Cone motion is proportional to both the
power and frequency of the amplifier signal.
Low frequencies have longer wavelengths than mid or high frequencies.
Thus, a low frequency or bass signal causes a longer displacement than
a mid or high frequency signal of the same power.
The suspension and centering device must be flexible enough to permit
large displacements during the reproduction of bass tones; at the same time
they must also offer some stiffness or damping
to prevent an exaggerated motion and a distorted sound. However, excessive stiffness hinders low frequency performance;
increasing the electromotive
damping with a larger magnet is a better solution. A large magnet permits a more flexible centering
device and suspension, with higher bass performance. Damping and the ability of the voice coil and
bobbin to withstand heat are the primary factors that determine the power
rating of a loudspeaker.
The loudspeaker frame has slots to prevent air from being trapped
behind the cone, which would resist cone motion. A negative amplifier signal causes the cone
to move backwards and radiate sound behind the loudspeaker through the slots.
A positive amplifier signal causes the cone to move forward and radiate sound
ahead of the loudspeaker. The forward and backward sounds from the cone are
equal in acoustic power but exactly opposite in phase. The sound cancellation that would normally result
is prevented by isolating the front and back of the cone with an airtight
box or enclosure (Fig. 5).
The enclosure is constructed of heavy fiberboard panels (usually 0.75
inch) and is internally braced to prevent vibration, which would add undesirable
noise to the sound from the loudspeaker.
Half of the enclosure volume is filled with glass fiber to improve
damping.
The performance of the loudspeaker is greatly improved with
the enclosure. However, the air that
is inside the enclosure is compressed by backward cone displacement, especially
at low frequencies. Air compression
resists cone motion and reduces bass performance from the loudspeaker. A vent or port helps to compensate for this loss by reversing the phase
of low frequency sound inside the enclosure and adding it to the forward output
of the cone. The output from a port
is added when the loudspeaker is receiving a negative amplifier signal and
the cone is moving backwards (Fig. 5). The
port consists of a long tube that is mounted through a hole in the enclosure
rear panel. The dimensions of the
port are largely dependant upon the area of the cone.
For example, an eight inch long tube of 1.5, 2.5 or 3.5 inch diameter
is used with four, eight and twelve-inch cones.
The internal volume of the enclosure is determined by the area of the cone, and can be calculated from the equation in Fig. 6. The ideal enclosure volume for each cone diameter (Fig. 6) minimizes the effects of air compression, but is not excessively large to control costs. In other words, the ideal volume permits the loudspeaker to yield its potential low frequency output with the smallest possible size of enclosure.
