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A description of the audio measurement lab here at Head-Fi HQ has been posted in a few different places on Head-Fi's forums, but I thought I'd post it here separately as we prepare to increase our measurement activity. We will also soon be creating a separate area on Head-Fi's forums for posting and discussing our audio measurements -- when we do that, I'll move this thread there.
We'll also use this thread to keep of a record of changes and updates to our audio measurement lab.
To start, here's a video showing how we position headphones on our current headphone measurement fixtures, including the use of real-time instruments:
In the above video we briefly show the gear we've been using to do headphone measurements. You'll find more detailed information about all of that gear below.
Also, since that video was made I have modified the real-time positioning method by reducing the number of FFT points while using the 30 Hz square wave stimulus. In doing this (which I'll shoot a video about later), I've been able to get a clear enough look at the frequency response at a given position, no longer needing to switch to the white noise stimulus (shown in the video above). Long story short, it eliminates one step in the positioning process.
We've been putting together our measurement systems and techniques for over three years now, with a lot of help, knowledge, and feedback from industry mentors that include acoustical engineers and others who make their livings in/around audio measurements. There's always going to be much more learning ahead, no matter how much we do, how much we read, how much we're taught. Also, like anyone else, we've made mistakes along the way, and I'm sure we'll make more going forward -- and while we'll always try our best to avoid making them, we'll own up to them when we do, and learn from them as they're discovered by us or others.
Here's the gear we're currently using in the audio measurement lab at Head-Fi's office for audio measurements:
At the center of our audio measurements -- whether we're doing electronic measurements (DACs, amps, etc.) or headphone measurements -- is an audio analyzer. The audio analyzer generates a stimulus of known characteristics, and then analyzes the response. (Wikipedia has an entry for "audio analyzer" which you can see at the following link for more general information about them: audio analyzer.)
We use the Audio Precision APx555 audio analyzer. In terms of its analog performance, the APx555 has a typical residual THD+N of -120 dB and over 1 MHz bandwidth, which exceeds the analog performance of all other audio analyzers. It will also do FFTs of 1.2 million points and full 24-bit resolution. The Audio Precision APx555 is an incredible tool, whether we're measuring DACs and amps or doing electro-acoustic tests, which is obviously more relevant to this thread's topic.
For some reading about the importance of low test system noise floor, Dan Foley from Audio Precision (one of my audio measurement mentors for over two years) wrote an article for audioXpress titled Test and Measurement: 'I Can Hear It. Why Can't I Measure It?' In this article, Dan uses headphones as a key example, as he discusses the human hearing threshold and how it relates to the noise floor of a sound card interface compared to an audio analyzer's noise floor. You can read this article at the following link:
"Test and Measurement: 'I Can Hear It. Why Can't I Measure It?'"
Is the level of precision of an Audio Precision analyzer needed when measuring audio electronics? Without a doubt. Many of today's DACs and amps -- even many affordable models nowadays -- have levels of distortion and noise below the threshold of most audio analyzers. As of the writing of this post, the Audio Precision APx555 may be the only audio analyzer able to get the full measure of any of the available DACs and amps on the market today.
Is this level of precision needed when doing electroacoustic measurements? Again, I would say yes. Some of the best headphones available today are built to nearly lab-grade standards. Consider the Sennheiser HD800, for example, and measuring its harmonic distortion (which is a routine measurement). When playing a 1 kHz sine stimulus at 90 dBSPL, the second harmonic (2 kHz) is at a measured voltage level of 270 nVrms (nanovolts RMS) -- one nanovolt is one billionth of a volt (0.000000001 volt). The third harmonic (3 kHz) is around 32 nVrms. The fourth harmonic (4 kHz) is at only 18.23 nVrms (at -19.932 dBSPL) -- and with enough isolation around the headphone to attenuate ambient noise, the APx555 (with the GRAS 45CA setup, as described later in this post) can detect that low fourth harmonic above the noise floor (see below):
With the low-noise GRAS 43BB simulators in the GRAS 45BB-12 KEMAR (also described below), the noise floor would be lower still, those harmonic spurs would stand out in even starker relief, and so we should be able discern even higher order harmonics. With the 45BB-12, then, we should be able to measure higher-order harmonic distortion than with the 45CA setup.
