The absolute threshold of hearing (ATH) is the minimum sound level of a pure tone that an average ear with normal hearing can hear in a noiseless environment. The absolute threshold relates to the sound that can just be heard by the organism (Durrant & Lovrinic 1984, Gelfand 2004). The absolute threshold is not a discrete point, and is therefore classed as the point at which a response is elicited a specified percentage of the time (Durrant & Lovrinic 1984).
The threshold of hearing is generally reported as the RMS sound pressure of 20 µPa (micropascals) = 2×10−5 pascal (Pa). This is equivalent to 2×10−4 dynes per square centimeter, or ~0.018195397 lbf/acre. It is approximately the quietest sound a young human with undamaged hearing can detect at 1,000 Hz (Gelfand, 1990). The threshold of hearing is frequency dependent and it has been shown that the ear's sensitivity is best at frequencies between 1 kHz and 5 kHz (Gelfand, 1990).
Measurement of the absolute hearing threshold provides some basic information about our auditory system (Gelfand 1990). The tools used to collect such information are called psychophysical methods. Through these, the perception of a physical stimulus (sound) and our psychological response to the sound is measured (Hirsh, 1952).
There are several different psychophysical methods which can be used for the measurement of absolute threshold. These methods may vary in many ways; however, certain aspects are identical. Firstly, the stimulus is defined, and the manner by which the person should respond is clearly specified. The sound is then presented to the listener and the level of the stimulus is manipulated in a predetermined pattern. The absolute threshold is defined statistically, often as an average of all obtained hearing thresholds (Gelfand, 1990).
Some procedures use a series of trials, with each trial using the ‘single-interval “yes”/”no” paradigm’. This means that sound may be present or absent in the single interval, and the listener has to say whether he thought the stimulus was there. When the interval does not contain a stimulus, it is called a "catch trial" (Gelfand, 1990).
Classical methods date back to the 19th century and were first described by Gustav Theodor Fechner in his work Elements of Psychophysics (Hirsh, 1952). Three methods are traditionally used for testing a subject's perception of a stimulus: the method of limits, the method of constant stimuli, and the method of adjustment (Gelfand, 1990).
Two intervals are presented to a listener, one with a tone and one without a tone. Listener must decide which interval had the tone in it. The number of the intervals can be increased, but this may cause problems to the listener who has to remember which interval contained the tone (Gelfand, 1990, Miller et al., 2002).
Unlike the classical methods, where the pattern for changing the stimuli is preset, in adaptive methods the subject's response to the previous stimuli determines the level at which a subsequent stimulus is presented (Levitt,1971).
Hysteresis can be defined roughly as ‘the lagging of an effect behind its cause’.
When measuring hearing thresholds it is always easier for the subject to follow a tone that is audible and decreasing in amplitude than to detect a tone that was previously inaudible. This is because ‘top-down’ influences mean that the subject will be expecting to hear the sound and will, therefore, be more motivated with higher levels of concentration. The ‘bottom-up’ theory explains that unwanted external (from the environment) and internal (e.g. heartbeat) noise will result in the subject only responding to the sound if the signal to noise ratio is above a certain amount.
In practice this means that when measuring threshold with sounds decreasing in amplitude, the point at which the sound becomes inaudible will always be lower than the point at which it returns to audibility. This phenomenon is known as the ‘hysteresis effect’.
Psychometric function ‘represents the probability of a certain listener's response as a function of the magnitude of the particular sound characteristic being studied’ (Arlinger, 1991).
To give an example, this could be the probability curve of the subject detecting a sound being presented as a function of the sound level. When the stimulus is presented to the listener one would expect that the sound would either be audible or inaudible, resulting in a 'doorstep' function. In reality a grey area exists where the listener is uncertain as to whether they have actually heard the sound or not, so their responses are inconsistent, resulting in a psychometric function.
The psychometric function is a sigmoid function which is characterised by being ‘s’ shaped in its graphical representation.
