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Headphone Measurements: The New Standard, Part 1
There is a new, substantially more human-like standard for measuring headphones. And as I will show you shortly, this new standard is a very important step toward more realistic, more meaningful headphone measurements. When it comes to headphone measurements, one of the biggest challenges we've seen in our community is correlation between subjective and objective analysis. I will show you, then, examples of how this more accurate hearing simulator will help close the gap between the headphones as we hear them and the headphone measurements we look at.
At the ALMA* annual conference in 2018, I saw one of Brüel & Kjær's earliest public presentations of the research and development behind their new High-frequency Head and Torso Simulator (HATS) Type 5128. I was sitting among many acoustical engineers in the audience who were bowled over by what Vince Rey (from Brüel & Kjær) had just shown us, which was the answer to the following question: How would you propose improving upon the 40-year-old standard of human hearing simulation to meet measurement needs that have evolved beyond that standard over the past several decades (and that will continue to evolve)?
The answer, simply put, was start over from scratch. And to do that meant finally establishing an entirely new standard that more accurately simulates human hearing across the full audio range. As we'll see later, accomplishing this was fantastically complex, taking over ten years, and involving technologies and techniques that were not available 40 years ago.
* ALMA is the Association of Loudspeaker Manufacturing & Acoustics, which is now known as Audio & Loudspeaker Technologies International (ALTI).
NOTE: You can click on any of the images below to see them full size.
When measuring headphones – whether around-ear/on-ear (AE/OE) or in-ear (IE) – we use a human hearing simulator, often simply called an "ear simulator." The typical ear simulators used in our industry are based on an international standard called IEC 60318-4. Because the IEC 60318-4 standard used to be called IEC 711 (and then IEC 60711), ear simulators based on this standard are often nicknamed simply "711 simulators," "711 couplers," or even just "711."
What many do not know is that the 711 standard is, again, 40 years old. What many also don't know is that these 711 ear simulators only simulate human hearing from 100 Hz to 10 kHz [1]. (As we'll see later, even that specified range of simulation was subject to improvement.). Below 100 Hz and above 10 kHz you can use a 711 ear simulator as an acoustic coupler, but you are not simulating average human response in those ranges outside the standard. (See Fig.2 below.)
40 years ago, that range was perfectly acceptable, as the focus for this type of measurement then was testing hearing aids and telecom devices. However, in the decades since the establishment of 711, the need to obtain realistic measurements across the full audio range (20 Hz to 20 kHz) has increased. This need has been particularly apparent in our segment of the audio industry, in which consumers have high expectations when it comes to audio quality from premium headphones – very high expectations.
In the last four decades, what has also evolved are the technologies and techniques available to characterize human hearing over the full audio range. While I'll discuss in some detail how Brüel & Kjær was able to characterize human hearing over the entire audio band to eventually develop the 5128, their methods were succinctly summarized by them in a video titled "Evolution of Hearing Simulation – Part 2." If you haven't already watched that video, please do so now, as the information in it will come in handy as we move further in this discussion.
The Brüel & Kjær 5128 arrived at Head-Fi HQ this past winter, and we started measuring with it immediately, to get to know this new measurement fixture and the results from it, and to try to determine what an entirely new, more human-like standard of hearing simulation represents. Even very early on, key findings and observations started to take shape:
When it comes to correlation between subjective and objective analysis, one example of a headphone I've found to sound quite different from its 711 measurements (found online) is the Westone W60 universal-fit in-ear monitor. (In each earpiece the W60 uses six balanced armature drivers with a 3-way passive crossover.) Here are the 711 measurements of the Westone W60 I found online:
Here are the Westone W60 "711" frequency response measurements from Figs. 4 to 7 shown together:
If you had not heard the Westone W60 before seeing these measurements, what you see (in each of the measurements in Figs. 4 to 7 above) would suggest that above 5 kHz, the W60's tonal balance is largely characterized by the 10 to 18 decibel peak centered at 8 kHz to 10 kHz, as well as the peak's effects through its lower and higher peripheries. This is due to a resonance that actually is part of the 711 standard.
