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Design Article

Introduction to the Six Basic Audio Measurements - Part 1

David Mathew, <A HREF="http://ap.com">Audio Precision</A>

11/14/2007 2:41 PM EST

Initial Setup
Initial Setup
Start with the DUT and control software setup instructions listed in Section 2.

Adjust DUT for Unity gain

  • Turn the Generator outputs ON. With the default New Test settings, this will output a 1 kHz sine wave at a level of 1 Vrms.
  • Observe the Level meters on the Analog Analyzer [ATS-2: Analyzer: Audio Analyzer], and slowly increase the DUT volume until you have a reading of about 1 V. Since our input is 1 Vrms, this volume setting produces unity gain.

    • Adjust DUT for 1 W

  • On the Analog Analyzer [ATS-2: Analyzer: Audio Analyzer], drop down the units menu for each of the Level meters, and select the watts units.
  • Observe the Level meters on the Analog Analyzer [ATS-2: Analyzer: Audio Analyzer], and slowly increase the DUT volume until you have a reading of about 1 W.

    • Adjust DUT for 1% THD+N

  • Double-click the title bar of the Analog Analyzer [ATS2: Analyzer: Audio Analyzer] to enlarge the panel, drop down the Function Reading [ATS-2: Measurement Function] menu and select THD+N Ratio.

  • Observe the Function Meter and slowly increase the DUT volume until you have a reading of about 1%. You may find that the distortion jumps suddenly from some ratio just below 1% to a very high ratio of distortion. This is caused by the onset of amplifier clipping. Find the volume that produces the highest distortion that is below 1% (see figure below).

    For our DUT, this output level was about 97 W (about 28 Vrms in 8 Ω). This level is often called Maximum Output Level, or MOL.

    CAUTION: At this level, the amplifier will be producing its greatest undistorted output. Depending upon the design of the DUT and its output rating, this condition may stress the amplifier, and may heat the amplifier heat sinks and the terminating load resistors. Be sure that your load resistors are designed to safely handle the rated output of your DUT, and that they are well ventilated. Leaving the amplifier at its maximum output may damage the amplifier, the load resistors and may be a fire hazard. Generally, you should make maximum output levels tests brief, and be sure you turn the generator OFF and / or the volume control down as soon as you have your measurement.

    Setting a reference testing level for subsequent measurements
    Once you have found a useful level using one of the above methods, you can set it as a reference in the 2700 or ATS-2 memory. To set the current Analyzer input levels as a references, select Edit > Set Analyzer dBr Ref or press F4 in the control software. Separate references are set for analyzer input channels A and B, called dBrA and dBrB. To set the current Generator output level as a reference, select Edit > Set Generator dBr Ref or press F3.

    Since the DUT gain setting affects level, you should note the DUT volume setting as a reference as well.

    Part 2 continues with how to test for frequency response, THD+N, phase, crosstalk, and signal-to-noise ratio (SNR).

    About the author
    David Mathew is technical publications manager at Audio Precision. He is an Emmy-winning sound mixer who can still align a 3M 24-track in his sleep. He enjoys playing electric bass badly.

    Related links:
    Tips and Tricks for Debugging Audio
    Analysis software tests playback-only audio gear
    Testing audio fidelity with absolute dynamic range
    Free audio analysis software
    Audio analyzer delivers faster testing speeds, more channels



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