Technical Description
DESIGNING FOR NATURAL BASS
Research programs have one common advantage
for the alert scientist and engineer. The thousands of
hours of effort, analysis, conjecture, and synthesis
uncover opportunities to make meaningful contribu-
tions. Most of these opportunities are discovered just
where lesser men saw only problems. One of the
great opportunities that the Mclntosh acoustics re-
search team uncovered was a design for natural
sound of bass instruments.
When a bass drum is struck, the motion of the
diaphragm (drum head) pumps a large volume of air.
The reproduction of such a sound image requires
the motion of the same volume of air or, at least, a
significant portion of it. Because the design objec-
tives of low distortion and widely dispersed radia-
tion dictate a maximum effective cone diameter of
10 inches the low frequency loudspeaker had to be
designed with large excursion (back and forth move-
ment) to achieve the air volume movement necessary.
Our design objective was the development of a low
frequency loudspeaker with a plus and minus one
half inch cone excursion. Even with this large ex-
cursion, a straight line relationship between driving
force and movement had to be maintained.
To achieve this linear relationship our engineers
employed a long voice coil winding, a very light edge
and centering suspension and an air spring. The air
spring is the natural result of mounting the speaker
in a relatively small air tight extremely rigid box.
When the cone moves inward, it reduces the volume
of air thereby increasing its pressure. The increased
air pressure returns the cone to the center of its
travel when the electrical signal returns to zero value.
When the cone moves outward, the volume of air in-
creases reducing the air pressure in the box. The air
pressure in the room can now restore the cone to its
center position when the electrical signal again
reaches zero. One of the properties of an air spring
is an almost perfectly straight line relation between
volume and pressure for such small changes in vol-
ume. This insures a low level of distortion at low
frequencies and also a low level of intermodulation
distortion.
The linear air spring was combined with a stiff
cone, a long voice coil winding and a very large and
massive magnet structure to achieve almost perfect
transient response for bass frequencies.
Design engineers, in the past, have not taken ad-
vantage of this opportunity to obtain excellent tran-
sient response because of the price that must be
paid. That price is a reduction in bass response.
There are two ways around this problem of loss of
bass. In the older technological approach to loud-
speaker system design, flat frequency response was
obtained by compromising the transient response
using mechanical resonances in the speaker and its
enclosure. This has been the traditional solution with
its emphasis on cost reduction rather than reproduc-
tion accuracy.
The other solution is the new approach. To restore
the system to a flat frequency response while pre-
serving nearly perfect transient response, an elec-
trical signal is supplied having an exactly opposite
curve to the curve of the reduction in bass response.
Supplying this electrical signal introduces an
equalizer into this system. The combination of the
loudspeaker system and the equalizer produces both
a flat response and an excellent transient character-
istic, free of ringing, down to 20 Hz.
As if this advantage were not enough, the equalizer
brings about another additional improvement to the
system. The position of the loudspeaker in the room
can substantially alter the loudspeaker's sound bal-
ance. The equalizer can be designed to compensate
for different types of rooms and for different positions
of the loudspeakers in these rooms.
DESIGNING FOR DIFFERENT LISTENING ROOMS
The power out of a bass speaker at very low fre-
quencies varies over an 8 to 1 range (9 decibels)
depending on where the speaker is positioned in the
room.
For example if the loudspeaker enclosure is sus-
pended in the center of a large, sound absorbing
room, a 20 hertz signal will radiate equally in all di-
rections from the loudspeaker. It will be radiating
into a sphere.
If the loudspeaker enclosure is then lowered to the
floor in the center of the room it would be radiating
into a hemisphere. The signal striking the floor would
be reflected upward and the speaker power output
will appear to double.
If the loudspeaker is then moved along the floor to
the center of the wall the power will double again.
The loudspeaker is then radiating into one quarter
of a sphere.
Then by moving the loudspeaker into a corner of
the room the radiation is concentrated into one
eighth sphere with another apparent doubling in
power.
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