In your study of microphones, you may have come across “measurement microphones.” However, all microphones are effectively instruments of measurement.
What do microphones measure and how do they measure it? Microphones essentially measure the sound pressure variations around their diaphragms within a range of frequencies. As the sound waves cause varying pressures around the mic diaphragm, the microphone produces a coinciding electrical mic signal.
In this article, we’ll discuss the sound pressure variations that microphones are designed to measure and how microphones do so.
Microphones Measure Sound Pressure Variations
Microphones are transducers that convert sound waves (mechanical wave energy) into audio signals (electrical energy).
In other words, microphones measure sound waves (also known as sound pressure or mechanical wave energy). Microphones do so with a transducer element that work with a moveable diaphragm.
For a detailed read into how microphones work, check out my article How Do Microphones Work? (A Helpful Illustrated Guide).
As sound waves hit the diaphragm and pass by, they cause varying amounts of pressure at one or both sides of the microphone.
The difference in pressure between the front and back sides of the microphone diaphragm cause it to move and the microphone then produces an electrical mic signal (AC voltage).
For more information on mic signals and analog signals in general, check out my article What Is A Microphone Audio Signal, Electrically Speaking?
If a microphone was in an anechoic chamber (or any other environment with no sound), there would still be pressure at either side of its diaphragm. This ambient pressure, however, would be equal and constant at either side. Therefore, the microphone diaphragm would not move and there would be no mic signal.
As a thought experiment, if a microphone was in a vacuum, there would also be equal pressure on both sides of the diaphragm (albeit a much lower pressure than atmospheric). Sound requires a medium to travel in and so the microphone would be incapable of measuring anything.
Thus, microphones measure the sound pressure in the medium around their diaphragms.
What Is Sound Pressure?
Sound pressure, as we’ve alluded to, is a deviation in pressure from the ambient pressure.
Sound pressure (or “acoustic pressure”) is caused by a sound wave and varies the ambient pressure above and below its set point. Acoustic pressure variation is typically very small relative to the ambient pressure.
The deviation in ambient pressure caused by a sound wave is typically measured in decibels of sound pressure level (dB SPL) or Pascals (Pa).
What Is A Decibel?
What is a decibel (dB) and what are dB SPL?
A decibel is a unit a relative measurement along a logarithmic scale.
dB SPL is defined as 20 log10 P1/P0
P1 is the sound pressure level of a given sound wave (the deviation from ambient pressure).
P0 is a reference value of 20μPa (the lowest hearing threshold of the human ears).
By the above equations, we see that 0 dB SPL is the lower limit of human hearing. At 0 dB SPL, most humans will not hear anything.
The theoretical maximum limit of sound pressure would be a pressure wave that deviates the entire ambient pressure, causing a vacuum at the wave’s trough and two times ambient pressure at its peak. At standard atmospheric pressure (101.325 KPa), this would equal 194 dB SPL.
A regular microphone can come close to measuring the decibels of sound pressure level.
What Are Pascals?
A pascal is an SI unit of pressure.
1 Pascal is equal to 1 Newton per square metre.
Unlike dB SPL, pascals are linear.
A common sound pressure level used to test microphones is 1 Pascal or 94 dB SPL.
1 Pascal = 94 dB SPL
Using Microphone Sensitivity To Estimate Sound Pressure
Microphone sensitivity is a specification that relates a microphones output signal strength to the dB SPL the mic is subjected to.
Typically, a microphone’s sensitivity rating is given as a signal strength in millivolts (mV) or decibels relative to 1 volt (dBV) at a 94 dB SPL tone of 1 kHz.
This specific measurement gives us a good idea of how a microphone will react to sound pressure, but doesn’t necessarily provide an ideal basis for the measurement of sound pressure.
For more information on microphone sensitivity, check out my article What Is Microphone Sensitivity? An In-Depth Description.
