Tuning Fork Tests

Tuning Fork Tests

Tuning forks were used to test hearing long before the development of the audiometer.

Schwabach Test

The Schwabach test is a technique for estimating a patient’s hearing sensitivity by bone-conduction. The test has two principal characteristics:

1.     It makes use of the fact that the tone produced by a tuning fork becomes softer with time after it has been struck due to damping, and

2.     The patient’s hearing is expressed in relative terms compared with the examiner’s hearing ability. This comparison is done by timing how long the tuning fork is heard by the patient and how long it is heard by the examiner. The basic procedure involves placing the base of the vibrating fork on the patient’s mastoid process until the tone fades away. The clinician then moves the fork to his own mastoid and times how long he can hear it. Compared with how long the examiner can hear the tone, it is expected that the patient will hear the tone

o    For a shorter period of time if she has a sensorineural loss;

o    For a longer period of time (or perhaps the same length of time) if she has a conductive loss; and

3.     For the same amount of time if she has normal hearing. Schwabach outcomes are problematic when dealing with mixed losses. The Schwabach test provides a relative estimate of hearing at best, and its validity is completely dependent on the tenuous assumption that the examiner really has normal hearing. It is not surprising this test is rarely if ever used.

Weber Test

The Weber test is used to help determine whether a unilateral hearing loss is sensorineural or conductive.

It is a lateralization test because the patient is asked to indicate the direction from which a sound

appears to be coming. Before starting this test, the patient should be advised that it is possible for the

tone to be heard from the good side or the poorer side, or any other location for that matter. The procedure involves putting the base of the vibrating tuning fork somewhere on the midline of the skull, most commonly on the center of the forehead or the top of the head. The audiometric Weber test uses the bone conduction vibrator instead of tuning forks. The patient is asked to indicate where the tone is heard. Hearing the tone in the better ear implies that there is a sensorineural loss in the poorer ear, whereas hearing the tone in the poorer ear suggests a conductive loss in that ear. The tone is heard in the middle of the head, “all over,” or equally in both ears when the patient has normal hearing, although some patients with sensorineural losses also report such midline lateralizations. If there is a mixed loss, the tone will be lateralized to the better ear if its level is below thepoorer ear’s bone-conduction threshold. The Weber test will fail to detect the conductive component of a mixed loss in such cases. The Weber test works for several reasons, all of which are related to the idea that the bone-conduction tone from the tuning fork reaches both cochleae at the same intensity.

The tone lateralizes to the better ear with sensorineural losses for either of two reasons:

1.     The tone will only be heard in the better ear if its level is lower than the bone-conduction threshold of the poorer ear.

2.     The second mechanism is due to the Stenger effect, which means that a sound presented to both ears is perceived only in the ear where it is louder. The intensity of the tone from the tuning fork will have a higher sensation level in the better ear than in the impaired ear. Hence, it will be louder in the better ear and will be perceived there. Several factors can explain why a bone-conduction tone would be louder in (and thus lateralized to) the poorer ear.  These mechanisms are just briefly mentioned because they are beyond the scope of an introductory text:

a.     Outer ear obstructions (e.g., impacted cerumen) may cause an occlusion effect,

b.    Mass loading of the middle ear system caused by effusions or ossicular chain interruptions may lower its resonance, and

c.     Phase advances may be caused by fixations or interruptions of the ossicular chain.

Bing Test

The Bing test is used to determine if closing off the patient’s ear canal results in an occlusion effect. The audiometric version of the test has already been discussed. In the traditional Bing test the patient is asked to report whether a tuning fork sounds louder with the ear canal open or closed. The base of a vibrating tuning fork is held against the patient’s mastoid process. The tester then presses the tragus down over the entrance of the ear canal to occlude it. The usual technique is to alternately occlude and unocclude the ear canal to help the patient make a reliable louder/softer judgment. It is desirable to make sure that the tuning fork sounds louder when the ear is closed and softer when the ear is open, instead of just asking whether the tone pulses between louder and softer. Unlike the audiometric version, which involves thresholds and thus quantifies the amount of the occlusion effect, the outcome of the tuning fork Bing test is based completely on a subjective judgment of louder versus not louder. If the occlusion effect is present, covering the ear canal should cause the tuning fork to sound louder. This is called a positive result and implies that the ear is either normal or has a sensorineural hearing loss. A negative result occurs if closing off the ear canal fails to make the tuning fork sound louder, and implies that there is either a conductive or mixed hearing loss.

