PURETONE AUDIOMETRY

PURETONE AUDIOMETRY

Audiometers are used to make quantitative measures of Air Conduction and Bone Conduction Pure Tone thresholds. AC thresholds assess the entire auditory pathway and are usually measured using earphones. When sound is delivered by an earphone, the hearing sensitivity can be assessed in each ear separately. BC thresholds are measured by placing a vibrator on the skull, with each ear assessed separately, usually by applying masking noise to the non test ear.  

Equipment

AUDIOMETERS

Puretones are generated within an audiometer. Audiometers have the ability to select tonal frequency and intensity level and to route tones to the left or right earphone. All audiometers also have an interrupter switch that presents the stimulus to the examinee.

The American National Standards Institute (ANSI) Specification for Audiometers (ANSI, 2010) describes four types of audiometers: Type 1 having the most features and Type 4 having the fewest features.

Type 1 audiometer

1.     Is a full-featured diagnostic audiometer.

2.     A Type 1 audiometer has earphones, bone vibrator, loud speakers, masking noise, and other features.

Type 4 audiometer

Is simply a screening device with earphones, but none of the other special features. Type 1 (full-featured, diagnostic audiometer) has the ability to assess puretone AC thresholds for frequencies ranging from 125 to 8,000 Hz and BC thresholds for frequencies ranging from 250 to 6,000 Hz. If an audiometer has extended high-frequency capability, air-conduction thresholds can be extended to 16,000 Hz. Maximum output levels for AC testing are as high as 120 dB HL for frequencies where hearing thresholds are most sensitive.  

Earphones

Earphones are generally used to test puretone AC thresholds. Supra-aural earphones, ones in which the cushion rests on the pinna, were the only choice for clinical audiology. The popularity of supra-aural phones was mainly due to their ease of calibration and the lack of other types of commercially available earphones. In the past few years, insert earphones and circumaural earphones have become available and provide some useful applications for puretone assessment.

Insert earphones are coupled to the ear by placing aprobe tip, typically a foam plug, into the ear canal. These earphones have gained popularity in the past few years because they offer distinct advantages over supra-aural earphones.

Advantages

Insert earphones yield higher levels of interaural attenuation than supra-aural earphones.

Interaural attenuation represents the decibel reduction of a sound as it crosses the head from the test ear to the nontest ear.

The average increase in interaural attenuation is roughly 20 dB. This reduces the need for masking the nontest ear and decreases the number of masking dilemmas, situations for which thresholds cannot be assessed, because the presentation level of the masking noise is possibly too high.

Another important advantage of insert earphones over supra-aural earphones is lower test–retest variability for thresholds obtained at 6 and 8 kHz; variability for other frequencies is comparable. Given that thresholds for 6 and 8 kHz are important for documenting changes in hearing due to noise exposure and for identifying acoustic tumors, lower variability should increase the diagnostic precision.

Insert earphones offer is elimination of collapsed ear canals. Supra-aural earphones cause the ear canal to narrow or be closed off entirely when the cushion presses against the pinna, collapsing the ear canal, resulting in false hearing thresholds, usually in the high frequencies. Because insert earphones keep the ear canal open, collapsed canals are eliminated.

Insert earphones is that they can be easily used with infants and toddlers who cannot or will not tolerate supra-aural earphones.

Insert earphones is the option of conducting middle-ear testing and otoacoustic emission testing without changing the earphones; some recently introduced diagnostic instruments use this approach. Although insert earphones offer a hygienic advantage over supra-aural earphones, because the foam tips that are placed into a client’s ear canal are disposable, the replacement cost of those tips is prohibitive for many applications. In addition to higher costs, insert earphones also yield errant thresholds in persons with eardrum perforations, including pressure-equalization tubes for additional

information about perforations.) Insert earphones also have maximum output levels that are lower than those produced by supra-aural earphones for some frequencies. Because of these differences, many diagnostic clinics keep both earphone types on hand and switch between them depending

on the application.

Speakers

AC thresholds can be measured using speakers as the transducer. Thresholds so obtained are known as sound-field thresholds. Sound-field thresholds are unable to provide ear-specific sensitivity estimates. In cases of unilateral hearing losses, the listener’s better ear determines threshold. This limitation and others dealing with control over stimulus level greatly limit clinical applications involving sound-field thresholds. Applications for sound-field thresholds are screening infant hearing or demonstrating to the parents their child’s hearing ability. Sound-field thresholds may also be desirable for a person wearing a hearing aid or cochlear implant. In sound-field threshold measures, the orientation of the listener to the speaker has a large effect on stimulus level presented at the eardrum. A person’s head and torso as well as the external ear affect sound levels. Differences in SPL at the eardrum are substantial for speaker locations at different distances and different angles relative to the listener. For this reason, sound-field calibration takes into consideration these factors. A mark is usually made on the ceiling (or floor) of the room to indicate the location of the listener during testing. Even at the desired location, stimulus level at the eardrum for some frequencies can vary as much as 20 dB or more by simply having the listener move his or her head. Calibration assumes the listener will always be facing the same direction relative to the sound source. Furniture and other persons in the sound field also affect the stimulus level at a listener’s eardrum. All of these factors add to the challenge of obtaining accurate sound-field thresholds. Another important consideration in sound-field

threshold measures is the stimulus type. Thresholds corresponding to different frequencies are desired for plotting an audiogram, but puretones can exhibit large differences in level at different positions in a testing suite as a result of standing waves. Standing waves occur when direct sound

from the speaker interacts with reflections, resulting in regions of cancellation and summation. Differences in stimulus level due to standing waves are minimized by using narrowband noise or frequency-modulated (FM) tones as the stimulus. FM tones, also known as warbled tones, are tones that vary in frequency over a range that is within a few percent of the nominal frequency. This variation occurs several times per second. Under earphones, thresholds obtained with these narrowband stimuli are nearly identical to thresholds obtained with puretones, with some exceptions in persons with steeply sloping hearing loss configurations. FM tones and narrowband noise are the preferred stimuli for sound-field threshold measures.

Bone Vibrators

A bone vibrator is a transducer that is designed to apply force to the skull when placed in contact with the head. Puretone BC thresholds are measured with a bone vibrator. A separation of 15 Db or more between masked AC and BC thresholds, with BC thresholds being lower than AC thresholds, is often evidence of a conductive hearing loss. Other possible explanations for air–bone gaps and bone–air gaps are equipment miscalibration, test–retest variability, and individual differences in anatomy that cause thresholds to deviate from the groupmean data used to derive normative values for relating AC

and BC thresholds. For threshold measurements bone vibrators are typically placed behind the pinna on the mastoid process or on the forehead. Placement on the mastoid process is preferred by 92% of audiologists. Mastoid placement is preferred mainly because it produces between 8 and 14 dB lower thresholds than forehead placement for the same power applied to the vibrator, depending on the frequency (ANSI, 2010). The median difference is 11.5 dB. Given that the maximum output limits for bone vibrators with mastoid placement are as much as 50 dB lower than that for AC thresholds, forehead placement yields an even larger difference. The inability to measure BC thresholds for higher levels means that a comparison of AC and BC thresholds is ambiguous in some cases. That is,

when BC thresholds indicate no response at the limits of the equipment and AC thresholds are poorer

than the levels where no response was obtained, the audiologist cannot establish from these thresholds whether the loss is purely sensory/neural or whether it has a conductive component.