnidirectional, Cardioid, Super-cardioid, and Shotgun.

Diagram 3 – Four diverse types of microphone polar responses, from left to right, Omnidirectional, Cardioid, Super-cardioid, and Shotgun.

Cardioid mics are more sensitive to signals from the front and sides, rejecting sound sources from the back. This will help where some separation is needed when recording adjacent instruments.

Variations on the cardioid are the super-cardioid and hyper-cardioid. They are only found in condenser type microphones and become more selective of sound signals in front of them. As they reject sounds from the sides and rear, they also tend to block out reverberant sound.

Shotgun Response

Shotgun mics are condenser cardioid types and are a tube approximately 12 inches (30 cm) long. They have the option of switching between super cardioid and hyper-cardioid types to make them more directional and are used in Electronic News Gathering (ENG) shoots where the highly directional response blocks out ambient noise. This is particularly useful in press conferences where there may be many other people talking near the presenter.

Boundary effect mics are specialist omnidirectional microphones that are attached to a surface such as a piano or wall. This is particularly useful where the ambience of a room needs to be recorded when recording a small acoustic performance such as a chamber orchestra.

Piezo Effect

Contact microphones are small microphones that use the piezo effect. They do not pickup vibrations in the air but do pickup vibrations from the instrument they are directly connected to. They can be placed under the bridge of an acoustic guitar to convert the vibrations of the strings directly into electrical signals.

Choosing the right microphone is a critical role performed by the sound engineer and is arguably the most important part of the sound recording chain. The first choice will always be some version of a condenser mic due to its superior performance. However, where extreme situations are experienced, such as a rock band on stage, dynamic mics will be used.

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