We've also added all of Audio Precision's Electro-Acoustic Test Options to the APx555 through their Electro-Acoustic Research & Development option (APX-SW-SPK-RD), which is a suite of measurements intended for designers and engineers who develop electro-acoustic audio products. You'll be seeing some measurements specific to this module from us in the near future, along with explanations of them, including waterfall (CSD) plots.
We're also interested in testing wireless headphones, as that headphone category continues to explode, so another update we're making to our measurement lab very soon is the addition of Audio Precision's new Bluetooth Duo Module to the APx555 audio analyzer. This new Bluetooth module can act as source and sink for AAC, aptX, aptX-HD, aptX-LL, and SBC. Yes, I said aptX-HD, and yes I'm very excited about that. Last year we had two aptX-HD-enabled wireless headphones in our office. Now we have many more, and it will be exciting to be able to measure these headphones at their wireless best. We'll be covering this module more over time.
EDIT 2018-03-12 10:16 EDT: We just received the APx555 back from Audio Precision, with the new Bluetooth Duo module installed (and the APx555 re-certified / re-calibrated in the process).
In the photo above, the Audio Precision APx555 Audio Analyzer (bottom) and Audio Precision APx1701 Transducer Test Interface.
For amplification (to drive the earphones and headphones), we use the Audio Precision APx1701 Transducer Test Interface. The APx1701's functions are directly integrated with and controlled by the APx500 audio measurement software and the APx555 audio analyzer (or other Audio Precision APx model analyzers).
The APx1701 has instrument-grade amplifiers and microphone power supplies, but we do not currently use its mic power supplies (we use GRAS's power modules). The APx1701 is made to drive both loudspeakers and headphones, includes current-sense resistors in the amplifier outputs for impedance measurements, has a signal-to-noise ratio of 134 dB, THD+N is ≤ -105 dB, output impedance is 0.13Ω, and it can drive up to 100W per channel into 8Ω. Given the APx1701's fixed gain of 20 dB, we will sometimes use the Rupert Neve Designs RNHP headphone amplifier (set to unity gain) to drive more sensitive headphones and earphones.
EDIT 2018-08-29 02:25 EDT: We added a THX AAA-888 audio measurement headphone amplifier custom-crafted for us by the team at THX. The THX AAA-888 is the lowest noise, lowest distortion amplifier we have so far measured (first measuring THX prototype AAA-888 boards and then with the Benchmark HPA4). I mentioned to THX's Patrick Flanagan and Andrew Mason that a unity gain THX AAA-888 amplifier would be an ideal headphone amplifier for measurement purposes. Much to our surprise, several months later, THX contacted us to tell us they had custom-built just such an amplifier for our measurement lab.
(Above) The THX AAA-888 audio measurement headphone amplifier custom-built by THX for the Head-Fi Audio Measurement Lab.
Here are some of the key specifications of this amplifier: Dual XLR-3 balanced inputs. XLR-4 (balanced) and 1/4" headphone outputs, and dual XLR-3 outputs for AP probes or other. Output power (clean, unclipped, <<1% THD) is 410 mW/channel into 300Ω; 3850 mW/channel into 32Ω; 6000 mW/channel into 16Ω. Noise is rated at 1.25 uVrms (A-wt), with a signal-to-noise ratio of 139 dB. THD (2V, 32Ω) is -147 dB at 1 kHz, and -137 dB at 10 kH. Gain is 0 dB (unity gain). Rated impedance is 8Ω to 1000Ω.
These are incredible specifications. I'll be saying more about this custom THX AAA-888 amplifier soon to explain its role in our measurement systems. Thank you, Team THX!
For headphone measurements, we're working with a head or fixture with more or less fixed dimensions (defined by international standards) representing average human dimensions, and those are the GRAS 45CA headphone test fixture and the GRAS 45BB-12 KEMAR test manikin with anthropometric pinnae for low-noise earphone and headphone test.
.
In the photo (above left), KEMAR is face-forward in our measurement lab, inside a custom Herzan acoustic/vibration isolation enclosure (more information below).
