- Special tests using pure tone
- Special tests using threshold level
- Special tests for assessing cochlear and retro-cochlear pathology
- Special tests requiring behavioural response
Special Audiometric Hearing Tests
Tone Decay Test
Tone Decay Test (TDT) is a subjective behavioural test that is used to detect and measure auditory fatigue and neural lesions such as acoustic neuroma. It’s an essentially powerful procedure for auditory nerve damage
Tone decay is measured either at or near threshold or well above it, and therefore is classified into 2 groups
- Threshold Tone Decay: Reduction in sensitivity arising from the presence of hardly audible tone
- Supra Threshold Tone Decay: Loss of audibility through stimulation at higher presentation level
In the case of patients with normal hearing, a tone slightly above their hearing threshold can be heard straight for 60 seconds. This test produces a measure of decibels of decay, which is the numbers of decibels above the patient’s hearing threshold.
Clinical Procedure for Tone Decay Test
Once the patient is relaxed, a tone with a frequency of 4000 Hz is passed through the patient’s ears at an intensity of 5 dB above their threshold of hearing. In case the patient stops hearing before 60 seconds, the tone’s intensity is increased by another 5 decibels.
The process is repeated until the decibel level reaches a point where the patient is able to hear the tone for straight 60 seconds, and finally, the resultant measure is produced as decibels of decay.
A decay of 0.5 dB indicates a normal hearing level, while that of 10-15 dB indicates a mild hearing loss. Any decay above 15 dB and up till 20 db suggests a cochlear hearing loss and that above 25 db suggests a vestibulocochlear nerve damage.
Short Increment Sensitivity Index Test
The Short Increment Sensitivity Index (SISI) Test is widely used to determine if a patient is having a cochlear pathology or not and is based on a phenomenon called recruitment, i.e. abnormal loudness growth.
This hearing test determines a patient’s capacity to detect a 1 decibel increment at a 20 decibel threshold tone, i.e. the carrier tone, in frequencies between 1000 and 4000 Hertz.