to preserve the article I waybacked it and uploaded here for those interested
Skullcandy's Director of Electrical & Acoustical Engineering, Dr. Tetsuro Oishi Visits InnerFidelity!
By
Tyll Hertsens • Posted: Mar 5, 2012
Dr. Tetsuro Oishi
Tet is from Japan and came to the U.S. to attend U. Mass at Dartmouth for his advanced degrees in acoustics and electrical engineering. There he designed and fabricated various prototype transducers for underwater applications such as directional broadband piezoelectric transducers and arrays, acoustic motion sensors, and bioacoustic transducers. He also developed a water pollution detection system using the acoustic resonance spectroscopy technique. Wow, cool stuff.
Out of school he went to work for Bose's Automotive division modeling audio in cars, and then into the headphone division where he worked on noise canceling technologies. He also had a hand in the AE2 design.
Early in 2010, Skullcandy recruited him to lead its effort to improve the audio design and quality of its headphones. Yet another piece of hard evidence that Skullcandy intends to become a strong and well-founded maker of headphones.
The Hesh 2.0
The Skullcandy Hesh ($50) has been around for some time. The Hesh 2.0 ($59-$69 MSRP, likely available in April at Target and Best Buy) is a complete revision, and is Tet's first full effort at Skullcandy. This headphone is a medium-low cost circumaural, sealed headphone; designed for a big-bass sound; and no doubt a potentially very popular headphone among the action sports crowd looking to upgrade from junker headphones.
I put the Hesh 2.0s through the measurements routine with Tet looking on, and we spent a good bit of time discussing how to get a good fit and seal on the head. Over the last year I've developed some techniques for getting the job done, and I was heartened to hear that Tet uses some of the very same methods in his testing. (Sorry, I've got some competing publications measuring headphones, so I'm not going to detail these techniques.)
The Hesh 2.0
measured fairly flat to 1kHz with, what I consider, an appropriate roll-off to 4kHz, and then a reduction in overall treble energy about 5-10dB reduced beyond what I would consider flat.
Subsequent to our visit, I spent a good bit of time listening to these headphones, and comparing them to the Sony MDR-XB500 and Audio-Technica ATH-WS55. I felt the Hesh and WS55 easily bested the XB500, which sounded woolly and bloated in comparison. The WS55 emphasized the bass and the mid/upper treble a bit more, but the Hesh seemed more even and coherent. I think the choice of having good bass extention; keeping the bass properly related to the mids; and then rolling off the highs to get rid of crappy harshness from compressed files, and give the impression of big bass, is the way to go for this type of can. (Overwhelming bass is overwhelming.) I think the Hesh 2.0 is going to be the go-to recommendation for the hoodie wearing, bass-heads out there.
General Headphone Talk
Tet gave me some super help interpreting headphone measurement data (and we'll get to that on the next page), but we spent quite a bit of time just talking about headphones in general. Well ... that's not quite right, we talked about a wide variety of very particular aspects of headphones. Because I'm free to opine, and he's constrained by the nature of his job, I'll quickly rattle through a few of the topics we covered and what my take was:
- Headphone mini-plugs - Should be slender and have enough "neck" on them to reach into the jack while the player has a protective cover on it. Plugs should not be straight as it tends to stress the jack more; and I believe plugs should not be 90-degree angles, as it tends to snag on things more easily. I like a 45-degree angle plug. I showed him the plugs on the V-Moda M-80 as an example of a great plug.
- Freedom of spin in "lay flat" headphones - If the headphones are laying flat with cushions down on a table in front of you, you should be able to pick the headphones up and spin the earpieces towards your ears and slightly beyond. But there should be a limit in how far they can spin so as to provide a little more pressure of the pads behind the ears than in front of the ears.
- Headphone cables should have plugs at both ends - Headphone cables should attach to the earpiece with a straight 3.5mm mini-plug, and without any weird mechanical attachments. This will allow the manufacturer to make one cable that fits all of its headphones, and alternate cables to turn the headphones into headsets for iPhones, Androids, and other phones easily. Even if a manufacturer doesn't provide those options, having a standard connector will allow the user to buy aftermarket cable to suit the customer's style of use.
- Tangle-free cables - It's a hot topic among headphone makers at the moment, and I showed Tet the cable on the Philips Citiscape Downtown. It's a flat cable you can wad to your heart's content, and when you let it go, nine times out of ten it just "sproings" open completely tangle free. Then I showed him a prototype I got from a company with a flat, woven-cover cable the is almost impossible to handle without tangling.
Most interesting to me was the feeling that we were two hard-core headphone enthusiasts, yammering on about headphones at about the highest possible level one could imagine. It was great fun, and immensely satisfying.
