Microphone Polar Plot Substitution Method Using LinearX LT360

A test sequence specifically written for the LinearX LT360 turntable

Introduction

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.

Required Software

• SoundCheck 18 or later
• 2004 – Post Processing
• 2006 – Time Selective Response
• 2011 – Polar Plot

Required 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 (or equivalent)
• Listen SCM-3 (or other reference microphone for calibration)
• Listen AudioConnect audio interface (or equivalent)

If testing past 20 kHz an AudioConnect 4×4 audio interface and SoundConnect 2 microphone power supply are recommended.

• B&K 4231 calibrator (or equivalent)

Setup Diagram

Hardware Setup and Calibration

1. Connect the turntable serial connection to computer Serial Port or USB adapter.
2. Setup your RS-232 connection in the External Tab of the Hardware Editor, make sure to establish the correct COM port.
3. 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.
4. Connect the microphone power supply outputs to Inputs 1 and 2 on your audio interface.
5. Connect the reference microphone to input 1 of the microphone power supply.
6. Connect the DUT microphone to Input 2 of SoundConnect 2. Use a phantom power supply if the mic requires 48V power.
7. Calibrate the reference microphone as per the instructions in the SoundCheck manual.
8. Position the reference microphone at the desired test location on the turntable stand. This must match the test position of the DUT microphone.
9. 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.
10. You are ready to start the sequence.
11. After measuring the reference mic at the 0 degree position, the sequence will prompt you to replace it with the DUT microphone.

Get Sequence

Download Sequence (zip file)
Download Sequence documentation (PDF)