Specifications
close to a large object. Behind the blocking object is a “zone of silence” in which reception of
the transmitted signal is not possible. Divers can reduce zones of silence by moving away from,
around, or above the blocking object until communication is reestablished (Fig. 15).
Most single-sideband signals are efcient enough to permit communications around
blocking objects. Reception is mainly through surface and bottom reections. Man-made
noise may be present underwater in busy harbors, shipping lanes, and many coastal
locations, particularly at lower frequencies. The outstanding characteristic of this coastal
ambient noise is its great variability from place to place in the same harbor and from
time to time at the same place.
Under some conditions, when your diving suit is directly between the transmitting source
and your Magnacom
®
, a small zone of silence may be created that prevents reception.
This effect becomes greater at longer ranges. Turning approximately 45 degrees in any
direction eliminates this zone of silence.
Since most divers are in constant motion, more than a momentary signal loss would be unlikely
when acoustic energy is blocked by air inside a dry suit, by gas bubbles within neoprene
wetsuit material, or when both are present—such as with a partially inated dry suit.
23
Figure 15: Communication through indirect and direct paths. Note kelp
scattering sound. Without surface and bottom bounce, the diver would lose
communications.
Figure 14
the water seems still. A
short distance away, the
current is owing again
(Fig. 14).
Similarly, acoustic
energy in the water
can be blocked if the
transmitting source is