Frequency Response Measurement with SignalScope Pro 3 (Mac)

Measuring the Frequency Response of Your Audio Device

Although SignalScope Pro does not include the Dual FFT Analyzer tool, found in Electroacoustics Toolbox, it is still equipped to perform basic frequency response measurements. This tutorial focuses on using SignalScope Pro’s FFT Analyzer and Signal Generator tools to measure the frequency response of the audio device that you use as an analog interface to measure other systems and devices. If you want to measure the frequency response of a listening room for example, that measurement will be affected by the quality of the audio interface that you are using to make the measurement. Therefore, it is important to know how your measurements will be influenced by your audio interface.

When measuring the properties of some device, like its frequency response, that device is commonly referred to as the “device under test” or DUT. In this case, the DUT is actually the audio device that would normally be used as part of the complete measurement system you use to measure other DUT’s.

Measuring Your Audio Device

  1. Connect your device to your Mac (if necessary, you may want to consult your device’s user guide or owner’s manual).
  2. Using a patch cable that is appropriate for the device you are using, connect one or more outputs of the device to one or more inputs of the same device. Figure 1 demonstrates the connections using an Echo AudioFire4 FireWire interface. It is important to keep in mind that what will be measured in this tutorial is actually the combined frequency response of the input channel, the output channel, and even the patch cable between them.
    AF4
    Figure 1: 1/4″ plug patch cable

     

  3. Launch SignalScope Pro, if necessary.
  4. Create a new project, if one was not created automatically when the program launched.
  5. Click the Device IO button in the project window’s toolbar to open up the Device IO Setup window.
  6. In the Device IO Setup window, click on the name of the device you would like to measure in the Available Devices list. This will display the device’s properties in the lower portion of the window.
  7. Make sure the nominal sample rate is set high enough to capture the desired frequency range. For this tutorial, select 44100 or 48000 Hz.
  8. Select your device from both the Input Device and Output Device popup menus in the project window. If your device does not have both input and output channels, you will need to select one that does, or use the Aggregate Device Editor in the Audio MIDI Setup application to create one. (Audio MIDI Setup is included with Mac OS X in the /Applications/Utilities/ folder.)
  9. Create a new FFT Analyzer tool. This can be accomplished by clicking the “+” button in the FFT Analyzer row of the project toolbox, selecting FFT Analyzer from the Tools menu in the project window’s toolbar, or by selecting New FFT Analyzer from the Tools menu.
  10. Select the Live Inputs tab in the controls drawer at the bottom of the Dual FFT Analyzer window.
  11. In the analyzer table below the Input Device label, the first row will already be pre-filled for Analyzer 1. For Analyzer 1, change the channel in the Input column to match the physical input channel connected in step 2.
    • The name of each analyzer can be edited by double clicking on it within the Channel column of the table, either in the Display tab or in the Live Inputs tab.
  12. If you connected multiple physical input/output channel pairs in step 2, click the “+” button above the analyzer table to add an additional analyzer for each additional channel pair and configure the input channels as in the previous step.
  13. In the FFT tab of the FFT Analyzer’s controls drawer, set the number of spectral lines to a value that will provide the frequency resolution you need. For the purposes of this tutorial, select 4410 spectral lines if your DUT’s sample rate is 44100 Hz or 4800 lines if your sample rate is 48000 Hz.
    • The frequency resolution of your measurement can be determined by the selected frequency span (which is dependent on the sample rate) and the number of spectral lines. You can calculate the frequency resolution by dividing the frequency span by the number of lines (if guardbanding is turned off). For example, if the selected frequency span is 24000 Hz, and the number of lines is 4800, the frequency resolution will be 24000/4800 = 5 Hz. You can also view the current frequency resolution of the analyzer inside the analyzer’s info drawer, which slides out of the right-hand side of the analyzer’s window.
  14. Create a new Signal Generator tool.
  15. Select your DUT in the Output Device popup menu of the Signal Generator’s signal drawer (on the lefthand side of the Signal Generator window).
  16. Select the output channels corresponding to the physical output channels that you connected in step 2. Select the first output channel in the Left Output Channel box, and the second output channel in the Right Output Channel box. If you have connected more than two output channels for a multichannel measurement, you will need to create a new Signal Generator tool for each pair of output channels to be measured.
  17. Click on the Swept Sine (Chirp) tab in the Signal Generator window to display controls for establishing a frequency sweep excitation signal. Configure the swept sine generator as follows:
    • Frequency Sweep: Linear
    • Sweep Direction: Up
    • Lower Frequency: 0
    • Upper Frequency: 22050 or 24000 Hz (The Upper Frequency should be half the selected sample rate, which corresponds to the Nyquist frequency–22050 for 44.1 kHz sampling or 24000 for 48 kHz sampling.)
    • Duration: 8820 samples for 44.1 kHz sampling or 9600 samples for 48 kHz sampling
    • Repeat: Yes
  18. Click the “On” check box to enable the swept sine generator.
  19. Create a new Meter Bridge tool.
  20. Select your device in the Input Device popup menu of the Meter Bridge’s controls drawer (in the Live Inputs tab).
  21. Start the Meter Bridge.
  22. Make sure the Peak level type is selected in the Meter Bridge’s controls, then look to be sure none of the input channels are in danger of clipping (colored red at the top of the meter bar). If any of the input signal levels are too high, reduce the level in the Signal Generator.
  23. Select the FFT Analyzer window again.
  24. Start the FFT Analyzer, either by clicking the start icon in the window’s toolbar, or by selecting Start Analyzer from the Control menu (or by typing Command-R).
  25. Start the generator(s), either by clicking the start icon in the window’s toolbar, or by selecting Start Generator from the Control menu (or by typing Command-R). If you have one or more Signal Generator tools in your project, you can start all of them by selecting Start All Tools from the Control menu.
  26. After everything is running and the measurements have stabilized, you can stop the tools. (If you choose to stop the tools individually, rather than with the Stop All Tools command, it would be best to stop the FFT Analyzer tool(s) first.) Figure 2 shows a plot which shows the frequency response of the Echo AudioFire4. The frequency response of the AudioFire4 is quite flat between 20 Hz and 20 kHz.
    • For more advanced frequency response measurements, including phase response, coherence, group delay, and SNR, or to measure multiple audio devices simultaneously, please consider downloading Electroacoustics Toolbox.
  27. Capture your measurement, either by clicking the capture button in the FFT Analyzer’s toolbar, or by choosing Capture Data from the Control menu.
  28. Save your project so you can review your measurement or export the data at another time.
    Figure 2: Echo AudioFire4 Frequency Response

