An introduction to measurement microphones
David Mathew of Audio Precision provides an overview of these convenient tools for accurate acoustic test and analysis.
Like all technological devices, loudspeakers, headphones, ear buds, MEMS speakers, sonar emitters and even police sirens need to be measured and tested, both in design and in production. Further, products that make any noise at all (motors, aeroplanes, wind turbines, coffee makers) are often measured for safety or environmental impact, or are continuously monitored, listening for signature acoustic signals that indicate correct performance, or failure.
The sensors required to accurately acquire the acoustic signals for test and evaluation are not the rock ’n’ roll mics a drummer has arrayed around his kit. A wide range of measurement microphones from a number of manufacturers have existed for many years, with a special solution for every acoustic test need.
A measurement microphone is like an ordinary microphone in terms of its superficial features: it is typically tubular, with a sensor at one end and a connector at the other, and the sensor itself is a lightweight diaphragm that is excited by changes in air pressure, responding in a way that can produce an electrical signal. But at this point the two microphone types diverge – you won’t see a singer’s wireless mic measuring loudspeaker drivers in an anechoic chamber, and you won’t see a comedian using a measurement microphone for the mic drop at the end of his routine.
Characteristics
Measurement microphones are optimised for superior performance in one or more of these characteristics: frequency response, frequency range, self-noise, maximum level and distortion. Further, some are designed to be robust in harsh environments, or to have characteristics that closely match in an array application. Sensitivity and frequency response are very stable over time. A measurement microphone is typically shipped with a calibration table or chart documenting its performance.
Frequency response – Typical measurement microphones are specified as ± 2 dB from 5 Hz to 20 kHz, but some models have useable response as low as 0.07 Hz, or as high as 140 kHz.
Low noise – Most measurement microphones have a noise floor of about 20–40 dBA, but specialised 1in models can spec a noise floor as low as –2.5 dBA.
Maximum level – For measurement microphones, 3% THD is considered overload. Typical measurement microphones might overload at 160 dB; specialised models will not overload until 184 dB or more.
Engineers with some experience in sound amplification or recording might be familiar with microphone directional patterns such as cardioid, figure of eight, shotgun and so on. These characteristics are accomplished by modifications to the basic diaphragm element, such as acoustic ports, additional diaphragms, or interference tubes.
Measurement microphones, on the other hand, are omnidirectional, without modifications for directionality. Measurement microphones are optimised for one of three acoustic applications: measuring sound pressure, measuring incident sound from one direction in a free-field (anechoic) acoustic space, and measuring sound that may arrive from any direction (random incidence) in a diffuse-field acoustic space.
Measurement microphones are offered in four nominal diaphragm sizes: 1”, ½”, ¼” and 1/8”. Generally speaking, the smaller the diaphragm, the greater the self-noise, the higher the frequency response and the higher the maximum level. Most general applications are satisfied with ½” measurement mics.
Sensor design
A number of methods have historically been used to convert sound pressure to an electrical signal: piezoelectric, using a crystal attached to a diaphragm; variable resistance, using packed carbon granules in a small container, attached to a diaphragm; dynamic, using a magnet and a coil to convert diaphragm movement to a current; and variable capacitance, where the diaphragm itself is one side of a capacitor, converting the movement of the diaphragm into a voltage.
As it turns out, the capacitive method will, in most applications, provide the most sensitive microphones, largely due to the low diaphragm mass that this method makes possible. A survey of measurement mics over the past 50 years reveals wide use of capacitive microphones. In microphone circles, capacitive microphones are often called condenser microphones, and that is the term we will continue with.
The one exception is an application where the sound levels are very high, such as near a blast or explosion. In this case, a piezoelectric measurement microphone is the correct choice.
Powering condenser microphones
A dynamic microphone can simply be connected via a shielded cable to an appropriate downstream amplifier and put to work. Condenser microphones, however, require more support:
• The capacitive sensor element requires a polarising voltage.
• The impedance of the sensor element is very high. Consequently, the signal current is so small that it must be amplified at the source before it is swamped by noise. Condenser microphones always have a preamplifier either built into the microphone body or connected directly to the microphone sensor capsule.
Prior to the introduction of solid-state amplifiers, the preamplifier in a condenser microphone was of a vacuum tube (valve) design. These microphones required custom power supplies and multi-conductor cables that provided the capacitor polarising voltage and also plate voltage and filament current for the tube.
Today, measurement microphone preamplifiers are solid-state and have modest power requirements. Depending upon applications, some microphones are externally polarised and require a 200V polarising voltage; many other designs are pre-polarised, with an electret capacitor as the sensor element, and require only preamp power. Early electrets were not suitable for high-performance applications, but modern electret microphones offer excellent specifications and long-term stability.
Effect on incident sound waves
The mere presence of a microphone in an acoustic space disrupts the sound pressure wave as it encounters the microphone. The wave reflects from and diffracts around the sensor element to varying degrees, depending on the dimensions of the microphone and the frequency and angle of incidence of the sound wave. This effect is avoided in the first case below.
Pressure mic – A microphone’s pressure response is flat when its presence does not disrupt the pressure wave. This occurs when the microphone is not in the sound field, but is a component of the barrier containing the sound field. Applications include flush mounting within an acoustic coupler, or flush mounting on a wall or barrier.
Free-field mic – A free-field microphone is compensated to produce a flat response when used in an anechoic space where the sound waves are arriving from one direction. Applications include loudspeaker testing, microphone testing, evaluations and monitoring of sound-emitting equipment, and sound-level meters. The sound field must be free of reflections, such as an anechoic chamber or use out-of-doors.
Diffuse-field mic – Compensated to produce a flat response in a reverberant space such as a church, a concert hall, or an aircraft or automobile cabin. Applications include room tuning, impulse-response testing and ambient industrial or environmental noise evaluation.
Microphone arrays
Some applications require a geometric array of two or more matched mics to capture temporal, directional and phase information for mathematical analysis. Array microphones are typically of the free-field type, with careful attention paid to close phase-matching among the microphones. Because a large number of microphones may be required, array microphones are usually of a general-purpose design.
David Mathew is technical publications manager and a senior technical writer at Audio Precision. He has worked as both a mixing engineer and as a technical engineer in the recording and filmmaking industries, and was awarded an Emmy in 1988.
