The basic purpose of a
sound room in audiometric testing is to provide the proper acoustical
environment so that tests can be conducted without interference from outside
noise. A room of this type should afford adequate ventilation and lighting
so that the subject will be comfortable while his hearing is being
evaluated. Not only does the room supply environmental control but it
eliminates distraction from changes on the visual horizon which may
invalidate an audiogram as readily as acoustical interference.
Hearing test rooms should
be located in as quiet a place as possible. Preferably they should be within
practical access but away from outside walls, elevators, heating and
plumbing noises, waiting rooms and busy hallways. If the highest noise
levels in a test room do not exceed the levels listing in Table
1, tests room noises will not affect test results.
To obtain these internal
room ambients one must know what the outside ambient is going to be. This
can be obtained with a noise survey by octave bands of the area where the
room is to be located. Once this is done, a room should be selected which
will provide ample noise reduction to bring the internal noise down to those
prescribed by the standard (Table
1).
Background Noise Levels
The American Standards
Assn., in its pamphlet on criteria for background noise levels in audiometer
rooms, has printed a very useful chart for depicting the outside background
noise levels allowable for the use of different types of sound rooms, i.e.,
regular duty single wall rooms and double walled rooms. This chart shows the
relationship between outside ambient levels and the amount of performance
required to bring down the internal noise levels to the acceptable levels
for testing without interference.
The following procedure
gives a rough indication of the kind of construction that may be necessary.
In the octave bands that will contain test tones, measure the sound pressure
level at the site of proposed test room and plot those on Figure
1. The highest range in which the measured levels fall determines which
of the three general classes of rooms is probably necessary.
Specifically the octave
band background noise at the room location should be measured at teach test
tone; 125, 250, 500, 1000, 2000, and 4000 Hz. From these measured levels the
published noise reduction of the proposed audiometric room should be
subtracted. To be acceptable the room must bring the noise down below that
prescribed in the standard listed in the table below that prescribed in the
standard listed in the table.
* Test Tone of 6000 Hz is
also required.
Types of Rooms
Basically there are three
types of prefabricated audiometric rooms. The first type, which is most
familiar to those in audiometric testing, is the single wall modular panel
type. It is fabricated from a 4" (100mm) thick, steel panel, which
weighs approximately 10 lbs. per sq. ft. This panel has a solid outer
surface and a perforated inner surface and is filled with high density
acoustical fill and damping material.
These panels require
assembly at the location where they are to be used. The components of such
rooms include sound doors, window panels and a panel with a jack plate for
connection of the audiometer. Separate floor and ceiling panels are
necessary.
The second type of room is
a hybrid heavy duty version of the first. It incorporates double wall panel
construction. These are rooms within rooms and are usually constructed using
the same 4" panels with a 4" air space between the inner and outer
rooms. Both of these types of modular panel rooms can be assembled to
construct a suite of rooms with capacity for many subjects.
The third type of room is
a unique development in the field of audiometric testing: compact or
mini-room. It is a smaller, single walled, one person occupancy unit which
incorporates the features of the larger room in a smaller pre-assembled
package, Figure 1. It
provides somewhat less noise reduction than the larger rooms but is more
that adequate in most industrial testing applications.
All three types of rooms
should include adequate lighting, (incandescent or fluorescent with remote
ballasts to avoid ballast hum), adequate ventilation (15 air changes per
hr.) which does not distribute background noise levels, as well as vibration
isolation mounts to isolate from structural vibration.
For precise medical and
clinical applications, where extremely low ambients are required and radio
frequency shielding may be a requirement for diagnostic work, the
single-wall and double-wall modular panel rooms are most practical in this
type of application and in speech and hearing research.
In industrial testing and
some clinical testing, the single-wall modular panel and smaller
pre-assembled compact rooms will be adequate depending on the background
noise. They afford sufficient noise reduction to satisfy these testing
requirements.
Selecting A Room