Microphone Models
Definition: A microphone is a transducer that converts acoustical sound pressure into an electrical signal.
Common in studio use either ribbon, moving coil or condenser transducer.
Ribbon Microphones
The first ribbon microphone was RCA 44A introduced in 1932
Construction:
Very simple, a ribbon or strip of very thin foil – usually aluminium – suspended within the field of a magnet.
The foil is corrugated to give lateral stiffness and longitudinal flexibility.
When the ribbon vibrates within the magnetic field, a current is generated across the length of the ribbon. This current is very small and all ribbon microphones require a step-up transformer to yield an output that is useful and compatible with the typical gain ranges available on most microphone preamps.
A simple ribbon microphone with no surrounding acoustic baffling or treatment is a pure pressure gradient transducer, meaning the ribbon moves and therefore generates its signal output in response to the instantaneous pressure difference between the front and rear sides of the ribbon. The nature of this response gives rise to the figure – 8 or bi – directional pickup pattern of ribbon microphones, as sound that arrives from the side direction and presenting ‘edge on’ to the ribbon will not cause a pressure difference between the front and rear sides of the ribbon. Thus, the ribbon doesn’t move and there’s no signal output. Ribbon microphones are capable of very deep nulls in their side – on response, making them useful for not capturing a particular sound source in a recording situation.
Ribbon microphones typically have a frequency response that tapers off at high frequencies, their definition and accuracy at middle frequencies sets them apart.
This is primarily why they’ve been ‘rediscovered’ in recent years. Much of this is to do with the simplicity of its design; the ribbon is freely exposed to the air with no baffling or acoustic treatment – this yields a pure and simple transducer.
- fragile; ribbon thickness ranges from 0.6 to 2 microns
- breath of singer can break it
- ribbon replacing is not trivial, factory returning recommended
- tuning of the ribbon; is lightly tensioned and thus tuned to a very low frequency
- step – up transformer contributes significantly to the quality of the mic
- the interaction of the complete ribbon mic and the input impedance of the
- mic – preamp to which it is connected can change the tonal characteristics of the microphone significantly, more so than any other type of microphone
- in response manufacturers integrate amplifiers, either transistor or tube to enable the best and most consistent frequency response
Moving coil or ‘dynamic’ microphones
- A moving coil produces a current flow when a coil of wire moves in a magnetic field.
- In simple terms the same construction as a loudspeaker
- Dynamic microphones are essentially pressure transducers where the diaphragm moves and generates a signal in response to sound wave pressure
- This makes a dynamic microphone omnidirectional in its pick-up pattern
Most microphones have acoustic cavities and porting from the rear of the capsule to modify rhe pickup response to achieve a cardioid (directional) pattern.
Thus a greater variation in frequency response and off-axis performance is yield.
A microphone with directional pattern will exhibit the ‘proximity’ effect whereby the bass response increases as the sound source moves closer to the microphone.
- moving coil microphones create a larger signal than ribbon microphones because of the large number of turns and the narrow gap between this coil and the magnet, thus no step-up transformer required
- some dynamic microphones have a second coil wired in a ‘humbucking’ way to eliminate hum and buzz
- dynamic microphones are very robust
Condenser Microphones
- The capsule of a condenser microphone consists of a thin membrane, usually in rhe order of three- to six-microns stretched and fixed closely to a solid metal ‘back-plate’.
- The diaphragm is coated with a metallic layer, often gold, creating in combination with the back-plate, a small capacitor
- the gap between the diaphragm and the capacitor needs to be very small in order to give the usable capacitance, it’s typically 15 to 30 microns
- The diaphragm is polarised with a high voltage typically ranging from 30 to 80 volts, creating a fixed range on the capacitor
- When exposed to sound pressure the diaphragm moves towards and away from the back-plate, causing a change in its capacitance and thus producing a voltage output
Overheads: X/Y vs. A/B
Both techniques have advantages and disadvantages. A/B exhibits a wide stereo image over the drum kit and X/Y is very phase accurate. On the other hand, A/B introduces phasing issues due to the time arrival difference and you can’t apply the 3:1 rule over the entire drum kit.
What can you do to get the best out of both worlds? I assume you are familiar with the 3:1 rule. The problem zone on a drum kit is the centre signal, mainly the snare. The trick is to keep the snare in phase by making use of the time arrival difference. Measure the distance for the snare skin to each mic. You could even use a microphone lead for that. And don’t worry if it looks a bit funny to you. Most likely the overhead mic above the ride cymbal is much lower positioned than the one above the hi hat. This will keep your snare phase accurate and for the outer parts of the kit the 3:1 rule kicks in. What you achieve from this is a wide stereo field due to the A/B setup and minimized phasing issues where it matters, at the centre of the drum kit.
And the X/Y technique? People often complain about the lack in stereo width. But X/Y doesn’t just work with a 90 degree setup. Try widening the angle to get more side signal and lower the mic position a bit. This will give you a much wider stereo image with no phasing issues on the overheads whatsoever. The X/Y technique works well for angles from 60-135 degrees.
At the end of the day you must use what works best for what you want to achieve. Have a bit of a think before you start recording and know what sort of sound you are going for.
