Microphone Directivity Testing: Measuring Sound Response at Different Angles

How does a microphone's orientation influence the sound it captures? Students from the IUT of Le Creusot designed an experiment combining precision motion control with Dewesoft data acquisition to investigate microphone response in an anechoic chamber. Their work explores the relationship between microphone rotation, acoustic wave propagation, and measurement accuracy.

Understanding microphone directivity and sound propagation

A microphone’s response to sound depends not only on frequency and sound pressure level but also on the direction from which the sound arrives. Understanding this directional response is important when evaluating microphone performance and measurement accuracy.

Students at the IUT of Le Creusot, part of Université Bourgogne Europe, investigated how microphone orientation affects the sound it captures. Their goal was to characterize the directivity of an MG370 measurement microphone by measuring its response at different rotation angles and frequencies.

The experiment was performed in an anechoic chamber using a controlled sound source and a motorized system that rotates the microphone around its own axis. An encoder measured the rotation angle, while a Dewesoft data acquisition system recorded and synchronized the acoustic and position data. 

By comparing sound pressure level with microphone angle, the students could determine how the microphone’s sensitivity changes with the direction of incoming sound and visualize its directional response.

This leads to the central question of the study: How does microphone orientation affect its response to sound at different frequencies?

Microphone directivity test setup

To carry out this study, the following equipment is used:

Dewesoft data acquisition system

The Dewesoft SIRIUS 6STG/25TG+/8AO is a data acquisition system designed to measure, record, and analyze signals simultaneously. It performs signal conditioning, analog-to-digital conversion, synchronization of all measurements, and data recording and analysis using DewesoftX data acquisition and signal processing software.

Measurement microphone

Microphone rotation angle measurement

Servo-controlled microphone rotation

Acoustic sound source

Microphone positioning system

Microphone directivity test procedure

The experimental setup is installed inside an anechoic chamber.

The setup requires generating a predefined frequency range (63 Hz to 8000 Hz, in octave bands) using a loudspeaker controlled via DewesoftX software.

Then, a command is sent to the servomotor, which drives a belt system that rotates the microphone. The encoder, connected to the microphone, allows the rotation angle to be measured and displayed in the software.

The software enables real-time monitoring using various widgets, including frequency (Hz), sound pressure level (dBA), rotation angle (°), angular velocity (RPM), and a live camera view. It also provides graphs showing sound pressure level (dBA) as a function of rotation angle (°).

Microphone directivity measurements

Analyzing microphone directivity at different frequencies

The first measurements showed a relationship between microphone orientation and measured sound pressure level. However, the variation was smaller than expected, resulting in a less pronounced directivity pattern.

To measure the microphone’s angular response, we rotated it around its own axis while keeping the sound source stationary. This allowed us to measure how the microphone’s sensitivity changed with the angle of incoming sound.

Rotating the microphone rather than moving it around the loudspeaker was an important part of the test method. If the microphone had moved around the source while continuously facing it, the experiment would primarily have characterized the loudspeaker’s sound distribution rather than the microphone’s directivity. 

We performed 10 measurements at different frequencies in octave bands, initially covering 31.5 Hz to 16 kHz. At lower frequencies, the excitation amplitude was increased to provide a sufficient sound level for the measurements.

Technical limitations reduced the usable frequency range. Below 250 Hz, the microphone did not capture a sufficient signal, while above 5 kHz, the loudspeakers could not generate a stable continuous tone. The final analysis was therefore limited to measurements between 250 Hz and 4 kHz. 

Investigating Sound Propagation Around a Rotating Microphone.pdf

Microphone response at 1 kHz

The 1 kHz measurement shows how the measured sound pressure level changes as the microphone rotates relative to the sound source.

The maximum measured level was 73.4 dB, while the minimum was 72.7 dB, a difference of approximately 0.7 dB. The lowest levels occurred when the microphone was oriented away from the sound source, indicating a directional response, although the variation at this frequency was relatively small. 

Microphone response at 2 kHz

A similar angular response was observed at 2 kHz. The microphone’s initial position was offset by approximately 90°. The lowest response occurred at approximately 250°, corresponding to a relative rotation of about 160° from the initial position.

This places the minimum response close to the position where the microphone is facing approximately opposite the sound source. 

Measurements at the other tested frequencies showed broadly similar angular behavior, although the measured sound pressure levels varied with frequency.

Visualizing microphone directivity with a polar plot

A polar plot provides a clearer way to visualize the relationship between microphone angle and measured sound pressure level.

Figure 10 shows the measurement at 4 kHz. The microphone’s starting position was offset by 30°, while the minimum response occurred at approximately 210°. The resulting difference of approximately 180° places the minimum response opposite the microphone’s initial orientation. 

This representation makes the microphone’s angular response easier to interpret and demonstrates how polar plots can be used to characterize microphone directivity.

Microphone directivity test results

The experiment successfully characterized how the MG370 microphone’s response changes with orientation and frequency. By rotating the microphone relative to a fixed sound source, the measurements revealed changes in sound pressure level as the angle of incoming sound varied.

The results showed a measurable directional response across the tested frequencies. Polar plots provided a clear visualization of this relationship, showing how microphone sensitivity changed with rotation angle.

Using Dewesoft data acquisition and synchronized angle measurement, the students were able to capture and analyze the microphone’s response throughout the rotation. The experiment demonstrates a practical method for measuring microphone directivity and evaluating how orientation can influence acoustic measurements.

Acknowledgements

We conducted this study with the project management and valuable support of Fabian Jeandenans from Dewesoft France.