Exam Prep Flashcards
What happens when 2 mics pick up the same sound, but with slightly different times of arrival?
3 to 1 rule:
Comb filtering!
To avoid comb filtering, the second mic that will pick up a sound must be at least 3 times further away than the first mic – this will make the ripples from the comb filtering practically inaudible.
- Have the second mic 3x further away than the first one
- Have the mics close together
- Or only use one mic
- Comb filtering becomes much less - 1dB
How is the 3:1 rule determined?
When you add a signal to its delayed replica at equal levels, you get severe comb filtering with deep notches.
SRA
If the SRA is wider than the ensemble width then the reproduced
stereo sound will be narrow.
If the SRA is narrower than the ensemble width then any source
outside of the SRA will be compressed at the loudspeakers.
Proximity Effect
When the sound is far from the mic, the extra distance to the rear of the diaphragm is tiny.
However, as the sound source gets closer to the mic, the distance to the rear of the diaphragm becomes significant. So the rear wave is lower amplitude than the front wave.
When the rear wave is much lower amplitude than the front wave, then there is a pronounced base boost.
Directional mic’s (pressure gradient) gets its output signal by responding to the difference in pressure from the front to the rear of the diaphragm..When a sound wave is on axis with the diaphragm, the sound wave must travel a further distance to get to the rear of the diaphragm, so the wave will be slightly delayed at the rear of the capsule.
Imagine a capsule 8.5mm deep.
The sound wave hitting the back of the diaphragm travels an extra 8.5mm, and is delayed from the front wave.
We can see the increase in delta p (output) across the 8.5mm capsule as frequency increases.
Graphing the red lines shows that the frequency response increases 6dB per octave, called the ‘gradient component’ of the response - fairly useless for recording … To compensate, the diaphragm is damped to create a 6dB.octave decrease. The combination of these two factors create an overall frequency response that is mostly flat, except for the bass and rough HF.
So the proximity effect is bass boost, caused by a sound source getting closer to a directional mic capsule, so that the distance from the sound source to the front of the capsule is significantly different from the sound source to the rear of the capsule.
Interaural Intensity Difference
This is the differing levels of intensity that result at each ear due to the ‘shading effect’ of the head. This effect shows that the levels at each ear are equal when the sound source is on the median plane, but that the level at one ear progressively reduces, and increases at the other, as the source moves away from the median plane. The level reduces in the ear that is furtherest away from the source
An object is not significant as a scatterer or shader of sound until its size is about two thirds of a wavelength (1/2), although it will be starting to scatter an octave below that frequency.
This means that there will be a minimum frequency below which the effect of intensity is less useful for localisation which will correspond to when the head is about one third of a wavelength in size (1/3). For a head the diameter of which is 18cm, this corresponds to a minimum frequency of about 630Hz.
Interaural Time Difference
The interaural
Interaural Time Difference
The interaural intensity difference is a cue for direction at high frequencies whereas the interaural time difference is a cue for direction at low frequencies. Note that the cross-over between the two techniques starts at about 700Hz and would be complete at about four times this frequency at 2.8kHz. In between these two frequencies the ability of our ears to resolve direction is not as good as at other frequencies.
Coincident mic technique characteristics
- Perfect mono-compatability
- Directional mics - proximity effect, uneven high frequency response, bass roll off
- Realistic
- Soundfield somewhat compressed left-right
- Off axis colouration not occurring:
- MS does not have this (the middle)
- Spaced AB is a great option - pointing directly at the
source - The blumlein pair produces an exceptionally realistic stereo image, but the quality of recordings highly dependent on the acoustics of the room and the size pf the sounds source
Advantages:
- Perfect mono-compatibility
- Good to excellent stereo image
Disadvantages: All of the problems of directional mics - Bass roll off - Proximity effect - Uneven high frequency response
Spaced mic technique characteristics
- Poor mono compatibility
- Wide stereo image - exaggerated (Poor to okay)
- Not as realistic
- Omnidirectional - no proximity effect, flat frequency response
- You can use cardioid with AB but it just depends of what reflections you want to be picking up - omni will pick up ceiling and floor
- Wide coverage for large sources
- Minimal colouration for off axis sources
Name the Spaced Microphone Techniques
- Spaced AB
- Decca Tree
Name the Coincident Microphone Techniques
- XY
- Mid-Side
- Blumlein
Name the Near Coincident Microphone Techniques
- ORTF
- NOS
- Jecklin Disc
- Binaural
All directional microphones have:
- Falling bass response
- “Rough” high frequency response
- Proximity effect problems
- Very poor frequency response off axis
XY
90˚ crossed SDC cardioids
Blumlein
90˚ crossed fig-8’s
Mid-Side
90˚ crossed fig-8 and cardoid
NOS
90˚ angled SDC cardioids spaced at 30cm
ORTF
110˚ angled SDC cardioids spaced at 17cm
AB
Omnis or Cardioids spaced at 30cm or more
Decca Tree
3 omnis, inverted T arrangement 2m x 1.5m
Near Coincident mic technique characteristics
Advantages:
- Good mono-compatibility
- Good to excellent stereo image
Disadvantages: All of the problems of directional mics - Bass roll off - Proximity effect - Uneven high frequency response - Front of soundfield is quite off-axis to the mics, which causes colouration
Stereo Recording Angle
Every stereo microphone technique has an angle in front or around it which will produce a stereo image on a stereo playback system.
