Microphone Polar Plot Substitution Method Using LinearX LT360
A test sequence specifically written for the LinearX LT360 turntable
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 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, 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.
Hardware
- LinearX LT360 turntable (Requires Serial Port on PC or USB to Serial Adapter and “Straight Thru” Serial Cable)
- Genelec 8331A (powered) or KEF Q300/350 (unpowered), (or equivalent full range loudspeaker)
- Listen SC Amp power amplifier Part # 4060 (or equivalent)
- Listen SCM-3 Part #4002kB (or other reference microphone for calibration)
- Listen AudioConnect audio interface Part # 4050 (or equivalent)
Software
- SoundCheck 16 or later
- Post Processing module Part # 2004
- Time Selective Response Part # 2006
- Polar Plot Part # 2011
Setup & Calibration
- Connect the turntable ethernet cable to the ethernet interface on the computer.
- Connect Output 1 of the audio interface to the input of the power amplifier and connect the output of the power amplifier to the loudspeaker.
- Connect SoundConnect 2 outputs 1 and 2 to audio interface inputs 1 and 2.
- Connect the reference microphone to input 1 of SoundConnect 2.
- Connect the DUT microphone to Input 2 of SoundConnect 2. Use a phantom power supply if the mic requires 48V power.
- Calibrate the SCM-3 reference microphone as per the instructions in the SoundCheck manual.
- Position the reference microphone at the desired test location on the turntable stand. This must match the test position of the DUT microphone.
- Equalize the Source Speaker according to the instructions in the SoundCheck Manual: Calibration Chapter > Speaker Equalization. Even though the equalization of the source is not used in the Mic Substitution method, the calibration of the playback level of the source is.
- You are ready to start the sequence.
- After measuring the reference mic at the 0 degree position, the sequence will prompt you to replace it with the DUT microphone.
You are ready to start the sequence.
Further Sequence Development
- To use this as a production sequence, remove the demo data recall steps:
- 2 – Mes – Recall Demo Data and 48 – Rec – Recall curves.
- The sequence can be modified to run a complete 360 degree polar without mirroring the data. The following step changes are required:
- 37 – Skip Last Acquisition – Change value from 179 to 359
- 41 – Dis – Microphone Polar Plot – Uncheck Mirror in the polar plot window preferences and add 190 to 360 Normalized Curves to Polar Window
- 42 – Mes – End of Loop – Step Configuration: Change 18 repetitions to 36
- 49 – Dis – Microphone Polar Plot Final – Uncheck Mirror in both polar plot window preferences and add 190 to 360 Normalized Curves to Polar Windows
