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Microphones and Acoustics

Frequently Asked Questions

Get your most commonly asked acoustics questions answered.

Section I: Definitions and Terminology
Section II: Microphone Recommendations
Section III: Calibration and Testing
Section IV: Specification Clarifications
Section V: Specialty Microphone Applications
Section VI: Maintenance and Handling

If you don’t see the answer to your question, call our 24/7 SensorLineSM 716-684-0001 to speak with an application engineer or visit Ask the Acoustics Experts.

Can I use a microphone designed for one response field, within a different field? And how do I correct for it manually?

When measuring sound pressure levels in a certain field, for example a random incidence field, we recommend using a random incidence microphone, PCB® Models 378C20 or 378A21. This eliminates the need for manual corrections and the possible errors that can stem from introducing a manual operation. It also provides a truer reading of the environment, because the random incidence (RI) field for end users testing may be slightly imperfect from that of a theoretical random incidence field.

If you do not have a random incidence microphone and can’t justify the purchase, you can use an existing microphone and manually correct for the field type, provided the manufacturer has furnished the corrections. Below is an example of a free-field (FF) microphone corrected to a random incidence field. You can also correct between the other microphone field responses, (FF, RI or pressure.)

The chart below provides corrections for the ½” PCB® 377B02 microphone for FF, RI and pressure responses. Corrections, like in the chart, when supplied would be added to the electrostatic actuator response of the microphone. The difference in the response between the three field types using a ½” microphone is negligible up to 1 kHz. At frequencies below 1 kHz you will get a similar response using a ½” microphone in any field, because the wavelength is large in comparison to the diameter of the microphone. The chart below shows that above 1 kHz you see a greater separation between the response curves.

It is important to note that the free-field curve has the largest correction, and the RI and pressure responses have smaller corrections. This will help you remember whether to add or subtract the correction values in order to manually correct for the desired field.



The chart below shows numeric values of these corrections at ten arbitrarily selected 1/12 octave band center frequencies.



PCB’s 377B02 microphone is designed to measure accurately in a free-field (an open area, free of reflections). The 377B02 free-field (column 2) requires a 4.99 dB correction at 10 kHz to be added to the actuator response in order to provide an accurate response in a free-field.

If you look at the 3rd column, which is the random incidence column, you see a 1.42 dB correction at 10,000 Hz.

In order to manually correct using a free-field microphone in a diffuse field, you take the net result from your measurement (let’s pick 90 dB as an example) and adjust it based on the corrections. If you measure a 90 dB signal at 10 kHz with a FF microphone in a random incidence field, you would subtract the difference between the fields from your measured response at the desired frequency.

The difference between the FF and RI field at 10 kHz is equal to 4.99 – 1.42 = 3.57 dB.

Since the RI curve is lower than the FF response, you know you need to subtract the RI calculated result from the FF measured result: 90 dB – 3.57 dB = 86.43 dB.

So if the free field mic measures 90.00 dB at a 10 kHz frequency in a free field, it will measure 86.43 dB in a random incidence field at the same frequency.

Can I use a microphone designed for one response field, within a different field? And how do I correct for it manually?

When measuring sound pressure levels in a certain field, for example a random incidence field, we recommend using a random incidence microphone, PCB® Models 378C20 or 378A21. This eliminates the need for manual corrections and the possible errors that can stem from introducing a manual operation. It also provides a truer reading of the environment, because the random incidence (RI) field for end users testing may be slightly imperfect from that of a theoretical random incidence field.

If you do not have a random incidence microphone and can’t justify the purchase, you can use an existing microphone and manually correct for the field type, provided the manufacturer has furnished the corrections. Below is an example of a free-field (FF) microphone corrected to a random incidence field. You can also correct between the other microphone field responses, (FF, RI or pressure.)

The chart below provides corrections for the ½” PCB® 377B02 microphone for FF, RI and pressure responses. Corrections, like in the chart, when supplied would be added to the electrostatic actuator response of the microphone. The difference in the response between the three field types using a ½” microphone is negligible up to 1 kHz. At frequencies below 1 kHz you will get a similar response using a ½” microphone in any field, because the wavelength is large in comparison to the diameter of the microphone. The chart below shows that above 1 kHz you see a greater separation between the response curves.

It is important to note that the free-field curve has the largest correction, and the RI and pressure responses have smaller corrections. This will help you remember whether to add or subtract the correction values in order to manually correct for the desired field.



The chart below shows numeric values of these corrections at ten arbitrarily selected 1/12 octave band center frequencies.



PCB’s 377B02 microphone is designed to measure accurately in a free-field (an open area, free of reflections). The 377B02 free-field (column 2) requires a 4.99 dB correction at 10 kHz to be added to the actuator response in order to provide an accurate response in a free-field.

If you look at the 3rd column, which is the random incidence column, you see a 1.42 dB correction at 10,000 Hz.

In order to manually correct using a free-field microphone in a diffuse field, you take the net result from your measurement (let’s pick 90 dB as an example) and adjust it based on the corrections. If you measure a 90 dB signal at 10 kHz with a FF microphone in a random incidence field, you would subtract the difference between the fields from your measured response at the desired frequency.

The difference between the FF and RI field at 10 kHz is equal to 4.99 – 1.42 = 3.57 dB.

Since the RI curve is lower than the FF response, you know you need to subtract the RI calculated result from the FF measured result: 90 dB – 3.57 dB = 86.43 dB.

So if the free field mic measures 90.00 dB at a 10 kHz frequency in a free field, it will measure 86.43 dB in a random incidence field at the same frequency.