Headphones Are Gaining Quality and Acceptance
One long-neglected audio component has been enjoying a
rehabilitation in recent years, although the process is nowhere near complete. Stereo
headphones have been with us for more than four decades, of course, and their predecessors
go back to the very beginnings of sound reproduction. Long before anyone came up with a
really practical moving-coil speaker, radio listeners of the 1920s huddled over their
crystal sets with primitive headphones securely clamped to their heads.
Then as now, however, headphones made for solitary
listening. On the other hand, the loudspeaker ("loud" because it put out a lot
more energy than a pair of headphones) could be enjoyed by the whole family. Headphones
continued to reign in areas where exclusion of outside noise was important -- in
communications, for instance -- but the use of speakers for entertainment became
universal. And as the radio/phonograph became the audio system, it relied on
ever-more-sophisticated speakers.
Phones didn't disappear completely, of course. The stereo
headphone appeared in 1957 -- somewhat before most hi-fi enthusiasts had abandoned mono --
and the granddaddy of all headphone manufacturers, Koss Corporation in Milwaukee,
introduced its first model the following year. From that time to this, there has been a
dedicated segment of the audio world who have preferred to do their listening on phones,
but speakers remained the preferred reproducer for the majority.
Still, special applications were particularly suited to
headphone use from the beginning. Listeners who wanted to block out noisy surroundings, or
to keep their music from disturbing the neighbors, used phones, and they were widely used
in broadcasting and recording studios as well. But even though some very fine phones were
developed over the years, they were never really accepted as "real" hi-fi
components. For one thing, they still tended to make listening a solitary pursuit; for
another, they were mostly bulky and vaguely claustrophobic.
The change came with the introduction of Sony's first
portable tape players in 1979. The miniaturization of cassette mechanisms that the Walkman
required was a remarkable achievement, to be sure, but the thing that made the portable
revolution possible was the development of a suitable headphone, one that would be light
and comfortable, but which would be sensitive enough to produce adequate listening levels
without draining a player's batteries in minutes.
Earlier phones required a lot of power compared to today's,
although that didn't matter very much as they could draw whatever power they required from
a regular hi-fi system. The new generation of headphones featured much stronger magnetic
materials (often samarium cobalt, although there are others), which could turn tiny
electrical signals into remarkably robust sounds without requiring a lot of mass.
Just as other manufacturers jumped on the Walkman
bandwagon, and instantly flooded the market with a bewildering array of equivalent
devices, they also embraced the new headphone technology. Today, there are hundreds of
models available, the majority lightweight models designed primarily for portable use.
Even many phones aimed primarily at the home or professional markets use the more powerful
magnets, and could therefore be used in portable applications as well.
Because headphones are very close to the ears, and tightly
coupled to them, practically none of the low-frequency information is lost to the
surrounding air, and therefore very little needs to be produced. Diaphragms can therefore
be small without sacrificing bass; in fact most headphones use single elements to produce
the whole audio spectrum, although there are exceptions. By the same token, what happens
to the rear wave produced by the transducer is relatively unimportant, so headphone makers
can dispense with the bulky enclosures that make up most of a speaker system's volume.
The manner of coupling a headphone's active element to the
ear canal varies quite widely. Early phones were circumaural: they used a foam pad that
sealed to the sides of the head, leaving the ear untouched -- and therefore undeformed --
but surrounded. Most of these had sealed backs, both to improve bass coupling and to
provide some sound isolation in noisy environments. Many of today's circumaural phones
have open backs so that ambient sounds are audible, but the ear remains untouched.
A later development, and one that is extremely popular for
use with today's portable equipment, is the supra-aural phone, in which the transducer and
the ear are separated by a foam pad that sits directly on the outer ear. These offer
little isolation, but improvements in magnet and transducer design now mean that little
bass must be sacrificed when using this style of headphone. The most recent variety is the
in-ear or earbud phone, a tiny device that plugs directly into the ear canal, removing the
necessity of a headband.
Useful as they are, both for portable use and private
listening at home, headphones have some distinct sonic characteristics that are often
pleasurable but not necessarily accurate. With the exception of a few binaural recordings
made specifically for headphone listening, practically all the music we buy is produced
under the assumption that it will be heard through speakers in a live acoustic
environment, where some of the sound from each speaker reaches both ears.