     

Frequency Response Measurement with Electroacoustics Toolbox 3

Measuring the Frequency Response of Your Audio Device

One of the most powerful tools in Electroacoustics Toolbox is the Dual FFT Analyzer, which is capable of measuring system transfer functions and even indicating the quality of the measurement. This tutorial focuses on using the Dual FFT Analyzer to measure the frequency response of the audio device that you use to measure other systems and devices. If you want to measure the frequency response, or impulse response, of a listening room for example, that measurement will be affected by the quality of the audio interface that you are using to make the measurement. Therefore, it is important to know how your measurements will be influenced by your audio interface.

When measuring the properties of some device, like its frequency response, that device is commonly referred to as the “device under test” or DUT. In this case, the DUT is actually the audio device that would normally be used to measure some other device or system.

Measuring Your Audio Device

  1. Connect your device to your Mac (if necessary, you may want to consult your device’s user guide or owner’s manual).
  2. Using a patch cable that is appropriate for the device you are using, connect one or more outputs of the device to one or more inputs of the same device. Figure 1 demonstrates the connections using an Echo AudioFire4 FireWire interface. It is important to keep in mind that what will be measured in this tutorial is actually the combined frequency response of the input channel, the output channel, and even the patch cable between them.
    AF4
    Figure 1: 1/4″ plug patch cable

     

