Loudness Flashcards
(Fletcher & Munson, 1933)
Loudness is a subjective attribute of sound and represents the perceived of the “magnitude of an auditory sensation” (Fletcher & Munson, 1933). AND Note that a given number of decibels represents an intensity or power ratio, not an absolute intensity (Fletcher & Munson, 1933).
Define Loudness
Loudness is a subjective attribute of sound and represents the perceived of the “magnitude of an auditory sensation” (Fletcher & Munson, 1933). In other words, loudness is the perceptual correlate of sound intensity which can be ordered on a scale ranging from quiet to loud.
Define Intensity
Intensity (an objective term) is the sound energy transmitted per second (i.e. the power) through a unit area in a sound field. The human auditory system can make sense of a wide range of sound intensities.
logarithmic scale / Bels
Due to this large range of perceived intensities, a logarithmic scale expressing the ratio between two intensities is used. The first intensity (I0) is a reference intensity for another intensity (I1). One Bel corresponds to a ratio of intensities of 10:1, two Bels corresponds to an intensity ratio of 100:1, and so on. The Bel corresponds to a rather large ratio of intensities for everyday use, so to obtain units of more convenient size, the Bel is divided into 10 decibels (dB). When the magnitude of a sound is specified in decibels, it is customary to use the word “level” to refer to its magnitude. Note that a given number of decibels represents an intensity or power ratio, not an absolute intensity (Fletcher & Munson, 1933). To specify the absolute intensity of a sound it is necessary to state that the intensity of the sound (I1) is some “X” dB above or below a reference intensity (I0).
dB intro
The dB is used to measure sound level, but it is also widely used in electronics, signals and communication. Since decibels represent the logarithm of a ratio of two values, the term is meaningless without a reference.
SPL
The reference level for dB SPL is 20 micropascal, a pressure value. This reference value for SPL was selected to correspond to the faintest pressure that is audible in the frequency region where hearing is most sensitive. The SPL scale is frequently used in audiology to compare the level of speech or other sounds at different frequencies. Such comparisons are critical for programming hearing aids based on a patient’s specific hearing loss and evaluating the function of hearing aids. Also of note, the perception of loudness changes with sensorineural hearing loss. In this case, low level sounds are no longer audible, while high level sounds remain the same. One desirable goal for a hearing aid is to amplify speech sounds in order to restore loudness to the most comfortable listening level for a patient with hearing loss. To do so, a hearing aid must use a compression system to accommodate the loss of hearing for soft sounds while not making loud sounds too loud.
HL
HL, a second decibel scale, is used to plot an audiogram, the accepted clinical representation of pure-tone thresholds as a function of frequency. The reference for HL is the median threshold for a particular frequency for young adults with no history of ear problems. Unlike dB SPL, the zero reference level for dB HL varies with frequency, because humans have more sensitive hearing at some frequencies than others. Because the reference is normal human hearing, thresholds that deviate from 0 dB HL at any frequency shows how much one’s hearing deviates from this normal value. When considering loudness and intensity during any evaluation of hearing, there are many things to keep in mind. For example, a tone presented to both ears via earphones is perceived louder than the same tone presented to only one ear (Fletcher & Munson, 1933). Similarly, there are differences in the SPL that arrives at the level of the ear canal when an equally intense sound is presented to both ears through earphones compared to through speakers in the sound field (Epstein & Florentine, 2009; Munson & Wiener, 1952).
nHL
A third clinically relevant dB scale is normalized hearing level (nHL), which pertains to measures of sound level obtained during objective electrophysiologic assessments. Electrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage changes or electric current. During these tests, the patient does not need to make a behavioral response. Data is obtained strictly from the output of the electrical response. One example of an electrophysiology measure is an auditory brainstem response (ABR) test, which is referenced in dB nHL. ABR threshold estimates (dB nHL) are higher than behavioral thresholds measured in dB HL with pure-tone audiometry. As such, estimated behavioral hearing thresholds (eHL) are used to denote a corrected nHL value that represents an estimated behavioral threshold. Since electrophysiology is an objective, it is interesting that the click-evoked ABR may be used to estimate loudness growth for individuals with normal hearing and those with a flat configuration of cochlear hearing loss; this is not applicable, however, to listeners with a sloping configuration of cochlear hearing loss (Serpanos, O’malley, & Gravel, 1997).
Loudness is affected by the frequency of the signal.
This is evident in that to produce the same sensation of loudness across different frequencies, different SPLs are needed for different frequencies. The human ear is most sensitive to sounds of about 2000 Hz, and the ear becomes progressively less sensitive to very high and very low frequencies. This phenomenon is beneficial to humans who use oral speech for communication, since the mid-frequencies contain most of the information from speech sounds. Robinson and Dadson (1956) are among many who have measured equal-loudness contours, which represents the sound pressure levels of a sound that give rise to a sensation of equal-loudness magnitude as a function of sound frequency; their contours have since been accepted into international standards for sounds measures in the sound field. For complex signals, loudness increases with an increasing number of frequency components, since the loudness of a complex signal is the summation of the individual components.
critical bandwidth
Fletcher (1940) established that the ear can be modeled as a bank of frequency filters with the filter bandwidth being termed ‘‘critical bandwidth”. A critical band is the summation of neural activity across a range of frequencies. For moderate and high level tones, when the spacing between a group of pure tones is increased, the loudness remains constant until a critical point is reached, after which the loudness increases (Zwicker, Flottorp, & Stevens, 1957). The same effect occurs when the width of a band of noise of constant SPL is made larger (Zwicker, Flottorp, & Stevens, 1957). For low level sounds, loudness does not change based on the sound’s bandwidth.
duration
Additionally, the ability to make loudness judgments is compromised for very brief sounds less than some critical duration. The critical duration varies with the intensity of the stimulus, but is roughly between 15-200 ms duration. Below this critical duration, the loudness is affected by the duration of the sound, such that a short burst will seem quieter than a longer burst (Small, Brandt, & Cox, 1962). Durations longer than the critical duration do not affect loudness judgment until the ear eventually fatigues through adaptation and no longer processes the sound as robustly after a long period.
(Epstein & Florentine, 2009; Munson & Wiener, 1952)
Similarly, there are differences in the SPL that arrives at the level of the ear canal when an equally intense sound is presented to both ears through earphones compared to through speakers in the sound field (Epstein & Florentine, 2009; Munson & Wiener, 1952).
Serpanos, O’malley, & Gravel, 1997
Since electrophysiology is an objective, it is interesting that the click-evoked ABR may be used to estimate loudness growth for individuals with normal hearing and those with a flat configuration of cochlear hearing loss; this is not applicable, however, to listeners with a sloping configuration of cochlear hearing loss (Serpanos, O’malley, & Gravel, 1997).
Robinson and Dadson (1956)
Robinson and Dadson (1956) are among many who have measured equal-loudness contours, which represents the sound pressure levels of a sound that give rise to a sensation of equal-loudness magnitude as a function of sound frequency; their contours have since been accepted into international standards for sounds measures in the sound field.
Fletcher (1940)
Fletcher (1940) established that the ear can be modeled as a bank of frequency filters with the filter bandwidth being termed ‘‘critical bandwidth”.