Is that resonance, however, part of 711's human hearing simulation range? No, it is not. Because these resonant peaks will often shift down and appear within 711's specified range of simulated human hearing when measuring headphones [2], I can understand how this might be a bit confusing. Let us examine this, then, by looking at some key parts of the 711 standard (emphasis by me):
Again, it might appear that the resonance in Figs.4 to 8 – because it appears at or below 10 kHz in these four measurements – is within the simulation range. However, let's look at another part of the standard:
Measured at the 711 simulator's reference plane, this resonance is outside the 711 standard's human simulation range, its purpose being to help specify the physical geometry of the principal cavity (the primary volume). It is very important to note a couple of things about this part of the specification, though:
So, while that resonance is not part of a 711 coupler's human simulation, it nevertheless largely defines much of the treble region of the Westone W60 measurements in Figs. 4 to 8. I feel very confident that of those who have heard the Westone W60, most would agree those previous measurements do not correlate well with subjective impressions. And the W60 is just one of many examples of headphones for which I could not previously reconcile what I was hearing with the measurements I was seeing.
Now let's look at measurements of the Westone W60 using the Brüel & Kjær 5128. Though we measured with two different ear tips (stock silicone and stock foam tips), the silicone tips are what comes installed on the W60 out of the box.
Here (below) is the Westone W60 measurement from the Brüel & Kjær 5128 compared to the previously shown "711" W60 measurements:
Following is a comparison of the W60 measured on the 5128 compared to the shaded-in measurement range of the 711 measurements shown together (below):
Notice also that differences between the 711 and the 5128 measurements of the W60 are not limited to the treble range but appear throughout most of the audio band. Compared to the 5128, all of the 711 measurements in Figs. 4 to 8 show more bass, with two of the four showing what could reasonably be called substantially higher bass levels versus the 5128. Again, if you've listened to the W60, I think there's a very strong chance the measurements of it from the 5128 would correlate much more closely with what you heard versus any of the 711 measurements shown.
We will discuss in Part 2 what causes these noteworthy measurement differences, and we will also look at research that is completely separate from the work behind the 5128 that helps corroborate its findings.
The Brüel & Kjær 5128 was developed to finally simulate average adult human hearing across the full audio range (20 Hz to 20 kHz) for the first time, based on research and development using more modern technologies and techniques developed in the past four decades, and taking 12 years to complete. Some of the names you'll see behind the 5128 research are also names you'll see in the references of the IEC 60318-4 standard. Brüel & Kjær was also instrumental in establishing the 711 standard, having contributed to its development, and being the first with a commercially available IEC 711 simulator with the Brüel & Kjær Type 4157 (which is still commonly in use today).
While, again, we'll discuss that research and development in greater detail in another post, the Brüel & Kjær High-frequency Head and Torso Simulator (HATS) Type 5128 (and the Type 4620 ear simulators within it) involves very important developments versus all previous such standards. To summarize those changes simply for now:
A new era of headphone measurement arrived with the advent of the Brüel & Kjær 5128. While the 711 standard will no doubt continue with the inertia of a 40-year-old industry standard for the foreseeable future, engineers and enthusiasts will increasingly seek out the new standard for more representative, more meaningful absolute measurements of headphones.
We will continue the discussion of the Brüel & Kjær 5128 in Part 2 of this series soon, including a closer look at the associated research and development, as well as, of course, more measurements. We will also look at separate, corroborating research, discuss measurement observations with the 5128 and a corresponding hypothesis that may help explain some of the key differences between the Harman AE/OE Target and the Harman IE Target.
References
[1] IEC 60318-4 Ed. 1.0 (2010) Simulators of human head and ear – Part 4: Occluded-ear simulator for the measurement of earphones coupled to the ear by means of ear inserts (International Electrotechnical Commission).
[2] Wille, M. (2017). High Resolution Ear Simulator. GRAS Sound & Vibration white paper.
[3] Darkner, S., Sommer, S., Baandrup, A. O., Thomsen, C., & Jønsson, S. An Average of the Human Ear Canal: Recovering Acoustic Properties with Shape Analysis. Cornell Univ. Libr., ArXiv e-prints (2018).
[4] Staab, W. April 2013. Tympanic Membrane – Anatomical Influence on Hearing Aid Fittings. Hearing Health & Technology Matters. https://hearinghealthmatters.org/wa...anatomical-influence-on-hearing-aid-fittings/
[5] S. Jønsson, A. Schuhmacher, H. Ingerslev Jørgensen, Wideband impedance measurement techniques in small complex cavities such as ear simulators and the human ear canal, ArXiv e-prints (2018).
[6] S. Jønsson, A. Schuhmacher, H. Ingerslev Jørgensen, Wideband impedance measurement in the human ear canal; In vivo study on 32 subjects, ArXiv e-prints (2018).
[7] Brüel & Kjær, Design of the new Bruel & Kjaer High Frequency Head and Torso Simulator (HATS) type 5128, presented by Vince Rey at the 2018 ALMA International Symposium & Expo (AISE).