The pinnae / ear canals we use on the GRAS 45BB-12 KEMAR are anthropometric, based on 260 three-dimensional scans of human ear canals. These pinnae include the first bend and the second bend of the canals, with "flesh" all the way to the mics. Because they're more anatomically representative than traditional measurement pinnae, they have (among other features) a more realistic, more oval-shaped entrance point. Here's a photo of a standard measurement pinna:
Here is a photo showing the new anthropometric pinna / canal:
Here's the same headphone on the new anthropometric pinnae on the GRAS 45BB-12 KEMAR:
Where in-ears are concerned, we've found the new pinnae/canals to help tremendously with more realistic and consistent placement, as the pinnae/canals are definitely more human-like now. With this improved realism we've found, for example, that characterizing the differences between eartip types via measurements (relative to our subjective experiences) is improved.
Additionally, I think the dimensional characteristics of the rig we're currently using (a GRAS KEMAR) might also contribute to some of the differences (especially versus the type of DIY rigs shown in the photos above). KEMAR has head shape characteristics that lead to more dimensional limitations on placement -- more human-like limitations, in my opinion. Whereas on a flat plate coupler you can place the headphone in any number of places and still maintain a seal (and thus maintain bass, the loss of which is one of the primary indicators that fit has gone wrong). On humans -- and on KEMAR -- if you go too far back, the curvature of our head can break the seal. Too far down on a human (and KEMAR) and you can also lose seal. In other words, the dimensional limitations of our anatomy -- and the larger the headphone, the more this may come into play -- play a role in guiding and limiting the placement range of the headphone over our ears in actual use. In the frequency ranges we're talking about (as the wavelengths get shorter), minor shifts in placement and dimensions can have substantial effects.
Unlike most DIY measurement rigs, the measurement manikin (GRAS 45BB-12) and fixture (GRAS 45CA) use ear simulators to simulate the input and transfer impedance of a human ear. The GRAS 43BB ear simulators in this specific KEMAR configuration are quite different than standard 60318-4 simulators. While they still meet the IEC 60318-4 tolerances, the single high-Q resonance above 10 kHz is replaced by two more balanced, more damped resonances. The splitting of the one high-Q resonance into two low-Q resonances may present an advantage in decreasing the uncertainty in the measurements around the resonance (above 10 kHz). Also, the GRAS 43BB is highly sensitive, and very low-noise, and extends the lower dynamic range below the threshold of human hearing. Given its extremely low-noise nature, the 43BB can be used to measure and characterize things like the self-noise of an active headphone (both with and without active noise canceling), which is something we'll be increasingly interested in with the growing prominence of high-fidelity wireless headphones and earphones. It can also help in measuring low-level distortion in headphones and earphones. NOTE: One thing to consider with this low-noise simulator is that it's not suited to very-high-SPL measurements, with an upper limit of the dynamic range to about 110 dBSPL. This hasn't been an issue for us, though, as most of our measurements are set at 90 dBSPL (at 1 kHz), and we're also now incorporating additional ear simulators with much higher dynamic range upper limit (the GRAS RA0401, discussed immediately below).
Here's a whitepaper about the GRAS 43BB Low Noise Ear Simulator
In late 2017, GRAS announced still another ear simulator designed specifically for measuring high-resolution headphones. We recently took delivery of the new GRAS RA0401 High Resolution Ear Simulators, and we've installed them on our GRAS 45CA, along with the new anthropometric pinnae for the GRAS 45CA.
This is still another very exciting development for headphone measurements, as obtaining meaningful measurements above 8 kHz with most systems can be enormously challenging. This new GRAS RA0401 High Resolution Ear Simulator also meets the IEC 60318-4 tolerances, but GRAS was able to design it so that its performance from 10 kHz to 20 kHz is substantially improved, that range through which it has a tolerance of +/- 2.2 dB. Here is a graph showing the RA0401's response (including the IEC 60318-4 tolerances) compared to a standard 60318-4 ear simulator:
Again, meaningful headphone measurements above 8 kHz or 10 kHz have been a major pain point for decades, so I think these new GRAS High Resolution Ear Simulators may prove an important development in the world of headphone testing.
