What is the difference between microphone types?

I’m often asked what is the difference between free field microphones, diffuse field microphones and pressure microphones.

For a run-of-the-mill ½ inch microphone the short answer is nothing.

However the long answer is a bit more involved.

Basically if the frequency range is below 16 kHz and an accuracy of ±2dB is acceptable then there is no real difference between the types.

So what is the difference between these types of microphone?

Well, first you have to understand the different types of sound field involved.

In theory a free field is a sound field where the sound waves are free to expand outwards forever from the source. That is, we assume that there are no reflections or reverbarations. In practice, we would consider an anechoic chamber to be a free field or an outdoor measurement, provided we measured at a sufficient distance from the ground.

A diffuse field or a random incidence field, is a sound field where the sound waves arrive equally from all directions. Another way to think of a diffuse field is that you have sounds coming from different directions in succession with no time inbetween their arrival. For example, in a diffuse field the sound waves that arrive at a person’s ears are so completely different that the brain finds it impossible to work out where the sound came from.

A pressure field is a sound field where the sound pressure has the same magnitude and phase at any position in the sound field.

It is important to understand that all of these microphone types are fundamentally the same. They are transducers that are designed to sense pressure levels in air. The differences between them are in the designs of the microphone heads. Each type of microphone will normally be supplied with a calibration data sheet that will show the frequency response against sound pressure. Comparing the frequency response against sound pressure for the three microphone types will show the differences clearly.

The differences between the three types of microphone generally occur at higher frequencies as we stated previously, below about 16 kHz the response from each will be similar. This is partly to do with the physical size of the microphone with respect to the wavelength of the sound being measured.

Free field microphones are designed to compensate for the effect of the microphone itself in the field. So you measure the sound as though the microphone was not there. They are designed this way as, of course, the presence of the microphone will affect the propagation of the sound wave. A free field microphone should be pointed towards the sound source at a 0° angle of incidence.

Diffuse field microphones are designed to respond in a uniform manner to any signal arriving on its measuring surface from any angle. Generally, these tend to be oriented at about 75° to the direction of the sound wave propagation in a free field.

Pressure microphones are designed to respond to a uniform frequency response to the sound level itself. When used in a free field a pressure microphone should be mounted at 90° to the direction of the sound wave propagation, effectively the sound passes the measuring face of the microphone.

 

I’m often asked what is the difference between free field microphones, diffuse field microphones and pressure microphones.

For a run of the mill ½ inch microphone the short answer is nothing.

However the long answer is a bit more involved.

Basically if the sample rate is 10 kHz or below, and in terms of accuracy within 2dB is ok then there is no real difference between the types.

So what is the difference between these types of microphone,

Well firstly you have to understand the different types of sound field involved.

A free field is a sound field where the sound waves are propagating freely through a medium, for example a large room or sound chamber with no objects to interfere with the movement of the sound waves.

A diffuse filed or a random incidence field, is a sound field where the sound waves arrive equally from all directions. For example in a diffuse field the sound waves that arrive at a person’s ears are so completely different that the brain finds it impossible to work out where the sound came from.

A pressure field is a sound field where the sound pressure has the same magnitude and phase at any position in the sound field.

An important point to understand is that all of these microphone types are fundamentally the same, they are transducers that are designed to sense pressure levels in air. The differences are physical in the design of the microphone head. The three different types of microphone will all normally be supplied with calibration data, this calibration data will show the frequency response against sound pressure. Comparing the frequency response against sound pressure for these three microphone types will show the differences clearly.

The differences between the three types of microphone generally occur at higher frequencies as we stated previously, below about 10 kHz the response from each will be similar. This is partly to do with the physical size of the microphone with respect to the wavelength of the sound being measured.

Free field microphones are designed to compensate for the effect of the microphone itself in the field. So you would be measuring the sound as though the microphone was not there. They are designed this way as of course the presence of the microphone will affect the propagation of the sound wave.

Diffuse field microphones are designed to respond in a uniform manner to any signal arriving on its measuring surface from any angle. Generally these tend to be oriented at about 75 degrees to the direction of the sound wave propagation in a free field.

Pressure microphones are designed to respond to a uniform frequency response to the sound level itself. When used in a free field a pressure microphone should be mounted at 90 degrees to the direction of the sound wave propagation, effectively the sound passes the measuring face of the microphone.

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James Wren

Application Engineer & Sales Manager at Prosig
James Wren is an Application Engineer and the Sales Manager for Prosig Limited. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a member of the Institution of Engineering and Technology. He has been involved with motorsport from a very early age with special interest in data acquisition. James is a founder member of the Dalmeny Racing team.

12 thoughts on “What is the difference between microphone types?

  1. Thomas Ahlersmeyer

    According to your graph the frequency responses start to differ at 1.6 kHz. not at 16 kHz as stated in the text. For a 1/2" mic the free field response is rather bad - at least for a B&K mic. A good 1/2" mic can be flat within +/- 1 dB up to 16 or even 20 kHz.

    BR
    Thomas A.

    1. James Wren Post author

      Hello Thomas,

      Thank you for posting on our blog.

      I'd like to respond to your comments if I may please.