By Jude Mansilla
Head-Fi.org
Head-Fi.org
There is a new, substantially more human-like standard for measuring headphones. And as I will show you shortly, this new standard is a very important step toward more realistic, more meaningful headphone measurements. When it comes to headphone measurements, one of the biggest challenges we've seen in our community is correlation between subjective and objective analysis. I will show you, then, examples of how this more accurate hearing simulator will help close the gap between the headphones as we hear them and the headphone measurements we look at.
At the ALMA* annual conference in 2018, I saw one of Brüel & Kjær's earliest public presentations of the research and development behind their new High-frequency Head and Torso Simulator (HATS) Type 5128. I was sitting among many acoustical engineers in the audience who were bowled over by what Vince Rey (from Brüel & Kjær) had just shown us, which was the answer to the following question: How would you propose improving upon the 40-year-old standard of human hearing simulation to meet measurement needs that have evolved beyond that standard over the past several decades (and that will continue to evolve)?
The answer, simply put, was start over from scratch. And to do that meant finally establishing an entirely new standard that more accurately simulates human hearing across the full audio range. As we'll see later, accomplishing this was fantastically complex, taking over ten years, and involving technologies and techniques that were not available 40 years ago.
* ALMA is the Association of Loudspeaker Manufacturing & Acoustics, which is now known as Audio & Loudspeaker Technologies International (ALTI).
NOTE: You can click on any of the images below to see them full size.
Fig.1: Brüel & Kjær High-frequency Head and Torso Simulator (HATS) Type 5128.
When measuring headphones – whether around-ear/on-ear (AE/OE) or in-ear (IE) – we use a human hearing simulator, often simply called an "ear simulator." The typical ear simulators used in our industry are based on an international standard called IEC 60318-4. Because the IEC 60318-4 standard used to be called IEC 711 (and then IEC 60711), ear simulators based on this standard are often nicknamed simply "711 simulators," "711 couplers," or even just "711."
What many do not know is that the 711 standard is, again, 40 years old. What many also don't know is that these 711 ear simulators only simulate human hearing from 100 Hz to 10 kHz [1]. (As we'll see later, even that specified range of simulation was subject to improvement.). Below 100 Hz and above 10 kHz you can use a 711 ear simulator as an acoustic coupler, but you are not simulating average human response in those ranges outside the standard. (See Fig.2 below.)
Fig.2: Human hearing simulation range of IEC 60711/60318-4 ("711") compared to the full audio range.
40 years ago, that range was perfectly acceptable, as the focus for this type of measurement then was testing hearing aids and telecom devices. However, in the decades since the establishment of 711, the need to obtain realistic measurements across the full audio range (20 Hz to 20 kHz) has increased. This need has been particularly apparent in our segment of the audio industry, in which consumers have high expectations when it comes to audio quality from premium headphones – very high expectations.
In the last four decades, what has also evolved are the technologies and techniques available to characterize human hearing over the full audio range. While I'll discuss in some detail how Brüel & Kjær was able to characterize human hearing over the entire audio band to eventually develop the 5128, their methods were succinctly summarized by them in a video titled "Evolution of Hearing Simulation – Part 2." If you haven't already watched that video, please do so now, as the information in it will come in handy as we move further in this discussion.
The Brüel & Kjær 5128 arrived at Head-Fi HQ this past winter, and we started measuring with it immediately, to get to know this new measurement fixture and the results from it, and to try to determine what an entirely new, more human-like standard of hearing simulation represents. Even very early on, key findings and observations started to take shape:
- The 5128 represents a very important step toward closing the gap between the measurements we look at and the headphones as we hear them.
- While I expected most of the differences to be above 8 kHz, with many headphones I also found measurement differences (between the old standard and the 5128) frequently occurred throughout the audio band, including all the way down to the bottom of our hearing range. (We will explore the reasons why later.)
- The 5128 forced me to reconsider many of the hundreds of measurements we had done here at Head-Fi HQ since 2015. (I'll be showing examples in our discussion.)
- Measuring with the 5128 led me to a hypothesis that may have a role in explaining at least some of the differences between the Harman AE/OE (Around-Ear/On-Ear) Target and the Harman IE (In-Ear) Target. (More on this later, too.)