The sonic perspective of most recordings is thus engineered
to sound natural in such a setting; with headphones, on the other hand, each ear receives
information from one transducer only, and this can cause an exaggerated perspective,
characterized by a wider-than-life soundstage and center-channel material seeming to be in
the middle of the head.
In addition, high frequencies roll off naturally in a real
room, even over quite short distances, and this sounds normal to most of us. There is no
such attenuation in the fraction of an inch between a headphone's transducer and the
eardrum, however, which means that phones often have an unnaturally bright sound, unless
care has been taken by the designer to correct it. Some do, some don't.
In fact, the achievement of the proper tonal balance in a
pair of headphones presents one of the greater challenges in audio design. Phones do not
behave the way other audio components do: To sound natural, they must have non-linear
frequency response. With an amplifier or tape deck, or even a speaker, the flatter the
frequency-response curve the better the equipment will sound. But the sound that actually
reaches your ear is anything but flat.
The outer ear is a sophisticated acoustic modification
system that radically alters the sound reaching it; the spectral balance at the eardrum is
very irregular. There's no problem in that -- since you hear all sounds modified this way,
they will sound natural, however bumpy the actual curve may be.
The headphone disturbs all this. For one thing, with
supra-aural models, the phone physically deforms the outer ear by sitting right on it,
changing its acoustic properties. Even with circumaural phones, the diaphragm is
acoustically coupled to the air in the ear canal -- the sound is, in effect, pumped
directly from the diaphragm to the eardrum, bypassing virtually all of the acoustic
modifications usually provided by the ear's physical shape. The result is that a truly
flat headphone -- one that would perform perfectly by the standards applied to other audio
equipment -- would sound terrible. The designer must instead build into the headphone a
frequency response curve that duplicates the modification normally provided by the twists
and folds of the ear itself.
That is not an easy task, partly because all ears differ,
and partly because there is no universal agreement as to how to measure the ear to
discover the modifications that must be made, nor on how to measure the headphones to see
whether they successfully duplicate the ear's effects. The result has usually been that
phones have mostly been designed and evaluated on a purely subjective basis.
The transducers in the majority of headphones are
"dynamic," as they are in most speaker systems, using the same combination of
voice coil and diaphragm to create acoustic energy. There are, however, a small number of
electrostatic models available, which usually require an external polarizing voltage and
take their audio feed from an amplifier's speaker outputs rather than a conventional
headphone jack. A variation on this technique, called the electret headphone, contains a
permanently charged plate making an outside power source unnecessary.
Whatever technique is used, the amount of sound output a
phone produces for a given signal level -- its sensitivity -- is often considered of prime
importance because it bears directly on battery use in portable applications. While
measuring methods vary somewhat from brand to brand, most headphone makers specify the
sensitivity of their models by stating the acoustic output (in decibels of sound pressure
level, or dB/SPL) for an input of 1 milliwatt. Older phones, and many current models
designed for non-portable use, tend to have sensitivities of about 95dB; portable phones
usually have sensitivities of at least 100dB, and there are lots in the 106-108dB range (a
100dB phone requires four times as much power as a 106dB phone to produce the same
acoustic output).
Headphones are the only audio component that we actually wear,
so it's important not only that they sound good but that they feel good as well. A
critical matter with portable phones is weight, because you are likely to have them
clamped on your head for fairly long periods. The upper limit should probably be about
half a pound, although even that might become fatiguing after a while. Fortunately, many
brands include extremely lightweight models, and most of these perform as well as heavier
ones. A number of companies have phones that weigh in under 3 ounces, which should be
light enough for all practical purposes. Make sure as well that the headband isn't painful
-- different heads have different shapes. Also make sure that they clamp securely to your
head and won't fall off. If you are choosing earbuds, make sure that they fit tightly
enough to stay in your ears, but not so tightly that they hurt.
