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Gaining Clarity: Approaches For Testing Headphone/Earbud Performance

Attaining accurate frequency response data from headphones and in-ear monitors has long been a 'missing link' in a quality monitoring signal chain.

By David Mathew and Joe Begin October 16, 2018

When considering the frequency response of headphones measured on an ATF with ear simulators, it’s important to keep in mind that the target response is not flat.

There are two options in this regard.

The first option is to display the design target curve(s) on the same graph as the measured (normalized) frequency response, as shown in Figure 3. With this approach, one can mentally compare the measured curve to the target curve when evaluating the measured frequency response.

Figure 9: Frequency response of a circumaural, closed headphone, with the Diffuse Field and Free Field response curves.

The second approach for evaluating measured headphone frequency response is to “correct” or refer the measured response to the target response. This is accomplished by inverting the target response curve and applying it as an EQ curve to the measured response, and is illustrated in Figure 10, in which the headphone response measured in Figure 9 was corrected to the diffuse field and the free field. With this approach, the corrected response of a headphone that matches the target response perfectly would be a flat line at 0 dB.

Figure 10: Frequency response of the circumaural headphones (as measured in Figure 9) corrected with Diffuse Field and Free Field equalization.

Further Options

Left/Right Tracking is a useful metric for stereo headphones because it measures the relative response of each earphone in a pair. It’s easily derived from frequency response measurements on an ATF with two ear simulators by comparing the response from the right and left ear.

Earphones that match perfectly will have a Left/Right Tracking response curve that is a flat line at 0 dB. As shown in Figure 11, the left and right earphones of the insert earphone are well matched from 20 Hz to 10 kHz.

Sound attenuation is a measure of how effective a headphone or earphone is at blocking ambient noise from entering the ear canal. This is of particular interest to manufacturers of headphones equipped with active noise cancellation (ANC).

Figure 11: Frequency response of the left and right earphones (left axis) and their Left/Right tracking response curve with ±3 dB limits (right axis).

Guided by the standard ISO 4869-1 and -3 (which are referenced in IEC 60268-7), a random incidence sound field is created around an isolating ATF, such as shown in Figure 7. A broadband signal such as pink noise is generated and sound levels in the ear simulators of the ATF are measured in 1/3-octave bands.

For headphones without ANC, the procedure requires first measuring the 1/3-octave sound level spectrum of the open ear (headphones removed), and then repeating the measurement with the headphones in place. The insertion loss is calculated as the difference between these spectra.

For headphones with ANC, an additional step is required: measurements are conducted with and without the ANC feature enabled, from which passive and active attenuation values are calculated. Measured spectra are typically normalized to the measured open ear spectrum, as shown in Figure 12. This graph indicates that the active noise attenuation is effective below about 1.5 kHz, and that it’s less effective than passive attenuation alone in the frequency range from about 1.5 kHz to 4 kHz.

Figure 12: Normalized spectra from one measurement of an ANC headphone showing passive and active attenuation.

Note: This article is largely based on the application note Headphone Electroacoustic Measurements, Audio Precision, which goes into much more detail and provides many references to standards. It’s available at www.ap.com


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About David and Joe

David Mathew and Joe Begin
David Mathew and Joe Begin

Joe Begin, PE (Acoustical Engineering) serves as director of applications and technical support at Audio Precision. He's a graduate of McGill University (B.Sc.) and the University of Canterbury (M.Sc.), has more than 35 years of experience in test and measurement. David Mathew is technical publications manager and a senior technical writer at Audio Precision. He's served as both a mixing engineer and as a technical engineer in the recording and film-making industries, and was awarded an Emmy f...
http://www.ap.com

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Dennis Buhrmann says

This is a great technical article! My compliments to the authors.

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