A tuning fork is a U-shaped acoustic resonator that produces a tone when set into vibration. This device functions by resonating at a fixed frequency, determined by the length and mass of its two prongs, or tines. John Shore, a British musician, invented the fork in 1711 for tuning musical instruments, but its application quickly expanded into science and medicine. The ability to produce a pure tone made it an ideal tool for early acoustic experiments and basic medical diagnostics. Today, it remains a quick, inexpensive tool used by health professionals to conduct preliminary tests of hearing and nerve function.
Fundamental Techniques: Striking and Holding
Properly preparing the tuning fork is the first step in any medical assessment, ensuring a clear and sustained tone. The correct way to hold the instrument is by the stem, or handle, using the thumb and forefinger. Holding the stem prevents contact with the vibrating prongs, which would immediately stop the sound.
To activate the fork, strike it lightly against a soft, resilient surface, such as the heel of the palm, a knee, or a rubber block. Striking the tines against a hard surface is avoided because this can generate nonharmonic frequencies, compromising the purity of the tone. A gentle tap is sufficient to set the metal into oscillation. If the vibration needs to be immediately stopped, the tines can be quickly pinched together near the tips to damp the sound.
Using the Fork for Hearing Assessments
In hearing assessments, the 512 Hz tuning fork is used to quickly differentiate between conductive and sensorineural hearing loss. The 512 Hz frequency is preferred because it offers the best balance between a sustained tone and tactile vibration. These tests compare the patient’s perception of sound transmitted through the air (air conduction) with sound transmitted through the bones of the skull (bone conduction).
The Rinne test compares a person’s hearing by air conduction versus bone conduction in one ear. The vibrating fork’s base is first placed firmly on the mastoid process, the bony prominence behind the ear, until the patient reports the sound is no longer heard. The still-vibrating fork is then immediately moved next to the external ear canal, and the patient is asked if they can hear it again. A normal result, or a “Rinne positive,” occurs when the sound is heard longer or louder next to the ear (air conduction) than on the bone (bone conduction).
The Weber test is performed by placing the vibrating fork’s base on the midline of the head, such as the forehead or the top of the skull. The vibrations travel through the skull to the inner ear, delivering sound symmetrically to both cochleae. The patient is asked in which ear the sound is heard louder, or if it is heard equally. If the sound is heard equally, the result is normal; if it is louder in one ear, it “lateralizes,” suggesting a hearing imbalance. Together, the Rinne and Weber tests help determine if a hearing problem is caused by conductive loss (outer or middle ear obstruction) or sensorineural loss (inner ear or auditory nerve issue).
Using the Fork for Vibration and Sensory Testing
The tuning fork is used in neurological examinations to assess a patient’s sense of vibration, testing the function of peripheral nerves. A lower-frequency fork, typically 128 Hz, is used because its stronger vibration is more easily felt through bony structures. This assessment is relevant in screening for conditions that cause peripheral neuropathy, such as diabetes, where the ability to perceive vibration often diminishes early.
The procedure involves tapping the 128 Hz fork and placing its stem on a bony prominence, such as the big toe joint, ankle, or wrist. The patient, usually with eyes closed, reports when they feel the vibration start and when it stops. The test is often conducted by comparing the patient’s perception time to the examiner’s, or by comparing sensation on both sides of the body. Loss or reduction of the ability to feel this vibration indicates sensory impairment, providing an early way to detect nerve damage.