tend to behave similar to organ pipes closed at both ends. Figure 10-1 shows
the fundamental resonant frequencies for a closed chamber as a function of
chamber length, atmosphere type, and total pressure. It can be seen that the
resonant frequency increases with pressure and decreases with length. Values
above 100 Hz are readily audible to most persons and may be excited by common
types of machinery. Accidental tuning of a chamber to 60 Hz could result in
annoying resonant vibration due to AC electrical power devices such as
transformers and motors in the area.
(2) Correction. Since a chamber must operate over a range of
pressures and with a variety of atmospheres, there is little the designer can
do to eliminate resonance problems. On the other hand, he can avoid obvious
difficulties such as having the chamber tuned near 60 Hz or its multiples at
atmosphere, and be aware of the presence of resonances and the factors
governing them.
c.
Helium Speech Distortion.
(1) Problem. A disadvantage of helium as an inert diluent is its
effect on speech. The distortion of human voicing in atmospheres of 90
percent helium and above can be so severe that listeners find less than half
the words intelligible. Practice can improve a person's ability to
understand the distorted speech, but cannot eliminate the problem.
Good commercial helium speech unscramblers are available for installation
into a communication system. Some of the systems are simple expansions of
portable diver communication systems and are packaged appropriately for easy
installation into an existing communication system. There are more
sophisticated systems also on the market which may be designed directly into
the intercom communications systems.
3.
INTERCOM POWER REQUIREMENTS. For very small volumes with reflective
acoustics, such as hyperbaric chambers, the estimated power required is on
the order of tenths of a watt. In practice, a good quality amplifier of a
few watts output should have no trouble providing sufficient sound energy for
communications in the chamber. The system power needs are more likely to be
determined by the acoustic conditions outside the chamber, at the monitoring
console. Since high quality audio amplifiers up to 100 watts are widely
available at low cost, there is no harm in somewhat over-powering the chamber
communication system and attenuating as required. This provides reserve
capacity for overcoming unusually high background noise and other acoustic
disturbances.
4.
PROTECTION OF AUDIO EQUIPMENT FROM PRESSURE EFFECTS. Most audio
equipment is either immune to damage from increased atmospheric pressure or
easily modified to be so.
a.
Receiving and Transmitting Devices. The conventional permanent
magnet speaker can be used as a speaker as well as a microphone in a chamber
atmosphere. Boom microphones as well as headsets utilizing crystal, ribbon,
and carbon elements may be used with no pressure effecting problems.
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