Shure's Microphone Techniques for Studio Recording Glossary and Appendix
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About Shure's Microphone Techniques for Studio Recording
This one of Shure's Educational Publications. This article is reprinted with the permission of Shure Incorporated and is not under the GNU Free Documentation License 1.2. It is protected from alteration and is provided as is.
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Glossary
3-to-1 Rule
When using multiple microphones, the distance between microphones should be at least 3 times the distance from each microphone to its intended sound source.
Absorption
The dissipation of sound energy by losses due to sound absorbent materials.
Active Circuitry
Electrical circuitry which requires power to operate, such as transistors and vacuum tubes.
Ambience
Room acoustics or natural reverberation.
Amplitude
The strength or level of sound pressure or voltage.
Audio Chain
The series of interconnected audio equipment used for recording or PA.
Backplate
The solid conductive disk that forms the fixed half of a condenser element.
Balanced
A circuit that carries information by means of two equal but opposite polarity signals, on two conductors.
Bidirectional Microphone
A microphone that picks up equally from two opposite directions. The angle of best rejection is 90 degrees from the front (or rear) of the microphone, that is, directly at the sides.
Boundary/Surface Microphone
A microphone designed to be mounted on an acoustically reflective surface.
Cardioid Microphone
A unidirectional microphone with moderately wide front pickup (131 degrees). Angle of best rejection is 180 degrees from the front of the microphone, that is, directly at the rear.
Cartridge (Transducer)
The element in a microphone that converts acoustical energy (sound) into electrical energy (the signal).
Clipping Level
The maximum electrical output signal level (dBV or dBu) that the microphone can produce before the output becomes distorted.
Close Pickup
Microphone placement within 2 feet of a sound source.
Comb Filtering
An interference effect in which the frequency response exhibits regular deep notches.
Condenser Microphone
A microphone that generates an electrical signal when sound waves vary the spacing between two charged surfaces: the diaphragm and the backplate.
Critical Distance
In acoustics, the distance from a sound source in a room at which the direct sound level is equal to the reverberant sound level.
Current
Charge flowing in an electrical circuit. Analogous to the amount of a fluid flowing in a pipe.
Decibel (dB)
A number used to express relative output sensitivity. It is a logarithmic ratio.
Diaphragm
The thin membrane in a micro- phone which moves in response to sound waves. Diffraction - The bending of sound waves around an object which is physically smaller than the wavelength of the sound.
Direct Sound
Sound which travels by a straight path from a sound source to a microphone or listener.
Distance Factor
The equivalent operating distance of a directional microphone compared to an omnidirectional microphone to achieve the same ratio of direct to reverberant sound.
Distant Pickup
Microphone placement farther than 2 feet from the sound source.
Dynamic Microphone
A microphone that generates an electrical signal when sound waves cause a conductor to vibrate in a magnetic field. In a moving-coil microphone, the conductor is a coil of wire attached to the diaphragm.
Dynamic Range
The range of amplitude of a sound source. Also, the range of sound level that a microphone can successfully pick up.
Echo
Reflection of sound that is delayed long enough (more than about 50 msec.) to be heard as a distinct repetition of the original sound.
Electret
A material (such as Teflon) that can retain a permanent electric charge.
EQ
Equalization or tone control to shape frequency response in some desired way.
Feedback
In a PA system consisting of a microphone, amplifier, and loudspeaker, feedback is the ringing or howling sound caused by amplified sound from the loudspeaker entering the microphone and being re-amplified.
Flat Response
A frequency response that is uniform and equal at all frequencies.
Frequency
The rate of repetition of a cyclic phenomenon such as a sound wave.
Frequency Response Tailoring Switch
A switch on a microphone that affects the tone quality reproduced by the microphone by means of an equalization circuit. (Similar to a bass or treble control on a hi-fi receiver.)
Frequency Response
A graph showing how a microphone responds to various sound frequencies. It is a plot of electrical output (in decibels) vs. frequency (in Hertz).
Fundamental
The lowest frequency component of a complex waveform such as musical note. It establishes the basic pitch of the note.
Gain
Amplification of sound level or voltage.
Gain-Before-Feedback
The amount of gain that can be achieved in a sound system before feedback or ringing occurs.
Gobos
Movable panels used to reduce reflected sound in the recording environment.
Harmonic
Frequency components above the fundamental of a complex waveform. They are generally multiples of the fundamental which establish the timbre or tone of the note.
Hypercardioid
A unidirectional microphone with tighter front pickup (105 degrees) than a supercardioid, but with more rear pickup. Angle of best rejection is about 110 degrees from the front of the microphone.
