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100 Things #35: Automate Measurements Using M-Noise with SoundCheck

The AES75 standard details a method for measuring maximum linear sound levels of a loudspeaker system or driver using M-Noise, a test signal specifically developed to emulate the dynamic characteristics of music. Our pre-written test sequence automates this measurement to the new AES 75 standard for Maximum SPL, removing subjectivity, increasing reliability, and saving time.

Automated AES75 Measurements in SoundCheck

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Video Script: Automate Measurements Using M-Noise with SoundCheck

Our pre-written test sequence automates measurement to the new AES 75 standard for Maximum SPL, removing subjectivity, increasing reliability, and saving time.

This standard details a method for measuring maximum linear sound levels of a loudspeaker system or driver using M-Noise, a test signal specifically developed to emulate the dynamic characteristics of music. Clearly defined limits for linear frequency response and coherence determine the Max SPL level and remove any measurement ambiguity.

Implementing the standard manually relies on an operator’s subjective judgment of real time spectrum analyzer data. It also requires multiple iterations of a measurement, therefore is labor intensive.

Our test sequence is fully automated. We use the same test signal and calculations outlined in the standard, but automated analysis steps objectively calculate the measurements and drive the next steps in the procedure.

Let’s take a look.

Here, you can see we are using the freely available M-Noise test signal introduced by Meyer Sound. This stimulus features a relatively constant peak level as a function of frequency, but a diminishing RMS level with increasing frequency.

First, we use a test signal approximately 20dB below our expected Max SPL to obtain a provisional linear frequency response, linearity, coherence and signal to noise ratio. We then increase the test level by 3dB, and compare the results to the initial value. The results must be within +/- 1dB, have a coherence of at least 97% and a signal to noise ratio 15dB or higher so that we know we are operating in the speaker’s linear region and our signal to noise ratio is sufficient for accurate measurements.

Next we automatically increase the test level by 3dB and compare it to the initial results, normalized to the current test level. Multiple measurement iterations take place until one of the ‘stop’ conditions is reached. These conditions are either:

  • the live measurement differs from the linear frequency response by at least 2 dB over at least two octaves
  • the live measurement differs from the linear frequency response by at least 3 dB anywhere, or
  • the Coherence Reduction Target is met –  this means the signal to noise ratio is 10dB or less and/or the coherence is 91% or less.

When one of these limits is reached, the sequence then reduces the test level to the last level that passed and repeats the measurements in 1dB increments to find the precise Level at which the response deviates from the base level.

Once this level is established, the device enters a burn-in process, where the M Noise stimulus is played through the DUT for five minutes and fifty-three seconds and again compared to the initial result. This long duration measurement is also used to generate peak, rms and A weighted RMS sound levels. If the response curve remains consistent, this curve is the Max SPL curve according to the standard. If it is not within the acceptable limits, the device is cooled down and the tests repeated with a longer stimulus duration.

All the operator needs to do is enter any stimulus limits based on the operating range of the DUT into the sequence before starting, then return when the test is complete.

As well as saving time, SoundCheck mathematically calculates the data in analysis steps within the sequence, which avoids the subjectivity of relying on operator interpretation of real-time spectrum analyzer outputs. This increases repeatability and confidence in the results. This sequence is available free of charge on our website. Check it out!