20. Nov. 2017
Audio and acoustic devices and systems are operated throughout the audible range and beyond. This range extends over the two dimensions of frequency and level. Accordingly, measurements on audio and acoustic systems must measure and evaluate the relevant parameters in these dimensions. The basic measurements used are sweep measurements.
For a meaningful and accurate measurement, the most important sweep parameters to be defined are the start and stop values for the frequency or amplitude as well as the steps (number of desired measuring points) or the duration of the measurement (in the case of a GlideSweep).
Measurements on acoustic components
For sweep measurements on acoustic components, such as loudspeakers, several items have to be considered. Apart from the acoustic frequency response in dBSPL (left), the impedance frequency response (right) may also be interesting. The start and stop frequencies must therefore be selected so that not only the specified response bandwidth of the speaker is covered, but that the sweep also begins far enough below the resonance frequency.
The duration of the GlideSweep depends on the start frequency. The lower the frequency, the longer the electromechanical system has to be excited. In the following measurement of a midrange speaker, 1.5 seconds of signal duration was used for a measurement bandwidth of 20 Hz - 20 kHz.
This video shows how this measurement procedure is applied to a smartphone speaker analysis.
Measurements on audio devices
These are devices with analog or digital audio inputs and outputs, e.g. Amplifier or mixing console. A closed-loop input and output connection to the audio generator and analyzer is possible. This example measures a microphone input stage of an audio mixer. Amplitude and distortion across the audible frequency range are measured with a 500 ms fast GlideSweep of 20 Hz - 20 kHz. The output level and distortion as a function of the input level is measured with an amplitude sweep in the range of -100 dBV to -20 dBV.
Particular attention is needed on systems with dynamic control behavior, such as Automatic Gain Control (AGC) or level limiter. In this case, to ensure that the system under test is in a stable state before the actual measurement, the sweep signal is prefixed with a "pre-tone" with, for example, 1 second duration.
Read the first part of the article.
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