More Microphone Design/Application

Transient Response to

Phantom Power

Transient Response

Measure of how quickly a microphone’s diaphragm will react to acoustic waveform.

No accepted measure. Dynamic mic’s usually have larger diaphragm, coil,

and core. All add up to slower transient response. Ribbon mic’s have thinner, lighter diaphragms.

Better transient response. Condenser mic’s have extremely lightweight

diaphragms, offering little mechanical resistance. Generally have most accurate transient response.

Microphone Output Characteristics

Sensitivity RatingEquivalent Noise RatingOverload characteristicsMicrophone Impedance

Sensitivity Rating

The output level (in volts) produced by a given (standardized) acoustic input signal (in dB SPL). Also specifies the amount of amplification

required to bring signal up line level (-10 dBV in consumer systems; +4 dBm in pro systems)

Allows comparison of mic’s. Sensitivity proportional to output-signal voltage.

Equivalent Noise Rating

Self-noise of device, measured in dB SPL or dBA (weighted).

Generally not as pronounced as self-noise of other stages in recording chain, but more noticeable with technological advancements.

Dynamic and Ribbon mic’s: s-n from movement of electrons in ribbon or coil.

Condenser: s-n from built-in preamp.

Overload Characteristics

Upper limit of SPL that won’t distort.Dynamic: typically around 140 dBCondenser: typically doesn’t distort except

at very high levels. However, mic preamp can be overloaded. Pad or attenuation switch. Pad reduces overall signal-to-noise ratio by

amount of pad.

Microphone Impedance

Output impedance (sometimes called “Z”) Impedance measures resistance to signal flow. Important to match output (signal providing) capability to

input (signal receiving) capability (flow). Typical low: 50 Ω, 150 to 250 Ω Typical high: 20 to 50 kΩ (high)

High impedance likely to pick up noise through cable. Low impedance can have long cable runs, but can pick

up electromagnetic noise. (shielded, twisted pair, balanced line, 200 Ω used to prevent this)

Balanced Lines

Balanced line: 3 wires to carry audio signal. Audio signal carried on two pins (wires), with different

polarities Third pin is a ground. Transformer/summing amp eliminates added noise

(equal polarity on both pins), responds to difference in voltage between two pins.

XLR: 2 (+), 3 (-), 1 (ground) TRS: tip-ring-sleeve 1/4”, ring (+), tip (-), sleeve

(ground)

Less susceptible to external noise.

Unbalanced Lines

Two wires to carry audio signal. XLR: 2 is still +, but (-) pin connected to ground

(along with 1). TS: tip-sleeve, tip (+), sleeve (ground)

More susceptible to external noise.

Microphone Preamps

Mic output levels too low to drive line level input of recording systems.

Boost needed (30 - 60 dB) Most mixing consoles have “stock” pre’s. Recording professionals usually use “outboard”

(separate from mixer) preamps. Higher quality (lower noise/distortion) Special sound characteristics Special circuitry, or tube designs More control over input gain, filtering, perhaps compression. With the general demise of large, analog consoles, “classic”

pre’s from these boards are being marketed separately.

Phantom Power

Power needed for condenser (and other “active”) microphones.

48V, DC, sent to mic condenser and impedance preamp over pins 2 and 3. Equally distributed through the use of identical value

resistors (often hand-chosen)

Input preamp transformer looks for voltage differences. Matched 48V over both leads is eliminated at preamp

transformer. Doesn’t reach preamp gain stage.