  3. Launch Electroacoustics Toolbox.
  4. Create a new project if one was not created automatically when the program launched.
  5. Click the Device IO button in the project window’s toolbar to open up the Device IO Setup window.
  6. In the Device IO Setup window, click on the name of the device you would like to measure in the Available Devices list. This will display the device’s properties in the lower portion of the window.
  7. Make sure the nominal sample rate is set high enough to capture the desired frequency range. For this tutorial, select 44100 or 48000 Hz.
  8. Create a new Dual FFT Analyzer tool. This can be accomplished by clicking the “+” button in the Dual FFT Analyzer row of the project toolbox, selecting Dual FFT Analyzer from the Tools menu in the project window’s toolbar, or by selecting New Dual FFT Analyzer from the Tools menu.
  9. Select the Live Inputs tab in the controls drawer at the bottom of the Dual FFT Analyzer window.
  10. Select the DUT (the device you previously configured in step 6) from the Input Device popup menu.
  11. In the measurement table below the Input Device menu, the first row will already be pre-filled for Measurement 1. For Measurement 1, change the channel in the Reference column to match the physical output channel connected in step 2, and change the channel in the Source column to match the input channel connected in step 2.
  12. If you connected multiple physical input/output channel pairs in step 2, click the “+” button above the measurement table to add an additional measurement for each additional channel pair and configure the Reference and Source channels as in the previous step.
  13. In the FFT tab of the Dual FFT Analyzer’s controls drawer, set the number of spectral lines to a value that will provide the frequency resolution you need. For the purposes of this tutorial, select 4410 spectral lines if your DUT’s sample rate is 44100 Hz or 4800 lines if your sample rate is 48000 Hz. The frequency resolution of your measurement can be determined by the selected frequency span (which is dependent on the sample rate) and the number of spectral lines. You can calculate the frequency resolution by dividing the frequency span by the number of lines (if guardbanding is turned off). For example, if the selected frequency span is 24000 Hz, and the number of lines is 4800, the frequency resolution will be 24000/4800 = 5 Hz. You can also view the current frequency resolution of the analyzer inside the analyzer’s info drawer, which slides out of the right-hand side of the analyzer’s window.
    DFFT FFT
    Figure 2: Dual FFT Analyzer FFT Parameters
  14. Click on the Display tab of the Dual FFT Analyzer’s controls drawer to choose which kind of measurement to display. The name of each measurement can be edited by double clicking on it within the Name column of the table, either in the Display tab or in the Live Inputs tab.
  15. From the Function popup menu, select Transfer Function (H1) Mag to measure the magnitude of the DUT’s frequency response. Figure 3 shows the Function configuration for measuring the Echo AudioFire4.
    DFFT Display
    Figure 3: Dual FFT Analyzer Function Selection
  16. If you are only using one output channel of the device, you can select that channel in the output channel popup menu in the Excitation tab of the Dual FFT Analyzer’s controls drawer. Then jump to step 22. Otherwise, follow steps 17 through 21 to configure as many Signal Generators as necessary to measure all the input/output channel pairs connected in step 2.
  17. Create a new Signal Generator tool.
  18. Select your DUT in the Output Device popup menu of the Signal Generator’s signal drawer (on the lefthand side of the Signal Generator window).
  19. Select the output channels corresponding to the physical output channels that you connected in step 2. Select the first output channel in the Left Output Channel box, and the second output channel in the Right Output Channel box. If you have connected more than two output channels for a multichannel measurement, you will need to create a new Signal Generator tool for each pair of output channels to be measured.
  20. Click on the Swept Sine (Chirp) tab in the Signal Generator window to display controls for establishing a frequency sweep excitation signal. Configure the swept sine generator similarly to that shown in Figure 4. The Upper Frequency should be half the selected sample rate, which corresponds to the Nyquist frequency.
  21. Click the “On” check box to enable the swept sine generator.
    SigGen Sweep
    Figure 4: Signal Generator Log Sweep Configuration
  22. Select the Dual FFT Analyzer window again.
  23. Go ahead and save the project now.
  24. Create a new Meter Bridge tool.
  25. Select your device in the Input Device popup menu of the Meter Bridge’s controls drawer (in the Live Inputs tab).
  26. Start the Meter Bridge.
  27. Make sure the Peak level type is selected in the Meter Bridge’s controls, then look to be sure none of the input channels are in danger of clipping (colored red at the top of the meter bar). If any of the input signal levels are too high, reduce the level in the Signal Generator (or the Excitation tab of the Dual FFT Analyzer).
  28. Start the Dual FFT Analyzer, either by clicking the start icon in the window’s toolbar, or by selecting Start Analyzer from the Control menu (or by typing Command-R).
  29. If you have one or more Signal Generator tools in your project, you can start all of them by selecting Start All Tools from the Control menu.
  30. After everything is running and the measurements have stabilized, you can stop the tools. (If you have Signal Generators running and choose to stop the tools individually, it would be best to stop the Dual FFT Analyzer tool first.) Figure 5 shows a plot, created by the Dual FFT Analyzer, which shows the frequency response of the Echo AudioFire4. The frequency response of the AudioFire4 is quite flat between 20 Hz and 20 kHz.
  31. Now that the frequency response magnitude has been measured, other measurements are just a menu selection away. Go back to the Display tab of the Dual FFT Analyzer and take a look at the different functions in the popup menu. All the data necessary to compute the various functions has already been acquired, so there is no need to run the analyzer again to measure the phase response. Once you have measured one of those quantities, you have essentially measured them all. All that’s left to do is change the selection in the popup menu.
  32. Capture your measurement, either by clicking the capture button in the Dual FFT Analyzer’s toolbar, or by choosing Capture Data from the Control menu.
  33. Save your project so you can review your measurement or export the data at another time.
    Figure 5: Echo AudioFire4 Frequency Response