Two methods can be used to measure the minimal audible stimulus (Gelfand 2004) and therefore the absolute threshold of hearing. Minimal audible field involves the subject sitting in a sound field and stimulus being presented via a loudspeaker (Gelfand 2004, Kidd 2002). The sound level is then measured at the position of the subjects head with the subject not in the sound field (Gelfand 2004). Minimal audible pressure involves presenting stimuli via headphones (Gelfand 2004) or earphones (Durrant & Lovrinic 1984, Kidd 2002) and measuring sound pressure in the subject's ear canal using a very small probe microphone (Gelfand 2004). The two different methods produce different thresholds (Durrant & Lovrinic 1984, Gelfand 2004) and minimal audible field thresholds are often 6 to 10 dB better than minimal audible pressure thresholds (Gelfand 2004). It is thought that this difference is due to:
Minimal audible field and minimal audible pressure are important when considering calibration issues and they also illustrate that the human hearing is most sensitive in the 2-5 kHz range (Gelfand 2004).
Standardisation of audiometric equipment must be carried out, so that hearing loss relative to “normal hearing” can be quantified. This is based on the SPL needed for the average person to detect a sound. Reference equivalent sound pressure levels (RETSPLs) are standards we use for normal hearing. They are reference values for pure tone signals presented from various kinds of earphones. The values are based upon a round robin of loudness-balance and threshold experiments involving American, British, French, Russian, and German test centres. The reference levels obtained here have been incorporated into the American National Standards Institute. Because the RETSPL at each frequency represents the populations’ average hearing threshold we may think of them as all representing the same hearing level. Thus each RETSPL may also be referred to as 0 dB hearing level (0 dBHL); see table below.
For example, a reference level for a 1000 Hz tone may be 7, so that 0 dBHL corresponds to 7.5 dBSPL at 1000 Hz. At 250 Hz, more sound pressure is required for the average listener to reach the normal threshold so 0 dBHL for this frequency is 26.5 dBSPL. When measuring bone conduction thresholds we use reference-equivalent threshold force levels (RETFLs) because they express the equivalent force on a measuring device called an artificial mastoid, which corresponds to 0 dBHL when the bone-conduction vibrator is placed upon a person's mastoid (Gelfand, 2004).
From http://www.gnresound-group.com/lossandcare/encyclopedia/decibel.htm [dead link]
| Frequency (Hz) | dBSPL | dBHL |
|---|---|---|
| 250 | 15.0 | 0.0 |
| 500 | 9.0 | 0.0 |
| 1000 | 3.0 | 0.0 |
| 2000 | -3.0 | 0.0 |
| 4000 | -4.0 | 0.0 |
| 8000 | 13.0 | 0.0 |
Temporal summation is the relationship between stimulus duration and intensity when the presentation time is less than 1 second. Auditory sensitivity changes when the duration of a sound becomes less than 1 second. The threshold intensity decreases by about 10 dB when the duration of a tone burst is increased from 20 to 200 ms.
For example the most quiet sound a subject can hear is 16 dB if the sound is presented at a duration of 200 ms. If the same sound at 16 dB is then presented for a duration of 20 ms only the most quiet sound that can now be heard by the subject goes up to 26 dB. In other words if a signal is shortened by a factor of 10 then the level of that signal must be increased by as much as 10 dB to be heard by the subject.
The ear operates as an energy detector that samples the amount of energy present within a certain time frame. A certain amount of energy is needed within a time frame to reach the threshold. This can be done by using a higher intensity for less time or by using a lower intensity for more time. Sensitivity to sound improves as the signal duration increases up to about 200 to 300 ms, after that the threshold remains constant (Gelfand 2004).
The timpani of the ear operates more as a sound pressure sensor. Also a microphone works the same way and is not sensitive to sound intensity.
The audible frequency range is usually quoted as 20 Hz to 20,000 Hz. Obtaining thresholds is reliant on stimuli being presented in frequencies within the Auditory Response Area.
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