Capture of Sound Flashcards
How Dynamic Microphones Work
Dynamic microphones have a movable coil positioned in a magnetic field attached to a diaphragm.
When Sound enters the mic the diaphragm vibrates
The coil moves in the magnetic field which leads to an electrical current which is proportional to the changes in air pressure
Features of Dynamic microphones
generally inexpensive Robust Can withstand high volume Resistant to moisture Good for live use Dose not require Phantom power Suitable for bass instruments.
How do Condenser Microphones work
They contain a capacitor which consists of two plates.
Sound causes the plates to vibrate which functions as the diaphragm
When the diaphragm vibrates the plates move apart
This causes a change in the capacitance which allows the current to flow.
Features of a Condenser Microphone
Sensitive, Effective for capture of quiet sounds
Accurate frequency response
able to capture a wide frequency range
Generally able to capture a brighter signal than DM’s
Good SNR: high output volume and thus low noise
Wide dynamic range
Suitable for most studio work
How do Ribbon Microphones work
Sound vibrations disturb a metal ribbon suspended in the magnetic field
This generates a voltage that is proportional to the movement of the ribbon
Warm sound when used as a close mic with emphaissed low frequencies.
Damaged or broken by phantom power
Phantom power (48)
48V from a mixing desk or Audio interface.
it provides power for condenser Mics and DI boxes
Gain
Used to boost signals to an usable level.
Can be used to dive a signal into distortion for musical purposes
Pad
A switch on equipment that attenuates the gain by a set amount to prevent clipping.
High pass filter
a filter which allows high frequencies through and removes lower frequencies
Low pass filter
a filter which allows low frequencies through and removes higher frequencies
notch Filter
A notch filter is a bandstop filter with a narrow bandstop bandwidth. Notch filters are used to attenuate a narrow range of frequencies.
A band-pass filter
also bandpass filter passes frequencies within a certain range and rejects frequencies outside that range
Polarity
Inverts the polarity of the signal
Signal-to-noise ratio
This is the signal you want to hear, versus the noise you don’t want to hear
Omnidirectional polar patterns
their diaphragms measured sound pressure at a single point in space.
Because they had no directional information, they were equally sensitive to sound from all directions.
Figure of 8 Polar patterns
Commonly known as “pressure gradient” microphones, they measured the difference in pressure between either side of an open diaphragm.
This means that they were very sensitive to sound from the front and rear, but almost completely deaf on the sides.
Hypercardioid Polar patterns
Polar pattern in the shape of a heart
Positives about cardioids
Miking up a drum kit – With so many instruments so close together, isolation might seem impossible. But it CAN be done, with the right cardioid mics, positioned in the right spots.
Live performances – On-stage, when sounds are coming at you from all directions, cardioid mics are great maintaining isolation and preventing feedback.
Untreated rooms – In rooms with poor acoustics, close-miking with cardioid mics can work wonders at minimizing reflected sound.
Negatives about cardioids
Off-axis coloration – With most cardioid mics, you see a drop in high frequency sensitivity as sounds move further off-axis. This could be bad, for instance, with an inexperienced singer unconscious of his head movements.
Proximity effect – A phenomenon exclusive to cardioid mics…proximity effect is a boost in bass frequencies that results from extreme close-miking. Using the same “inexperienced singer” example, you can see how this might also cause problems.
When to Use Omni’s
When recording the sound of the room – such as with room mics for drums.
When recording a wide sound source – such as an orchestra, choir, or grand piano.
When recording a moving target – such as an acoustic guitar player who can’t sit still.
When recording in stereo – such as with the common A/B technique.
Positive’s about Omni’s
immunity to proximity effect
lower self noise
a frequency range that typically extends a full octave lower
less coloration of off-axis sounds
When to Use Figure-8
for stereo recording
with ribbon mics
for maximum isolation of off-axis sounds
Frequency Response
A mic’s Frequency response tell us the frequencies it picks up
It’s common to see a graph of a Microphone’s response
The flatter the graph the more “true” sound there is
The peaks on the graph means the sound at that point have been captured louder than they are in real life
Transient Response
How quickly the diaphragm moves when disturbed by a vibration
Small diaphragm condenser mics tend to have the fastest transient response as their diaphragm is the lightest and easiest to move
a condensers high frequency response is usually better
Dynamic mic’s diaphragm is connected to a metal coil so it doesn’t move as quickly
SNR
the difference in volume between the signal you want to capture and the noise captured
A poor ratio will mean noise is more prevalent in the recording
Proximity Effect
The increase of low frequencies depending on how close the mic is to the sound source.