     

Measuring Time Delay with Electroacoustics Toolbox

Measuring Time Delay in an Audio System Using Electroacoustics Toolbox

Although Electroacoustics Toolbox is not limited to measuring audio systems, the measurement of audio systems is a common use that offers convenient ways to demonstrate the capabilities of the software. These examples can then be applied to other applications in which the Toolbox’s measurement capabilities may be considered useful.

At times it is desirable to measure the delay of a signal through some component, or group of components, in an audio system. For example, when a digital signal processor (DSP) is employed in an audio system, to optimize or otherwise alter the listening experience, it may be desirable to measure the delay introduced by the DSP’s algorithms in the audio signal. Alternatively, users may be interested in measuring the difference in time delay as an audio signal passes through each of two different loudspeakers to a reference microphone.
This tutorial requires two free software packages in addition to the Toolbox: Soundflower and AU Lab. AU Lab is distributed with Tiger (Mac OS X, 10.4), although it will run on Panther (Mac OS X, 10.3).
Soundflower is a free system extension for Mac OS X, version 10.2 or later, that allows for convenient routing of audio signals between applications on the Mac. More information regarding Soundflower, as well as a download link may be found at cycling74.com. (Version 1.2 was employed at the time this tutorial was written.)
AU Lab is a digital mixing application, designed as a reference Audio Unit host for those wishing to develop their own Audio Unit plug-ins or Audio Unit hosting applications. AU Lab is provided free of charge with the Apple Developer Tools. After installing the Apple Developer Tools, AU Lab can be found in the directory /Developer/Applications/Audio (that’s the Developer directory at the root level of the hard drive). Documentation for AU Lab should be available from the same directory. (Version 1.0.3 was employed at the time this tutorial was written.)
This tutorial will focus on the configuration and use of Electroacoustics Toolbox to make time delay measurements, rather than on how to tie in to the audio system itself. The reader is encouraged to examine the tutorial “Measuring Audio Unit Effects Plug-ins Using Electroacoustics Toolbox, Soundflower, and AU Lab” since this tutorial starts where that one left off. The Audio Unit tutorial can be downloaded in PDF format, or viewed online in this forum.

AU Lab Configuration
1. Open SFOut12In34.trak, created in the AU Lab tutorial, in AU Lab.
2. In AU Lab, remove all effects from the Audio 2 input track (and any effects that may have been added to other inputs or outputs).
3. In the Audio 2 input track, select the AUSampleDelay effect from the first Effects popup menu. The document window should look like Figure 1.
4. In the Audio Unit interface window, change the delay to 48 samples, as shown in Figure 2.

