IEC-60268-7 Headphone Sequences

IEC-60268-7: Sound System Equipment – Part 7: Headphones and Earphones is an international standard intended to characterize the performance of headphones and earphones. The standard itself is a lengthy document, 9 Sections and 3 Annexes covering 46 printed pages. These SoundCheck sequences focus on the electro-acoustic tests which are detailed in Section 8 “Characteristics to be specified and their method of measurement”.

Five separate sequences are provided, each designed to measure specific characteristics. This approach provides the user with the flexibility to measure all or some of the characteristics of their headphone.

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Triggered Record Using WAV File (Version 16.0 and earlier)

triggered_record_screenshotThis sequence allows you to test devices without an analog input such as tablets, cellphones and MP3 players. A stimulus WAV file is created in SoundCheck, and copied to the device under test, where it is played and the response recorded in SoundCheck as if the stimulus were played directly from SoundCheck. The stimulus WAV file to be used on the device under test (DUT) may be customized in the stimulus step.

Note that this sequence uses the level-based trigger available in SoundCheck 16.0 and earlier. If you are using version 16.1 or later, please see the frequency-trigger based sequence which takes advantage of new functionality to offer more robust triggering.

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Measuring Digital MEMS Microphones: Frequency, Sensitivity and Power Supply Rejection (PSR) Performance

seq_dig_mic_final_display_substitution_methodThis test suite contains 3 sequences to enable comprehensive testing of digital MEMS microphones.

The first measures the frequency and sensitivity and displays two graphs: absolute level in dBFS, and the same response curve but normalized to 0 dB at 1 kHz.

The second sequence uses the substitution method to test a digital MEMS microphone frequency response with a source speaker that is not or cannot be equalized. The MEMS microphone is simultaneously measuring with a reference microphone , and by subtracting the response of the reference microphone from the DUT microphone the response and sensitivity of the device under test is revealed.

Measuring Digital Microphone PSR (Power Supply Rejection)
The third sequence demonstrates a method for measuring a digital MEMS microphone’s power supply rejection performance (PSR). This sequence measures PSR at 217 Hz (the 217 Hz GSM TDM pulse often of concern) but is easy to modify to test at any frequency. A DC supply with a calibrated AC signal, simulating electrical interference is applied to the MEMS microphone. SoundCheck then records the audio from the DUT, analyzes it with a spectrum analyzer and extracts the RMS energy at the specific frequency of the simulated electrical interference and returns the PSR value. The setting of frequency, waveform type and amplitude of the simulated electrical interference is controlled entirely from within SoundCheck.

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Microphone Polar Plot Sequence

mic_polar_plot_screenshotThis sequence measures the directional response of a microphone and graphs the result as a polar plot. A log sweep stimulus is played from 100 Hz to 10 kHz at each angular increment, and the acquired waveform is analyzed using the Time Selective Response algorithm. This method allows the test to be performed in a non-anechoic environment by placing a window around the direct signal, eliminating the influence of reflections. Commands are sent automatically to the LT360 turntable via an RS-232 connection, instructing it to move in 10 degree increments after each measurement. The sequence measures the response every 10 degrees from 0 to 180 and mirrors the polar image, simulating a full 360 degree test while saving time. The response at each angular increment is compared against the on-axis response to create a normalized curve. This removes the influence of the device’s frequency response and sensitivity, such that the polar plot only shows the directional response. The final display also contains a graph of the directivity index in decibels versus frequency.

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Microphone Self Noise Test

The purpose of this sequence is to measure the self noise of a condenser microphone. To do this we use the spectrum analysis step and a power sum calculation to derive an RMS rating for the unit under test.

The sequence has several parts, a few of which are optional. The first section begins by prompting you to enter your mic preamp’s gain. If your preamp does not have a gain setting with labeled detents you may wish to use the “Amplifier THD + N” sequence located in C:\SoundCheck 18.1\Sequences\Electronics\ to determine this value. The following part tests the preamp’s self noise so that the system can determine whether you have a high enough signal (signal being the microphones self noise) to noise ratio to accurately measure your microphone.