Here's a whitepaper about the GRAS High Resolution Ear Simulator
We've transitioned almost entirely to the GRAS 45CA with the RA0401 simulators as our primary headphone test fixture for now, and will keep it that way for the next couple of months, to build up a bigger database of measurements with these simulators. That said, we still have plans to take advantage of the GRAS 43BB low-noise ear simulators for more progressive measurements that their extreme sensitivity affords.
To help improve the quality of the measurements, we wanted to maximize environmental isolation. Though we obviously do not have the space (not to mention the budget) to build a full walk-in anechoic chamber, we still wanted to achieve as much acoustic and vibration isolation as reasonably possible. Skylar Gray (formerly of AudioQuest, now with Definitive Technology) recommended we contact Herzan. We worked with Herzan to carefully spec out a custom-built acoustic and vibration isolation enclosure. Our Herzan acoustic / vibration isolation enclosure has thick walls, made with 11 variable density layers of sound-damping material, and the interior of the enclosure is lined with acoustic sound absorption foam. This enclosure is (by design) fairly massive, weighing around 1200 pounds -- the more mass there is, the more energy it takes to excite the system. The enclosure has two cable ports, both covered with solid machined metal screw-down blocks that are damped, and also have soft gaskets that allow full sealing around the cables. (See photo below.)
The headphone measurement manikin or fixture being used at the time is placed on a Herzan Onyx-6M vibration isolation table inside the enclosure to further help isolate the system from vibrations caused by foot traffic, HVAC systems, vehicle traffic, etc. The Onyx-6M is essentially a steel tabletop supported by pneumatic isolators, and with a resonant frequency of 3.5 Hz, it provides vibration isolation beginning at 4.5 Hz (improving as the frequency goes up).
(Above left) Closed left-side cable port on the Herzan acoustic/vibration isolation enclosure.
Even with the Herzan enclosure, we still make sure to keep it as quiet as we can in the office while doing measurements. Both of the office HVAC systems are switched off when we measure. You'll frequently hear the tongue-in-cheek cry, "Measurement in 3...2...1...fire in the hole!" before we start a measurement, and everyone remains quiet until an all-clear is given. We're particularly careful about this when using the GRAS 43BB ear simulators because, again, their dynamic range extends below the threshold of human hearing -- so even the very faintest sounds you can hear can be heard by the 43BB's.
System Calibration
Keeping all of the measurement equipment calibrated and maintained is obviously important. We do the calibrations we can do at the office. We use GRAS pistonphones to regularly calibrate the measurement microphones and systems. Ideally, we would do this before every measurement session -- we do it no more than once per month, corrected for temperature and ambient static pressure at the time of the calibration. We run Audio Precision's APx Self Test utility on the APx555 from time to time to confirm proper operation of its analog and digital sections. (We have not yet run that utility on the APx1701, which is still quite new.) We do occasionally measure the Neve RNHP on the APx555 just to confirm the consistency of its performance and to confirm unity gain. We intend to adhere to the manufacturers' maintenance and calibration schedules, and will send the gear to them for check-up and calibration accordingly.
We're in the process of shooting multi-part Head-Fi TV videos to discuss the components of the measurement systems in Head-Fi's audio measurement lab, including some examples of them in actual use. One of the things I'm excited to show is the use of the real-time instruments available in the Bench Mode of Audio Precision's APx software to aid in headphone placement (for measuring). While others may use the instruments and techniques we're using to set headphone positioning, we're not specifically aware of anyone who does -- it is something we hadn't seen previously and something we've received positive feedback about from the engineers we've showed it to.
Like I said earlier, there's always going to be much more learning ahead, no matter how much we do, how much we read, how much we're taught. And like humans do, we've made mistakes along the way, and I'm sure we'll make more going forward -- and while we'll always try our best to avoid making them, we'll own up to them when we do, and learn from them as they're discovered by us or others.
We'll also use this thread to keep of a record of changes and updates to our audio measurement lab.
To start, here's a video showing how we position headphones on our current headphone measurement fixtures, including the use of real-time instruments:
In the above video we briefly show the gear we've been using to do headphone measurements. You'll find more detailed information about all of that gear below.
