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

     

Are you looking for a measurement microphone for your iPhone?

Since iOS 6 finally remedied the low frequency roll-off problem of the headset mic input of the iPhone, iPod touch, and iPad, the headset jack has become a suitable option for measurement microphone input. Prior to the release of iOS 6, the only way to connect an external measurement microphone, without sacrificing low-frequency information, was to go through the dock connector. Dock connector devices can still provide higher quality solutions, but working with the headset jack offers a level of portability (i.e. compact size) that cannot be matched when a 30-pin dock connector is involved (we’ll see what comes along to take advantage of the new Lightning connector).

MicW i436

Some time ago, I was made aware of the i436 measurement microphone from MicW. It looked like exactly what was needed to turn any iOS device into a quality sound level meter, or acoustical analysis tool, that you could truly carry around in your pocket. However, it was limited in its utility by that pesky low-end roll-off that plagued earlier versions of iOS. Some developers attempted to perform software correction for the input filters, but when an input signal is driven into the noise floor by the hardware (or firmware), there’s nothing app software can do to restore the lost signal at those lower frequencies. Now that iOS 6 has solved the issue, the i436 has become the attractive measurement microphone solution for iOS that it should have been when it was first introduced.

i436i436

Last weekend, I carried the i436 around the Denver Tech Center Marriott at the Rocky Mountain Audio Fest (RMAF), along with my iPhone 5. The i436 is small enough that it could slip into my pocket and remain there unnoticed or it could stay connected to the iPhone, which was perched in my shirt pocket when it wasn’t in my hand. In short, I was quite pleased to confirm that the i436 does indeed make for a portable measurement solution that you can carry in your pocket all day long.

MicW i436 Noise Measurement

As for quality, the i436 looks and feels like a proper measurement microphone. It was designed to meet the Class 2 standard for sound level meters, which addresses issues like environmental stability in addition to frequency response. The i436 also fits a standard microphone field calibrator, with a 1/4″ adapter, which you would also expect from a measurement microphone. A field calibrator makes microphone sensitivity calibration very easy with measurement software like SoundMeter or SignalScope Pro.

i436 Typical Frequency Responsei436 Typical Polar Pattern

The i436 is available in a package with just the mic, or in a kit. The kit includes a wind screen, extension cable, splitter cable (to connect headphones or an audio cable to the headphone output), a small clip, and an aluminum storage tube that doubles as a holder for the i436 that mounts to the top of a standard microphone stand (very handy).

i436 Single Packagei436 Kit Package

If Class 2 compliance meets your needs, then I highly recommend the MicW i436, especially in the kit. Either option is quite affordable for a quality measurement microphone. If you need a microphone that conforms to the Class 1 standard, then another hardware solution will be necessary.

Finally! iOS 6 kills the filter on headset and mic inputs!

So, iOS 6 has finally arrived and the biggest news for SoundMeter, SignalScope, and SignalScope Pro users may just be that the high-pass filter which used to plague the built-in microphone and headset microphone inputs now gets bypassed. This exciting improvement to iOS 6 will significantly improve the quality of acoustical measurements that can be made with the iPhone, iPad, or iPod touch, without requiring a dock connector accessory for audio input.

Below are 1/3-octave headset input frequency response comparisons for the various iOS devices that support iOS 6. More details regarding the headset input and built-in microphone will be presented in the days ahead.

Update (9/19/20120, 7:32 PM): It should be noted that these are electrical frequency response measurements. When making acoustical measurements, the overall frequency response will depend also on the microphone that is used.

iPhone 4S Before iOS 6/Now

iPhone 4S Headset Leq Pre-iOS 6  iPhone 4S Headset Leq iOS 6

iPhone 4 Before iOS 6/Now

iPhone 4 Headset Leq Pre-iOS 6  iPhone 4 Headset Leq iOS 6

iPhone 3GS Before iOS 6/Now

iPhone 3GS Headset Leq Pre-iOS 6  iPhone 3GS Headset Leq iOS 6

iPod touch 4 Before iOS 6/Now

iPod touch 4 Headset Leq Pre-iOS 6  iPod touch 4 Headset Leq iOS 6

iPad 3 Before iOS 6/Now

iPad 3 Headset Leq Pre-iOS 6  iPad 3 Headset Leq iOS 6

iPad 2 Before iOS 6/Now

iPad 2 Headset Leq Pre-iOS 6  iPad 2 Headset Leq iOS 6

 

iOS 6 is now highly recommended for all SoundMeter, SignalScope, and SignalScope Pro users.

