Headphone & earbud audio testing
01 December 2022
Figure 1. Headphone types: (a) circumaural; (b) supra-aural; (c) intra-concha; and (d) insert (adapted from [c])
EPDT readers may be aware of the basic method used for loudspeaker testing: at its simplest, a measurement microphone is placed a standard distance from a loudspeaker in an acoustically appropriate space, while the speaker is driven by a test signal. The microphone acquires the acoustic output of the speaker & passes it to an analyser.
This article was originally featured in the December 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
Testing headphones and earbuds is similar to loudspeaker testing: drive an acoustic transducer, pick up the result with a microphone and analyse it. But, unlike loudspeakers, headphones and earbuds are designed to be coupled to the ear, and this complicates the issue. As David Mathew, Senior Technical Writer & Joe Begin, Director of Applications & Technical Support at audio T&M experts, Audio Precision explain here, the measurement microphone must be placed in a structure that acoustically models the human ear, and the driver under test (DUT) must be mounted in a way that represents actual use…
There are circum-aural headphones, which surround the ear; supra-aural, which rest on the pinnae (the external part of the ear); and intra-concha insert earphones (or earbuds). The resonance of the ear canal and the reflections from the body and the pinnae greatly affect the response at the eardrum (referenced as the Drum Reference Point, or DRP, which is where the measurement microphone diaphragm must be located). Earphones and headphones can be tested with a variety of acoustic test fixtures (ATF), selected by headphone characteristics and applications.
Figure 2. Occluded Ear Simulator (Ear Simulator with Occluded Ear Canal Extension), Audio Precision
IEC 60318-4 (IEC 60711) Ear Simulator
The IEC (International Electrotechnical Commission, based in Geneva, Switzerland) has specified an Ear Simulator to model the human ear canal (the IEC 60318-4 Ear Simulator), with a microphone at the DRP. An Ear Canal Extension and an Occluded Ear Canal Extension are specified accessories, so that the IEC 60318-4 Ear Simulator can be used in multiple fixtures and applications.
When fitted with an Ear Canal Extension and an artificial pinna, the Ear Simulator can be used to characterize headphones; with an Occluded Ear Canal Extension, it is used for intra-concha and insert earphones (earbuds).
Figure 3 shows a diagram representing a loudspeaker response (where the loudspeaker is equalized to be flat) measured in free-field, and the same acoustic signal as measured at the DRP of the Ear Simulator in a HATS (see HATS manikin, below). The altered response due to the resonance and reflections is obvious.
Figure 3. Frequency response of a loudspeaker equalized flat in a free field as measured by a free-field microphone & an in-ear microphone at the Drum Reference Point
For insert headphones, only an Ear Simulator with an Occluded Ear Canal Extension may be necessary, as external ear, head or torso effects are not relevant. For earphones and headsets that involve pinna, head or chest reflections, the Ear Simulator can be mounted in a larger fixture such as a HATS.
Ear & cheek fixture
This fixture has an Ear Simulator and artificial pinna mounted on a plane representing the cheek or side of the head, with an arm to set an adjustable pressure to the headphone-to-pinna coupling.
Figure 4. Occluded Ear Simulator with insert earphone
A Head and Torso Simulator (HATS) manikin models the reflections and acoustic shadow of the head, and the reflections and absorptions of the neck, shoulders and chest. Ear Simulators and pinnae are mounted in each side of the head. For headset microphone testing, a mouth simulator transducer can be added. A HATS is particularly important for accurate microphone testing in headsets that incorporate a microphone.
Binaural headphone test fixture
Less expensive than a HATS, but a more complete solution than some other options, a Headphone Test Fixture uses two Ear Simulators and artificial pinnae mounted in a massive aluminum head. Like a HATS, left and right headphones can be tested simultaneously.
Figure 5. 43AG Ear & Cheek Simulator, G.R.A.S. Sound & Vibration
For production-floor testing, the pinnae can be replaced with conical aluminum fittings for fast and repeatable headphone seating.
Additionally, the massive tubular head provides substantial acoustic isolation, required for measuring the effectiveness of ear-muff style hearing protectors and ANC (active noise cancelling) headphones in reducing ambient noise.
An Ear Simulator in any of these fixtures paired with a modern analyser, capable of Farina log-swept-sine chirps, can provide a wide range of measurement results, including:
Figure 6. Type 5128 HATS, Brüel & Kjær Sound & Vibration Measurement
• Frequency response
• Electrical impedance
• Input voltages
• Sound pressure level
• Harmonic & intermodulation distortion
• Noise attenuation
Figure 7. ISO 4869-3 Headphone Test Fixture, model AECM206, Audio Precision
• Crosstalk attenuation
• Rub & buzz
• Left/right tracking
HRTF (Head Related Transfer Function)
The reflections from the pinnae and the impedance and resonances of the ear canal are faithfully modelled by the Ear Simulator and its microphone, providing the response curve at the DRP. However, as seen in Figure 3, this raw measurement data does not easily correlate with published data, standards or engineering specifications for the device under test (DUT) that cite free field or diffuse field response curves. This can be accomplished by converting the measured data using free field or diffuse field HRTFs (Head Related Transfer Function curves).
Figure 8. Frequency response of circumaural headphones for 5 cycles of placing the headphones on the ATF
An HRTF is created by first measuring the response curve of an Ear Simulator at DRP (in a HATS, for example), and then removing the HATS and placing a microphone at the same location, before measuring a second response curve. These measurements can be made in either a free field (anechoic space) or diffuse field (reverberant space). The difference between the two measurements is the HRTF, plotting the characteristics of the acoustic test fixture being used.
Once the HTRF is obtained, the free field or diffuse field response of the headphone under test can be obtained by dividing the frequency response at the DRP by the appropriate HTRF.
The importance of fit
Figure 9. Frequency response of a circumaural, closed headphone, with the Diffuse Field & Free Field response curves
The fit of an earphone to the head and/or pinna can have a dramatic effect on performance. This is especially true for the bass response of closed headphones; leaks of any sort will reduce the ability of the earphone to generate sound at low frequencies. For example, Figure 8 shows five frequency response measurements, where the headphones were removed from the ATF and then reapplied before each measurement. To account for this variation with fit, it is a good practice to average the results of several measurements (typically 3 to 5) with the headphones being removed and re-applied between measurements.
Design targets for headphone frequency response
In the past, headphone target responses were designed to match the free-field HRTF for the position directly in front of the listener. Later, a diffuse field HRTF was added as an ideal for studio monitor headphones. Recent work on this subject indicates that listeners prefer alternatives to the free field or diffuse field headphone target frequency response curves described above, and that in general, trained listeners preferred a headphone target response that corresponds to a flat loudspeaker calibrated in a reference listening room.
Compensation of ear simulator responses with HRTF
Figure 10. Frequency response of the circumaural headphones (as measured in Figure 9) corrected with Diffuse Field & Free Field equalization
When considering the frequency response of headphones measured on an ATF with ear simulators, it is 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.
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. This 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.
Left/Right Tracking is a useful metric for stereo headphones, because it measures the relative response of each earphone in a pair of headphones. It is 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.
Figure 11. Frequency response of the left & right earphones (left axis) and their Left/Right tracking response curve with ±3 dB limits (right axis)
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).
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 is 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 & active attenuation
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