Max SPL vs Frequency Sequence
This sequence measures the Max SPL of a transducer versus frequency that a device can play back with acceptable distortion
This sequence measures the Max SPL of a transducer versus frequency that a device can play back with acceptable distortion. It is particularly valuable for designers using DSP algorithms to optimize the performance of their speakers.
It characterizes the Max SPL of a transducer by setting limits on specific metrics (THD, Rub & Buzz, Perceptual Rub & Buzz, Input Voltage and Compression) and then driving the transducer at a series of standard ISO frequencies, increasing the stimulus level until the one of the limits is surpassed. The sequence begins by measuring the frequency response and impedance of the DUT. The user is asked if they wish to use the -3dB from resonance frequency as the test Start Frequency or manually enter another value. The user is then prompted to enter a Stop Frequency, initial test level and limit values for the metrics of interest. The sequence then plays the stimulus Start Frequency in a loop, increasing the level +3dB with each loop iteration until one of the limits is exceeded. The stimulus level is then adjusted -3dB and the sequence continues to a second loop which increases the stimulus level +0.5 dB with each loop iteration until the limit is exceeded. At this point, the limit results are saved to an Excel file, the stimulus frequency is incremented by a constant multiplication step and the process is repeated until the Stop Frequency is achieved. Every time the main loop is completed, the individual SPL and Stimulus Level x-y pairs are concatenated to master curves. At the end of the sequence, the Max SPL and Stimulus Level curves are autosaved in .dat format.
- p/n 4002 – Listen SCM 3 reference microphone or equivalent
- p/n 4050 – Listen Audio Connect audio interface/microphone power supply or equivalent
- p/n 4060 – Listen SCAmp audio test amplifier or equivalent
- p/n 4009 – Listen Impedance Box (if not using Listen hardware containing internal impedance reference)
- SoundCheck Basic version 17.0 or later
Setup & Calibration
Caution: Take care when setting the limits for the various metrics. You may damage your DUT if the limits are set too high. It is good practice to set a practical Vmax value that will guarantee that the thermal or mechanical limits of the DUT won’t be exceeded.
- Calibrate your amplifier and reference microphone per the instructions in the SoundCheck user manual
- Connect your audio interface output 1 to the input of your amplifier
- Connect the output of your reference microphone’s power supply to input 1 of your audio interface
- Connect your DUT to the output of the amplifier
You are ready to start the sequence.
Additional Sequence Notes
- The sequence default settings will measure the DUT using standard ISO 1/3 octave frequency resolution.
- Stimulus resolution is controlled by Step 25 Stimulus Resolution Multiplier. You can use different constant values in this step to produce different stimulus resolutions:
|Stimulus Resolution||Constant value|
|1/3 octave (default)||1.259|
- If the stimulus resolution is changed, Step 32 Resolution must also be modified to reflect the stimulus resolution. The purpose of this step is to coerce the stimulus frequency to standard ISO values as the resolution multiplier sometimes produces non-ISO frequency values.
- If you are using Listen Hardware that supports Input Auto Range/Auto Gain (e.g. AmpConnect, AudioConnect or SoundConnect 2) we recommend that you enable these features to ensure that you achieve good Max FSD values in Acquisition. Since the sequence may present the DUT with a broad range of test levels, Auto Ranging will prevent input clipping during Acquisition by automatically adjusting the input signal path gain while optimizing Max FSD.
Further sequence development
This sequence has been designed to be accessible to 100% of SoundCheck customers. Ways in which you could modify or further develop the sequence include:
- Headphone/earphone testing
- Modify the sequence to use a higher resolution stimulus (Steps 25 and 32 as noted above) and save it as a new sequence
Changes from Max SPL vs. Frequency V1 (2018)
- Added initial response and impedance measurement
- Added option to use f0 -3dB as Start Frequency
- User defined test level (that actually works)
- User defined Stop Frequency – previous version relied on loop index logic to halt the sequence which varied depending on stimulus frequency range and resolution
- User defined Vmax
- Stimulus frequency is now coerced to standard ISO frequency points (in the past there was the potential for +/- 20 Hz error)
- Stimulus type changed from Amplitude Sweep to Stweep
- Incrementing Stimulus Level is no longer achieved by multiplying the stimulus waveform by a constant