20. November 2017

Let’s Clear Up Some Things About Sweeps

Part 2 of 2: Practical measurements

Let’s Clear Up Some Things About Sweeps

The second part of this article (read part one here) deals with the practical applications of sweep measurements on audio and acoustic systems. In addition to choosing the correct sweep method, it is important to adjust the parameters of the sweep to suit the test object being tested. This article provides some valuable tips.

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.

Frequency response

The measurement was carried out in non-free-field conditions. The resulting reflections appear in the frequency response (green curve) in the form of ripples. These are removed by a sliding curve averaging (red curve). Alternatively, the reflections can also be eliminated by the use of a time window.

Measurements on playback devices
Playback devices are identifiable by the absence of a closed signal path. i.e. It is not possible to play a test signal generated by the audio generator directly into the device while simultaneously reproducing the signal as an output from the device. Without this closed loop, the audio analyzer cannot synchronize with the signal. Typical examples of playback devices are mobile phones or tablet computers, as well as most devices with built-in speakers.

For these devices, the GlideSweep test signal is recorded and pre-loaded on the device as a wav or mp3 file for playback and analysis. To overcome the lack of synchronization between the audio generator and the analyzer, the GlideSweep test signal can be preceded by a short audible trigger, at a fixed interval before the sweep signal, that is recognized by the analyzer.  

When the trigger is detected by the audio analyzer, the measurement starts automatically. Since the duration of the GlideSweep is known, the analyzer is exactly synchronized with the test signal being played.

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.

Mic input

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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Categories: Quality Control