MUSC1010 – WEEK 7

Microphones

Shure 565 SD Rode K2Hand held/stand mounted Stand mounted

Dynamic Condenser

Small diaphragm Large diaphragm

Robust fragile

Solid state circuitry Tube circuitry

Fixed polar pattern Variable polar pattern

Tone? Tone?

Transient response? Transient response?

Recording with different microphones can have a significant effect on the sound

SOUND WAVES ELECTRICAL SIGNAL

How is vibration stimulated in the diaphragm?For a diaphragm to move there must be a pressure difference between its sides. There are two common ways of creating this pressure difference, resulting in two typical different styles of transducer;

•Pressure Operated Microphones•Pressure Gradient Microphones

PRESSURE (operated) mics

One side of the diaphragm is exposed to the acoustic environment. The other side forms part of a sealed chamber. The diaphragm moves as a result of the difference in pressure between the sound wave and the air pressure in the sealed chamber.Pressure microphones are analogous to a barometer.

PRESSURE GRADIENT MICS

Both sides of the diaphragm are exposed to the acoustic environment. The diaphragm will only move if there is a different pressure at either side of the diaphragm. Microphones create the necessary “pressure gradient” by delaying the sound that hits the back of the diaphragm. Notice that the pressure difference is greater for higher frequencies. Microphone that use this principle must compensate for this bias in the spectral response.

Pressure Operated Microphones

Pressure operated microphones operate much like a barometer. One side of the diaphragm is sealed while the other is exposed to pressure fluctuations in the atmosphere. The difference in pressure causes the diaphragm to move. A pressure operated microphone recognises fluctuations in air pressure regardless of the direction of the sound. As such they are said to have an omnidirectional polar response pattern. Sometimes they are just called an “omni”.

Diagram from Mic Techniques for Live Sound Reinforcement by Waller, Vear and Boudreau. Niles, Il: Shure Inc., 2005.

The phase difference between A and B is created by delaying the incident sound wave to the rear of the diaphragm. The resulting difference in pressure (∆p) results in movement of the diaphragm. Higher frequencies give a greater value for ∆p, up until the point where phi is greater than 180º.

Pressure Gradient Microphones

• When the wavelength is very short (the frequency is very high), it approaches the distance between A and B. If the distance between A and B equals half the wavelength then the maximum pressure difference is reached and the diaphragm will not move with greatest force for a given amplitude of wave. The pressure gradient principle is therefore not effective for frequencies above ft

• The force exerted on the diaphragm increases with frequency up to ft

• For most microphones ft is no more than 12000Hz above which other principles such as interference come into effect.

The pressure gradient characteristic is achieved via acoustic delay to the back of the diaphragm. The time delay depends on the direction of the incident sound wave. Waves that approach the front of the diaphragm result in the most significant delay.

This construction results in a cardiod (heart-shaped) polar response

Microphone Polar Patterns

omnidirectional figure of eight cardioid Hyper-cardioid

Transducer concepts

1. Dynamic Microphones

• the diaphragm is attached to a coil of electrical wire.• movement of coil within a magnetic field produces electrical current (law of induction).• diaphragm needs to be relatively thick/strong enough to support and move the coil. This means that

dynamic mics don’t have the extent of frequency response or “transient pick-up” that other mic types can have.

• Diaphragm can be pressure operated or use the pressure gradient

Ribbon Microphones

• Ribbon mic is a type of dynamic microphone in that there is a conductive material (often an extremely thin strip of aluminium) moving within a magnetic field.

• An important difference is that a ribbon (often an extremely thin strip of aluminium) is much freer to move than the relatively heavy diaphragm of other dynamic microphones.

• Blowing into a ribbon mic can damage the ribbon

2. Condenser Microphones

• A difference in charge is created between the diaphragm and the backplate. The charge is provided by a battery or by “phantom power”. Phantom power comes from the mixing desk.

• the diaphragm vibrates while the position of the back plate is fixed. The output voltage varies as a function of the varying distance between the two surfaces.

• The diaphragm of a condenser microphone can be very thin as it does not need to support the weight of a coil. The lower weight results in increased response to fast transients.

• condenser microphones can be pressure operated or use the pressure gradient principle.• you can have single and dual-diaphragm condenser mics

The proximity effect is not evident in omnidirectional mics

The Proximity Effect

Microphones that have directional characteristics, i.e. those other than omnidirectional microphones tend to exhibit the proximity effect. This is an exaggeration of the bass frequencies as the microphone approaches the sound source. Microphones that show the proximity effect rely on delaying air pressure fluctuations to the back of the diaphragm. It is the difference in air pressure at the front and rear of the diaphragm that causes it to vibrate.

XLR balanced leads

XLR leads are usually balanced. Unless cables are balanced they are prone to picking up interference such as radio, mobile phones and electrical noise from nearby devices. Balanced leads carry a duplicate signal from the microphone that is inverted. A difference amplifier at the other end effectively cancels out any extraneous signal that is picked up along the way.