Table 3.4 shows the maximum ambient sound-pressure levels allowable for air-conduction and bone-conduction testing. Rooms in which such standards can be met are not always utilized. This is true in the case of hearing test sites in industry or in the public schools.
Regardless of the practical limitations imposed by a given situation, the person responsible for audiometric results must realize that background noise may affect audiometric results by elevating auditory thresholds or causing unnecessary screening failures. There are three major ways in which ambient room noise may be attenuated: by using specially designed earphone enclosures, by testing through receivers that insert into the ear, and by using sound-treated chambers.
Earphone Attenuation Devices
Audiometer earphone and cushion combinations do not provide sufficient attenuation of most background noises to allow determination of threshold down to 0 dB HL for people with nor-mal hearing. Several devices are available that allow the supra-aural audiometer earphone and cushion to be mounted within a larger cup, which assists in the attenuation of background noise (see Figure 4.2). Most use a fluid-filled cushion to achieve a tight seal against the head. Such enclosures may be effective but differences exist in the efficiency of different models (Franks, Engel, & Themann, 1992).
Problems with regard to calibration are encountered in the use of some earphone enclo-sures. The phones cannot be placed on the usual 6 cm3coupler of an artificial ear. Even if the earphone is checked and found to be in proper calibration before it is placed into the enclosure, mounting may alter the calibration slightly, especially in the low frequencies. Because bone-conduction testing is done with the ears uncovered, the masking effects of room noise may affect these test results without affecting the air-conduction results, possibly causing a misdiagnosis.
Children often find these earphone devices heavy and uncomfortable.
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Insert Earphones
Testing hearing with receivers that insert directly into the ear has a number of advantages audiometrically, including further reduction of background noise compared to supra-aural earphones and better infection control because the inserts are disposed of following testing. If the foam is inserted deep into the ear, just short of causing discomfort, even more attenuation is obtained. It is desirable to have patients open and close their mouths three or four times to ensure proper seating of the cushion. Insert earphones can be used for testing children as well as adults. Problems with room-noise masking remain unsolved for bone-conduction testing, even if insert earphones are used for air conduction.
Clinical Commentary
During all patient contact for hearing testing and subsequent treatment, audiologists should be conscious of effective infection-control procedures. Harmful organisms can be passed from person to person through direct patient contact or indirect contact when test instruments are used without proper cleaning. The importance of this area of clinical care cannot be overemphasized.
The American Academy of Audiology’s recommended protocol for infection control within audiological practice may be found in the Professional Resources section of the Companion Website.
FIGURE 4.2 Commercial earphone enclosure device used to attenuate room noise during threshold audiometry. (Audiocups courtesy of Amplivox, Ltd.)
Sound-Isolated Chambers
The term “soundproof room” is often used erroneously. Totally soundproofing a room—that is, removing all sound—is impossible. All that is necessary in clinical audiometry is to keep the noise in the room below the level of masking that would cause a threshold shift in persons with normal hearing. Sound-isolated rooms may be custom built or purchased commercially in a prefabricated form.
The primary objective in sound-treating a room is to isolate it acoustically from the rest of the building in which it is housed. This usually involves the use of mass (such as cinder blocks), insulating materials (such as fiberglass), and dead air spaces. The door must be solid and must close with a tight acoustic seal. Sometimes two doors are used, one opening into the room and the other opening out. The inside walls are covered with soft materials, such as acoustic tile, to help minimize reverberation. Some such chambers contain large wedge-shaped pieces of soft material such as fiberglass on all walls, ceilings, and floors, with a catwalk provided for the subject. Rooms in which reverberation is markedly diminished are called anechoic chambers.
An example of such a room is shown in Figure 3.17.
Audiometric suites may be designed for either one-room or two-room use. In the one-room arrangement, examiners, their equipment, and the patient are all together in the same room, or the patient is within the room and the clinician just outside. In the two-room arrangement, the examiner and audiometer are in the equipment room and the patient is in the examining room.
Windows provide visual communication between the rooms. As a rule there are several panes of glass to attenuate the sounds that emanate from the equipment room. Moisture-absorbing materials must be placed between the panes of glass to keep the windows from fogging. Electri-cal connections between the rooms are necessary so that signals can be directed from the audiometer to the earphones. In addition, a talkback device, consisting of a microphone, ampli-fier, and speaker and/or earphone, enables the examiner to hear patients when they speak.
In customizing sound-isolated rooms, a great deal of attention is often paid to attenuating sound from adjoining spaces outside the walls of the room, but insufficient care is given to building-borne vibrations that may enter the room from the floor or ceiling. It does little good to have four-foot walls of solid concrete when footsteps can be heard from the floor above.
When sufficient space, money, and architectural know-how are available, custom sound rooms may be the proper choice. Contemporary audiology centers, however, lean more toward the commercially prefabricated sound room, which is made of steel panels and can be installed with wiring included for a two-room operation (Figure 4.3). The manufacturer does not guarantee the noise level within the sound room after installation, but only the amount of attenuation the room will provide under specific laboratory conditions, a fact often not fully un-derstood by many purchasers. It is therefore necessary to prepare the room that will enclose the prefabricated booth by making this area as quiet and nonreverberant as possible.
Prefabricated sound booths are available in one- and two-room suites. Windows with several panes of glass are installed to enable the examiner to observe the patient. The inside walls of the booth are constructed of perforated steel and filled with sound-absorbing materials. Some booths are double-walled; that is, there is one booth inside another, larger one. Prefabricated booths are freestanding, touching none of the walls of the room in which they stand, and are isolated from the ceiling by air and from the floor by specially constructed sound rails.
One of the great weaknesses of audiometric rooms, whether custom or commercial, is their ventilation systems. Rooms that are to be tightly closed must have adequate air circulation, requiring the use of fans and motors. Sometimes the air-intake system is coupled directly with the heating and air-conditioning ducts of the building. In such cases, care must be taken to min-imize the introduction of noise through the ventilation system.
Lighting for both kinds of rooms should be incandescent, but if the use of fluorescent light-ing is desired, the starters must be remotely mounted. Starters for fluorescent tubes often put out an annoying 60 Hz hum that can be heard by the patient or picked up by the audiometer.
2. The most popular
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