Can be used positively for kick drums and Bass Guitars
Directional mics exhibit the proximity effect
EQ or a Rumble filter can be used to reduce it’s effect . Or you can alter the mic’s position,
Hiss and Hum
Low pass filter is used to remove hiss
High pass filter could be used to remove hum
Phase
If sound waves are in phase then their peaks and troughs line up
If sound waves are out of phase then the peaks and troughs do not line up and cause destructive influence
Direct Injection
A DI box converts a signal at instrument level to mic level and unbalanced signals to balanced
DI boxes are used to eliminate the need to mic up electronic instruments and they give a direct connection to the audio interface
DI boxes rquire phantom power(48V)
Plosive sounds
Plosive sounds have a strong initial transient which creates a large disturbance in air pressure on the diaphragm and creates a pop sound
A pop filter is used to disperse the air more evenly to avoid such a quick and large diaphragm movement
it is possible to reduce the impact of plosive sounds using EQ and compression, but by far the best solution is to re-record part of the song
What is Clipping??
When an amplifier is pushed to create a signal with more power than its power supply can produce, it will amplify the
signal only up to its maximum capacity, at which point the signal will be clipped. The extra signal is
cut off, resulting in a sine wave becoming a distorted square-wave
Avoiding Clipping
To avoid this clipping, the overall level of a mix can be lowered, or a limiter can be used to dynamically bring the levels of the loud parts down (for example, bass and snare drums).
It is not simple to eliminate all clipping, as filtering (e.g. a high-pass filter) can align various frequencies in such a way as to create excessive peak outputs.
Repairing a clipped signal
. The goal of repair is to make up a plausible replacement for the clipped part of the signal. Complex hard-clipped signals (recorded at
CD resolution or less) cannot be restored to their original state because the information contained in the peaks that are clipped is completely eliminated.
Soft-clipped signals can be restored to their original state to within a case-dependent tolerance because no part of the original signal is completely eliminated.
Radio Frequency Interference
RFI is caused by CB transmitters, computers, lightning, radar, radio and TV transmitters, industrial machines, cell phones, auto ignitions, stage lighting, and other sources.
Cables and Interference
Another cause of interference is audio cables picking up magnetic and electrostatic hum fields radiated by power wiring in the walls of a room.
Also keep audio equipment and cables away from computer monitors, power amplifiers, lighting dimmers and power transformers.Use balanced cables wherever possible.
Ground Loops
Another major cause of hum is a ground loop: a circuit made of ground wires. It can occur when two pieces of
equipment are connected to the building’s safety ground through their power cords, and also are connected to each other through a cable shield.
Polarity
Used to invert the waveform of the signal to help deal with out of phase issues.
A/B Stereo Recording
Mics used: Two omnidirectional mics, usually small diaphragm condensers.
Positioning: Point both mics toward the instrument, at a distance of a foot, and spaced two feet apart.
How to mix the signals: The mono signalsfrom each microphone areassigned to the left and right channels of a stereo track to create a sense of width in the recording.
How it should sound: The stereo image is created because the time of arrival at each microphone is slightly staggered. The frequency balance is different as well, which will provide an added level of stereo width.
The downside of A/B stereo recording is that because of the timing offset between each microphone, you will be likely to have issues with phase cancellation when combining the stereo signal to mono.
X/Y Stereo Recording
Mics used: Two directional mics, usually small diaphragm condensers
Positioning: at an angle between 90-135 degrees so that their capsules coincide at a single point. The wider the angle, the wider the stereo image.
How to mix the signals: The mono signalsfrom each microphone areassigned to the left and right channels of a stereo track to create a sense of width in the recording.
How it should sound: Compared to A/B stereo recording, this technique will have less of a stereo effect. The reason is that since both microphones are positioned at the same point in space, there will be no differences in timing.
The entire stereo effect will be created from the differences in frequency balance. The upside to this is there are no issues with mono phase cancellation either.
ORTF Stereo Recording
Mics used: Two directional mics, usually small diaphragm condensers.
Positioning: Spread outward at an angle of about 110 degrees, with the capsules spaced 17cm apart.
How to mix the signals: The mono signalsfrom each microphone areassigned to the left and right channels of a stereo track to create a sense of width in the recording.
How it should sound: The technique is basically a combination of the previous two. The microphones are physically spaced apart, like with A/B recording, which will yield a wider stereo image.
Then it uses directional mics, like with X/Y recording, so it should pick up less of the ambient room sound.
