Bone Conduction Oscillator Placement

Bone conduction technology has been making waves with bone conduction headphones and bone conduction hearing aids. Proponents of bone conduction headsets advocate many advantages and use including safety during outdoor sports or even for military uses. However, it has been around for longer than you may think!

What is bone conduction and how do bone conduction headphones work?

Humans typically hear sounds through their ears. Bone conduction bypasses the eardrums and vibrations from sound are transmitted through the bones of the head and jaw directly to the cochlea (inner ear). Bone conduction headphones and hearing aids work in this manner helping individuals with damaged eardrums to hear by transmitting sounds through the bones, bypassing the eardrums entirely. Beethoven used a primitive form of this technology when he started going deaf way back in the 1800s by placing a stick on his piano and clenching it in his jaws. The vibrations from the piano were transferred from his jaw to his inner ear allowing him to hear.

A brief history of bone conduction audiometry

Although Beethoven may have used a form of bone conduction centuries ago, the foundations of modern bone conduction audiometry can be attributed to the work of two gentlemen – Georg von Békésy and Raymond Carhart. Békésy explained how the cochlea converts vibrations into neural impulses while Raymond Carhart, also known as the Father of Audiology, researched the clinical applications of bone conduction testing in otosclerosis patients.

Is bone conduction safe?

Bone conduction testing is completely safe provided it is carried out by a trained clinician. The issue with bone conduction testing lies not with its safety, but its accuracy. Over the years, researchers have questioned the validity of bone conduction tests mainly due to the many variables involved like different skull structures and placement of the bone oscillator, which in turn can affect test and retest measurements.

Quality of evaluation relies on bone-conduction test accuracy

Hearing loss may be sensorineural, conductive, or mixed. A form of pure-tone testing, bone conduction tests can be vital in diagnosing the type of hearing loss an individual has. The correct diagnosis procured through bone conduction tests will have a great impact on any further test recommendations, medical or surgical treatments, or hearing assistive devices recommended to the individual. With a lot resting on the results of the audiogram, it becomes imperative that bone conduction test assessments are accurate.

Bone conduction testing can indicate abnormal middle ear functionality as the inner ear is stimulated identically with air and bone conduction. Patients with conductive or mixed hearing loss are likely to show differences between their air and bone conduction audiometric thresholds. This is known as an Air-Bone Gap (ABG). Accurate and reliable ABG readings can be obtained via a computer-based audiometer which ensures that the individual receives the medical attention they need preventing inappropriate diagnoses, treatment, or further complications.

Placement of the bone conductor oscillator during testing

The placement of the bone oscillator can play a role in obtaining reliable and accurate test results. Researchers have been exploring the effects of bone vibrator placement for decades – most commonly on the mastoids and forehead.

Mastoid placement

There are many advocates for placing the bone vibrator on the mastoid behind the ear pinna as this placement has a wider dynamic range than on the forehead. However, this positioning does pose several problems:

  • If the vibrator touches the ear pinna, the test is invalidated as the subject feels the vibration.
  • Additionally, the oscillator cannot be placed on the hair, which can be difficult in the mastoid area.
  • Keeping the bone oscillator stable can also prove to be difficult due to slippage in this area.
  • Small shifts in the placement of the bone vibrator can cause larger threshold shifts as compared to similar shifts on the forehead resulting in inaccurate results.

Forehead placement

Unlike the mastoid, the forehead bone has a uniform thickness and a relatively flat shape which produces a regular form of vibration. This makes the center of the forehead the ideal place for placing the bone oscillator. The forehead placement of the bone vibrator is not a new concept and has been around for decades.

Forehead placement requires 10dB more force to reach the threshold when compared to mastoid placement which is why many audiologists favor mastoid placement. However, as technology has progressed, so have hearing screen equipment and audiometers negating this issue.

Advantages of forehead placement of the bone oscillator with the KUDUwave

Forehead bone vibrator placement can overcome many of the difficulties of mastoid placement. When placed on the forehead and both ears are covered by circumaural earphones there is no acoustic radiation from the oscillator to the ear canal.

Occluded bone conduction thresholds tend to be lower than unoccluded thresholds. This occlusion effect occurs as a ‘stronger’ bone conduction signal reaches the cochlea when ears are occluded. The KUDUwave minimizes this occlusion effect with the insert earphones being placed in the bony part of the ear canal. You can read more about insert earphone placement in our article here. Circumaural cups over the insert earphones provide additional attenuation.

Studebaker in 1962 found that ‘the forehead offers increased reliability and validity through a reduction of intersubject variability and a reduction of the effect of the middle ear on the threshold of audibility.

While the verdict is still out about the placement of the bone vibrator, KUDUwave has won favorable reviews from researchers testing the efficacy of boothless audiometry and hearing telemedicine:

  • The KUDUwave system has the capability of performing a complete diagnostic hearing evaluation, including pure-tone air- and bone-conduction testing, speech audiometry, and masking, in addition to tympanometry.
  • Test-retest threshold differences using the computer-based system outside the sound booth were similar to those measured by standard audiometry (Swanepoel et al., 2015).

The KUDUwave Pro TMP is designed to allow you to improve the quality of patient care while doing away with the necessity of a bulky sound booth.  Click here to learn more about the features and capabilities of the KUDUwave Pro TMP.