Absolute threshold of hearing - AbsoluteAstronomy.com

The absolute threshold of hearing (ATH) is the minimum sound level of a pure tone

Pure tone

A pure tone is a tone with a sinusoidal waveshape.A sine wave is characterized by its frequency, the number of cycles per second—or its wavelength, the distance the waveform travels through its medium within a period—and the amplitude, the size of each cycle...

that an average ear with normal hearing

Hearing (sense)

Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...

can hear with no other sound present. The absolute threshold relates to the sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

that can just be heard by the organism. 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. This is also known as the auditory threshold.

The threshold of hearing is generally reported as the RMS

Root mean square

In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...

sound pressure

Sound pressure

Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...

of 20 µPa (micropascals) = 2×10⁻⁵ pascal

Pascal (unit)

The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

(Pa). It is approximately the quietest sound a young human with undamaged hearing can detect at 1,000 Hz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

. The threshold of hearing is frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

dependent and it has been shown that the ear's sensitivity is best at frequencies between 1 kHz and 5 kHz.

Psychophysical methods for measuring thresholds

Measurement of the absolute hearing threshold provides some basic information about our auditory system

Auditory system

The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...

. The tools used to collect such information are called psychophysical methods. Through these, the perception

Perception

Perception is the process of attaining awareness or understanding of the environment by organizing and interpreting sensory information. All perception involves signals in the nervous system, which in turn result from physical stimulation of the sense organs...

of a physical stimulus (sound) and our psychological response to the sound is measured.

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.

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".

Classical methods

Classical methods date back to the 19th century and were first described by Gustav Theodor Fechner in his work Elements of Psychophysics. 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.

Method of limits: In the method of limits, the tester controls the level of the stimuli. Single-interval “yes”/”no” paradigm’ is used, but there are no catch trials.

There are several series of descending and ascending runs.

The trial starts with the descending run, where a stimulus is presented at a level well above the expected threshold. When the subject responds correctly to the stimulus, the level of intensity of the sound is decreased by a specific amount and presented again. The same pattern is repeated until the subject stops responding to the stimuli, at which point the descending run is finished.

In the ascending run which comes after, the stimulus is first presented well below the threshold and then gradually increased in two decibel

Decibel

The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

(dB) steps until the subject responds.

As there are no clear margins to ‘hearing’ and ‘not hearing’, the threshold for each run is determined as the midpoint between the last audible and first inaudible level.

The subject's absolute hearing threshold is calculated as the mean of all obtained thresholds in both ascending and descending runs.

There are several issues related to the method of limits. First is anticipation, which is caused by the subject's awareness that the turn-points determine a change in response. Anticipation produces better ascending thresholds and worse descending thresholds.

Habituation

Habituation can be defined as a process or as a procedure. As a process it is defined as a decrease in an elicited behavior resulting from the repeated presentation of an eliciting stimulus...

creates completely opposite effect, and occurs when the subject becomes accustomed to responding either “yes” in the descending runs and/or “no” in the ascending runs. For this reason, thresholds are raised in ascending runs and improved in descending runs.

Another problem may be related to step size. Too large a step compromises accuracy of the measurement as the actual threshold may be just between two stimulus levels.

Finally, since the tone is always present, “yes” is always the correct answer.

Method of constant stimuli:

In the method of constant stimuli, the tester sets the level of stimuli and presents them at completely random order.

Thus, there are no ascending or descending trials.

The subject responds “yes”/”no” after each presentation.

The stimuli are presented many times at each level and the threshold is defined as the stimulus level at which the subject scored 50% correct. “Catch” trials may be included in this method.

Method of constant stimuli has several advantages over the method of limits. Firstly, the random order of stimuli means that the correct answer cannot be predicted by the listener. Secondarily, as the tone may be absent (catch trial), “yes” is not always the correct answer. Finally, catch trials help to detect the amount of a listener's guessing.

The main disadvantage lies in the large number of trials which are needed to obtain the data and therefore long time required to complete the testing.

Method of adjustment: Method of adjustment shares some features with the method of limits, but differs in others. There are descending and ascending runs and the listener knows that the stimulus is always present.

