APx500 has sequence automation tools that allow us to characterize an acoustic setup with minimal operator interaction. Before performing acoustic measurements of a microphone, the measurement equipment must be characterized. The power amplifier, loudspeaker, and room all contribute to the acoustic signal measured. To accurately measure the microphone, we must characterize these factors using a reference mic with a known flat response. Once we have this response, we can use the measurement EQ feature in APx500 to compensate the output, creating a flat response. This process has been explored in detail in Technote 127: Leveled Acoustic Output.
In this short article, we will discuss some of the sequence automated tools built into APx500 that allow us to automate this process, only requiring the operator to step in to physically manipulate the setup.
The hardware required to perform leveled acoustic measurements of a mic are as follows:
a. An APx Analyzer and APx500 v4.2 or later
b. A power amplifier to drive the loudspeaker (we used an APx1701)
c. A loudspeaker with relatively low distortion
d. A small acoustic chamber, or a room
e. A reference mic with a known flat response, and accompanying power supply (we used a 378M31 1/2″ Measurement Microphone System and drove it with the CCP Power option of the APx1701)
f. A sound level calibrator or pistonphone (we used a Larson Davis CAL200)
1. Create a new approjx file. Set the generator to drive the speaker. Since we are using an APx1701 Transducer Interface, we set the output to Transducer Interface.
2. Check the Acoustic option, to enable generation in acoustic units (dBSPL and Pa). Note that at this point, you will receive the following warning:
Figure 1: Warning before setting the output Voltage Ratio
This is because we have not set the acoustic sensitivity of the monitor. We will do this in step 6.
3. Set the analyzer to measure the microphone. Since we were powering the mic with a 1701, we set this to Transducer Interface.
4. Check the Acoustic option, to enable analysis in acoustics units.
5. Click Signal Path1 in the sequence navigator and hit Ctrl+C then Ctrl+V to copy and paste the signal path. We now have two identical signal paths. We will use the second signal path to implement measurement EQ, giving us a flat response.
6. In the first Signal Path Setup, expand out the Sequence Steps, and click Add Step. Add a Prompt… and the Prompt Step dialog box will open. In this step, we will ask the operator to couple the microphone to the calibrator which will allow us to set the sensitivity of the microphone. Make sure that the message is appropriately detailed for the physical setup, then check the options Show OK Button and Set dBPSL. Optionally, check the Timeout (sec) option:
Figure 2: Prompt to ask the operator to calibrate the reference mic
7. Add a second Prompt sequence step after the first. Here we will ask the operator to place the mic in the test position (in this case, inside the acoustic test fixture). Only check the Show OK Button option:
Figure 3: Prompt the operator to put the mic in testing position
8. We can now add an Acoustic Reponse measurement. In this measurement, we will perform the Relative Level measurement and export the result.
In the Sequence Steps for this measurement, add an Export Result Data… sequence step. Make sure it is after the Measure Acoustic Response step. Configure it to export the Relative Level result from the Measured 1 dataset. Be sure to set the Data Specification to All Points. This will ensure the data is not decimated upon export. In the To File field, you can hard-code a location, or select Use Variables… . This opens the Edit Text and Use variables dialog box, which allows you to use a variety of variables to define the path. For example, you can export to the $(Temp) location, which will export to the temp folder of the currently logged-in Windows user.
The advantage of this is that it will work regardless of the PC being used. In this example, we will use the $(Temp) variable, and the file name FR1.xls. This will export an Excel file to the same directory as the approjx file’s location:
Figure 4: Configure the Relative Level result to export to the project directory using variables
9. Go to the second signal path and navigate to the Signal Path Setup. We will apply the response measured in step 4, invert it, and use it to generate a leveled response.
In the Sequence Steps, add an Import Output EQ Curve step. This will allow us to specify the FR1.xls file from the last measurement. Use the same path used to export the data. The use of variables allows us to set this once, and transport this project file to multiple computers. Be sure to check Invert Data.
Figure 5: Sequence step to import the EQ from the last measurement, using variables to reference the file
10. We have now created a project file that walks the operator through the leveling process. If you run this sequence, you will observe that the RMS Level of the second Acoustic Response measurement is flatter than in the first measurement. We can now measure the microphone being tested, and use the same generator EQ file. Please see the attached project file for an associated example.