More Microphone Design/Application

1. More Microphone Design/Application Transient Response to Phantom Power

2. 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.

3. Microphone Output Characteristics • Sensitivity Rating • Equivalent Noise Rating • Overload characteristics • Microphone Impedance

4. 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.

5. 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.

6. Overload Characteristics • Upper limit of SPL that won’t distort. • Dynamic: typically around 140 dB • Condenser: 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.

7. 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)

8. 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.

9. 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.

10. 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.

11. 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.