Many companies have given some thought to what becomes of
their phones when you are not actually using them. When not on the head, a pair of phones
can be awkward to store, so more and more models have collapsible headbands that allow you
to fold up the phones and store them in your pocket or in a box, usually no larger than a
cassette case. This could be dangerous if you carry extra tapes around with you, however,
as the strong magnets in the phones are quite capable of erasing the tapes if brought too
close.
Other companies offer cases that allow you to wind up the
cord, rather than leaving it as a tangle in your pocket. And almost all headphone
companies assume you will use their products at home occasionally, and provide an adaptor
that lets you plug the normal mini-plug into the 1/4-inch jacks on conventional hi-fi
gear. A few even offer adapters for use with equipment with mono outputs, such as portable
TV sets.
Normally, however compact they may be, portable headphones
must be connected to a signal source, and that usually is by means of a cable supplied
with the phones. These can range from three to ten feet, the choice being mainly dictated
by how you intend to use the phones: a three-foot cord would be restricting at home, while
a ten-footer might be hazardous on your mountain bike.
One alternative is the cordless headphone, which uses a
transmitter attached to the signal source, and a sensor somewhere on the phones
themselves, which drive a small built-in amplifier. The original versions of the cordless
used infrared light, like your TV remote, and like the remote had to be able to
"see" the transmitter. More recently, several companies have adopted a radio
system, that makes line-of-sight positioning unimportant, and greatly extends the range of
operation.
It may be that the headphone still has some way to go
before it is accepted as a "standard" high-fidelity component. But it has had a
huge amount of technology lavished on it in a very short time, and that shows no sign of
stopping. Phones have definitely become fixtures in one very important branch of audio,
which depends on them entirely, and it doesn't take a lot of prescience to predict that
they will find their way into more and more home systems as well. They're a worthy
addition.
A Headphone Glossary
Binaural: The way we hear, using two ears. Also, a
specialized type of audio recording using closely spaced microphones to simulate the
positions of a listener's ears, and designed to be listened to through headphones. In some
cases the microphones are mounted in the ear canals of a replica human head, in which case
the technique is called "kunstkopf" or dummy-head recording.
Cordless: A headphone with no physical connection to
the source of its signal. May use infrared light or radio to transmit the signal.
Coupling: The intimate acoustic interaction between
a headphone's diaphragm, the air inside the ear, and the eardrum.
Circumaural: A class of headphone in which the case
is supported by a foam ring that surrounds the ear without deforming it. May be closed,
for better isolation from outside sounds, or open.
Diaphragm: The flexible panel that moves back and
forth in the headphone's earcup, transmitting sound directly into the ear.
Dynamic: A method of converting an audio signal into
diaphragm motion using magnets and coils, like most speakers. The most common technique in
headphones.
Earbud: A tiny headphone that fits into the opening
of the ear canal. Also called an in-ear phone.
Electret: A form of electrostatic that uses a
permanently charged plate. Much cheaper and lighter than a conventional electrostatic.
Electrostatic: A transducer that converts audio to
motion using charged plates that attract and repel each other. Usually the most expensive
sort of design, and only suitlable for stationary use as it requires an external
polarizing voltage. Often must also be fed from a speaker, rather than a headphone,
output.
Lightweight: Headphones designed primarily for
portable listening. May weigh anywhere from less than 1 ounce to 6 or 7 ounces.
Mini-plug: The standard connector for portable
phones. Requires an adapter to fit the 1/4-inch phone jack on most home equipment; this
may be in the form of a cable that lets you have a short wire for portable use and a
somewhat longer one at home.
Outer ear: The fleshy part of the ear protruding
from the head. Its folds significantly alter the sound as it enters the ear, so headphones
that distort the shape must compensate for the changes in tonal balance.
Sensitivity: The amount of acoustic output produced
for a given audio signal level; primarily affects battery life in portables, as low
sensitivity uses up power more quickly.
Supra-aural: A headphone design in which the phone
is covered by a foam pad that sits right on the outer ear, deforming it to some extent.
The most popular design, it tends to be small and light.
Transducer: A device that changes one form of energy
to another, such as electricity to sound. Most headphone transducers are dynamic, but a
few are electret or electrostatic.
...Ian G. Masters
ian@mastersonaudio.com
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