As I have mentioned in the previous posts about optimizing audio in our Mercedes GLK, I used Sennheiser Ambeo Headset as a measurement device in the car. In this challenging acoustical environment it allowed to achieve better channel matching than a conventional measurement microphone. I decided to make a dedicated post about this headset because I've found some interesting applications for it.
Overview
I discovered this headset at the AES Headphones conference where it was used in conjunction with Magic Leap's One AR glasses. By the time when I decided to buy it for my experiments, Sennheiser had already abandoned its production. Nevertheless, it's still possible to buy leftovers from the stock and used gear.
This is how this device looks:
By comparing it with the image on the packing box it's easy to spot a marketing trick. On the box the controlling unit is pictured from the side, making an impression that it's thin and long. However, in reality this unit is pretty thick and looks a bit ugly:
The headphones themselves are designed to be worn around ears, sports-style. They don't however feel sturdy enough like a real sport-style headphone should—yet another perceptual mismatch. Overall, the look of these headphones isn't too exciting, certainly not as appealing as "iconic" Apple earbuds.
Speaking of the technical side, the only connection option offered is Apple Lightning connector. There is also a companion iOS app, however so far I was only using this headset with Android devices and laptops. This becomes possible using Anker's Lightning-to-USB-C adapter which is a must have device if you happen to own any good Lightning headsets and plan to connect them to other devices besides your iPhone. Anker's connector tech specs explicitly lists the Ambeo headset as a compatible device. As a side note, the adaptor also works great with Lightning cables by Audeze.
The controlling unit has a lot of buttons. Besides three usual media controls, there is also a rocking switch toggling between active noise cancelling, "normal" mode, and "transparent hearing"—when the device uses its built-in microphones to allow any external sounds in. This mode is useful because the headphones are designed for in-ear insertion and actually provide a good noise isolation even without active noise cancelling.
Another switch on the controlling unit activates "padding" for the stereo microphones. The designers intended it for use at concerts to avoid clipping during recording.
Speaking of the microphones, since this device was conceived for "3D" recording, besides the usual headset style mono microphone on the right earphone wire, it also has a microphone housed inside left and right earphone:
Before I bought this device I was thinking that the microphones are behind the grilles on the sides of the earphones, but actually the microphone is placed on the inner side of the earphone and faces the reflecting cavity of the pinna:
Overall, from a regular consumer's point of view, the appealing features of this headset are its noise cancelling function and the ability to create entertaining 3D "dummy head"-style recordings. However, the build of the earphones and bulkiness of the controlling unit (and probably relatively high price) most likely worked against its wide adoption.
Earphones
I didn't plan to actively use this headphone for listening to music, but it's still interesting to check what it is capable of. For comparison I'm using very well known and widespread Shure SE215 in-ear phones.
What you will immediately notice with the Ambeo headset is that it's very bright, up to the point when listening to vocal recordings with a bit of extra sibilance becomes unpleasant. My usual tracks for checking this are "Little Wing" performed by Valerie Joyce on "New York Blue" album, and Madonna's "Hang Up" from "Confessions on a Dance Floor".
On the other hand, this brightness also provides a very strong sense of spatiality that can be heard on Hol Baumann's "Endless Park" theme from "Human" album which sounds much duller and more two-dimensional on SE215.
I don't have a rig for measuring headphones, however I was able to capture reliably the high-frequency part of the transfer function of both Ambeo and SE215 by moving them in a free air close to a measurement microphone (a variant of MMA averaging)—the measurements are only valid starting from about 2 kHz. Then I simply divided these transfer functions and found this huge bump around 9 kHz on Ambeo:
To validate my finding, I used an equalizer first to add more high-end to SE215 and then to reduce the harshness of Ambeo, and it worked. The setting of the high-end equalization on Ambeo is extreme. The right setting seems to be somewhere in the middle between Ambeo and SE215—to add a wide peak of +6 dB Q 0.7 centered at 9 kHz to playback via SE215, and to apply a good dip when playing via Ambeo.
The difference in 1–5 kHz region can also be seen and it results in a more "distanced" perception of vocals. I tried adding a -2.5 dB Q 0.7 filter centered at 2 kHz and this helped adding some "depth" to the sounding of SE215 trading for some loss of clarity. Looks like these two settings result in a more "ambient" perception of an audio program. I suppose the reason for this equalization on the Ambeo headset is due to intention to use it primarily for immersive audio playback—playing back the "3D" sound captured with its microphones.
As a side note, I also liked that I found this equalization curve for Shure SE215, which by default sounds more "closer" and two-dimensional. It works even better if crossfeed is added. This experiment has rekindled my interest in SE215.
One problem that I've found at least with my particular Ambeo headset is the mismatch of the earphones transfer function at high frequencies. First I thought that this was due to my bad measurements—I used a DIY coupler to simulate an ear canal, so positioning of the earphone wasn't super precise. But then I also tried the averaging measurement method mentioned above. With both methods, I was always able to match left and right speakers on other in-ear headphones, except for Ambeo which always yielding rather different curves for the left and right earphones (below is the MMA measurement):
So I came to a conclusion that it must be the headset's fault. However, I can't say that I can hear this mismatch clearly, (especially the one in high frequencies). Still, for a headset of this price which has built-in DSP processing leaving this fairly obvious (via measurements) mismatch between left and right channels seems strange to me.