Figure 1: AU Lab - AUSampleDelay Effect on Audio 2 Input Track

Figure 2: AU Lab - AUSampleDelay Parameters

Measuring Time Delay with Cross Correlation
1. Open SFOut12In34.featproj, created in the AU Lab tutorial in Electroacoustics Toolbox.
2. In the Dual FFT Analyzer, change the Function to Cross Correlation in the Function tab of the controls drawer.
3. In the Function tab of the controls drawer, double click the name of the current measurement to edit it. Change the name to “Sample Delay AU” as shown in Figure 3.

Figure 3: Electroacoustics Toolbox - Dual FFT Analyzer Function Settings

4. In the Cursors tab, select “Sample Delay AU” in both the Horizontal Bar and Peak Track popup menus for Cursor 1, and turn the cursor on, as shown in Figure 4.

Figure 4: Electroacoustics Toolbox - Dual FFT Analyzer Cursor Settings

5. Now, the Dual FFT Analyzer should display a single spike in the correlation at a time offset of 1.0 milliseconds (ms), since the sample rate is nominally 48 kHz and the sample delay was set for 48 samples. The time offset of the peak in the correlation corresponds to the time delay through the system or device under test (DUT). The time offset at the cursor location can be read in the info drawer as the “X” value, as shown in Figure 5.
6. Doubling the sample delay in AU Lab to 96 samples will cause the peak in the cross correlation to move to 2.0 ms, as one would expect. This illustrates how easy it is to measure time delay through a system, in this case a sample delay Audio Unit, using the Cross Correlation function in the Dual FFT Analyzer. One of the keys for such ease in time delay measurements is the use of a spectrally rich excitation signal, or one that has an autocorrelation function approaching a delta function, which, in this case, is a swept sine. The Dual FFT Analyzer’s built-in excitation can also produce random and pseudorandom noise signals with white or pink frequency weightings, each of which would work well for this type of measurement.

Figure 5: Electroacoustics Toolbox - Cross Correlation Time Delay Measurement

Measuring Time Delay with the Impulse Response
A time delay measurement may also be made by measuring the impulse response function of a system or device under test (DUT). Once again, by using an excitation signal with an autocorrelation function characterized by a dominant peak, approaching a delta function, and using the cursor’s peak track capability, a time delay can be measured very easily by the Dual FFT Analyzer.
1. With all else unchanged from the cross correlation measurement, change the function to Impulse Response (H1) in the Function tab of the Dual FFT Analyzer’s controls drawer.
2. Although the display changes to represent an impulse response, the measured time delay is the same. Figure 6 demonstrates the impulse response measurement of the AUSampleDelay Audio Unit with a delay of 96 samples.

Figure 6: Electroacoustics Toolbox - Impulse Response Time Delay Measurement

Measuring Group Delay
Group delay is essentially a frequency dependent measure of time delay. For a linear-phase system, like the sample delay effect being used in this tutorial, the group delay is constant over frequency, and is equal to the overall time delay measured previously. If the phase response of the system is not linear, the group delay at frequency f indicates the amount of time delay experienced by a small group of frequencies, which are centered about f.
1. In the Function Tab, select Group Delay as the current measurement under the Function tab.
2. The group delay, in this case, should appear as a flat line, indicating a delay of 2 ms (assuming the sample rate is still 48 kHz, and the sample delay is set to 96 samples). This is shown in Figure 7.

Figure 7: Electroacoustics Toolbox - Group Delay Measurement

This article was originally published in the forums for an earlier version of Electroacoustics Toolbox. The content still applies to the latest version of the Toolbox.

Measuring Audio Unit Effect Plug-ins With Electroacoustics Toolbox

Measuring Audio Unit Effects Plug-ins Using Electroacoustics Toolbox, Soundflower, and AU Lab

A valuable use of Electroacoustics Toolbox is measuring Audio Unit effects. Measuring Audio Units can be useful to developers of such plug-ins, or to anyone employing Audio Unit effects in their digital audio recording, editing, or reproduction system.

