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."
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!