IEC-60268-7 Headphone Sequences

seq_IEC-60268-7_distortion_impedance_no borderIEC-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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AGC Hearing Aid – Reference Gain & EIN Test Sequence

This sequence performs two of the measurements from the ANSI hearing aid test standard S3.22-1996.  The first part, in accordance with section 6.7 of the standard, helps the user set the reference test gain for the hearing aid, which is used for multiple measurements in the standard.  The second part, from section 6.12, tests the equivalent input noise (EIN) of the device.

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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

microphone self noise test screenshotThis sequence measures the self noise of a condenser microphone, using a spectrum analysis step and a power sum calculation to derive an RMS rating for the unit under test.
The sequence has several parts (some optional). The sequence intitally determines  whether or not you have a high enough signal (signal being your microphone’s self noise) to noise ratio to accurately measure your microphone. It then takes a measurement from your microphone, creates a spectrum, accounts for the preamp gain, applies an A-weighting, and finally calculates the power sum.  You are then prompted to enter the sensitivity of the microphone if it is known.  The resulting display provides you with the equivalent input noise of your preamp, the self noise of your microphone in dBV, and also a result in dB(A) after factoring in the microphone’s sensitivity.  You’re also provided with the waveform, a maximum voltage level, and the crest factor to check for sharp transients.

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Active Noise-Cancelling Headphone Battery Life Test

ANC battery life sequence screenshotThis sequence is designed to measure performance characteristics of Active Noise Cancelling (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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Noise Cancelling Headphones

noise cancelling headphone measurementWhen measuring noise cancelling headphones there are three important pieces of data to generate. 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 feature. 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 sequence performs three separate measurements using a Head and Torso Simulator and a small speaker which serves as a noise source. The alternative to using the small speaker would be to develop a diffuse environment with multiple speakers playing uncorrelated noise. This is a far more complicated arrangement and would require additional steps in the sequence.

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Headphone Test Sequence

headphone test screenshotThis sequence performs a comprehensive headphone test on a stereo headphone. Both left and right earphones are measured simultaneously using a standard 1/12th Octave stepped-sine sweep from 20 to 20 kHz.

The analysis is then performed using the HarmonicTrak™ algorithm that measures harmonic distortion and fundamental frequency response simultaneously, and the diffuse-field and free-field corrected curves are calculated. The diffuse-field correction curve compensates for the overall frequency response from the diffuse-field (sound in every direction) to the eardrum and includes the effects of the head, torso, pinna, ear-canal and ear simulator. The free-field correction curve compensates for the overall frequency response from the free-field (sound at 0 degree incidence to the nose of the Head and Torso Simulator – HATS) to the eardrum. Further post-processing of the signal compares left and right earphone responses to show the difference curve (magnitude and phase are available). The average sensitivity from 100 to 10 kHz for both left and right earphone is calculated and the total harmonic distortion displayed.

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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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