Introduction to measurement microphones
31 August 2017
Like all technological devices, loudspeakers, headphones, MEMS microphones, sonar emitters and police sirens all need to be measured and tested, both during design and production. Furthermore, products that generate any noise at all (such as motors, airplanes, wind turbines, coffee makers or HVAC) are often measured for safety or environmental impact, or are continuously monitored, listening for signature acoustic signals that indicate correct performance – or failure.
This article originally appeared in the July 2017 issue of Electronic Product Design & Test; to view the digital edition, click here – and to register to receive your own printed copy, click here.
This tutorial from Audio Precision provides a guide on the basics of measurement microphones.
The sensors required to accurately acquire acoustic signals for test and evaluation are not the same as the rock ’n’ roll mics a drummer might have arrayed around their kit. A wide range of measurement microphones from a variety of manufacturers exist, with a specialised solution available for every acoustic test need.
A measurement microphone resembles an ordinary microphone in 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 stand-up comedian performing the mic drop at the end of their routine with a measurement microphone!
Measurement microphones are usually optimised for superior performance in one or more of these characteristics: frequency response, frequency range, self-noise, maximum level, and distortion. Some are designed to be robust in harsh environments, or to have characteristics that closely match in an array application. Microphone sensitivity and frequency response are designed to be very stable over time and they are typically shipped with a calibration table or chart that document their performance (see example below).
If you’re accustomed to using sensors that are TEDS-enabled, you’ll be pleased to know that most modern measurement microphones also carry identification and calibration information as TEDS data. TEDS (transducer electronic data sheets) data is functionality integrated into sensors to store identification, calibration, correction data and manufacturer-related information. It’s a key element of IEEE 1451, a set of smart transducer interface standards outlining open, common, network-independent communication interfaces for connecting transducers to microprocessors, instrumentation systems and control/field networks.
Typical measurement microphones are specified as ± 2 dB from 5 Hz to 20 kHz, but some models have usable response as low as 0.07 Hz, or as high as 140 kHz.
Most measurement microphones have a noise floor of about 20 to 40 dBA, but specialised 1” models can spec a noise floor as low as -2.5 dBA.
For measurement microphones, 3% THD is considered overload. Typical measurement microphones may overload at 160 dB; specialised models will not overload unless 184 dB or more is reached.
A number of methods are 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
- variable capacitance, where the diaphragm itself is one side of a capacitor, converting
- the movement of the diaphragm into a voltage
The capacitive method will, in most applications, provide the most sensitive microphones – largely due to the low diaphragm mass that this method facilitates. A survey of measurement microphones over the past 50 years reveals wide use of capacitive microphones (often called condenser microphones).
One exception is where sound levels are very high, such as near a blast or explosion. In this case, a piezoelectric measurement microphone is the best 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 overwhelmed 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, which provided the capacitor with polarising voltage, as well as 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 200 V polarising voltage; many other designs are pre-polarised – with an electret capacitor as the sensor element – and require only preamplifier power. Early electrets were not suitable for high-performance applications, but modern electret microphones offer excellent specifications and long-term stability.
Measurement microphones are offered in four nominal diaphragm sizes: 1”, 1/2”, 1/4” 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 can be satisfied with ½” measurement microphones.
Directional and sound field characteristics
Engineers with experience in sound amplification or recording may 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 usually 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.
Effect of the microphone 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, dependent upon the microphone’s dimensions and the frequency and angle of incidence of the sound wave. This effect is avoided in the first case below: the pressure microphone.
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.
A free-field microphone is compensated to produce a flat response when used in an anechoic space, where the sound waves arrive 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 those found outdoors or in an anechoic chamber.
A diffuse-field microphone is compensated to produce a flat response when used in a reverberant space, such as a church, a concert hall, a room, or an aircraft or automobile cabin. Applications include room tuning, impulse-response testing, and ambient industrial or environmental noise evaluation.
Some applications require a geometric array of two or more (sometimes many more) matched microphones 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 (and therefore less expensive) design.
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