CAPTURE OF SOUND

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27 Terms

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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.

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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

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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

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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

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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

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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

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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

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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.

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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

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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.

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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

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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

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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

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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

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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

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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.

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Setting the gain

  • You

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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

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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.

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Microphone, line, and instrument level

  • Signals have a difference in impedance (sometimes labelled on equipment as ‘z’)

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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.

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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

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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.

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Microphone Switches

  • Polar Pattern

  • High Pass Filter

  • Pad - Changes sensitivity of mic.

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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

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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

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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.