This tutorial requires two free software packages in addition to the Toolbox: Soundflower and AU Lab. AU Lab is distributed with Tiger (Mac OS X, 10.4).

Soundflower is a free system extension for Mac OS X, version 10.2 or later, that allows for convenient routing of audio signals between applications on the Mac. More information regarding Soundflower, as well as a download link may be found at cycling74.com. (Version 1.2 was employed at the time this tutorial was written.)

AU Lab is a digital mixing application, designed as a reference Audio Unit host for those wishing to develop their own Audio Unit plug-ins or Audio Unit hosting applications. AU Lab is provided free of charge with the Apple Developer Tools. After installing the Apple Developer Tools, AU Lab can be found in the directory /Developer/Applications/Audio (that’s the Developer directory at the root level of the hard drive). Documentation for AU Lab should be available from the same directory. (Version 1.0.3 was employed at the time this tutorial was written.)
The rest of this tutorial assumes that Soundflower and AU Lab are installed and ready for use. Please refer to the Soundflower, AU Lab, or Electroacoustics Toolbox documentation for further assistance with each program.

Configuring AU Lab
1. Launch AU Lab.
2. If the Create New Document window appears, continue to step 3. If not, select New from the File menu, or type Command-N.
3. In the Audio Device popup menu, select Soundflower (16ch).
4. In the Outputs tab, change the Output Channels popup menu selection to Mono.
5. Click the Add Output button.
6. Place the two mono output channels under channels 1 and 2.
7. Make sure the output settings look like those in Figure 1.

Figure 1: AU Lab - Creating New Output Channels

8. Click on the Inputs tab to view the Audio Input Settings.
9. Click the Add Input button twice to create two new input channels.
10. Drag the two red squares so that both of the two input channels appear under channel 3.

Figure 2: AU Lab - Creating New Input Channels

11. Click OK to create the new AU Lab document.
12. In the AU Lab document window, click the small “1” and “2” buttons under Volume in the Audio 2 input track. This will remove Audio 2 from Master Output 1 and add it to Master Output 2.
13. Go ahead and save the new AU Lab document by selecting Save from the File menu, or typing Command-S. Save it with the name SFOut12In34.trak for easy reference in other Electroacoustics Toolbox tutorials.

Figure 3: AU Lab - Document Window

Configuring Electroacoustics Toolbox
1. Launch Electroacoustics Toolbox.
2. Create a new project if one was not created automatically when the program launched.
3. Click the Device IO button in the project window toolbar to open up the Device IO Setup window.
4. In the Device Setup window, click on Soundflower (16ch) in the Available Devices list. This will display the Soundflower (16ch) properties in the lower portion of the window.
5. Make sure the Soundlfower (16c h) nominal sample rate is set to 48 kHz.
6. Create a new Dual FFT Analyzer tool. This can be accomplished by clicking the “Add” button () in the Dual FFT Analyzer row of the project toolbox, selecting Dual FFT Analyzer from the Tools menu in the project window’s toolbar, or by selecting New Dual FFT Analyzer from the Tools menu (or by typing Command-1).
7. Select Soundflower (16ch) from the Input Device popup menu in the signal drawer of the Dual FFT Analyzer.
8. Select Soundflower (16ch) channels 1 and 2 in the Live Data Sources box (they will probably already be selected). Recall that Soundflower (16ch) channels 1 and 2 were chosen as output channels in the AU Lab document. This will allow AU Lab to route signals directly to the Dual FFT Analyzer’s input channels via Soundflower.
9. In the Excitation tab of the Dual FFT Analyzer’s controls drawer, select output channel 3 of the Soundflower device in the Output Channel popup menu.
10. Start the analyzer, either by clicking the start icon in the window’s toolbar, or by selecting Toggle Tool On/Off from the Control menu (or by typing Command-R).
11. At this point, a flat line should be visible in the Dual FFT Analyzer’s display, since the default measurement is Transfer Function Magnitude and AU Lab is just sending the two (identical) signals back to Electroacoustics Toolbox without altering them in any way.
12. Go ahead and save the Electroacoustics Toolbox project by selecting Save Project from the File menu, or typing Command-S. Save it with the name SFOut12In34.featproj for easy reference in other Electroacoustics Toolbox tutorials.