However, unlike in the method of limits, here the stimulus is controlled by the listener. The subject reduces the level of the tone until it cannot be detected anymore, or increases until it can be heard again.

The stimulus level is varied continuously via a dial and the stimulus level is measured by the tester at the end. The threshold is the mean of the just audible and just inaudible levels.

Also this method can produce several biases. In order to avoid giving cues about the actual stimulus level, the dial must be unlabeled. Apart from already mentioned anticipation and habituation, stimulus persistence (preservation) could influence the result from the method of adjustment.

In the descending runs, the subject may continue to reduce the level of the sound as if the sound was still audible, even though the stimulus is already well below the actual hearing threshold.

In contrast, in the ascending runs, the subject may have persistence of the absence of the stimulus until the hearing threshold is passed by certain amount.

Forced-choice methods

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.

Adaptive methods

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.

'Staircase’ methods (up-down methods):

The simple ‘1-down-1-up’ method consists of series of descending and ascending trials runs and turning points (reversals). The stimulus level is increased if the subject does not respond and decreased when a response occurs.

Similarly, as in the method of limits, the stimuli are adjusted in predetermined steps. After obtaining from six to eight reversals, the first one is discarded and the threshold is defined as the average of the midpoints of the remaining runs. Experiments showed that this method provides only 50% accuracy.

In order to produce more accurate results, this simple method can be further modified by increasing the size of steps in the descending runs, e.g. ‘2-down-1-up method’, ‘3-down-1-up methods’.

Bekesy's tracking method:

Bekesy's method contains some aspects of classical methods and staircase methods. The level of the stimulus is automatically varied at a fixed rate. The subject is asked to press a button when the stimulus is detectable.

Once the button is pressed, the level is automatically decreased by the motor-driven attenuator and increased when the button is not pushed. The threshold is thus tracked by the listeners, and calculated as the mean of the midpoints of the runs as recorded by the automat.

Hysteresis effect

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

Amplitude

Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

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

Noise

In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical 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 of absolute hearing threshold

Psychometric function

A psychometric function describes the relationship between a parameter of a physical stimulus and the responses of a person who has to decide about a certain aspect of that stimulus. Usually these sensory decisions take the form of a two-alternative forced choice . The psychometric function usually...

‘represents the probability of a certain listener's response as a function of the magnitude of the particular sound characteristic being studied’.

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

Psychometric function

.

The psychometric function is a sigmoid function

Sigmoid function

Many natural processes, including those of complex system learning curves, exhibit a progression from small beginnings that accelerates and approaches a climax over time. When a detailed description is lacking, a sigmoid function is often used. A sigmoid curve is produced by a mathematical...

which is characterised by being ‘s’ shaped in its graphical representation.

Minimal audible field (MAF) vs minimal audible pressure (MAP)

Two methods can be used to measure the minimal audible stimulus 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. The sound level is then measured at the position of the subjects head with the subject not in the sound field.
Minimal audible pressure involves presenting stimuli via headphones or earphones and measuring sound pressure in the subject's ear canal

Ear canal

The ear canal , is a tube running from the outer ear to the middle ear. The human ear canal extends from the pinna to the eardrum and is about 35 mm in length and 5 to 10 mm in diameter....

using a very small probe microphone.
The two different methods produce different thresholds and minimal audible field thresholds are often 6 to 10 dB better than minimal audible pressure thresholds. It is thought that this difference is due to:

monaural vs binaural
Binaural
Binaural literally means "having or relating to two ears." Binaural hearing, along with frequency cues, lets humans and other animals determine direction of origin of sounds...

hearing. With minimal audible field both ears are able to detect the stimuli but with minimal audible pressure only one ear is able to detect the stimuli. Binaural hearing is more sensitive than monaural hearing/
physiological noises heard when ear is occluded by an earphone during minimal audible pressure measurements. When the ear is covered the subject will hear body noises, such as heart beat, and these may have a masking effect.

Minimal audible field and minimal audible pressure are important when considering calibration

Calibration

Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

issues and they also illustrate that the human hearing is most sensitive in the 2–5 kHz range.

Temporal summation

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

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

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.

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.

Frequency variation

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.

External links

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