Calibrating a speaker/mouth simulator

This knowledgebase article outlines the "Equalized Source Speaker" method for calibrating a sound source (like a loudspeaker or a mouth simulator) using SoundCheck and an audio interface.

The goal of this procedure is to generate an Equalization (EQ) curve that flattens the acoustic output of the source speaker, as well as a Correction curve to compensate for any residual non-flatness.

1. Hardware Connections & Schematic

If you are using a Listen interface such as the AmpConnect 621, it is an all-in-one HW device that combines microphone power supply, audio interface, and power amplifier. If not, you many need separate pieces of HW for connecting your microphone and the speaker.

1. Connect the Source: Connect your mouth simulator or source speaker to the Power Amplifier Output. This power amplifier is playing the signal from your audio interface.
2. Connect the Microphone: Connect your calibrated reference microphone to audio interface inputs. If the audio interface by itself doesn't provide the required Bias (Voltage/IEPE) and Gain, you may require a separate mic power supply before you connect to the audio interface.
3. Positioning: Place the reference microphone at the exact test point where your Device Under Test (DUT) will be positioned during normal testing.

2. Prerequisites: Reference Microphone Calibration

Before equalizing a speaker, your reference microphone must be fully calibrated.

• Run the standard Microphone Calibration (Link) using an acoustic mic calibrator to measure the sensitivity.
• If you are using microphone with TEDS, you can automatically pull the sensitivity and serial number directly from the mic. Open Setup > Calibration, select your Reference Mic input path, and click Read TEDS.

3. Setting Up the Calibration Editor

Once your microphone is ready, configure the output path for the mouth simulator:

1. On the SoundCheck Main Screen, go to Setup > Calibration to open the Calibration Editor.
2. Select the Output tab.
3. Select your output signal path (e.g., Mouth Sim or Source Speaker).
4. Set the Output Hardware Channel to the output..
5. In the Output Calibrated Device drop-down, select your device file (e.g., Mouth Simulator.dat).
6. Change the Units field to dB re 20u Pa (dB SPL).
7. In the Calibration Sequence drop-down, select Speaker Equalization.
8. Set the Input Signal Path to your calibrated Reference Mic.

4. Running the Speaker Equalization Sequence

1. Click the Calibrate Device button at the bottom of the Output tab.
2. A prompt will remind you to ensure the microphone is calibrated and positioned up to 1m away. Press Enter to begin.
3. Set the Test Level: You will be prompted to enter a test level in dB SPL. Important: Always calibrate the speaker at the exact level you intend to test your DUT at, because loudspeakers are non-linear and their response changes at different volumes.
4. Set the Frequency Range: Enter the lowest and highest frequencies you wish to equalize.
   • Caution: Do not try to equalize beyond the designed range of the loudspeaker or mouth simulator, as forcing the speaker to produce extreme frequencies can cause damage.
5. Initial Measurement: SoundCheck will play a brief tone to adjust the level, followed by a sine sweep. The initial, un-equalized response of the speaker will be displayed on an X-Y graph. As long as the curve fits within the broad ±20 dB limits, press Enter to continue.
6. Equalized Measurement: SoundCheck will take the reciprocal of that response, apply it as an EQ curve, and run the sweep again. The new response should look significantly flatter, fitting within tighter ±1 dB or ±3 dB limits.
7. Fine-Tuning: A prompt will ask, "Is the EQ'ed Response flat enough?"
   • If there is still some ripple, select No. SoundCheck will run another pass of equalization to "fine-tune" the correction curve. You can run this iterative process until the curve is as flat as possible given your acoustic environment.
   • Once satisfied, select Yes.
8.      
9. The sensitivity and calibration date are automatically updated. Click Save in the Calibration Editor to save the EQ and Correction curves to your system.

5. Applying the Equalization in Your Test Sequence

To actually use this flat response during your measurements:

• In the Stimulus Editor: Open the Stimulus step in your test sequence. Under the advanced view, check the Apply EQ box. This tells SoundCheck to pre-shape the test signal using the eq-out.dat curve before it leaves the interface.
• In the Analysis Editor: Open your Analysis step and ensure Apply Correction Out is checked. The EQ step accounts for the vast majority of the speaker's response, and this correction out mathematically removes any tiny residual variance (the corr-out.dat curve) from the final measurement.

Your speaker/mouth simulator is now calibrated.

If you are new to SoundCheck, we recommend you start with one of our pre-written test sequences in the C:\SoundCheck 23\Sequences folder.