By not including the driver symmetry info though, what you're effectively doing is asking your readers to put all of their faith in your ears on that issue....
Actually,
@ADUHF, I have what is not so much a counter to what you're saying, as much as it is something else to consider.
While the readers/viewers would be expected to put their faith in you in this specific regard, they'd also need to put some faith in you and your equipment to be able to properly tease out and show symmetry. Even if a pair of headphones has perfect left-right symmetry, it does require both the instruments and methods to show that reliably, consistently. Even the slightest changes in position -- especially in the higher frequencies, but also at times in other parts of the audio band -- can show significant changes. Also, there's the importance of making sure the measurement system -- from end to end -- is as perfectly calibrated as possible.
It's one thing if we could reasonably expect people who've never done a headphone measurement to understand this, but most will just interpret any lack of perfect symmetry as being on the side of the headphone (which it may be), not the measuring process (which it also may be, but a variable often (and understandably) overlooked, especially by those who've not yet measured a headphone).
Regarding the equipment:
It
definitely helps if you can measure both sides at the same time. Again, assuming a given headphone is perfectly symmetrical, that symmetry
can be teased out measuring one ear at a time, but it would be
much more challenging to do this way. It would also be much more challenging to do
precisely one ear at a time over several seatings, absent the ability to monitor both channels in real time during placement.
Also, some of the more popular one-ear-at-at-a-time setups use GRAS KEMAR pinnae. While the latest KEMAR pinnae are available in symmetrical pairs, they're symmetrical from the pinnae flanges into the canals, but there's some asymmetry at the pinnae bases to accommodate for KEMAR's slightly asymmetrical head shape. While this shouldn't be an issue measuring in-ears, it could lead to asymmetry when measuring over-ears that do engage those differences in the pinnae bases. (I would expect these differences to be small, but, nevertheless, differences that result from this would certainly be attributed to the headphones by most readers.)
One advantage to the one-ear-at-a-time method is it's not as expensive, and you're guaranteed symmetry with the simulator itself (independent of the pinnae base differences mentioned above), as (obviously) you'd be using the same simulator/coupler for both sides. That said, at the level of precision (that you pay mightily for) of the type of simulators made by Brüel & Kjær and GRAS, you can expect symmetry in a pair. Keep in mind, though, that level calibration will still be necessary, as sensitivity varies even in shipped simulator pairs (even though their transfer functions are identical). In addition to the price of the additional simulator, then, there's the price of something like a piston phone to consider (which can also be quite expensive).
Regarding the method(s):
Even if you have the symmetry of the simulators accounted for, one must also keep in mind the methods (and instrumentation) necessary to take advantage of a hypothetically perfectly left-right symmetrical headphone. It helps to be able to see the frequency response (or some approximation of it)
in real time for both channels as you're positioning the headphones.
We use an Audio Precision analyzer (with its APx software) which helps a great deal with that. With APx we can use a low frequency square wave during placement (which is not novel). What
is novel is that APx lets us monitor at least three key things at once
during placement:
- The RMS level of the low frequency square wave (to help confirm a seal);
- The view of the waveform in scope view (again, to help confirm a seal, and to give us one more thing to get symmetrical);
- And crucially it also gives us a real-time (downward-slanting) approximation of the frequency response of both channels at the same time in the FFT Spectrum Monitor window. NOTE: It helps to reduce the FFT length (2000 points is usually good) to force contours instead of spurs (of the odd-order harmonics), and also to engage a little bit of averaging to settle things down a bit. If you understand what square waves are, then you won't be surprised that the frequency response appears downward-slanted when doing this.
While it's a simple method, I had not seen it used before, and presented a slightly different version of it at the ALMA (now ALTI) annual conference back in 2018 to a roomful of engineers (from Harman, Brüel & Kjær, Audio Precision, GRAS, Knowles, and many other companies). I also showed it in a video here back then, too. Since then I simplified it a bit and have again shown it to many engineers in the industry with positive feedback.
Long story short, I pretty much agree with what
@Resolve is saying about this.
Let's take a look at a headphone (the Dan Clark Audio Stealth) that has excellent symmetry, but whose excellent symmetry might be challenging to tease out measuring one channel at a time, or with a less than perfectly calibrated two-ear setup, or with a method/setup that doesn't allow for monitoring frequency response in real time.
Fig.1 (above): Dan Clark Audio Stealth, Seat 01, Measured 37 (unsmoothed).
Fig.2 (above): Dan Clark Audio Stealth, Seat 02, Measured 44 (unsmoothed).
Fig.3 (above): Dan Clark Audio Stealth, Seat 03, Measured 52 (unsmoothed).
Fig.4 (above): Dan Clark Audio Stealth, Seat 04, Measured 56 (unsmoothed).
Fig.5 (above): Dan Clark Audio Stealth, Seat 05, Measured 59 (unsmoothed).
Fig.6 (above): Dan Clark Audio Stealth, five-seat average (smoothing = 1/12 octave) versus the five individual seatings (unsmoothed). The smoothed average is what appeared in our video about the Stealth.
Fig.7 (above): Dan Clark Audio Stealth, five-seat average (unsmoothed) versus the five individual seatings (unsmoothed).
As you can see in Seat 03 (Measured 52) in Figure 3 above, our left-right symmetry wasn't as tight as it was in the other seatings. Still, we found this a reasonable seating, and it was used in the average. If we published that seating alone, I can assure you most would conclude the right channel of the tested Stealth has more treble than the left. You can see from the other seatings that would be an unfair conclusion.
I know a couple of people found our measurement notes interesting (when I posted them in at least one other thread), so here they are for these seatings:
Measured 37: DCA Stealth FINAL - 5128 - RAW - 390.7 mVrms WN80 - seat 01 (brian) - test 01 - cups maybe slightly back of center - went back to 0 dBRG after last measure (Measured 36) Measured 44: DCA Stealth FINAL - 5128 - RAW - 434.0 mVrms WN80 - seat 02 (jude) - test 02 - pulled forward from previous seating - pushed cups a touch vertically up Measured 52: DCA Stealth FINAL - 5128 - RAW - 374.0 mVrms WN80 - seat 03 (brian/jude) - test 02 - neutral-back seating (brian's 5128 photo seating, jude squeezed it tighter) Measured 56: DCA Stealth FINAL - 5128 - RAW - 402.2 mVrms WN80 - seat 04 (jude) - test 02 - neutral seating, maybe down a bit vertically from previous seating Measured 59: DCA Stealth FINAL - 5128 - RAW - 404.4 mVrms WN80 - seat 05 (brian) - test 01 - neutral seating |
Anyway, again, I pretty much agree with what
@Resolve is saying about this. There's definitely more to consider than might be obvious, even with something as seemingly straightforward as left-right channel symmetry.
2021-08-23 1724 EDT EDIT: I should have probably clarified (since we've not shown this before) that when I show
"80 dBSPL white noise" in the measurements, we're using white noise to set the voltage level, after which we run a sweep to get the frequency response. I'll try to make this clearer going forward. Some of you probably figured out why I'm playing with this, and it's something I'll talk more about later.
The measurements in this post were made in our measurement lab at Head-Fi HQ using the following equipment: