258 Chapter 8
Subsonic ( infrasonic ) information , comprising both music content and ambient
information, may occur below the high-pass “ turnover ” frequency (or low-end roll off) of
the bass loudspeaker(s). It will not be reproduced effi ciently.
Note: While potentially within humans ’ aural perceptive range, subsonic signals are
“ below hearing ” (strictly infrasonic ) in the sense of being “ out-of-band ” to, and only
faintly or at least reducingly reproducible by, the sound system.
Loudspeakers vary in their ability to handle large subsonic signals. Small ones may
or may not be heard but won’t ever cause damage. Large subsonic signals are more
risky with some kinds of loading. An approximate ranking of subsonic signal handling
robustness is shown in Table 8.5. Individual designs can vary widely , however.
8.4.2 Subsonic Stresses
Other than straining the speaker(s), if the amplitude of the subsonic (really infrasonic)
signal(s) is large enough, then signifi cant amplifi er capability will be wasted. At the very
least, the unrealizable portion will cause unnecessary amplifi er heating and electricity
consumption.
If the amplifi er is also being driven hard, the presence of a large subsonic signal will
reduce the threshold for clipping and also thermal shutdown. The amplifi er will behave as
if rated at only a fraction of its actual power capability. There are broadly two approaches
to the problem.
Table 8.5 : Loudspeaker Subsonic Handling (Infrasonic Handling)
More robust ⇑ Transmission lines
Differentially loaded cabs a
Properly arrayed bass horns
Sealed boxes
Open-backed cabs
Large cone-vented enclosures
Less robust ⇓ Small cone-vented enclosures
a Alias band pass or push–pull.