Audio For Broadcast: Microphones Part 2 - Types Of Mic

Polar patterns play a big part in designing the sound for a programme, and they apply to all microphones. The next step is to ascertain what kind of microphone to use.

In broadcast there are two types of mic in general use: dynamic mics and condenser mics. While both have membranes that vibrate to capture changes in air pressure and covert these changes into electrical energy, they are very different beasts. Dynamic mics work via electromagnetic induction, while condenser mics work using electrostatic principles, and both have inherent properties which can be used to help define how a broadcast will sound.

Dynamic Mics

A dynamic microphone converts sound waves into electrical signals using electromagnetism, and this can be done in two ways.

The most common method is with a moving coil. A dynamic mic with a moving coil uses a diaphragm (a thin membrane which registers changes in air pressure) attached to a metal coil suspended in a magnetic field. Sound waves cause this membrane to move and these vibrations are converted into electrical signals.

Dynamic mics are excellent all-rounders. They have no internal electronics, do not require power and are not prone to overloads and distortion. This makes them extremely rugged and can handle a wide range of sound pressure levels – a reliable workhorse for broadcast environments. Its Achilles heel is that due to the weight of the coil it is unable to respond fast enough to capture especially high or low frequencies.

This principal doesn’t apply to ribbon mics, which is the second kind of dynamic mic. Although designed around the same principal a ribbon mic uses an extremely thin metal foil ribbon suspended in a magnetic field instead of a heavy metal coil.  This makes it much more sensitive to vibration.

Ribbon Mic ‘Fig 8’ Polar Pattern.

Ribbon Mic ‘Fig 8’ Polar Pattern.

Ribbon mics also have a polar pattern which is different to what we have discussed. The nature of the transducer means that ribbon mics are inherently bi-directional (often referred to as “figure of eight”), so they capture sound from the front and back and have phenomenal rejection from the sides. This has benefits for some forms of broadcast (in fact, as you’re asking, ribbon mics have been used in broadcast environments since 1931 – they were the first kind of mic to be used) and are especially suitable for podcasters who can use a single mic to record a host and a guest sat opposite at the same time.

Ribbon mics have a high frequency roll off which mimics the way human beings listen to sound, which makes recordings sound very naturalistic. Another benefit is how well they respond to vocals; the proximity effect, which is where lower frequencies become more pronounced the closer the subject is to the mic, is stronger than on any other type of microphone. Combined with its naturalistic sound this makes it perfect for vocals and a good choice for talk radio or voice over booths.

Condenser Mics

Designed on electromagnetic principals, a condenser mic is completely different. It works by pairing its diaphragm with a parallel backplate and applying electricity to both to create a static charge. When the diaphragm vibrates it alters the distance between the two plates to create a change in the voltage, which mimics the changes in the sound waves.

This process requires condenser microphones to have internal electronics, which in turn means they need power to function. In a broadcast environment, this is provided by phantom power delivered via a standard 3-pin XLR cable. All broadcast consoles have built-in phantom power, often labelled as +48V, which can usually be switched in and out on a per channel basis. While we’re on the subject, while mostly passive, some ribbon mics also contain active electronics to allow them to output a higher signal. These will also require phantom power.

While we are still on the subject, phantom power is also required to drive another class of condenser mic. RF Capacitor microphones are highly regarded for extremely accurate sound reproduction, rugged nature, wide frequency response and low inherent self-noise. They differ to traditional condenser mics in that they use capacitive capsules as the tuning element of a radio-frequency oscillator; sound waves arriving at the capsule change the capacitance, which changes the frequency of the RF oscillator. After demodulation, an audio frequency signal with low source impedance is produced, capable of driving bipolar transistors which produce less random noise and can capture a wider dynamic range. The advantage of this approach is that the capsule works in a very low-impedance environment (as opposed to the very high-impedance environment of a traditional DC-biased and Electret capacitor mics), making it immune to the effects of humidity which can cause unwanted noise in conventional capacitor mics.

Both types of condenser mic outperform dynamic mics at higher and lower frequencies, and they are far more sensitive and detailed. The reason for this is the same reason as why moving coil mics struggle beyond a certain frequency range; it is much easier to vibrate a strip of metal than it is to move a heavy moving coil.

This makes them a good choice for a variety of broadcast environments, and an excellent choice for music and vocals as its wider range captures more of the nuance and character of the human voice.

Broadcast Use

This huge variety of mics, with multiple polar patterns and multiple mic types, are also available in a variety of form factors which can be employed in hundreds of different ways.

This all points to our initial assertion: microphone choice must always be dependent on the application. There will always be a variety of ways to mic up any environment, but knowing the basics about how microphones work will give you a head start.

For example, is it appropriate to see a mic? At a press conference it is part of the visual expectation to see mics with colourful mic flags, but this may not apply to an evening news broadcast.

Lavalier mics, otherwise known as lapel mics, can be a good solution for this environment. A lavalier is a tiny microphone that can be clipped to a person’s clothing and is a good option for on-camera shoots where the person needs to move around. Because the distance between the microphone and the talent’s mouth is reasonably consistent, the sound level is also reasonably consistent.

But there are still decisions to be made, with lavalier mics available in both dynamic and condenser versions. They can also have different polar patterns: a cardioid lavalier can isolate the talent from the environment, which may be desirable, but as it may have less reach it will require fitting higher up the chest.

An omni lavalier doesn’t need to be fitted as conservatively as it will pick up sound from all around, so sound supervisors can be more creative with its location. As it captures more of the room ambiance, it can provide a sense that the viewer is eavesdropping which may add an additional element to the storytelling. It that appropriate? It is also more forgiving if the talent moves their head or the mic slips from its mounting, so it might work better with more inexperienced presenters or subjects in more active environments.

Outside Influences & Noise Pollution

There may be factors outside of your control. Shotgun mics can be either condenser or dynamic as they are only categorized by their lobar - or super-cardioid - pickup patterns, but are the conditions suitable for both kinds?Durability should play a part in the application decision.

If the broadcast is in a noisy environment like a live venue, a reporter may prefer a dynamic handheld cardioid mic due to its ability to reject unwanted noise, but is there be a way to isolate the talent to enable them to use a condenser mic with a different polar pattern?

Does the broadcast need a backup mic? Could that be met with a nearby condenser mic, and if so, does that need to be hidden? Are there any stipulations in the rider of a guest regarding what kind of mic they are willing to use? Even in more controlled environments, the surrounding environment will also play a role.

For example, live entertainment shows require a lot of thought as the on-stage talent may need foldback to hear inserts from the studio and the production shouldn’t have any bleed from this picked up an any mics. Audience bleed might also play a part in the sound design.

On a panel show or a political debate, guests might be sat close to each other so they can hear each other, but they may be spaced apart and cross foldbacks might be necessary to the host can be heard by the guests and vice versa.

Plan And Plan And Plan

With such a variety of options that there are many ways of using microphones to provide value to the broadcast output, but there is seldom a one-size fits all solution.

“You don’t want to end up carrying around a bag with all your microphones, but you need to be confident that you’ve got enough variation,” says Bafta award-winning sound supervisor Robert Edwards. “On a fixed installation there might be 20 mics in the rack and that’s the show done. But you’ve got to know what your production is and what it might need visually as well as from an audio point of view.”

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