 

 

Stereo 1/3-octave analysis now available in SignalScope Pro

With the release of version 2.2, the Octave tool in SignalScope Pro for iOS can now analyze two input channels simultaneously. This new capability is only available on devices running iOS 5 or later, and it requires a stereo or two-channel dock connector audio accessory.

     

The Octave tool offers whole and 1/3-octave real-time spectral analysis with A, C, or flat frequency weighting and fast, slow, or impulse time weighting. Time exponential averaged level (Lp) and equivalent level (Leq) measurements are both supported.

The Octave tool available via in-app purchase in SoundMeter 3.0 and SignalScope 3.2 also offers two-channel measurement capability on iOS 5 or later. SoundMeter 3.0, SignalScope 3.2 and SignalScope Pro 2.2 are all available for immediate download on the App Store.

Improvements in SignalScope 3.2 and SignalScope Pro 2.2 include:

  • For users running iOS 5, or later, the Octave analyzer tool can now be operated as a stereo RTA, analyzing two input channels simultaneously (with compatible input hardware). (The Octave tool is available within SignalScope via in-app purchase.)
  • Data can now be exported to CSV files from the FFT, Octave, and Oscope tools. CSV files can be opened directly in Numbers, or other spreadsheet apps that support it, from within SignalScope. Tab-delimited text files can still be created by giving the file name a .txt extension. (Data file export requires the data acquisition upgrade available within SignalScope via in-app purchase.)
  • Corrects a potential crash when accessing the image picker on iPad.
  • SignalScope uses less memory on iPhone and iPod touch.
  • Twitter support has been restored (in iOS 5) for tweeting level meter data. (The Level Meter tool is available within SignalScope via in-app purchase.)

SignalScope Pro and friends offer enhanced iOS hardware support

SignalScope Pro 2.1, SignalScope 3.1, and SoundMeter 2.1, which are now available for download on the app store, feature better support for audio input hardware, including direct interaction with GuitarJack (models 1 and 2) from Sonoma Wire Works.

  • Each app offers enhanced support for audio accessories, connected via the 30-pin dock connector.
  • Input gain adjustment is available for any audio input hardware that supports it.
  • Software-selectable options for the GuitarJack and GuitarJack model 2, from Sonoma Wire Works, may be adjusted directly from within each app.
  • Each app now supports multi-channel USB audio devices, connected to any iPad model via the iPad Camera Connection Kit.
  • Default input sensitivities are now managed for each iOS device.
  • iPad retina display resolution is now fully supported.
  • SignalScope and SoundMeter also now support viewing saved PDF images within the app, in addition to retrieving them via iTunes File Sharing on a Mac or PC.

 

SignalScope Pro 3.0 is in the Mac App Store

SignalScope Pro 3.0 for Mac OS 10.6 is now available for download on the Mac App Store.

Faber Acoustical has announced the release of SignalScope Pro 3.0. The latest release of Faber Acoustical’s most popular acoustical test and measurement toolset for Mac is now available for purchase in the Mac App Store. Version 3 of SignalScope Pro features an enhanced user interface, better control over audio device configurations, and more comprehensive user preferences.

SignalScope Pro offers a suite of tools on the Mac for dynamic and real-time signal analysis, data acquisition, and automated testing. SignalScope Pro was designed for researchers, educators, consultants, and advanced hobbyists. Despite its power and flexibility, SignalScope Pro provides an intuitive user interface that enables even complex measurements to be configured within a few minutes. Those measurement configurations can then be saved and reloaded when the need arises.

 

New functionality in SignalScope Pro includes the option to perform arithmetic operations, such as addition, subtraction, and multiplication, on two arbitrary input channels of an audio input device. Users can also specify arbitrary FFT lengths and view live spectrogram data in 3D with a logarithmic frequency scale. Two of each type of analysis tool can be opened and operated simultaneously.

Minimum Requirements:
* Intel-based Mac
* Requires Mac OS 10.6 or later

Pricing and Availability:
SignalScope Pro 3.0 is now available for download on the Mac App Store for $199.99 (USD) in the Utilities category. Those who wish to try SignalScope Pro before they buy are welcome to download a free 30-day trial. SignalScope Pro 2 users can upgrade to version 3 for $99 at our online store.

Learn more about SignalScope Pro 3.0

Download SignalScope Pro from the Mac App Store

Download SignalScope Pro 3 trial

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