Impedance
In an electrical circuit, opposition to the flow of alternating current, measured in ohms. A high-impedance microphone has an impedance of 10, 000 ohms or more. A low impedance microphone has an impedance of 50 to 600 ohms.
Interference
Destructive combining of sound waves or electrical signals due to phase differences. Inverse Square Law - States that direct sound levels increase (or decrease) by an amount proportional to the square of the change in distance.
Isolation
Freedom from leakage; the ability to reject unwanted sounds.
Leakage
Pickup of an instrument by a micro- phone intended to pick up another instrument. Creative leakage is artistically favorable leakage that adds a “loose”or “live”feel to a recording.
Maximum Sound Pressure Level
The maximum acoustic input signal level (dB SPL) that the microphone can accept before clipping occurs.
Microphone Sensitivity
A rating given in dBV to express how “hot”the microphone is by exposing the microphone to a specified sound field level (typically either 94 dB SPL or 74 dB SPL). This specification can be confusing because manufacturers designate the sound level different ways. Here is an easy reference guide:
94 dB SPL = 1 Pascal = 10 microbars.
To compare a microphone that has been measured at 74 dB SPL with one that has been measured at 94 dB SPL, simply add 20 to the dBV rating.
NAG
Needed Acoustic Gain is the amount of gain that a sound system must provide for a distant listener to hear as if he or she was close to the unamplified sound source.
Noise
Unwanted electrical or acoustic interference.
Noise Canceling
A microphone that rejects ambient or distant sound.
NOM
Number of open microphones in a sound system. Decreases gain-before-feedback by 3dB everytime NOM doubles.
Omnidirectional Microphone
A microphone that picks up sound equally well from all directions.
Output Noise (Self-Noise)
The amount of residual noise (dB SPL) generated by the electronics of a condenser microphone.
Overload
Exceeding the signal level capability of a microphone or electrical circuit. PAG - Potential Acoustic Gain is the calculated gain that a sound system can achieve at or just below the point of feedback.
Phantom Power
A method of providing power to the electronics of a condenser microphone through the microphone cable.
Phase
The “time”relationship between cycles of different waves. Pickup Angle/Coverage Angle - The effective arc of coverage of a microphone, usually taken to be within the 3dB down points in its directional response.
Pitch
The fundamental or basic frequency of a musical note.
Polar Pattern (Directional Pattern, Polar Response)
A graph showing how the sensitivity of a microphone varies with the angle of the sound source, at a particular frequency. Examples of polar patterns are unidirectional and omnidirectional.
Polarization
The charge or voltage on a condenser microphone element.
Pop Filter
An acoustically transparent shield around a microphone cartridge that reduces popping sounds. Often a ball-shaped grille, foam cover or fabric barrier.
Pop
A thump of explosive breath sound produced when a puff of air from the mouth strikes the microphone diaphragm. Occurs most often with “p”, “t”, and “b”sounds.
Presence Peak
An increase in microphone out- put in the “presence”frequency range of 2, 000 Hz to 10, 000 Hz. A presence peak increases clarity, articulation, apparent closeness, and “punch.”
Proximity Effect
The increase in bass occurring with most unidirectional microphones when they are placed close to an instrument or vocalist (within 1 foot). Does not occur with omnidirectional microphones.
Rear Lobe
A region of pickup at the rear of a supercardioid or hypercardioid microphone polar pattern. A bidirectional microphone has a rear lobe equal to its front pickup.
Reflection
The bouncing of sound waves back from an object or surface which is physically larger than the wavelength of the sound.
Refraction
The bending of sound waves by a change in the density of the transmission medium, such as temperature gradients in air due to wind.
Resistance
The opposition to the flow of current in an electrical circuit. It is analogous to the friction of fluid flowing in a pipe.
Reverberation
The reflection of a sound a sufficient number of times that it becomes non-directional and persists for some time after the source has stopped. The amount of reverberation depends on the relative amount of sound reflection and absorption in the room.
Rolloff
A gradual decrease in response below or above some specified frequency.
Sensitivity
The electrical output that a micro- phone produces for a given sound pressure level.
Shaped Response
A frequency response that exhibits significant variation from flat within its range. It is usually designed to enhance the sound for a particular application.
Signal to Noise Ratio
The amount of signal (dBV) above the noise floor when a specified sound pressure level is applied to the microphone (usually 94 dB SPL).
Sound Chain
The series of interconnected audio equipment used for recording or PA.
Sound Reinforcement
Amplification of live sound sources.
Speed of Sound
The speed of sound waves, about 1130 feet per second in air.
SPL
Sound Pressure Level is the loudness of sound relative to a reference level of 0.0002 microbars.
Standing Wave
A stationary sound wave that is reinforced by reflection between two parallel surfaces that are spaced a wavelength apart.
