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Capture of sound - Microphones
A microphone is a transducer which convert different types of energy - Converting sound (air pressure) into electrical energy
Transducer element of a mic is called its capsule
Different types of microphones work in different ways. - Dynamic, Condenser and ribbon mics.
Dynamic Microphones - Mechanics
Dynamic mics can be known as moving coil mics
Moving coil is position in a magnetic field - Attached to a diaphragm
Diaphragm is a thin piece of material that moves in response to the changes in air pressure
When sound enters the mic, the diaphragm vibrates
Coil moves in the magnetic field and a varying electrical current is induced that is proportional to the changes in air pressure
Called Electromagnetic induction
Dynamic Microphones - Characteristics
Generally inexpensive
Robust
Can withstand high SPL / volume
Resistant to moisture
Good for live use
Does not require phantom power
Limited HF response: Suitable for bass instruments
Condenser Microphones - Mechanics
Condenser mics contain a capacitor which in turn consist of two plates
Sound causes one plate to vibrate which functions as diaphragm
When diaphragm vibrates, the gap between two plates change
Capacitor plates are powered. Moving the diaphragm causes a change in capacitance and thus a current flows
Condenser Microphones - Characteristics
Sensitive, giving effective capture of quiet sounds
Flat and accurate frequency responses
Able to capture a wide frequency range
Generally able to capture a wide frequency range
Generally captures a brighter signal than DMs
Good signal-to-noise ratio. High output volume and thus low noise
Wide dynamic range
Suitable for most studio work
Ribbon Microphones - Mechanics
Sound vibrations disturb a metallic ribbon suspended in a magnetic field
This generates a voltage that is proportional to the movement of the ribbon.
‘Warm’ sound when used as a close mic with emphasised low frequency
Fragile and often very extensive
Damaged or broken by phantom power
Pre-amps
A pre-amp converts a signal into a workable line level
There are a number of controls on a pre-amp that help us to work with microphones and recording equipment
Controls on a pre-amps:
Phantom Power
Gain
Pad
High Pass Filter
Polarity
Clip / Activity LEDs
Phantom Power
Phantom power is required to charge the capacitor of a condenser mic, and for its internal pre-amp
Some condenser mics can also be powered by a battery
Many mixing desks and audio interfaces allow you to switch phantom power (48v) on and off.
Polar Patterns
A mic’s polar pattern describes how it picks up sound from around the capsule
Polar patterns including:
Omnidirectional
Cardioid
Hypercardioid
Figure-of-8
Polar patterns all exhibit the proximity effect
Polar patterns - Omnidirectional
Picks up sound from all around the capsule
Captures room ambience
Provides little isolation so can lead to spill being captured
Useful if the space sounds nice or the instrumentalists are all around the mic.
Polar patterns - Cardioid
Rejects sound from behind the microphone
Minimises the amount of reverb and / or noise from behind the microphone that is captured
Used for most close-mic work
Polar patterns - Hypercardioid
Picks up sound from in front of the capsule and gives some capture of those behind
Effective at providing partial isolation from other instruments but capturing some of the ambience of the room
Polar Patterns - Figure-of-8
Picks up sound from in front of and behind the capsule
They are used in M/s (mid-side) recording techniques to give a stereo image of the sound
Figure-of-8 microphones strongly reject sound from the sides of the capsule
Polar patterns - Avoiding unwanted noise and spill
Performers wear (closed-back) headphones
Keeping the monitor mix relatively quiet in headphones
Using acoustics screens / isolation booths
Making use of overdubbing and directional microphones
Frequency Response
A microphone’s frequency response tells us the frequency it picks up
It is common to see a graph of a mic’s frequency response
The flatter the graph, the more ‘true’ the sound is
If a graph has any particularly peaks, this means that those frequencies are captured louder than they are in real-life
Transient Response
How quickly the diaphragm can move when disturbed by a vibration
Small diaphragm condenser microphones tend to have the fastest transient response as they have the lightest and easiest to move diaphragm.
High frequency response is often better
Diaphragm in DM is connected to a heavy coil, it does not move as quickly. Particularly true of large diaphragm DMs.
Can introduce a form of acoustic compression.
Setting the gain
You
Signal-to-noise Ratio (SNR)
SNR ratio of a recording is difference in volume between the signal you want to capture and the noise
A poor signal-to-noise ratio will mean that noise is more prevalent in a recording
Because the signal is quieter and is thus closer to the volume of the noise, the engineer will ha
Headroom
Headroom is the gap between the loudest peaks of your mix or audio and the point at which digital clipping begins
Sometimes, analogue soft clipping is used to add warmth to a recording, but digital clipping can sound harsh and unmusical.
Microphone, line, and instrument level
Signals have a difference in impedance (sometimes labelled on equipment as ‘z’)
Proximity Effect
Increase of low frequencies depending on how close the microphone is to sound source. When closer, there will be an increase in the captured signal’s lower frequencies.
Apparent when recording male voices and acoustic guitars
Can be used positively for kick drums and bass guitars
Directional microphones exhibit the proximity effect
EQ or an HPF/rumble filter can be used to reduce its effect, or you can alter the mic position, moving it further away when capturing the sound.
Hiss and Hum
Filters can be used to remove unwanted low or high frequency noise
Low pass filters could be used to remove hiss
High Pass filters could be used to remove hum
A parametric EQ with a narrow Q could be used to remove a specific frequency that is causing a problem
Phase
Phase is when sound waves’ peaks and troughs line up.
If sound waves are out of phrase then the peaks and troughs do not line up and cause destructive interference, which sometimes can completely cancel out the sound of the wave.
Phase must be considered in any situation where you are using lots of mics to record the same sound source.
Problematic when recording using two mics on opposite sides of a drum.
Phase switches are used to invert polarity of signal.
Microphone Switches
Polar Pattern
High Pass Filter
Pad - Changes sensitivity of mic.
On and Off Axis Microphone Placement
Placing the mic at different angles changes the range of frequencies that are captured
Placing the mic on-axis will capture a brighter sound and off-axis will make it duller
Direct Injection
Converts a signal at instrumental or line level to mic level and unbalanced signals to balanced signals.
Used to eliminate the need to mic up electronic instruments giving a direct injection to an audio interface or mixer
Active DI boxes require phantom power or a 9V battery, whereas passive DI boxes do not require external power
Plosive sounds
Strong initial transient (p, d) which can create a large disturbance in air pressure on the diaphragm and a ‘pop’ sound
Pop filter is used to disperse the air more evenly to avoid such a quick and large diaphragm movement
Possible to reduce impact of plosive sound using EQ and compression, but best solution is to re-record or comp in the word or phrase from another part of the song.