Also, since that video was made I have modified the real-time positioning method by reducing the number of FFT points while using the 30 Hz square wave stimulus. In doing this (which I'll shoot a video about later), I've been able to get a clear enough look at the frequency response at a given position, no longer needing to switch to the white noise stimulus (shown in the video above). Long story short, it eliminates one step in the positioning process.
We've been putting together our measurement systems and techniques for over three years now, with a lot of help, knowledge, and feedback from industry mentors that include acoustical engineers and others who make their livings in/around audio measurements. There's always going to be much more learning ahead, no matter how much we do, how much we read, how much we're taught. Also, like anyone else, we've made mistakes along the way, and I'm sure we'll make more going forward -- and while we'll always try our best to avoid making them, we'll own up to them when we do, and learn from them as they're discovered by us or others.
Here's the gear we're currently using in the audio measurement lab at Head-Fi's office for audio measurements:
The Audio Analyzer: Audio Precision APx555 and APx1701
At the center of our audio measurements -- whether we're doing electronic measurements (DACs, amps, etc.) or headphone measurements -- is an audio analyzer. The audio analyzer generates a stimulus of known characteristics, and then analyzes the response. (Wikipedia has an entry for "audio analyzer" which you can see at the following link for more general information about them: audio analyzer.)
We use the Audio Precision APx555 audio analyzer. In terms of its analog performance, the APx555 has a typical residual THD+N of -120 dB and over 1 MHz bandwidth, which exceeds the analog performance of all other audio analyzers. It will also do FFTs of 1.2 million points and full 24-bit resolution. The Audio Precision APx555 is an incredible tool, whether we're measuring DACs and amps or doing electro-acoustic tests, which is obviously more relevant to this thread's topic.
For some reading about the importance of low test system noise floor, Dan Foley from Audio Precision (one of my audio measurement mentors for over two years) wrote an article for audioXpress titled Test and Measurement: 'I Can Hear It. Why Can't I Measure It?' In this article, Dan uses headphones as a key example, as he discusses the human hearing threshold and how it relates to the noise floor of a sound card interface compared to an audio analyzer's noise floor. You can read this article at the following link:
"Test and Measurement: 'I Can Hear It. Why Can't I Measure It?'"
Is the level of precision of an Audio Precision analyzer needed when measuring audio electronics? Without a doubt. Many of today's DACs and amps -- even many affordable models nowadays -- have levels of distortion and noise below the threshold of most audio analyzers. As of the writing of this post, the Audio Precision APx555 may be the only audio analyzer able to get the full measure of any of the available DACs and amps on the market today.
Is this level of precision needed when doing electroacoustic measurements? Again, I would say yes. Some of the best headphones available today are built to nearly lab-grade standards. Consider the Sennheiser HD800, for example, and measuring its harmonic distortion (which is a routine measurement). When playing a 1 kHz sine stimulus at 90 dBSPL, the second harmonic (2 kHz) is at a measured voltage level of 270 nVrms (nanovolts RMS) -- one nanovolt is one billionth of a volt (0.000000001 volt). The third harmonic (3 kHz) is around 32 nVrms. The fourth harmonic (4 kHz) is at only 18.23 nVrms (at -19.932 dBSPL) -- and with enough isolation around the headphone to attenuate ambient noise, the APx555 (with the GRAS 45CA setup, as described later in this post) can detect that low fourth harmonic above the noise floor (see below):
With the low-noise GRAS 43BB simulators in the GRAS 45BB-12 KEMAR (also described below), the noise floor would be lower still, those harmonic spurs would stand out in even starker relief, and so we should be able discern even higher order harmonics. With the 45BB-12, then, we should be able to measure higher-order harmonic distortion than with the 45CA setup.
We've also added all of Audio Precision's Electro-Acoustic Test Options to the APx555 through their Electro-Acoustic Research & Development option (APX-SW-SPK-RD), which is a suite of measurements intended for designers and engineers who develop electro-acoustic audio products. You'll be seeing some measurements specific to this module from us in the near future, along with explanations of them, including waterfall (CSD) plots.
