742 Chapter 25
25.4.1 Velocity
The open or free-air headphone, known as the velocity type, sits just away from the ear
fl ap, often resting on a pad of light, acoustically transparent reticulated foam. This type
of headphone cannot exclude outside sounds, which can intrude on the reproduced music,
but the best models can produce a very light, open, airy sound. This type of headphone
has been very successful in recent years, particularly since the growth in personal
stereos— “ Walkman ” clones—where their light weight and compact dimensions have
made them ideal.
A theoretically perfectly-stiff diaphragm operating in free air, like a velocity headphone,
has a frequency response that falls away at low frequencies at 6 dB per octave and so you
would expect the system not to work particularly well. However, by juggling with the
parameters of mechanical diaphragm stiffness, mass, and acoustic damping, it is possible
to get a perfectly adequate low-frequency performance from velocity headphones, which
is not dependent on the exact position of the headphones on the ears. The rear face of
the diaphragm in velocity headphones is essentially open to the air, which helps impart a
more open airy sound, but does not exclude outside noises.
25.4.2 Pressure
The other category is the closed or circumaural headphone, known as the pressure type,
which totally encloses the ear fl ap and seals around it with a soft ear cup. The principal
advantages of this type of headphone are that sealing around the ear makes it possible
to pressure couple the diaphragm to the ear drum from about 700 Hz down to very low
frequencies, with a linear response down to 20 Hz easily achievable, as long as the seal is
effective ( Figure 25.7 ). The frequency response of the pressure type, unlike the velocity
type, is essentially fl at down to a low frequency, which is dependent only on the degree of
sealing. A poor seal due to inadequate headband pressure, or the wearing of spectacles,
for instance, can cause a marked deep bass loss.
The principal disadvantages are that closed headsets tend to be heavier, require greater
headband pressure, and can make the ears hot and uncomfortable. Pressure-type
headphones can be either closed backed or open backed. Closed-backed headphones offer
the exclusion of outside sounds as a distinct advantage in situations that require this, such
as recording studios, for instance. However, there is a body of opinion that judges the
sound of closed-back headphones to be closed-in compared to open-backed types.