Rinne Test

The Rinne test is a tuning fork procedure that compares hearing by air-conduction and by bone-conduction; however, the approach used is different from the one used in pure tone audiometry. The Rinne test is based on the idea that the hearing mechanism is normally more efficient by air-conduction than it is by bone-conduction. For this reason, a tuning fork will sound louder by air-conduction than by bone conduction. However, this air-conduction advantage is lost when there is a conductive hearing loss, in which case the tuning fork sounds louder by bone conduction than by air-conduction.

Administering the Rinne test involves asking the patient to indicate whether a vibrating tuning fork sounds louder when its base is held against the mastoid process (bone conduction) or when its prongs are held near the pinna, facing the opening of the ear canal (air-conduction). After striking the fork, the clinician alternates it between these two positions so that the patient can make a judgment about which one is louder. The bone-conduction vibrator is used instead of the tuning fork in the audiometric version of the Rinne test, and the patient indicates whether the vibrator sounds louder on the mastoid or in front of the ear canal. Masking noise must be put into the opposite ear to make sure that the Rinne results are really coming from the test ear.

The outcome of the Rinne test is traditionally called “positive” if the fork is louder by air-conduction, and this finding implies that the ear is normal or has a sensorineural hearing loss. The results are

called “negative” if bone-conduction is louder than air-conduction, which is interpreted as revealing the presence of a conductive abnormality. This terminology is confusing because the examiner is often concerned with identifying a conductive loss with this test. Consequently, many clinicians prefer to describe Rinne results as “air better than bone” (AC > BC) versus “bone better than air” (BC > AC). In these terms, AC > BC implies normal hearing or sensorineural impairment, and BC > AC implies a conductive disorder.

Sometimes, the air and bone-conduction signals sound equally loud to the patient (AC = BC). This equivocal outcome can usually be overcome by usingthe timed Rinne test (Gelfand 1977). This more accurate way to administer the Rinne test involves timing how long the patient can hear the tuning fork at the two locations.  

In this case, the results are Positive (AC > BC) when the tone is heard longer by air-conduction, and

a.     Negative (BC > AC) when it is heard longer by bone-conduction.

Another variation of the timed Rinne test involves holding the tuning fork at the mastoid until the tone has faded away, and then moving it to the ear canal (and vice versa).  Here, the result is

a.     AC > BC if the tone can still be heard by air-conduction after it faded away by boneconduction, and

b.    BC > AC if the tone can still be heard by bone-conduction after it faded away by air conduction.

Some Comments on Tuning Fork Tests

Tuning fork tests are quick and easy to administer and do not require special instrumentation, so they

can provide general, on-the-spot clinical insights, and they provide important (albeit limited) diagnostic information, especially when an audiogram is not available. However, tuning fork tests fall short of audiological measures in their ability to assess the patient’s hearing status. This is due to the greater

precision of audiometric tests made possible by calibrated electronic equipment and systematic testing strategies. Moreover, tuning fork tests are subject to considerable variability in administration and subjectivity in interpretation. Several limitations have already been mentioned with respect to individual tests, and others are worthy of mention. Which frequencies are tested varies among clinicians; some test only at 512 Hz and others use various combinations of frequencies. The intensity of the tone produced by the tuning fork depends on how hard it is struck each time, which causes stimulus levels to be inconsistent. Subjective patient responses can be a confounding variable, especially when dealing with younger children, and when the perception is “not logical” (e.g., when a tone is heard in the bad ear or gets louder when the ear is closed off). Also, tuning fork tests are usually done in examination rooms and clinics that are not sound isolated. Hence, noises that can mask the test signal and/or distract the patient are often a real issue.

It is therefore not surprising that carefully done

studies have shown that tuning forks are less accurate than audiometric methods. Wilson and Woods

(1975) found that both the Bing and Rinne tests failed to achieve a high level of accuracy in properly

identifying conductive versus nonconductive losses. Gelfand (1977) studied the diagnostic accuracy of the Rinne test. All of the tuning fork tests were done with masking of the opposite ear. He found that the Rinne test cannot identify a conductive loss with reasonable accuracy until the size of the air-bonegap becomes at least 25 to 40 dB wide. Thus, mild conductive hearing losses that are easily revealed audiometrically are frequently missed by tuning fork tests like the Rinne. Some tuning fork test problems are exacerbated by how and where the test is done. For example, the need for masking with certain tuning fork tests, particularly the Rinne. However, few physicians actually perform the Rinne test this way.