Supercardioid Microphone
A unidirectional microphone with tighter front pickup angle (115 degrees) than a cardioid, but with some rear pickup. Angle of best rejection is 126 degrees from the front of the microphone, that is, 54 degrees from the rear.
Timbre
The characteristic tone of a voice or instrument; a function of harmonics.
Transducer
A device that converts one form of energy to another. A microphone transducer (cartridge) converts acoustical energy (sound) into electrical energy (the audio signal).
Transient Response
The ability of a device to respond to a rapidly changing input.
Unbalanced
A circuit that carries information by means of one signal on a single conductor.
Unidirectional Microphone
A microphone that is most sensitive to sound coming from a single direction-in front of the microphone. Cardioid, supercardioid, and hypercardioid microphones are examples of unidirectional microphones.
Voice Coil
Small coil of wire attached to the diaphragm of a dynamic microphone.
Voltage
The potential difference in an electric circuit. Analogous to the pressure on fluid flowing in a pipe.
Wavelength
The physical distance between the start and end of one cycle of a soundwave.
Appendix A: The Decibel
The decibel (dB) is an expression often used in electrical and acoustic measurements. The decibel is a number that represents a ratio of two values of a quantity such as voltage. It is actually a logarithmic ratio whose main purpose is to scale a large measurement range down to a much smaller and more useable range. The form of the decibel relationship for voltage is: dB = 20 x log(V1/V2) where 20 is a constant, V1 is one voltage, V2 is a reference voltage, and log is logarithm base 10.
Examples:
What is the relationship in decibels between 100 volts and 1 volt? (dbV) dB = 20 x log(100/1) dB = 20 x log(100) dB = 20 x 2 (the log of 100 is 2) dB = 40 That is, 100 volts is 40dB greater than 1 volt.
What is the relationship in decibels between .0001 volt and 1 volt? (dbV) dB = 20 x log(.001/1) dB = 20 x log(.001) dB = 20 x (-3) (the log of .001 is -3) dB = -60 That is, .001 volt is 60dB less than 1 volt.
Similarly:
If one voltage is equal to the other, they are 0dB different.
If one voltage is twice the other, they are 6dB different.
If one voltage is ten times the other, they are 20dB different.
Since the decibel is a ratio of two values, there must be an explicit or implicit reference value for any measurement given in dB. This is usually indicated by a suffix on the dB. Some devices are measured in dBV (reference to 1 Volt = 0 dBV), while others may be specified in dBu or dBm (reference to .775V = 0dBu/dBm). Here is a chart that makes conversion for comparison easy:
Audio equipment signal levels are generally broken into 3 main categories:Mic, Line, or Speaker Level. Aux level resides within the lower half of line level. The chart also shows at what voltages these categories exist. One reason that the decibel is so useful in certain audio measurements is that this scaling function closely approximates the behavior of human hearing sensitivity. For example, a change of 1dB SPL is about the smallest difference in loudness that can be perceived while a 3dB SPL change is generally noticeable. A 6dB SPL change is quite noticeable and finally, a 10dB SPL change is perceived as “twice as loud.”
Appendix B: Transient Response
The ability of a microphone to respond to a rapidly changing sound wave. A good way to understand why dynamic and condenser mics sound different is to understand the differences in their transient response. In order for a microphone to convert sound energy into electrical energy, the sound wave must physically move the diaphragm of the microphone. The speed of this movement depends on the weight or mass of the diaphragm. For instance, the diaphragm and voice coil assembly of a dynamic microphone may have up to 1000 times the mass of the diaphragm of a condenser microphone. The lightweight condenser diaphragm starts moving much more quickly than the dynamic’s diaphragm. It also takes longer for the dynamic’s diaphragm to stop moving in comparison to the condenser’s diaphragm. Thus, the dynamic’s transient response is not as good as the condenser’s transient response. This is similar to two vehicles in traffic:a truck and a sports car. They may have engines of equal power, but the truck weighs much more than the car. As traffic flow changes, the sports car can accelerate and brake very quickly, while the semi accelerates and brakes very slowly due to its greater weight. Both vehicles follow the overall traffic flow but the sports car responds better to sudden changes. The picture below is of two studio microphones responding to the sound impulse produced by an electric spark: condenser mic on top, dynamic mic on bottom. It is evident that it takes almost twice as long for the dynamic microphone to respond to the sound. It also takes longer for the dynamic to stop moving after the impulse has passed (notice the ripple on the second half of the graph). Since condenser microphones generally have better transient response then dynamics, they are better suited for instruments that have very sharp attacks or extended high frequency output such as cymbals. It is this transient response difference that causes condenser mics to have a more crisp, detailed sound and dynamic mics to have a more mellow, rounded sound.
