Headphones: From Ancient to Modern - Etymotic Research

Headphones:
From Ancient to
Modern
f you are like me and have been at this business for a while,
you may have followed the same path I did for headphone
(or earphone) listening, because I think it was quite
common. I started in the ’60s and wore Permoflux headphones
for a short time while I used a Nagra III. These
phones were designed for bomber crews in WWII, and the
name shows the development: they were an early application
of permanent magnets, new to the game. Did you
know that 1930s theater loudspeakers had to be powered
up to energize the “field” coil and make a magnetic field
for the moving coil to work against? Permanent magnets
had yet to be perfected. Technical developments made
for the war effort were spun
off to help our industry post
war. Among these were the A4
Voice of the Theater, which used
permanent magnets of the type
developed for the Permoflux
phones, and Cinerama, which
started out as WWII tail-gunner
training films, like a giant video
shooter game.
The Permoflux sounded pretty
bad, so I moved on as soon
as I bought a Nagra IV to Beyer
Photo courtesy National Institute of
Standards and Technology
1 Of course, that was only a wiring
difference between the mono DT-48s
and the DT-480s.
KEMAR manikin
by Tomlinson Holman, CAS
DT-48s, the standard for the time. These put a cylindrical
metal piece into your concha, the major opening in the
outer ear, and boy did one’s ears get red and sore wearing
those. When I traded up to the beautiful Nagra IV-S, I
got Beyer DT-480s—a lot more comfortable, and stereo
to boot 1 . None of these solutions had much bass it was
clear, but at least there was increasing comfort from one
to the next.
I left production sound and went to work first in consumer
electronics then at Lucasfilm as chief engineer of
post. The headphone “problem” was a lingering one—it
would affect us in production but only remotely, but more
close to home in transfer rooms and
QC. By this time, Henrik Staffeldt,
a Danish researcher, had proposed
the theory that all you had to get
right is the frequency response at
your eardrum, and all will be well
in transferring from one room to
another. I read the literature and
found out about a research tool,
the Knowles Electronics Manikin
for Acoustic Research (KEMAR),
which mimics the whole enchilada:
head, outer ear (pinna), middle
ear, and then places microphone
elements where the eardrum would
be. I thought that if I got a good
standardized room and measured
it, I could equalize other rooms
for the same ear canal response in
a dummy, and get good translation.
CAS QUARTERLY SPRING 2010 25

Because of the THX efforts, I was able to buy a KEMAR and
measure the frequency response of a properly-aligned dubbing
stage sound system in its ear canal. I realized that this would
also be useful for headphone listening, because if the reference
is your eardrum, the frequency response should match there
whether the sound field is external or from phones. I got a number
of headphones and compared them to the dubbing stage
response and found they were mostly off, significantly. This
was some years ago, and Grant Imahara made the headphone
measurements. Grant went on to reality television stardom—he
plays himself on Discovery Channel’s MythBusters.
Here’s what the Permoflux delivered compared to the
“proper” response, which is the ear canal response measured on
a dubbing stage aligned correctly to SMPTE 202.
Permoflux
And here’s the DT-48s
Bey DT-48
Not much better, but note that the scale has changed. Instead
of -46 +4 dB of the Permoflux, now we’re -38 +5 dB. Yikes,
these things are pretty bad, and yes they lack bass, a lot of bass.
26 SPRING 2010 CAS QUARTERLY
Here’s the next gen, a couple of AKG models, widely used in
music studios:
AKG K141
AKG K240
They measure -20 to +8 dB in one case, a distinct improvement,
but an ear canal resonance around 2.5 kHz is strongly
evident, and the lows are still not up to snuff.
The film industry kind of moved on to settle, more or less,
on the Sony MDR-7506 model. It measures thusly:
Sony MDR
While it still shows a strong ear canal resonance, the bass has
been improved, and the response is -5 +10 dB, and we have
moved a long ways from Permofluxes.
Then, more recently, I discovered a company with a very
familiar philosophy: earphones should be matched in response
at your eardrum to the external sound field produced by a good
sound system. They came at it from high-end consumer electronics
and loudspeakers, not cinema, so I was interested to see
how they’d match. A little surprisingly, they matched beyond
my expectations. They are the Etymotic ER-4P model, and here
is their response:

Etymotic
These show some roll off in the bass due to the degree of
seal we were able to get to the (metal) ear canals of the dummy.
I think on an actual ear and tightly coupled the bottom end
would be flat. And the response is a spectacular ±1 dB from
mid-bass through a quite high frequency, above 12 kHz! Note
graphically, we’ve gone from a 60 dB range to a 30 dB one, and
in measurement from a 50 dB response range to a