Threshold

A threshold, by definition, is the lowest quantity that elicits a certain response. In audiology, threshold is commonly used to assess hearing in diagnostic test procedures. The threshold of audibility is the minimum effective sound pressure level of a signal that is capable of evoking an auditory sensation. Threshold assessment is used in both clinical and research protocols, and threshold itself has historically been considered a somewhat complex concept. Research over the years has shown that true threshold sensitivity can vary from moment to moment based on both intrinsic factors, such as breathing or heartbeat, and extrinsic factors, such as room temperature and ambient noise. In a clinical test situation, extrinsic factors can be largely controlled by using test environments that meet ambient noise standards (sound-treated booths). These fluctuations in threshold typically affect audiology test protocols only minimally, and the variability that occurs from uncontrollable factors is usually negligible. This entry examines various ways of measuring hearing and speech threshold in a clinical setting.

Most often, hearing is assessed using a diagnostic test known as pure-tone audiometry. The goal of pure-tone audiometry is to establish hearing threshold sensitivity across the range of audible frequencies that are important to communication. In clinical pure-tone audiometry, the generally accepted and most commonly used protocol for finding audiometric threshold is based on Raymond Carhart and James Jerger’s suggestion and is known as a modified Hughson–Westlake [Page 1967]technique. The procedure begins by testing the perceived better hearing ear first, if there is one. If neither ear is perceived as better, testing should begin with the right ear. Testing is typically performed by presenting a 1000-Hz pure tone at 30 dB hearing level (dBHL). This can be modified, if necessary, for individuals with known or suspected hearing loss. If a response is obtained, this suggests that the presentation level is suprathreshold, and the intensity should be lowered. Therefore, the intensity is lowered in 10-dB steps until the individual no longer responds. At this point, the intensity is then increased in 5-dB steps until the tone becomes audible and then decreased again by 10 dBHL and raised by 5 dBHL until a 50% response criterion is met. This down 10, up 5 procedure is repeated for each test frequency until a full pure-tone audiogram is obtained. Threshold responses are considered to be the intensity level at which the individual responds (at least) 50% of the time. If an individual does not respond initially at the 30-dBHL presentation level, the intensity is increased in 20-dB steps until a response is obtained, and then the same down 10, up 5 bracketing procedure is observed to find threshold. Clinically, audiometric thresholds are considered to be estimated thresholds due to the fact that testing is typically performed using only a 5-dB step size. Smaller intensity increments are generally available on most audiometers, but due to time and efficiency, the 5-dB step size is most often used clinically.

Threshold measurements are also obtained clinically using speech stimuli. The truest threshold measurement using speech stimuli is called the speech awareness/detection threshold. This test method simply requires a response from the individual that the speech stimulus was heard and is commonly used in very young pediatric or cognitively impaired populations. Any acknowledgment of the sound, be it a head turn, change in sucking behavior, or other behavioral response, can be accepted as a positive response. In older pediatric and adult populations, the most common clinical threshold test used for speech is called the speech reception threshold, which requires the individual to repeat a word, answer a question, point to a picture, or otherwise identify or recognize the speech stimulus that was presented. While this protocol is essentially testing higher level auditory skills of identification/recognition rather than simply threshold detection, it is widely acknowledged clinically as a threshold measure for speech stimuli. Both speech-threshold and pure-tone threshold tests are commonly considered to be part of a basic clinical comprehensive audiometric test battery.

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