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Tone Decay Test (TDT)

Identifying Auditory Fatigue and Nerve-Related Hearing Loss

The Tone Decay Test (TDT), also known as the Threshold Tone Decay Test, is used to detect signs of auditory fatigue and assess the integrity of the auditory nerve. At Denoc, we use this test to determine if hearing loss is due to cochlear damage or issues related to the vestibulocochlear nerve.

What Does It Measure?

In normal hearing, a tone just above the hearing threshold can be heard continuously for 60 seconds. TDT helps us measure the “decibels of decay”—the additional intensity needed for the tone to remain audible for the full duration.

  • 15–20 dB decay: May suggest cochlear pathology
  • >25 dB decay: Often associated with vestibulocochlear nerve damage
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Clinical Procedure at Denoc

  • You will be seated comfortably with earphones placed.
  • A 4000 Hz tone is played at 5 dB above your hearing threshold for 60 seconds.
  • If the tone fades, intensity is gradually increased in 5 dB steps.
  • The lowest level at which you can hear the tone continuously is recorded as the decibel decay.
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Short Increment Sensitivity Index (SISI)

Detecting Cochlear Pathology through Loudness Growth Response

The SISI test at Denoc evaluates how your ear responds to very small changes in sound intensity. It is particularly useful in identifying cochlear (inner ear) disorders, as it takes advantage of a phenomenon called recruitment—abnormal loudness growth often associated with sensorineural hearing loss.

Clinical Procedure at Denoc

  • You’ll be seated comfortably and fitted with earphones.
  • A steady pure tone at 20 dB is played in one ear.
  • At regular intervals, slight increments (as small as 1 dB) are superimposed on this tone.
  • Patients with cochlear pathology can detect these small changes, while those with normal hearing or conductive loss typically cannot.
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Average Testing Time

A combined session of TDT and SISI takes approximately 45 minutes to 1 hour. You will receive your clinical report immediately after the tests.
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Why Choose Denoc for Special Tests?

  • Expert Audiologists
  • Advanced Diagnostic Equipment
  • Same-Day Results
  • Personalized Hearing Care Plans

Frequently Asked Questions (FAQs)

The TDT is used to evaluate auditory fatigue and identify possible nerve-related hearing loss. It helps determine if your hearing problem originates from the cochlea (inner ear) or the auditory nerve.

The SISI test detects a patient’s ability to hear very small (1 dB) changes in sound intensity. This sensitivity is often seen in people with cochlear (sensorineural) hearing loss and is absent in those with normal or conductive hearing loss.

Not at all. Both TDT and SISI are completely non-invasive and painless. You simply need to sit comfortably with headphones on while the audiologist conducts the tests.

Special Test


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