Since the adoption and publication in March 2022 of the AES75-2022 "Standard for Acoustics - Measuring Loudspeaker Maximum Linear Sound Levels Using Noise,” multiple manufacturers have been embracing the use of what was originally the M-Noise test signal used in music reinforcement applications. The AES75 test standard relies on multiple iterations of the same measurement at different levels to identify where the frequency response linearity, compression and coherence deviate from baseline levels.
When implemented manually, the user starts each individual measurement and subjectively compares responses to determine the next action. Listen’s test sequence uses the same test method and stimulus, but carries out the entire test automatically, from initial baseline measurement to final burn-in analysis. The SoundCheck software objectively analyzes and compares each response signal using automated analysis steps and uses logic to drive the subsequent measurements in the procedure. As well as saving time, this ensures more consistent results as it overcomes the subjectivity introduced in manual operation where the operator must interpret and compare real time spectrum analyzer outputs.
The test sequence uses the M-Noise test signal, a stimulus specifically developed to emulate the dynamic characteristics of music. It begins by verifying that the test is starting in the speaker’s linear region by using test signals well below the expected Max SPL to compare 2 baseline measurements of the speaker’s frequency response, linearity, coherence and signal to noise ratios.
The test level is then sequentially incremented by 3dB. At each level, the response is compared to the baseline level until certain ‘stop conditions’ - differences in frequency response linearity, compression and coherence - are met. The level is then reduced to the last passing level and increased in 1dB increments to find the precise level at which the response deviates from the base level.
The device then enters a burn-in phase, where the stimulus is played for several minutes and again compared to the initial result. This longer measurement also generates peak, RMS and A weighted RMS sound levels. If this response curve is within defined limits, it is recorded as the Max SPL curve determined by the standard; if not, then the device is cooled down and the tests repeated with a longer stimulus duration.
The only operator inputs required are the initial test level and the frequency range over which the measurements should be made. The frequency range is included to ensure that the sequence is flexible for all products, but the user can easily adapt the sequence to already include this information, if preferred. It should be noted that to run this sequence, SoundCheck Plus Version 20 or newer with the Waveform Filter and Transfer function modules is needed.
The sequence can be downloaded here: www.listeninc.com/AES75
In a related announcement, the new Portland Tool & Die MDT-4000 audio measurement turntable, is now available exclusively from Listen and is fully controllable from within SoundCheck. Designed specifically for audio measurements, the MDT-4000 is one of the most accurate and feature-filled turntables available, and packs a host of usability and safety features, along with high precision and low maintenance in a compact and lightweight package. With flexible control options (USB, RS-232, LAN) it fits easily into any audio test setup and it supports the LinearX LT-360 command set, making it a drop-in replacement for this much-loved, but now discontinued product.
The turntable has a built in level, along with 4 adjustable feet, features clearly visible degree markings for quick visual confirmation of position and accessories alignment, offers a variety of mounting options, and retains position memory across power cycles. The turntable variable torque enables slower rotation speeds to be used for quieter operation when measuring during rotation - ideal for audio applications.
Competitively priced, the MDT-4000 supports weights of up to 100kg in either compression or inverted orientation, making it suitable for the majority of audio applications.
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