Microphones
Since my primary intended use of this headset was for "dummy head"-style measurements, I was curious to see how well the left and right microphones are matched and how they are tuned. Note that when this headset is connected to a PC (or Mac), it offers both "mono" and "stereo" recording modes. My expectation was that the "mono" mode uses the headset microphone (located on the right earphone wire) which is intended for communications. However, it turned out that the "mono" mode simply uses the left earphone microphone only. So I'm not sure how to activate the headset mic—perhaps when this headset is connected to an iOS device directly, it uses some special mode not available via the Anker adapter. Not a big loss though.
After seeing the mismatch between the outputs of the left and right earphones I was worried whether left and right "3D" mics are suffering from the same issue. I validated them by placing as close as possible to each other in a fixture (not on my head) and measuring the same sound source. Turned out that the mics are actually matched quite well, and we can see very close measurements when coherence is good. On the picture below the measurements are blanked out when coherence is less than 85%:
The tuning of the mics seems to be for "diffuse field"—with a prominent bump at high frequencies. This is important to know as would I try to tune a sound system to a "flat" curve using these mics, this will result in an excessively bright sound. Here is the comparison of measuring the same sound source in the same conditions using a Beyerdynamic MM-1 microphone with "free field" (0 degrees) calibration:
We can see that microphones of Ambeo start sloping up after 2 kHz at approximately 2 dB/octave rate. I wouldn't be paying much attention to other differences as they are likely due to differences in the microphones placement.
The next validation was to see how the transfer function of the microphones differs when they are inserted into ears. Due to the microphone placement, the incoming sound is now transformed by reflections from the pinna and torso. Below is the graph comparing freestanding vs. in ear microphone placements for the same sound source:
As we can see, the main difference is the prominent dip at approximately 4.8 kHz. I'm not a big specialist on anatomy of human hearing, so I can't say what it is caused by exactly. I tried putting a sound absorbing material on my shoulder and this changed nothing, so I suppose this dip is caused by some interference within the pinna. The wavelength corresponding to 4.8 kHz is approx. 7 cm, so half and quarter wavelength fit ear size.
There is a 2–3 dB boost in the speech range (300 Hz to 4 kHz)—I suppose this is thanks to the design of the pinna. And also noticeable a significant loss in high frequencies starting from approximately 14 kHz. This can actually explain why I'm not hearing well the mismatch between the left and the right earphones.
The differences in low frequencies are most likely due to variations of placement of the freestanding vs. in-ear and need to be ignored.
While writing this post I've looked up other reviews of Ambeo headset and found that on iOS it's possible to record at 24/96. Unfortunately the Anker adapter only supports 24/48. However, that's enough for my applications.
Applications
Now let's consider a couple of applications for this headset.
Sound System "offline" Evaluation
It can be useful to capture the produced sound field of a sound system for evaluating it later, perhaps in a more comfortable setting. This is similar to the original function of this headset—capturing 3D sound fields for realistic playback recreating the original environment.
There are great notes by S. Linkwitz of how much our perceptual system can ignore the room and focus on the direct sound of the speakers. However, if we reproduce a binaural recording of the system in a room back using the same system, we immediately start noticing all the room contributions (see the paper "Room Reflections Misunderstood?", Section 5). This is a really interesting experiment to try with this headset.
Note that since Ambeo is a binaural headset, not a spherical stereo microphone, the pinnaes of the person making the recording inevitably color the sound. As we have seen in the section above, the filtering by the pinna is non-negligible. I found that it's best to play back these recordings either on Ambeo itself (no surprise here), or on IEMs with close to direct field equalization. Playing on over-ear headphones or via speakers will "apply" the pinnae filter once again.
"Dummy Head" or "Spherical Microphone" Measurements
This is what I was doing when tuning audio in the car. Since the "room" is very small, and the presence of a human body introduces a significant change in the acoustic environment, using in-ear microphones for left and right speakers alignment produced better results than use of a measurement microphone.
To reiterate, I was using Ambeo only for matching the sound arriving into the left ear from the left speaker to the sound arriving into the right ear from the right speaker by equalizing the speakers. The final tonal adjustment was done using MMA averaging and double-checking with known music tracks. As we saw from the measurement of Ambeo's stereo microphones are well matched and are equalized for diffuse field. The dip around 4.8 kHz that occurs when they are inserted into ears (at least, my ears) must be ignored during sound sources matching.
A note of caution here. Ambeo headset is a digital device, not an analog microphone, and unlike pro audio interfaces it lacks external clock input. Since the audio output from Ambeo only goes into its earphones, one will need to use another digital audio interface for audio output. This is where the problem comes in—with two digital devices not synchronized via "world clock" feed there inevitably will be clock drift between them. To illustrate how bad the resulting measurements can be affected check the graph below:
The red trace is the original EQ filter (BTW, it's the SE215 "improvement" filter I was discussing in the Earphones section, with a bit of bass added), the magenta trace is the same filter as measured by Ambeo headset from a playback done via a separate audio interface. As we can see, there is a very serious spectral shift.