When it comes to correlation between subjective and objective analysis, one example of a headphone I've found to sound quite different from its 711 measurements (found online) is the Westone W60 universal-fit in-ear monitor. (In each earpiece the W60 uses six balanced armature drivers with a 3-way passive crossover.) Here are the 711 measurements of the Westone W60 I found online:
Fig.4: Westone W60 frequency response measurement using a 711 ear simulator, example 1 of 4
Fig.5: Westone W60 frequency response measurement using a 711 ear simulator, example 2 of 4
Fig.6: Westone W60 frequency response measurement using a 711 ear simulator, example 3 of 4
Fig.7: Westone W60 frequency response measurement using a 711 ear simulator, example 4 of 4
Here are the Westone W60 "711" frequency response measurements from Figs. 4 to 7 shown together:
Fig.8: Westone W60 "711" frequency response measurements from Figs. 4 to 7 shown together (normalized at 1 kHz)
If you had not heard the Westone W60 before seeing these measurements, what you see (in each of the measurements in Figs. 4 to 7 above) would suggest that above 5 kHz, the W60's tonal balance is largely characterized by the 10 to 18 decibel peak centered at 8 kHz to 10 kHz, as well as the peak's effects through its lower and higher peripheries. This is due to a resonance that actually is part of the 711 standard.
Is that resonance, however, part of 711's human hearing simulation range? No, it is not. Because these resonant peaks will often shift down and appear within 711's specified range of simulated human hearing when measuring headphones [2], I can understand how this might be a bit confusing. Let us examine this, then, by looking at some key parts of the 711 standard (emphasis by me):
Above 10 kHz, the device does not simulate a human ear, but can be used as an acoustic coupler at additional frequencies up to 16 kHz. Below 100 Hz, the device has not been verified to simulate a human ear but can be used as an acoustic coupler at additional frequencies down to 20 Hz [1].
Again, it might appear that the resonance in Figs.4 to 8 – because it appears at or below 10 kHz in these four measurements – is within the simulation range. However, let's look at another part of the standard:
The length of the principal cavity shall be such as to produce a half-wavelength resonance of the sound pressure at (13.5 ± 1.5) kHz [1].
Measured at the 711 simulator's reference plane, this resonance is outside the 711 standard's human simulation range, its purpose being to help specify the physical geometry of the principal cavity (the primary volume). It is very important to note a couple of things about this part of the specification, though:
- Depending on the headphone being measured (and how it's coupled to the 711 ear simulator), this resonance can shift down to frequencies well within 711's specified human simulation range.
- The IEC 60318-4 (711) standard does not specify the magnitude of this peak, which may be why you see it at four different levels in the four different measurements in Figs. 4 to 8, which can add to measurement uncertainty.
So, while that resonance is not part of a 711 coupler's human simulation, it nevertheless largely defines much of the treble region of the Westone W60 measurements in Figs. 4 to 8. I feel very confident that of those who have heard the Westone W60, most would agree those previous measurements do not correlate well with subjective impressions. And the W60 is just one of many examples of headphones for which I could not previously reconcile what I was hearing with the measurements I was seeing.
Now let's look at measurements of the Westone W60 using the Brüel & Kjær 5128. Though we measured with two different ear tips (stock silicone and stock foam tips), the silicone tips are what comes installed on the W60 out of the box.
Fig.9: Head-Fi-measured Westone W60 frequency response using the Brüel & Kjær Type 5128 HATS
Here (below) is the Westone W60 measurement from the Brüel & Kjær 5128 compared to the previously shown "711" W60 measurements:
Fig.10: Head-Fi-measured Westone W60 frequency response using the Brüel & Kjær Type 5128 HATS compared to the previously shown "711" W60 measurements.
Following is a comparison of the W60 measured on the 5128 compared to the shaded-in measurement range of the 711 measurements shown together (below):
Fig.11 Head-Fi-measured Westone W60 frequency response using the Brüel & Kjær Type 5128 HATS compared to the measurement range from the four previously shown "711" W60 measurements.
Notice also that differences between the 711 and the 5128 measurements of the W60 are not limited to the treble range but appear throughout most of the audio band. Compared to the 5128, all of the 711 measurements in Figs. 4 to 8 show more bass, with two of the four showing what could reasonably be called substantially higher bass levels versus the 5128. Again, if you've listened to the W60, I think there's a very strong chance the measurements of it from the 5128 would correlate much more closely with what you heard versus any of the 711 measurements shown.
We will discuss in Part 2 what causes these noteworthy measurement differences, and we will also look at research that is completely separate from the work behind the 5128 that helps corroborate its findings.