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Active Noise-Cancelling Headphone Battery Life Test

ANC battery life sequence screenshotThis SoundCheck test sequence measures the performance characteristics of Active Noise Cancelling headphones (ANC headphones) while monitoring the DC voltage and current provided to the headphone by its batteries. The sequence first measures the passive attenuation of the headphone before moving into a loop. The loop plays a 2 minute pink noise stimulus at high output level to accelerate battery drain. During this stimulus period, a current measurement is made by Listen’s DC Connect. Immediately following the stimulus, battery voltage is measured followed by acquisition and analysis of audio parameters (response, THD and THD Normalized). The active attenuation of the headphone is then measured followed by a series of post processing and Autosave steps. The looping continues until no output is detected from the headphone, when the device shuts down due to insufficient battery capacity.

 

 

 

 

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Headphone Active Noise Cancellation Test Sequence

Screenshot of the final test result for active noise cancellation headphone test, showing active, passive and total attenuation.This SoundCheck test sequence measures headphone active noise cancellation (ANC). It measures both the active and passive noise cancellation and calculates total attenuation over the operating frequency range of the headphones. Passive Attenuation is the amount of noise that is reduced at the ear simply by the headphones being worn. Active Attenuation is the amount of noise that is further reduced by turning on the device’s active cancellation circuits. Lastly, Total Attenuation is the combined reduction in noise from passive and active sources and is what the end user of the product will experience.

To calculate these metrics this SoundCheck test sequence performs three separate measurements using a Head and Torso Simulator and a small speaker which serves as a noise source. This is a simple, low-cost alternative to using a diffuse background noise environment with multiple speakers playing uncorrelated noise, although the sequence could be user-modified to support this.

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Thiele-Small Parameters

Loudspeaker system performance can be quantitatively related to a set of electro-mechanical parameters. These parameters are known in the industry as Thiele-Small parameters. They were first introduced by A.N.Thiele and Richard H.Small in a series of famous articles published in the 1971-72 Journal of AES (Audio Engineering Society).  Over the years these parameters have become standards in the industry, and are used by loudspeaker designers worldwide. This package contains SoundCheck sequences for measuring measuring Thiele-Small Parameters by Added Mass, Known Volume, Known Driver Mass methods.

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Impedance Measurement – Dual Channel Method Using Math Post-Processing

This sequence demonstrates an alternative to the traditional SoundCheck single channel impedance measurement method. A stepped sine sweep from 20 Hz to 20 kHz is played through the speaker while the signal across the loudspeaker terminals is recorded by Direct In 1 and the signal across the sense resistor (impedance box) is recorded by Direct In 2. A heterodyne analysis step is then applied to calculate the fundamental response from both inputs and a math post-processing step divides Fundamental A (speaker terminal voltage) by Fundamental B (voltage across sense resistor). A post-processing step corrects for the value of the reference resistor before displaying the final impedance curve. The curve is then post-processed to calculate resonance frequency, maximum impedance and Q of the resonance peak. A set of arbitrary limits steps are also provided to generate pass/fail results.

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Perceptual Rub & Buzz

Perceptual Rub & Buzz ScreenshotThis sequence uses the CLEAR algorithm for perceptual Rub & Buzz measurement to detect AUDIBLE Rub & Buzz. It uses a simplified auditory perceptual model to measure the loudness of Rub & Buzz distor­tion in phons rather than the more traditional dB SPL and % distortion units. These better identify whether distortion due to manufacturing defects can be heard by the listener than conventional measurements. In addition to a result which corresponds more accurately to the human ear, this new test method also offers two significant advantages for use on the production line. It is less sensitive to transient background noises than traditional methods, therefore is reliable in noisy environments, and it is much simpler to set limits than when us­ing conventional distortion measurements. The sequence includes saved data that can be loaded from disk, so even if you don’t have a speaker handy you can still listen to the wav. file and see how SoundCheck displays the data.

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