Incomplete Loudspeaker Stepped Frequency Sweep Measurements

Created on 2009-02-12 15:00:00

Issue

I am testing the frequency response of a loudspeaker using the stepped frequency sweep measurement on an APx520. The measurement doesn’t return all frequency points, i.e., there is no data at several of the frequencies included in the sweep. What is wrong?

Here are the measurement details: The loudspeaker is in ananechoic chamber and the response is being measured with a measurement microphone connected to Analog Input Ch1 of the APx520. The loudspeaker amplifier is being driven with a CD player using the track Slow_freq_sweep_61_0dB(Track #29) from the APx CD1 Audio Test Signals CD. I have used the Measurement Recorder to verify that the test signal is correct.

Resolution

In the configuration you described above, the measurement is probably missing points because you are conducting the test in open-loop mode. Using an external signal such as a CD player to drive a DUT is typically referred to as open-loop testing. When the audio analyzer’s internal generator is used to provide the stimulus signal, the test is referred to as closed-loop.

The Stepped Frequency Sweep measurement in APx500 is much faster and won’t miss points when it is run in closed-loop mode, because the analysis is internally synchronized to the generator. In contrast, when the Stepped Frequency Sweep is run in open-loop mode, the instrument has to determine the onset of each frequency step by analyzing the measured response signal. In the case of a loudspeaker, the acoustic response signal may be distorted at some frequencies,or it could vary significantly in level from step to step. As a result, the APx probably can not discern all the frequency steps. Therefore, some frequencies are essentially missed and no data is returned at those steps.

To resolve this issue, we recommend that you test the loudspeaker in closed-loop mode. In addition to eliminating possible data dropouts, the test will be faster overall, because the analyzer knows the frequency it is generating and does not need to determine it for each step.

If, for some reason, you must conduct the test in open-loop mode, we recommend switching the microphone signal to Input Channel 2, and connecting one channel of the CD player to Input Channel 1. Since the analyzer uses Channel 1 to synchronize the measurement, it will be able to use the clean,undistorted signal direct from the CD player, while measuring the loudspeaker response on Channel 2. You can even remove Channel 1 from the graph display by unchecking its check box. This will still not be as fast as a closed-loop test,but it should return data for all frequency steps in the sweep.