The Speech Recognition Threshold (SRT) is a foundational measurement in audiology that determines the softest sound level at which a person can correctly identify speech stimuli. This threshold provides a functional measure of hearing sensitivity for speech, which is often more relevant to daily communication than testing with pure tones alone. It is formally defined as the lowest intensity level, measured in decibels (dB), at which an individual can correctly repeat 50% of the presented speech material. The SRT serves as a necessary cross-check for the reliability of pure-tone audiometry results, providing a comprehensive view of hearing ability.
Preparing for SRT Measurement
The measurement of the Speech Recognition Threshold begins with careful preparation of the patient and the testing materials. The standard stimuli are Spondee words, which are two-syllable compound words spoken with equal stress on both syllables (e.g., “hotdog,” “baseball,” or “ice cream”). These words are chosen because their audibility increases rapidly with a small increase in volume, making it easier to pinpoint the exact threshold.
Before the test starts, the patient must be familiarized with the list of Spondee words. This ensures their responses are based purely on hearing the word, not on guessing unfamiliar vocabulary. Familiarization is often done visually or auditorily at a comfortably loud level, and the patient is instructed to repeat each word immediately, even if they are unsure. All equipment must be properly calibrated to ensure the sound intensity levels presented are accurate.
Step-by-Step Threshold Determination
The SRT is typically determined using a modified ascending method, a standardized procedure that systematically brackets the threshold. The examiner first selects an initial presentation level, usually 30 to 40 dB above the patient’s anticipated SRT or 20 dB above their Pure Tone Average (PTA) at 1000 Hz, to ensure the patient can easily hear and respond to the first few words. Once the words are clearly heard, the intensity is decreased in large steps, often 10 dB, until the patient misses a word or fails to respond.
The main threshold search begins by presenting a word at a level 10 dB below the level where the first incorrect response occurred. If the patient correctly repeats the word, the intensity is lowered by 10 dB; if the response is incorrect, the intensity is raised by 5 dB. This bracketing technique, often called the down-10, up-5 method, focuses the search on the true threshold quickly and efficiently. The examiner continues this pattern, presenting one word per step, meticulously recording correct and incorrect repetitions.
The search is complete when the patient provides a specific number of correct responses at the lowest intensity level they can hear. The threshold is defined as the lowest intensity at which the patient correctly repeats at least 50% of a set of presented words, such as two out of three or three out of six words. The intensity level in decibels at which this criterion is met is the measured Speech Recognition Threshold.
Calculating the Final SRT Value and Clinical Significance
Once the procedural steps are complete, the final SRT value is the intensity level in decibels at which the 50% correct response criterion was met. This final number, expressed in dB Hearing Level (HL), represents the calculated speech threshold for that ear. For example, if the lowest level where the patient correctly repeated three out of six Spondee words was 25 dB HL, the calculated SRT is 25 dB HL.
This final SRT value holds clinical importance because it verifies the accuracy of the overall hearing evaluation. The SRT should correlate closely with the patient’s Pure Tone Average (PTA), which is the average of the hearing thresholds measured at 500 Hz, 1000 Hz, and 2000 Hz. For the test results to be considered reliable, the SRT and the PTA must agree within a range of about \(\pm\) 6 to 10 dB.
A significant discrepancy between the measured SRT and the calculated PTA can indicate potential issues such as test unreliability, the presence of non-organic hearing loss, or a misunderstanding of the task by the patient. The comparison between the two values acts as a quality control step, ensuring that the foundational measurements of the patient’s ability to hear both pure tones and speech are consistent. This consistency provides confidence in the diagnostic conclusions drawn from the entire hearing test battery.