We're also interested in testing wireless headphones, as that headphone category continues to explode, so another update we're making to our measurement lab very soon is the addition of Audio Precision's new Bluetooth Duo Module to the APx555 audio analyzer. This new Bluetooth module can act as source and sink for AAC, aptX, aptX-HD, aptX-LL, and SBC. Yes, I said aptX-HD, and yes I'm very excited about that. Last year we had two aptX-HD-enabled wireless headphones in our office. Now we have many more, and it will be exciting to be able to measure these headphones at their wireless best. We'll be covering this module more over time.
EDIT 2018-03-12 10:16 EDT: We just received the APx555 back from Audio Precision, with the new Bluetooth Duo module installed (and the APx555 re-certified / re-calibrated in the process).
In the photo above, the Audio Precision APx555 Audio Analyzer (bottom) and Audio Precision APx1701 Transducer Test Interface.
For amplification (to drive the earphones and headphones), we use the Audio Precision APx1701 Transducer Test Interface. The APx1701's functions are directly integrated with and controlled by the APx500 audio measurement software and the APx555 audio analyzer (or other Audio Precision APx model analyzers).
The APx1701 has instrument-grade amplifiers and microphone power supplies, but we do not currently use its mic power supplies (we use GRAS's power modules). The APx1701 is made to drive both loudspeakers and headphones, includes current-sense resistors in the amplifier outputs for impedance measurements, has a signal-to-noise ratio of 134 dB, THD+N is ≤ -105 dB, output impedance is 0.13Ω, and it can drive up to 100W per channel into 8Ω. Given the APx1701's fixed gain of 20 dB, we will sometimes use the Rupert Neve Designs RNHP headphone amplifier (set to unity gain) to drive more sensitive headphones and earphones.
EDIT 2018-08-29 02:25 EDT: We added a THX AAA-888 audio measurement headphone amplifier custom-crafted for us by the team at THX. The THX AAA-888 is the lowest noise, lowest distortion amplifier we have so far measured (first measuring THX prototype AAA-888 boards and then with the Benchmark HPA4). I mentioned to THX's Patrick Flanagan and Andrew Mason that a unity gain THX AAA-888 amplifier would be an ideal headphone amplifier for measurement purposes. Much to our surprise, several months later, THX contacted us to tell us they had custom-built just such an amplifier for our measurement lab.
(Above) The THX AAA-888 audio measurement headphone amplifier custom-built by THX for the Head-Fi Audio Measurement Lab.
Here are some of the key specifications of this amplifier: Dual XLR-3 balanced inputs. XLR-4 (balanced) and 1/4" headphone outputs, and dual XLR-3 outputs for AP probes or other. Output power (clean, unclipped, <<1% THD) is 410 mW/channel into 300Ω; 3850 mW/channel into 32Ω; 6000 mW/channel into 16Ω. Noise is rated at 1.25 uVrms (A-wt), with a signal-to-noise ratio of 139 dB. THD (2V, 32Ω) is -147 dB at 1 kHz, and -137 dB at 10 kH. Gain is 0 dB (unity gain). Rated impedance is 8Ω to 1000Ω.
These are incredible specifications. I'll be saying more about this custom THX AAA-888 amplifier soon to explain its role in our measurement systems. Thank you, Team THX!
GRAS KEMAR, 45CA, Ear Simulators, and Pinnae / Canals
For headphone measurements, we're working with a head or fixture with more or less fixed dimensions (defined by international standards) representing average human dimensions, and those are the GRAS 45CA headphone test fixture and the GRAS 45BB-12 KEMAR test manikin with anthropometric pinnae for low-noise earphone and headphone test.
.
In the photo (above left), KEMAR is face-forward in our measurement lab, inside a custom Herzan acoustic/vibration isolation enclosure (more information below).