Michael Williams (“Stereo Zoom” concept) approaches mic angle and spacing based on a desired SRA, he defines the SRA as:
“That sector of the sound field in front of the microphone system which will produce a virtual sound image between the loudspeakers.”
The SRA of a recording could be considered analogous to the framing of a photograph: the angle between the extreme left and right of the stereophonic image is considered to be the SRA. Sound sources that are at or beyond this angle will be reproduced at the extreme left or right speaker.
Michael Williams’ paper demonstrated that there are a number of combinations of angle and spacing between microphones for a given SRA, depending on the pickup pattern of the mic’s being used.
Points to Note:
- If the SRA is wider than the ensemble width then the reproduced stereo sound will be narrow
- If the SRA is narrower than the ensemble width then any source outside of the SRA will be compressed at the loudspeakers.
XY SRA
195˚
Blumlein SRA
76˚
Mid-Side SRA
195˚ ( same as XY, but variable in post)
ORTF SRA
96˚
NOS SRA
81˚
AB SRA
180˚ (for 50cm spacing, LESS WHEN WIDER)
Inverse Square Law
The rule that explains why sound gets softer as it gets further away from the sound source. As the wave expands in 3D from a source, the energy from the original sound expands over a larger and larger area, thus getting quieter.
At 1 meter from the sound source, the blue square contains a certain amount of energy. At 2 meters from the sound source, that same amount of energy must now be spread over the larger green square.
For a microphone placed at the blue square, the distance from the source to the rear os the microphone is 1.0085m. For example, if the source has an output of 1 watt, the difference in pressure between the front and rear os the microphone at the blue square, is 0.048 Pa. Calculating the same difference at 2 meters (green square), we get 0.012 Pa. So, as the microphone moves closer to the source, this inverse square component becomes larger. The inverse square component is equal for every frequency.
If we plot the inverse square component and the gradient component when the microphone is far away from the sound source then the pressure difference from the gradient component is larger than the pressure difference from the inverse square component - This is with an ‘undamped diaphragm’. Once we damp the diaphragm, the graph will tilt.
As the microphone moves close to the sound source, the inverse square component keeps rising on the graph.
Depth of a Pressure Capsule - Does it make a difference?
No.
The depth of a pressure capsule makes no difference to its response
Depth of a Pressure Gradient Capsule - Does it make a difference?
Yes.
The depth determines the frequency of the first (and subsequent) notches in the native frequency response.
For a pressure gradient capsule, a deeper capsule will have a more pronounced proximity effect, noticeable at a further distance.
Width of Capsules
The width of both pressure and pressure gradient capsules will affect the high frequency response.
As the diameter of the capsule approaches one half wavelength of the sound impinging sideways on it, there will be both high and low pressure points on the diaphragm simultaneously, so the deflection of the diaphragm is less, with nulls at some frequencies.
Thus, a larger diaphragm will exhibit polar pattern irregularities (lobing) at a lower frequency than a small diaphragm capsule
- A larger diaphragm will have a better signal to noice ratio because it will collect more sound pressure.
- However, a larger capsule will also collect more noise from the random air molecule movement - sound waves are correlated so double the diaphragm area will add 6dB output, but noise is decorrelated so it adds only 3dB of noise.
- So doubling the diaphragm area provides a 3dB signal-to-noise improvement
HF Lobing of the frequency response
Lobing: Colouration of high frequencies (HF first but can go into lows occasionally) and more directional the pattern becomes
“Smaller diameter capsules are more reliable”
Diameter of pressure gradient:
HF lobing of the HF response This is the SAME
HF Lobing of the polar response:
By making the capsule diameter smaller - the better
Advantages/Disadvantages of the Binaural mic technique
+ Excellent stereo reproduction over headphones
+ All advantages of omni mics
- Poor stereo reproduction over speakers
Binaural SRA
360˚
Advantages/Disadvantages of the Jecklin Disk
2 mics like an ORTF setup, with a head-sized baffle between them.
It sacrifices some stereo imaging on speakers to improve the stereo image over headphones
+ Gives a reasonable stereo image over headphones and speakers, although less than ideal for both
- All of the problems of directional mics: Bass roll off, proximity effect, uneven HF response
- Front of the soundfield is quite off-axis to the mics, causing colouration
Stereo presentation is less than ideal for speakers and headphones
SRA is similar to ORTF
