Loudspeaker Diaphragm Displacement Measurement Using a Laser

Laser Displacement Measurement final screenshotThis pre-written test sequence demonstrates how to use a laser and SoundCheck to measure the peak (max and min) displacement of a loudspeaker diaphragm. SoundCheck is easily configured to include a laser signal path, making it simple to correlate diaphragm displacement with electrical impedance and audio artifacts.

In this laser displacement measurement, the user is prompted to enter parameters including test level, start and stop Frequency and stimulus frequency resolution. The sequence then plays the Start Frequency through the DUT and records the resulting diaphragm motion with the laser. Post-processing functions create maximum and minimum curves across the frequency range and display them on a graph.

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VR Headset Leakage Measurement Sequence

VR Headset Leakage measurement screenshotVR headset leakage measurement is a useful parameter for VR headset characterization. While they are sometimes connected to headphones, most VR headsets also contain built-in speakers, often on the strap close to the ear. These small speakers often have considerable audio leakage due to their positioning on the band, where there is some transmission through air before reaching the ear. This is annoying to others in the room, so efforts are made to minimize this. 

For this measurement, the headset is positioned on a head and torso simulator mounted on a turntable, and a log sweep played from 20Hz-20kHz at user-defined level and distance. The sequence measures leakage and frequency response for one ear in 10° increments from 0 to 180°, and mirrors it to provide a complete 360° polar plot. The final display produces a polar plot for four frequencies, and all eighteen measurements are shown on a frequency response graph.

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ANSI/CEA 2010-B 2014 Sequence for Subwoofer Maximum SPL

This sequence measures the maximum peak SPL of a subwoofer according to ANSI/CEA 12010-B 2014. In this test, 1/3 octave band limited tone bursts are presented to the subwoofer across a 3 octave range from 20 Hz to 160 Hz. At each frequency, the stimulus level is increased in +3 dB increments until the harmonic (and non-harmonic) distortion and noise (HD+N) exceeds the specified threshold. The level is then decreased by 3 dB and the test continues with level increments of +1 dB until the HD+N threshold is again exceeded. The peak SPL of the fundamental at the last passing test level is recorded and the sequence continues to the next frequency. Peak SPL values are weighted according to the power spectrum defined in the standard and the Average Weighted SPL and final Broadband Peak SPL calculated as specified.

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In-Car Audio Measurements

Screenshot showing in-car audio measurement sequence final display showing frequency response, distortion and Max SPL

Final display of in-car audio measurement sequence showing frequency response, distortion and Max SPL

This in-car audio test sequence measures the transient distortion (also known as buzz, squeak, and rattle, Rub & Buzz, or impulsive distortion), frequency response, and maximum sound pressure level of a vehicle infotainment system to the methods outlined in the Audio Engineering Society Technical Committee on Automotive Audio (TC-AA) in-vehicle measurements draft white paper.  The three measurements are incorporated into one overall test sequence, making it fast and simple to run the entire suite of tests. This sequence facilitates evaluation of the committee’s proposals, and also serves as a basis for any similar in-house measurements. The white paper (linked above) outlines both measurement methods and physical configuration such as microphone and seat positioning in an effort to simplify comparison between vehicles. This test sequence may, of course, be used with your own in-house physical configuration if adherence to the TC-AA guidelines is not essential.

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Polar Plot (MDT-4000 Turntable) Sequence

This sequence measures the polar response of a loudspeaker in both the vertical and horizontal dimensions. It is designed to work with the Portland Tool & Die MDT-4000 turntable, and has all the necessary commands to automatically rotate it via RS-232. The sequence uses a log sweep stimulus with the Time Selective Response algorithm so that the measurements can be run in a non-anechoic environment. Note that the time window needs to be adapted to the user’s measurement space.

The sequence plays the stimulus and measures at 10 degree increments from 0 to 180 degrees. This process is repeated with the speaker positioned horizontally. The two results are mirrored to display full 360 degree polar plots for each axis. A directivity index curve is also calculated for each axis and is displayed at the end of the test.

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Receive Loudness Rating with ITU Real Speech Test Sequence

The purpose of this sequence is to measure the Receive Loudness Rating (RLR) following the ITU-T P.79 standard using a Head and Torso Simulator (HATS). First, real speech from the ITU-T P.501 standard is sent to the Device Under Test (DUT) speaker by an electrical interface. The HATS right ear captures the DUT‟s speaker response. SoundCheck calculates the frequency response and then RLR based on that recording.

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Send Loudness Rating with ITU Real Speech Test Sequence

The purpose of this sequence is to measure the Send Loudness Rating (SLR) following the ITU-T P.79 standard. This sequence can be used with handsets, headsets, and conference call devices. First, real speech from the ITU-T P.501 standard is played out of a mouth simulator. The Device Under Test (DUT) microphone then captures the signal and transmits this back to SoundCheck. SoundCheck calculates the frequency response function in 1/3 octaves and calculates SLR based on that frequency response.

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Microphone Frequency Response and Sensitivity Test Sequence

This is sequence is a basic example of the two most common microphone measurements: frequency response and sensitivity. A stepped sine sweep is played from 10 kHz to 100 Hz through a source that has previously been calibrated to produce 1 Pascal across the frequency band. The recorded signal is analyzed with a Heterodyne analysis step, which calculates the response curve. A post processing step is then used to extract the level at 1 kHz, the sensitivity value. Limits are set around both the frequency response and the sensitivity, but the default values are arbitrary and should be adapted to your particular device.

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Loose Particles Sequence

This sequence demonstrates how to use SoundCheck to detect loose particle defects in loudspeakers. Loose particles typically reveal themselves as randomly spaced impulses, so they may not be detected when performing frequency based measurements such as THD, even though they can be clearly heard as undesirable artifacts. The loose particle algorithm, which is an available function in all analysis algorithms, analyzes a time waveform to detect these impulses. The user sets a customized threshold level for detection.

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Complete Test Sequence

The purpose of this sequence is to perform a full suite of basic measurements for a loudspeaker. A 500 mV stepped sine sweep from 20 kHz to 50 Hz is played through the speaker and measured via two channels of the audio interface. A calibrated reference microphone is connected to one of the channels, and an impedance reference is connected to the other.

A HarmonicTrak™ analysis step analyzes the recorded waveform from the reference microphone, and outputs frequency response, THD, Rub & Buzz, and various harmonic curves. A second analysis step analyzes the waveform from the impedance reference and outputs a curve of impedance versus frequency. A post processing step is used to estimate the characteristics of the impedance curve and calculates the max impedance, resonance frequency, and the Q of the resonance peak.

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