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100 Things #62: Make Directional Measurements with Polar Plots in SoundCheck

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SoundCheck’s Polar Plots make directional measurements simple. No matter what device you are testing, whether it be a VR headset, spatial audio, or headset sound leakage, SoundCheck can automate the measurement and turntable control. This means long high resolution measurements become as simple as starting a sequence! We even have a free, prewritten directional measurement test sequence using the new Portland Tool & Die MDT-4000 turntable, available here.

Make Directional Measurements with Polar Plots in SoundCheck

Try our loudspeaker polar plot measurement sequence for yourself!

Our polar plot sequence measuring a loudspeaker is pre-written and ready to use. This sequence measures the polar response of a loudspeaker in both the vertical and horizontal dimensions and displays the measurements on polar plots. This sequence is designed to work with the Portland Tool & Die MDT-4000 turntable.

Video Script:

Making directional measurements in SoundCheck is simple! SoundCheck supports turntables from a variety of manufacturers including Outline, Linear X, B&K and of course, Portland Tool and Die. These can all be controlled through a custom step in SoundCheck to be operated as part of an automated test sequence. Let’s take a look. 

Here I have a Portland Tool and Die MDT-4000 turntable – this a great turntable, by the way.  I’m going to use this to rotate a speaker, and I have a stationary measurement microphone to capture the recorded waveform. And of course I have SoundCheck on my laptop, along with an AmpConnect 621 Audio interface.

Here’s a  simple loudspeaker test sequence that plays a test signal through a loudspeaker and measures the response. Now let’s say I want to measure the response of the speaker every 10 degrees for a full 360 degree rotation. I just need to open up the test sequence….And I am going to add a custom step telling it to rotate the speaker by 10 degrees and re-measure, saving the results in the memory list and plotting them on a polar chart, now I’m going to loop that whole measurement procedure so we continue moving it and measuring until we do a full rotation. And now I’ll edit the display step to get the results output to a polar plot. Now let’s run the sequence…and there you are – fully automated directional measurements! 

There are many ways to use this functionality, for example directional measurements on smart devices with microphone and speaker arrays. You can also put headphones or VR headsets on a rotating head and torso simulator and measure the sound leakage that occurs when the noise inside the headphones leaks outside to the point where it’s audible to others around.  There are also many applications in spatial audio measurements. 

If you want a quick way of getting started with directional measurements, head on over to our website where you can download basic directional measurement sequences for speakers and microphones for a variety of different turntables.

Polar Plot (Linear X Turntable) Loudspeaker Sequence

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This sequence measures the polar response of a loudspeaker in both the vertical and horizontal dimensions. It is designed to work with the Linear X turntable, and has all the necessary commands to automatically rotate it via RS-232. The sequence uses a log sweep stimulus with the Time Selective Response algorithm so that the measurements can be run in a non-anechoic environment. Note that the time window needs to be adapted to the user’s measurement space. The sequence plays the stimulus and measures at 10 degree increments from 0 to 180 degrees. This process is repeated with the speaker positioned horizontally. The two results are mirrored to display full 360 degree polar plots for each axis. A directivity index curve is also calculated for each axis and is displayed at the end of the test.

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Microphone Polar Plot Substitution Method Using Outline ET250-3D

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This sequence measures the directional response of a microphone and graphs the result as a polar plot. A log sweep stimulus is played from 100 Hz to 10 kHz at each angular increment, and the acquired waveform is analyzed using the Time Selective Response algorithm. This method allows the test to be performed in a non-anechoic environment by placing a window around the direct signal, eliminating the influence of reflections. Commands are sent automatically to the Outline ET250-3D turntable via an ethernet connection, instructing it to move in 10 degree increments after each measurement. The sequence measures the response every 10 degrees from 0 to 180 and mirrors the polar image, which simulates a full 360 degree polar and saves test time. The response at each angular increment is compared against the on-axis response to create a normalized curve. This removes the influence of the device’s frequency response and sensitivity, such that the polar plot only shows the directional response. The final display also contains a graph of the directivity index in decibels versus frequency.

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Microphone Polar Plot Sequence

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mic_polar_plot_screenshotThis sequence measures the directional response of a microphone and graphs the result as a polar plot. A log sweep stimulus is played from 100 Hz to 10 kHz at each angular increment, and the acquired waveform is analyzed using the Time Selective Response algorithm. This method allows the test to be performed in a non-anechoic environment by placing a window around the direct signal, eliminating the influence of reflections. Commands are sent automatically to the LT360 turntable via an RS-232 connection, instructing it to move in 10 degree increments after each measurement. The sequence measures the response every 10 degrees from 0 to 180 and mirrors the polar image, simulating a full 360 degree test while saving time. The response at each angular increment is compared against the on-axis response to create a normalized curve. This removes the influence of the device’s frequency response and sensitivity, such that the polar plot only shows the directional response. The final display also contains a graph of the directivity index in decibels versus frequency.

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