CAPTURE OF SOUND

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.