The coolest thing of all, though, was getting Tet's run-down on headphone measurements.
Skullcandy's Director of Electrical & Acoustical Engineering, Dr. Tetsuro Oishi Visits InnerFidelity! Page 2
Tet Teaches Me About Headphone Measurements
Most interesting of all, to me, was our dialog about headphone measurements and what they mean. First, it's terribly complicated with many variables coming into play, so looking at a set of measurements will never let you diagnose exactly what's going on. But there are a few things that tend to show up on the graphs, which, depending on where they are, can indicate the likelihood of particular problems.
Main Spring - Well ... that's my word for it. The "springiness" of the earcushions and the enclosed air volume allows for a low frequency resonance to develop. Think of it like the earphones bouncing gently on and off your ears. This will typically appear as some frequency response feature between 50Hz and 150Hz. In the Frequency plots to the right, the initial feature in the lows of each headphone is probably this "main spring" effect. You can see in the incredibly well engineered Sennheiser HD 800 the merest blip at 60Hz; all heaven and earth moves between 60Hz and 80Hz on the DT48.
Poor Ear-pad Seal - If the ear-pad is not sealing well, you will see a second-order (dramatic) drop-off in the lows. In the headphones to the right, the AKG K272 is likely not sealing properly. I'll probably have to go through my measurements and identify poorly sealing cans for re-measurement using my improved headphone positioning methods.
Voice Coil Wobble - If the magnetic field strength or voice coil weight isn't perfectly distributed around the circle of the voice coil, it will be accelerated with more force on one side than another. This might be from misaligned pole pieces or an eccentric voice coil, for example. At some frequency, this wobble will hit a resonance and create a blip in the frequency or impedance response, typically somewhere between 300Hz and 1kHz. In the frequency response plots to the right, the features above 300Hz on the T5p and K272 are likely candidates. Tet said coil wobble features are typically high-Q--meaning they are typically quite spiky looking.
Internal Driver Resonances - Behind the diaphragm is the magnet assembly. Small volumes of air can be partially trapped in this space creating the opportunity for resonance. Because these volumes are very small, the resonant frequency is quite high--typically between 2kHz and 8kHz. Headphone frequency response tends to become noisy in this range, so it may be difficult to separate internal driver resonances from resonances elsewhere in the headphone. But because these resonances exert a direct load on the driver diaphragm, and therefore voice coil, the reactive load of these resonances will readily appear on the impedance response plot. It is likely the fine features above 2kHz in the impedance response plots to the right are due to internal driver resonances.
There are, of course, many other characteristics of headphones that may manifest as wiggles in plots. The magnitude and combinations are endless, and as Tet looked at the measurements there was always great caution in his judgments. He'd always suggest some sort of test we could try to affect the plotted measurement to identify its origin. That's his job. Unfortunately, it's not mine, and while I'll continue to learn, my time with Tet tells me I'll really never be able to diagnose accurately what's going on in a particular pair of headphones by measurements alone.
There is good news though: while we can't know for certain the cause of particular bumps and wiggles, we can say for certain that less is more. A well designed headphone that is well manufactured with quality parts will have fewer of these bumps and wiggles than a poorly-executed headphone of the same type. It's important to note here that some headphone types will naturally have more features in their plots than others. A well designed circumaural, open headphone will always be flatter than an equally well designed supra-aural sealed headphone.
Thanks for the Visit, Tet!
Funny story: Tet and I are going through the book of graphs just for giggles and to point stuff out, and we stumble upon the Beyerdynamic DT48. He says, "Oh stop! Look at that: that has got to be a sealed circumaural headphone with a very small and tightly sealed chamber behind the driver."
"How do you know," I wondered?
"Well, it's virtually a text book plot. The huge main spring dip and peak at 60Hz and 80Hz means its a tightly sealed enclosure between the driver and the side of the head, and that it has very springy cushions. And the fact that the bass recovers flat, but very low in amplitude means the enclosure behind the driver is tightly sealed and small."
"Yuppers."
I am familiar with the headphones, and Googled a picture of the DT48. Of course, they fit Tet's description to a T.
Man, I am so glad to have had the opportunity to talk to a real expert on headphone design and learn this stuff. I'll no doubt be rummaging through all my measurements to start a list of plots that I think I can improve on; and I'll also be looking at data in the future with wiser eyes. I'm sure all you headphone geeks out there will feel the same after reading this far.
Thanks so much, Tet!
(Awesome individual, BTW - I highly recommend that seller.) My plinth coupler measures like your red line, not the green. Would be very interested in tracking down the origin of your coupler that produced the green curve, if you can...? 
I'll shift discussions on that topic back to the other thread...