The Brüel & Kjær 5128 was developed to finally simulate average adult human hearing across the full audio range (20 Hz to 20 kHz) for the first time, based on research and development using more modern technologies and techniques developed in the past four decades, and taking 12 years to complete. Some of the names you'll see behind the 5128 research are also names you'll see in the references of the IEC 60318-4 standard. Brüel & Kjær was also instrumental in establishing the 711 standard, having contributed to its development, and being the first with a commercially available IEC 711 simulator with the Brüel & Kjær Type 4157 (which is still commonly in use today).
While, again, we'll discuss that research and development in greater detail in another post, the Brüel & Kjær High-frequency Head and Torso Simulator (HATS) Type 5128 (and the Type 4620 ear simulators within it) involves very important developments versus all previous such standards. To summarize those changes simply for now:
- The anatomical ear canal used in the 5128 is based on average human geometry, from the canal entrance all the way to the eardrum. This allows for more human-like response-shaping, resonances, and damping characteristics [3] [6]. This canal even includes a more realistic smooth soft-to-hard transition (simulating the transition to the bonier condition nearer the eardrum) to help reproduce the correct damping; and an angled coupler attachment to simulate a human's slanted eardrum [3] [4].
- In contrast, the 711 ear simulator's canal geometry is commonly realized as a 7.5mm × 22mm metal tube, with a half-inch microphone terminating one end of that tube, perpendicular to the tube's axis.
- In contrast, the 711 ear simulator's canal geometry is commonly realized as a 7.5mm × 22mm metal tube, with a half-inch microphone terminating one end of that tube, perpendicular to the tube's axis.
- The 5128's middle ear simulator uses more precise, thorough wideband impedance modelling for more detailed, more accurate characterization of the complex acoustical loading of the human ear [5] [6].
- Compared to the 711 simulator's two-branch coupler, the 5128 uses a far more complex four-branch eardrum for more accurate, more detailed, more human-like frequency, resonance, and damping simulation [7].
- The 5128 uses a newly developed prepolarized microphone with a diaphragm that better simulates the dimensions of the human eardrum [7].
- The microphone's diaphragm is also at the front of the microphone/coupler assembly. This allows the diaphragm to terminate the canal at an incline, simulating the slant of the tympanic membrane in relation to the canal, as in a real human ear [3] [4].
- Compared to the 711 simulator's two-branch coupler, the 5128 uses a far more complex four-branch eardrum for more accurate, more detailed, more human-like frequency, resonance, and damping simulation [7].
A new era of headphone measurement arrived with the advent of the Brüel & Kjær 5128. While the 711 standard will no doubt continue with the inertia of a 40-year-old industry standard for the foreseeable future, engineers and enthusiasts will increasingly seek out the new standard for more representative, more meaningful absolute measurements of headphones.
We will continue the discussion of the Brüel & Kjær 5128 in Part 2 of this series soon, including a closer look at the associated research and development, as well as, of course, more measurements. We will also look at separate, corroborating research, discuss measurement observations with the 5128 and a corresponding hypothesis that may help explain some of the key differences between the Harman AE/OE Target and the Harman IE Target.
References
[1] IEC 60318-4 Ed. 1.0 (2010) Simulators of human head and ear – Part 4: Occluded-ear simulator for the measurement of earphones coupled to the ear by means of ear inserts (International Electrotechnical Commission).
[2] Wille, M. (2017). High Resolution Ear Simulator. GRAS Sound & Vibration white paper.
[3] Darkner, S., Sommer, S., Baandrup, A. O., Thomsen, C., & Jønsson, S. An Average of the Human Ear Canal: Recovering Acoustic Properties with Shape Analysis. Cornell Univ. Libr., ArXiv e-prints (2018).
[4] Staab, W. April 2013. Tympanic Membrane – Anatomical Influence on Hearing Aid Fittings. Hearing Health & Technology Matters. https://hearinghealthmatters.org/wa...anatomical-influence-on-hearing-aid-fittings/
[5] S. Jønsson, A. Schuhmacher, H. Ingerslev Jørgensen, Wideband impedance measurement techniques in small complex cavities such as ear simulators and the human ear canal, ArXiv e-prints (2018).
[6] S. Jønsson, A. Schuhmacher, H. Ingerslev Jørgensen, Wideband impedance measurement in the human ear canal; In vivo study on 32 subjects, ArXiv e-prints (2018).
[7] Brüel & Kjær, Design of the new Bruel & Kjaer High Frequency Head and Torso Simulator (HATS) type 5128, presented by Vince Rey at the 2018 ALMA International Symposium & Expo (AISE).
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. The only mod is internal and external wirings/cables mod