      The 16kHz mentioned in the article was intended to show a band, for example with a +/-3dB tolerance the microphones are similar in the frequency range of 5Hz to 15kHz or 16kHz. If the tolerance band you can accept is wider then the frequency range at which any type of microphone can be used it wider still. Conversely if the accuracy you require is narrower then the frequency range will be further restricted, well below the 1.6kHz you mention.

      So it depends on the required tolerances of your test.

      I am quite sure that several microphone manufactures make many different types of microphones with different advantages and disadvantages and importantly different amplitude ranges and different frequency ranges.

      The objective of this article is to demonstration the difference between the types of microphone, not to critique any particular type.

  2. Aitor

    Hello James,

    Thank you for your explanation but i continue having some doubts. Imagine that you want to measure noise to a motor or generator in a test bench. The motor/generator is rigidly fixed to a steel floor and around the motor/generator you have some walls-->the distance between the walls and machine varies in each face of the machine.
    What kind of microphone would be the best to measure the noise of the machine?
    Thank you
    Regards

    1. James Wren Post author

      Hello Aitor,

      Thank you for posting a question on our blog.

      This article shows the frequency ranges where different microphones types under test either behave or do not behave in the same way.

      So to begin you must know the frequency ranges that you are interested in before you start testing. Because you have studied the motor/generator and understand the expected frequency ranges you should already have this information. That is, you have asked yourself the question 'what am I going to measure?'

      Therefore, once you have answered this question you can view the datasheet of a given microphone and check that it behaves the way you expect it to across your frequency ranges of interest.

      I would suggest that you start with a free field microphone as this type is often the most accurate when measuring sound pressure levels that radiate from a single source.

      The free field microphone is best for situations where there is a single sound source.

      The pressure microphone is best for situations where there is a continuous sound wave and uniform sound wave, the microphone will take account of its own presence in the sound field.

      The random field microphone can be used in situations where sound is coming from any direction or all directions towards the microphone.

      1. Aitor

        My frequency range of interest is between 500Hz a 2500Hz.

        My first impulse would be to measure with a free field microphone because I have an only source that is the machine. But because of the reflections of the walls and the floor the microphone is going to receive different noise from different direction, although we only have one sound source that is the machine

        What would you do?

        Thank you!
        regards

        1. James Wren Post author

          Hello Aitor,

          Thank you for again posting on our blog.

          In short I would use the free filed microphone.

          At those frequencies it is unlikely to make a huge difference.

  3. Reet

    Hi,

    I have some doubts about the microphone position and wave length. For example, I use microphone which have frequency range 20 Hz - 5 kHz measure sound from single sound source that generate sound frequency 100 Hz. Do I need to position the microphone away from the source sound more than 3.43 m ? (sound speed in the air 343 m/s and wave length at 100 Hz is 3.43 m) if not why ?

    Many thank in advance.

    Regards,
    Reet

    1. James Wren Post author

      Hello Reet,

      Thank you for asking a question on our blog.

      I understand the objective of your question.

      You do not need to leave a full wavelength between a source and a measurement microphone. Wherever you place the measurement microphone it will measure the complete wave form as the wave passes the microphone.

  4. Ejaz

    Hello James!

    I have an application wherein I am trying to run and test a motor and gearbox assembly for Vibration and Noise. Now, this assembly is going to be tested on a traditional shop. So there are going to be other equipment and activities going on as well that are going to generate their own noise, which I don't want to interfere with my measurement greatly. I don't intend to have an anechoic chamber to carry out these analysis. So, in your opinion which mic is better suited for my application, a free field or pressure? Is there any difference in the directional response of these microphones. I studied datasheets of multiple manufacturers, but strangely, none of them have any directional characteristics specified. Also, in your opinion, do you think 1 mic will be sufficient to give me a good enough response or will I require more? I have decided that I will require a frequency range of not more than 10 Hz to 20 kHz.

    Thanks in advance!

    Regards,
    Ejaz

    1. Prosig

      Hi Ejaz

      That sounds like a fairly complex problem. We would recommend speaking to an expert in noise measurement. They would be able to take some test measurements an advise on the best method of performing your test. Prosig offer this type of service or you could find someone local to you. If you think we can be of assistance please contact us at sales@prosig.com.

  5. Ernest

    To my opinion, the explanation is not correct. The pictures shows a FF mic in a FF, a random mic in a diffus field etc - and not a comparison of the 3 mic types in the same sound field type. So the picture shows what level of perfection and upper limit can get achieved for each type.

    If a FF mic gets tested against constant pressure (by truning it 90 degrees to the wave direction, or in a dedicated test adapter called electrostatic actuator), the frequency response drops starting about at 3 kHz and resulting in about 10 dB drop at 20 kHz (for a 1/2 inch mic). FF mics need such a response to can compensate for the sound level increase in the front of the mic when the wavelength comes down to the diameter of the mic and due to reflection on the mic front the pressure increases (similar to waves in a harbor: slow waves go up and down the harbor wall; fast waves shoot up the wall)

    When to use what type:
    - Testsing speakers (in FF or anechoic chamber): a FF mic
    - Measuring reverberation, speech transmissions etc in room: Diffus (or pressure, since the difference is small). Here the sound comes from all directions
    - Pressure mic get used mainly in small closed chambers, e.g. in test adapters for testing hearing aids.

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