First, a microphone is subjected to sounds across the entire audible spectrum (20 Hz – 20,000 Hz) along with infrasound (<20 Hz) and ultrasound (>20,000 Hz). Though we know how the mic will respond to a 1 kHz tone, we cannot expect the microphone to behave the same at all other frequencies.
Second, microphones are typically connected to a mic preamp, which provides gain to our mic signal to boost it to line level. However, preamps do not explicitly tell us the voltage of the mic signal at their inputs.
Therefore, it’s not really easy to get an exact measurement reading of sound pressure with a microphone.
In order to measure dB SPL with a microphone, then, we would need a voltmeter at the mic’s output. But we would still have the problem of relying on a sensitivity rating at a specific frequency.
So even though microphone measure sound pressure, they are not the greatest instruments to measure dB SPL. This comes down to the all-encompassing term “colouration.”
Microphone colouration is a broad term to describe the differences between the sound waves at the microphone diaphragm and the way in which the microphone converts those sound waves into electrical signals.
In other words, microphone colouration affects the way in which a microphone measures the sound pressure around it. The more “coloured” a microphone is, the less accurate it is at measuring the true sound waves around it.
A perfectly uncoloured microphone would convert a sound wave perfectly into an electrical signal. A perfectly uncoloured speaker could then recreate this electrical signal as a perfect recreation of the original sound wave. Of course, these are idealities that never happen in the real world.
An uncoloured microphone will have, at the very least:
- Flat frequency response (equal sensitivity across the audible frequency spectrum).
- Omnidirectional polar pattern (equal sensitivity to sounds from all directions).
For in-depth articles on frequency response and polar patterns, please check out the following My New Microphone articles:
Complete Guide To Microphone Frequency Response (With Mic Examples).
The Complete Guide To Microphone Polar Patterns.
These, too, are idealities that are not achievable in the real world.
There are plenty of factors that influence the colouration of a microphone. The main factors include:
- Frequency response.
- Polar response.
- Physical components (diaphragm, capsule, body, etc.).
- Internal circuitry.
What About Measurement Microphones?
On top of just measuring sound pressure, measurement microphones are used to measure environmental noise and the overall acoustic properties of a space.
Calibrated measurement mics are also used to test loudspeakers.
Measurement mics work on the same principles as other microphones. The vast majority of measurement mics are condensers. Measurement mics, however, are much more accurate with flat frequency responses and calibrated sensitivity.
In order to use measurement microphones properly, they must be properly calibrated. With proper calibration, we know the microphone’s sensitivity (in volts per pascal) along its entire frequency response.
With known sensitivity, measurement mic signals are sent through audio analysis systems which allow us to measure sound pressure and room acoustics.
Measuring dB SPL With A Decibel Meter
If we really want to easily and accurately measure dB SPL, we could use a compact decibel meter.
A decibel meter (aka sound pressure meter) measures the sound pressure level at its position. It does so with a microphone!
The microphone of a decibel meter, like any other microphone, has a diaphragm that moves according to the varying sound pressure around it. This diaphragm movement causes an electrical AC signal.
However, rather than outputting this signal (like a regular microphone), the decibel meter accurately converts the electrical signal back to a sound pressure level value, which is then displayed in dB SPL.
There are plenty of decibel meters on the market. Amazon has a great selection from budget to high-end decibel meters. I’ll share links to check the prices of the 4 decibel meters pictured above:
- BAFX BAFX3370 (link to check the price on Amazon).
- VLIKE VL6708F2 (link to check the price on Amazon).
- REED R8050 (link to check the price on Amazon).
- TES 1151 (link to check the price on Amazon).
What is a measurement microphone? A measurement mic is specially designed with a flat, uncoloured response to sound waves. These mics are mostly omnidirectional and are intended for use with audio analysis systems in order to measure the acoustic characteristics of a room.
How are microphones calibrated? Microphones are typically calibrated via reciprocity calibration, where three uncalibrated microphones are used to calibrate one another. A secondary method includes relating an uncalibrated mic to a calibrated one. Changes are made primarily to the capsule and circuitry to achieve calibration.