What to do about it? In REW the solution is to use the shortest test impulse (128k):
Using a shorter impulse has worse signal-to-noise ratio (there is more visible noise on the green trace) but at least there is almost no spectral shift. In fact, it's a well known problem with REW when it's used with USB microphones like miniDSP UMIK-1. I've seen several threads on forums where people were wondering why the results of their measurements using different log sweep lengths but otherwise the same setup didn't match. I'm really curious why REW allows for multi-device measurements by default.
The clock drift problem is the reason why Acourate only allows using a single device for input and output. With Acourate it's recommended to use even longer sweeps than REW uses, so attempts to "work around" single device limitation by using drivers like ASIO4All will inevitably lead to a severe spectral shift.
SMAART has a very useful feature for tracking the impulse response delay changes automatically. This is in fact the technique I used when aligning car speakers via Ambeo. I had to experiment with averaging settings to find the one that allowed for more reactive compensation of clock drift. Usually, the shorter the averaging is, the better.
Over-ear Headphones Equalization
Here is the full story of how I obtained the graphs above. I put on the Ambeo headset and then put Audeze EL-8 closed back over-ear headphones on top of it. Then I was playing test sweeps via EL-8 and measuring their output using Ambeo. Crazy, right? I don't think anyone at Sennheiser were considering this application of the Ambeo headset. However, as the last graph demonstrates, this setup can actually be used for measuring acoustically the effects of headphone equalization.
Does it mean this $200–$300 headset plus your own head can replace a head simulator for over-ear headphone measurements? Not quite. The trick with the measurement above was that I didn't move or replace the headphones while doing it, I simply was toggling equalization on and off. This allowed for quite reliable comparison of the measurements before and after equalization. What happens if I remove the headphones from the head, put them back again, and make another measurement? The measurement will be different. As people in the headphone industry know, in order to obtain a reliable measurement of headphones one needs to re-mount them several times and then average all the measurements taken.
This is how the derived EQ looks like when I actually re-installed EL-8 several times and used averaged measurements:
The results at high frequencies are not that reliable anymore, the tolerance is only within 2 dB, which is a lot for headphone measurements. This is what will happen if one will try to compare equalization of different over-ear headphones. So, Ambeo isn't a very precise tool for this task, at least for the whole operating frequency range.
However, Ambeo still provides a good reliable output for low frequencies. And in fact, it's the low frequencies where use of a head simulator is required because headphone drivers can only deliver their full bass output when there is a closed chamber between the driver and the ear drum. That brings an idea—we can use a combined measurement of MMA for high frequencies plus Ambeo for low frequencies.
As I've mentioned in the Earphones section, this is a variation of MMA where we slowly move the headphones near a microphone and wait for the RTA measurement with infinite averaging to stabilize. To demonstrate that the produced measurement can be reliable used, here is the EQ derived from this technique, I was holding EL-8 close to MM-1 and moving it slowly, waiting for 100 sampled measurements to accumulate:
As we can see, there is much better tolerance at high frequencies, but below 1 kHz the data is unreliable. And here is where Ambeo comes to the rescue. By merging together low frequency measurement done by Ambeo on a head with the rest of measurement obtained via MMA we can measure over-ear headphones output reliably.
A note of caution—this method is only good for comparing headphones. We measure one headset, then another, then derive the differences in their equalization. There is no way for measuring absolute frequency response of headphones using this method.
For a practical demonstration I measured filtering applied by Audeze Cipher cable for EL-8. Everyone heard the debates whether or not headphone cables make the sound different. Well, in the case of the Cipher cable vs. analog cable the difference is real because Cipher cable is digital and contains a DSP in it. I noticed that even when the EQ in Audeze app is set to 0 dB at all bands, the sound via Cipher still differs from the sound via analog cable. And I was able to measure this using the technique described above:
For the analog cable measurements I used SPL Phonitor Mini, which has very low output impedance and thus provides adequate bass output. The section of the graph below 1 kHz was obtained from comparing measurements done by the Ambeo headset on my head. There is noise because I turned off FTW gating in SMAART to get a full bass extension. However, we can clearly see that the Cipher cable boosts the bass by almost 3 dB (remember, this is with a "flat" EQ setting in the controlling app!). The section of the graph after 1 kHz was obtained with MMA technique. A "scoop" at middle frequencies can be seen clearly.
I found electrical measurements of the Cipher cable done by user KaiSc on Head-Fi.org. It confirms the 3 dB boost, but not the middle section scoop. Although KaiSc also mentions compression effects from the Cipher DSP at high volume. Since I was testing EL-8 at high volume to obtain adequate free-field output, it's possible that the DSP has thrown in some compression at this point. UPDATE: the "scoop" is a measurement error, see my post.