Figure 4: Electroacoustics Toolbox – Signal Generator 's Swept Sine Configuration

Measuring an Audio Unit Effect
Now that AU Lab and Electroacoustics Toolbox are both configured for the measurement, measuring any available Audio Unit effect is quite simple.
1. In AU Lab’s Audio 2 track (in the SFOut12In34 document created previously), select the desired Audio Unit effect from the first Effects popup menu. The document window should look like Figure 5.
2. As the effect’s parameters are adjusted, the changes will be immediately reflected in the Dual FFT Analyzer. As an example, Figure 6 includes a screenshot of the AUGraphicEQ effect being measured by Electroacoustics Toolbox.

Figure 5: AU Lab -- Final Document Window Configuration

Figure 6: Measuring the Frequency Response of the Graphic EQ

This article was originally published in the forums for an earlier version of Electroacoustics Toolbox. The content still applies to the latest version of the Toolbox.

Audio Interface Frequency Response Measurement with Electroacoustics Toolbox

This article was originally published in the forums for version 2 of Electroacoustics Toolbox. Please visit the following link for an updated tutorial for version 3.

Frequency Response Measurement with Electroacoustics Toolbox 3

Measuring the Frequency Response of Your Audio Device

One of the most powerful tools in Electroacoustics Toolbox is the Dual FFT Analyzer, which is capable of measuring system transfer functions and even indicating the quality of the measurement. This tutorial focuses on using the Dual FFT Analyzer to measure the frequency response of the audio device that you use to measure other systems and devices. If you want to measure the frequency response, or impulse response, of a listening room for example, that measurement will be affected by the quality of the audio interface that you are using to make the measurement. Therefore, it is important to know how your measurements will be influenced by your audio interface.

When measuring the properties of some device, like its frequency response, that device is commonly referred to as the “device under test” or DUT. In this case, the DUT is actually the audio device that would normally be used to measure some other device or system.

Measuring Your Audio Device

  1. Connect your device to your Mac (if necessary, you may want to consult your device’s user guide or owner’s manual).
  2. Using a patch cable that is appropriate for the device you are using, connect one or more outputs of the device to one or more inputs of the same device. Figures 1 and 2 demonstrate the connections using an Echo Indigo io PCMCIA card interface and an Echo AudioFire4 FireWire interface, respectively. It is important to keep in mind that what will be measured in this tutorial is actually the combined frequency response of the input channel, the output channel, and even the patch cable between them.
    Figure 1: Stereo mini-plug patch cable

    Figure 1: Stereo mini-plug patch cable

    AF4

    Figure 2: 1/4″ plug patch cable

  3. Launch Electroacoustics Toolbox.
  4. Create a new project if one was not created automatically when the program launched.
  5. Click the Device IO button in the project window’s toolbar to open up the Device IO Setup window.
  6. In the Device IO Setup window, click on the name of the device you would like to measure in the Available Devices list. This will display the device’s properties in the lower portion of the window.
  7. Make sure the nominal sample rate is set high enough to capture the desired frequency range.
  8. Create a new Dual FFT Analyzer tool. This can be accomplished by clicking the “Add” button () in the Dual FFT Analyzer row of the project toolbox, selecting Dual FFT Analyzer from the Tools menu in the project window’s toolbar, or by selecting New Dual FFT Analyzer from the Tools menu.
  9. Select the DUT from the Input Device popup menu in the signal drawer of the Dual FFT Analyzer.
  10. In the Live Data Sources box, select the input and output channels corresponding to the physical channels connected in step 2. (Hold down the Command/Apple key to select multiple non-adjacent channels.)
  11. In the FFT tab of the Dual FFT Analyzer’s controls drawer, set the number of spectral lines to a value that will provide the frequency resolution you need. The frequency resolution of your measurement can be determined by the selected frequency span (which is dependent on the sample rate) and the number of spectral lines. You can calculate the frequency resolution by dividing the frequency span by the number of lines (if guardbanding is turned off). For example, if the selected frequency span is 24000 Hz, and the number of lines is 6000, the frequency resolution will be 24000/6000 = 4 Hz. You can also view the current frequency resolution of the analyzer inside the analyzer’s info drawer, which slides out of the right-hand side of the analyzer’s window.

