The physics of sound and vibration - Measurement microphones explained

Microphones are present in almost all our electronic devices - from mobile phones, PCs and portable speakers, to TV, tablets and smart watches. In this article, we walk you through the physics of microphones that are specifically designed to be used in systems that quantify sound: measurement microphones.

By: Dr. Rémi Guastavino
Domain Specialists, Acoustics 
Brüel & Kjær

Brüel & Kjær has developed and produced high-end measurement microphones since 1945. We have uninterruptedly worked on research and development and on calibration methodology to ensure continuous improvements in microphone accuracy and performance. Today, this approach contributes to a wide portfolio of measurement microphones covering all kinds of applications from acoustic seismic detection systems to airbag measurement and ultra-sonic applications.

Physics
Measurement microphones are based on a very simple physical principle: capacitance. The capacity of a microphone is inversely proportional to the distance between the backplate (a stiff plate), and the diaphragm (a thin, highly tensioned metal foil). When exposed to sound pressure, the diaphragm deforms and moves closer to or further away from the backplate, changing the capacity of the system.

These variations in capacitance are converted to voltage variation. Most measurement microphones are based on this principle.

Size
Measurement microphones come in diameters of 1-, ½-, ¼- and 1/8 inch. The bigger the diaphragm, the less stiff it is and better to detect smaller sound pressure variations. On the other hand, the size of the diaphragm limits detecting frequencies with wavelengths that are in the same range or bigger than the diaphragm. Large diaphragms give lower noise, while small diaphragms allow high frequency. Small-sized microphones allow better omnidirectional characteristics at high frequencies. 


The displacement of the diaphragm for a typical ½-inch microphone is only in the magnitude order of 5 nanometres for an excitation of one pascal. For comparison, a diaphragm with a diameter the size of planet earth, would barely move 5 metres

The wavelength of a 20 kHz sound wave (the upper limit of the audio range) is 1.7 cm. That is approximately the width of the index finger


Did you know?
Using a free-field microphone in a pressure-field environment gives an error close to 9 dB around 20 kHz.

 


Sensitivity
The sensitivity is the voltage produced by microphones under a defined sound pressure. It is defined in V/Pa and is frequency dependent.

A sound pressure of one pascal corresponds to a level of 94 dB SPL. That is why most microphone calibrators (for example, Brüel & Kjær’s Sound Calibrator Type 4231) produce 94 dB SPL or 1 Pa. 

Frequency response
Because sensitivity is frequency dependent, the frequency response is defined as an expression of the sensitivity in the entire frequency range. This is generally expressed in decibels relative to the 250 Hz sensitivity. 

Dynamic range
The dynamic range of measurement microphones (for example, 16 dBA - 143 dB) is the range where the microphone acts as a perfect linear transducer.

The first number is the inherent noise. Both condenser microphones and pre-amplifiers have an inherent noise caused, among other things, by limitations in the electronics and Brownian movements. This number indicates the sound pressure level that would create the same voltage as the noise generated by the microphone itself. The noise is historically measured in third-octave band, A-weighted in the human hearing range (between the 22.4 Hz and 22.4 kHz band), if nothing else is specified. 

The second number is the highest sound pressure level measurable with less than 3% total harmonic distortion. 

The dynamic range of a microphone is often limited by the preamplifier. For example, a CCLD (constant-current line drive) preamplifier is designed to deliver a maximum of 7 V peak for frequencies lower than 20 kHz. This gives a maximum of 134 dB SPL for a microphone sensitivity of 50 mV/Pa. It is possible in this case to extend the upper limit of the dynamic range to 146 dB by using a classical preamplifier.

A microphone’s dynamic range

 


Did you know?
Laboratory Standard Microphone Types 4180 and 4160 are so extremely stable that all absolute acoustical measurements worldwide are relative to their reference sensitivity. There are no direct sound pressure sources that can produce the pascal with sufficiently low uncertainty over a wide frequency range. 

Brüel & Kjær’s ½-inch Free-field Microphone Type 4191 has a dynamic range of 20 dBA to 162 dB (this is 142 dB). If we convert this ratio to distance, the measurement would stretch from a strand of hair to more than three Eiffel Towers stacked on top of each other

Sound field explained
Measurement microphones, by their size and shape, influence the sound pressure. This influence, which depends on the type of sound field, is considered in each microphone’s design and the microphone responses are optimized to compensate for the influence. This allows us to always give a flat response for the chosen sound field. Measurement microphones are divided in three main types, each type optimized for one of the three main types of sounds field, and it is important to choose the microphone that is best suited for the sound field.

Free-field
A free-field is a sound field where sound waves can propagate freely without any disturbing objects. This means a space with no reflections. Sound fields with a close resemblance to free-field can be achieved in an anechoic chamber or emulated using a time-selective response (TSR) algorithm.

Free-field microphones are typically used to measure, for example, loudspeakers or outdoor sound. 


Diffuse-field
A diffuse-field is created by sound waves arriving simultaneously from all directions with equal probability and level. Sound fields with a close resemblance to diffuse-field can be achieved in environments such as buildings with hard walls, where many simultaneous sound or noise sources exist; in-cabin measurements and churches. If you are not sure about the sound field you are in, it is best to assume that it is diffuse to minimize measurement error.

Diffuse-field microphones are typically used to measure vehicle interior noise or building acoustics.


Pressure-field
A pressure-field is sound pressure that has the same magnitude and phase at any position within the field. Sound fields with a close resemblance to pressure field can be achieved in small cavities (small compared to the wavelength) such as artificial ears. 

Pressure-field microphones are typically used in couplers, wind tunnels or in any flush-mounted measurement.

 


Securing stability
Brüel & Kjær measurement microphones are designed to be extremely stable, especially over time, temperature, humidity and ambient pressure. To achieve maximum stability, we use carefully selected, high-quality material; we apply controlled heat treatment to artificially age and release any tension in the cartridge and we continuously test each microphone at every step of the production process.

The microphones are exposed to multiple cleaning processes during fabrication in a class 10 clean room. In a class 10 clean room, there are less than 10 particles larger than 0.5 µm and less than 2 particles between 1  and 5 µm per cubic foot. 

As a comparison, ordinary room air is approximately ‘class 1 million’. Considering the distance between the backplate and the diaphragm being generally around 20 µm, any particles bigger than this that are present in the cavity would cause stability issues – especially when condensation or changes in temperature occur. 

If the membrane is punctured, contamination will occur, and particles and residue will enter the cartridge. Achieving the same level of cleanliness during a repair may not guarantee a microphone with optimal response. This is the reason why Brüel & Kjær does not offer microphone repair.


Did you know?
We have been carefully monitoring the sensitivity of our laboratory standard microphones Type 4160 and Type 4180 since 1984. The sensitivity stays in the ±0.02 dB range (this means less than 0.2% changes). These microphones are used all over the world in calibration systems.


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