The pinnae / ear canals we use on the GRAS 45BB-12 KEMAR are anthropometric, based on 260 three-dimensional scans of human ear canals. These pinnae include the first bend and the second bend of the canals, with "flesh" all the way to the mics. Because they're more anatomically representative than traditional measurement pinnae, they have (among other features) a more realistic, more oval-shaped entrance point. Here's a photo of a standard measurement pinna:
Here is a photo showing the new anthropometric pinna / canal:
Another advantage of the new pinnae is their increased realism externally. If you've ever felt measurement pinnae, they're typically stiffer than human pinnae and don't readily compress against the head, which is why many who measure headphones often have difficulty measuring supra-aural (on-the-ear) headphones with them. (They can also present problems with measuring shallow-cup circumaurals.) The new pinnae feel and move much more like human pinnae and compress against the head much more like (most) people's pinnae do. Here is a photo I took of the supra-aural Audeze Sine on the standard measurement pinnae on a GRAS 45CA:Here's the same headphone on the new anthropometric pinnae on the GRAS 45BB-12 KEMAR:
Where in-ears are concerned, we've found the new pinnae/canals to help tremendously with more realistic and consistent placement, as the pinnae/canals are definitely more human-like now. With this improved realism we've found, for example, that characterizing the differences between eartip types via measurements (relative to our subjective experiences) is improved.
Additionally, I think the dimensional characteristics of the rig we're currently using (a GRAS KEMAR) might also contribute to some of the differences (especially versus the type of DIY rigs shown in the photos above). KEMAR has head shape characteristics that lead to more dimensional limitations on placement -- more human-like limitations, in my opinion. Whereas on a flat plate coupler you can place the headphone in any number of places and still maintain a seal (and thus maintain bass, the loss of which is one of the primary indicators that fit has gone wrong). On humans -- and on KEMAR -- if you go too far back, the curvature of our head can break the seal. Too far down on a human (and KEMAR) and you can also lose seal. In other words, the dimensional limitations of our anatomy -- and the larger the headphone, the more this may come into play -- play a role in guiding and limiting the placement range of the headphone over our ears in actual use. In the frequency ranges we're talking about (as the wavelengths get shorter), minor shifts in placement and dimensions can have substantial effects.
Unlike most DIY measurement rigs, the measurement manikin (GRAS 45BB-12) and fixture (GRAS 45CA) use ear simulators to simulate the input and transfer impedance of a human ear. The GRAS 43BB ear simulators in this specific KEMAR configuration are quite different than standard 60318-4 simulators. While they still meet the IEC 60318-4 tolerances, the single high-Q resonance above 10 kHz is replaced by two more balanced, more damped resonances. The splitting of the one high-Q resonance into two low-Q resonances may present an advantage in decreasing the uncertainty in the measurements around the resonance (above 10 kHz). Also, the GRAS 43BB is highly sensitive, and very low-noise, and extends the lower dynamic range below the threshold of human hearing. Given its extremely low-noise nature, the 43BB can be used to measure and characterize things like the self-noise of an active headphone (both with and without active noise canceling), which is something we'll be increasingly interested in with the growing prominence of high-fidelity wireless headphones and earphones. It can also help in measuring low-level distortion in headphones and earphones. NOTE: One thing to consider with this low-noise simulator is that it's not suited to very-high-SPL measurements, with an upper limit of the dynamic range to about 110 dBSPL. This hasn't been an issue for us, though, as most of our measurements are set at 90 dBSPL (at 1 kHz), and we're also now incorporating additional ear simulators with much higher dynamic range upper limit (the GRAS RA0401, discussed immediately below).
Here's a whitepaper about the GRAS 43BB Low Noise Ear Simulator
In late 2017, GRAS announced still another ear simulator designed specifically for measuring high-resolution headphones. We recently took delivery of the new GRAS RA0401 High Resolution Ear Simulators, and we've installed them on our GRAS 45CA, along with the new anthropometric pinnae for the GRAS 45CA.
This is still another very exciting development for headphone measurements, as obtaining meaningful measurements above 8 kHz with most systems can be enormously challenging. This new GRAS RA0401 High Resolution Ear Simulator also meets the IEC 60318-4 tolerances, but GRAS was able to design it so that its performance from 10 kHz to 20 kHz is substantially improved, that range through which it has a tolerance of +/- 2.2 dB. Here is a graph showing the RA0401's response (including the IEC 60318-4 tolerances) compared to a standard 60318-4 ear simulator:
Again, meaningful headphone measurements above 8 kHz or 10 kHz have been a major pain point for decades, so I think these new GRAS High Resolution Ear Simulators may prove an important development in the world of headphone testing.
