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How microphones work

• A microphone is a transducer – it converts between types of energy; the mic’s capsule contains the transducer
• Dynamic mics use electromagnetic induction; a movable coil attached to a diaphragm is in a magnetic field
• Sound causes the diaphragm to vibrate and the coil moves through the magnetic field inducing a varying electrical current proportional to the changes in air pressure
• Dynamic microphones are generally inexpensive and robust, they can withstand high SPL, are resistant to moisture, do not require phantom power and thus are very suitable for live use
• They do not capture high frequencies as well as condenser microphones
• Compared to condenser microphones, they have a heavy diaphragm and introduce a form of acoustic compression – this means that the transient response is much slower than for a condenser microphone
• Condenser microphones consist of two capacitor plates; sound causes one plate to vibrate
• Because the capacitor is powered (by phantom power or a battery), moving the diaphragm causes a change in capacitance as the gap between plates changes and thus a current flows
• Condenser microphones are sensitive and capture quiet sounds well; they also have a flat and accurate frequency response and can capture a wide frequency range – they are used for most studio work
• The signal from a condenser is brighter than a dynamic because of its fast transient response; because condensers are powered, the output volume is higher giving a good signal to noise ratio
• Ribbon microphones are less commonly used nowadays than dynamic or condenser microphones
• Like dynamics, ribbon microphones also work using electromagnetic induction
• Because the ribbon much lighter than a coil, a ribbon mic is able to follow the movements of the sound waves more accurately than a moving coil capsule, giving a faster transient response than a dynamic microphone
• They are fragile, often very expensive and are damaged or broken by phantom power.

Frequency and transient response

• A microphone’s frequency response graph shows us the frequencies it picks up in relation to the human hearing range; the flatter the graph, the more ‘true’ the sound is
• A transient is a short duration, high level peak, such as a handclap or snare drum hit
• Transient response refers to the ability of a microphone to respond to these transient peaks – i.e. how quickly the diaphragm can move when disturbed by a vibration
• The transient response for a condenser microphone is quick because the diaphragm is light
• In contrast, dynamic microphones have a heavy diaphragm along with the additional weight of the coil of wire
• It therefore takes longer for the diaphragm to react to a sound wave due to the effects of inertia hence causing a less accurate recording; a ribbon mic sits somewhere in between
• Small diaphragm condenser microphones tend to have the fastest transient response as they have the lightest and easiest to move diaphragm; large diaphragm dynamic microphones have the slowest.

Polar patterns

• A microphone’s polar pattern describes how it picks up sound from around its capsule
• Directional mics (cardioid, hypercardioid, figure-of-8) are affected by the proximity effect, a boost in low frequencies that increases closer to the microphone (shown as a dotted line on a frequency response graph)
• Omnidirectional mics pick up sound from all around the capsule; they are used to pick up sound with ambience
• Cardioid mics reject sound from behind, minimising reverb and room noise
• Hypercardioid mics pick up a little sound from behind the capsule and provides partial isolation; figure-of-8 mics reject sound from the sides and pick up from the front and behind the capsule.

Switches and practical considerations

• On more expensive mics, you might see a polar pattern switch that changes between different capsules
• HPF / rumble filter is a common switch which cuts all frequencies below e.g. 100 or 150Hz
• A pad switch reduces the input signal by e.g. 10dB and is used to avoid clipping at the capsule e.g. for high SPL
• You will see phantom power switches on interfaces, used for condenser mics and DI boxes; switch it off for ribbon mics or you will break them
• Shock mounts are used to avoid vibrations travelling up the mic stand and giving rumble on your recording
• Pop shields can be used to avoid plosive sounds; you can also move away from the microphone.

Revision checklist

Characteristics and suitability of microphones / advantages & disadvantages

Dynamic, condenser and ribbon microphones: properties and benefits of each

Directional microphones: cardioid, hypercardioid and figure-of-8

Omnidirectional microphones

Advantages and disadvantages of different polar patterns

Proximity effect

Microphone frequency responses: comparing, and how they affect sound

Microphone transient responses: comparing, and how they affect sound

Sensitivity

How microphones work

Electromagnetic induction

Capacitance

Diaphragms: moving coils and plates

Phantom power

Switches: pad, HPF, polar pattern

Accessories: pop shield, elastic suspension mount

Microphone techniques

Eliminating plosives