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.
    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



  • I’m getting a 3dB shelf in signal generator output using this method with the FFT analyzer in Toolbox 3. The shelving frequency is dependent on sample rate, ranging from ~250Hz at 48K to ~1KHz at 192KS/s.

    Is there an explanation somewhere of the magic relation between sample rate, sweep duration, Spectral lines, etc.? There seems to be only one combination that works.

    • Henry,
      If you change the Synthesis Domain in the Swept Sine generator to Frequency, instead of Time, you should see that shelf disappear.