Here's a whitepaper about the GRAS High Resolution Ear Simulator
We've transitioned almost entirely to the GRAS 45CA with the RA0401 simulators as our primary headphone test fixture for now, and will keep it that way for the next couple of months, to build up a bigger database of measurements with these simulators. That said, we still have plans to take advantage of the GRAS 43BB low-noise ear simulators for more progressive measurements that their extreme sensitivity affords.
Herzan Acoustic / Vibration Isolation Enclosure
To help improve the quality of the measurements, we wanted to maximize environmental isolation. Though we obviously do not have the space (not to mention the budget) to build a full walk-in anechoic chamber, we still wanted to achieve as much acoustic and vibration isolation as reasonably possible. Skylar Gray (formerly of AudioQuest, now with Definitive Technology) recommended we contact Herzan. We worked with Herzan to carefully spec out a custom-built acoustic and vibration isolation enclosure. Our Herzan acoustic / vibration isolation enclosure has thick walls, made with 11 variable density layers of sound-damping material, and the interior of the enclosure is lined with acoustic sound absorption foam. This enclosure is (by design) fairly massive, weighing around 1200 pounds -- the more mass there is, the more energy it takes to excite the system. The enclosure has two cable ports, both covered with solid machined metal screw-down blocks that are damped, and also have soft gaskets that allow full sealing around the cables. (See photo below.)
The headphone measurement manikin or fixture being used at the time is placed on a Herzan Onyx-6M vibration isolation table inside the enclosure to further help isolate the system from vibrations caused by foot traffic, HVAC systems, vehicle traffic, etc. The Onyx-6M is essentially a steel tabletop supported by pneumatic isolators, and with a resonant frequency of 3.5 Hz, it provides vibration isolation beginning at 4.5 Hz (improving as the frequency goes up).
(Above left) Closed left-side cable port on the Herzan acoustic/vibration isolation enclosure.
Even with the Herzan enclosure, we still make sure to keep it as quiet as we can in the office while doing measurements. Both of the office HVAC systems are switched off when we measure. You'll frequently hear the tongue-in-cheek cry, "Measurement in 3...2...1...fire in the hole!" before we start a measurement, and everyone remains quiet until an all-clear is given. We're particularly careful about this when using the GRAS 43BB ear simulators because, again, their dynamic range extends below the threshold of human hearing -- so even the very faintest sounds you can hear can be heard by the 43BB's.
System Calibration
Keeping all of the measurement equipment calibrated and maintained is obviously important. We do the calibrations we can do at the office. We use GRAS pistonphones to regularly calibrate the measurement microphones and systems. Ideally, we would do this before every measurement session -- we do it no more than once per month, corrected for temperature and ambient static pressure at the time of the calibration. We run Audio Precision's APx Self Test utility on the APx555 from time to time to confirm proper operation of its analog and digital sections. (We have not yet run that utility on the APx1701, which is still quite new.) We do occasionally measure the Neve RNHP on the APx555 just to confirm the consistency of its performance and to confirm unity gain. We intend to adhere to the manufacturers' maintenance and calibration schedules, and will send the gear to them for check-up and calibration accordingly.
We're in the process of shooting multi-part Head-Fi TV videos to discuss the components of the measurement systems in Head-Fi's audio measurement lab, including some examples of them in actual use. One of the things I'm excited to show is the use of the real-time instruments available in the Bench Mode of Audio Precision's APx software to aid in headphone placement (for measuring). While others may use the instruments and techniques we're using to set headphone positioning, we're not specifically aware of anyone who does -- it is something we hadn't seen previously and something we've received positive feedback about from the engineers we've showed it to.
Like I said earlier, there's always going to be much more learning ahead, no matter how much we do, how much we read, how much we're taught. And like humans do, we've made mistakes along the way, and I'm sure we'll make more going forward -- and while we'll always try our best to avoid making them, we'll own up to them when we do, and learn from them as they're discovered by us or others.
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