    Figure 3: Dual FFT Analyzer FFT Parameters

  12. Click on the Function tab of the Dual FFT Analyzer’s controls drawer to set up the measurement. The number of individual measurements that appear in the Function table will be one less than the number of channels selected in the Live Data Sources box of the signal drawer (unless only 1 channel is selected, in which case they will be equal). For each input/output channel pair that is connected by a patch cable, the output channel should be selected in the Reference popup menu, and the input channel should be selected in the Source popup menu.
  13. The name of each measurement can be edited by double clicking on it within the Name column of the Function table.
  14. From the Function popup menu, select Transfer Function (H1) Mag to measure the magnitude of the DUT’s frequency response. Figure 4 shows the Function configuration for measuring the Echo AudioFire4.

    Figure 4: Dual FFT Analyzer Function Configuration

  15. If you are only using one output channel of the device, you can select that channel in the output channel popup menu in the Excitation tab of the Dual FFT Analyzer’s controls drawer. Then jump to step 21. Otherwise, follow steps 16 through 20 to configure as many Signal Generators as necessary to measure all the desired channels.
  16. Create a new Signal Generator tool.
  17. Select your DUT in the Output Device popup menu of the Signal Generator’s signal drawer.
  18. Select the output channels corresponding to the physical output channels that you connected in step 2. Select the first output channel in the Left Output Channel box, and the second output channel in the Right Output Channel box. If you have connected more than two output channels for a multichannel measurement, you will need to create a new Signal Generator tool for each pair of output channels to be measured.
  19. Click on the Swept Sine (Chirp) tab in the Signal Generator window to display controls for establishing a frequency sweep excitation signal. Configure the swept sine generator similarly to that shown in Figure 5. The Upper Frequency should be half the selected sample rate, which corresponds to the Nyquist frequency.
  20. Start the generator, either by clicking the start icon in the window’s toolbar, or by selecting Toggle Tool On/Off from the Control menu (or by typing Command-R).

    Figure 5: Signal Generator Log Sweep Configuration

  21. Go ahead and save the project now.
  22. Create a new Meter Bridge tool.
  23. Select your DUT in the Input Device popup menu of the Meter Bridge’s signal drawer.
  24. Start the Meter Bridge.
  25. Make sure the Peak level type is selected in the Meter Bridge’s controls, then look to be sure none of the input channels are in danger of clipping (colored red at the top of the meter bar). If any of the input signal levels are too high, reduce the level in the Signal Generator (or the Excitation tab of the Dual FFT Analyzer).
  26. Start the Dual FFT Analyzer and your measurement will be underway. After a few seconds, the measured curve will stabilize and you can stop the analyzer. Figure 6 shows a plot, created by the Dual FFT Analyzer, which shows the frequency response of the Echo AudioFire4. The frequency response of the AudioFire4 is quite flat between 20 Hz and 20 kHz.
  27. Now that the frequency response magnitude has been measured, other measurements are just a menu selection away. Go back to the Function tab of the Dual FFT Analyzer and take a look at the different functions in the popup menu. All the data necessary to compute the various functions has already been acquired, so there is no need to run the analyzer again to measure the phase response. Once you have measured one of those quantities, you have essentially measured them all. All that’s left to do is change the selection in the popup menu.
  28. Capture your measurement, either by clicking the capture button in the Dual FFT Analyzer’s toolbar, or by choosing Capture Data from the Control menu.
  29. Save your project so you can review your measurement or export the data at another time.

    Figure 6: Echo AudioFire4 Frequency Response