The effects of ear canal acoustics on hearing ability
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The effects of ear canal acoustics on hearing ability
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[post_date] => 2025-01-09 08:23:55
[post_date_gmt] => 2025-01-09 13:23:55
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
The acoustic properties of the human ear canal can be predicted by modeling the ear as a rigid tube that has been sealed at one end (Figure 1). Such a tube naturally amplifies sounds that are at or near its resonant frequencies, and it dampens sounds distant from its natural frequencies---the closer a given frequency lies to the resonant frequency of the tube, the more it will be amplified. The physical mechanism for this effect is the formation of standing waves in the tube as sounds enter the ear, pass to the end, and then reflect off the wall back towards the entry point. As a result of this effect, the perceived intensity of a given sound frequency often does not match its actual intensity before it enters the ear canal. This effect is deliberate on the part of the body, and it allows humans to retain maximum sensitivity to particular sounds, like speech.
This effect is observable in equal-loudness curves (Figure 2). The x axis gives the frequency of a given sound (in Hz), the y axis gives the laboratory-measured amplitude of the sound waves (in dB, or decibels), and the separate curves correspond to the perceived “loudness” of sounds with a given frequency and amplitude, as reported by a human observer. The units for loudness are called “phons.” The perceived loudness of a sound can be found by finding the point on the graph corresponding to the given frequency and intensity (in dB) of the sound, and then finding the equal-loudness curve that lies closest to that point and recording its corresponding loudness level (in phons). Thus, for a given perceptual loudness curve, a peak corresponds to a frequency that the ear canal filters out, such that a higher intensity is needed to reach a given perceived loudness — resulting in human hearing being less sensitive to that frequency.
Figure 1: A simple model of the ear canal as a tube with an opening at one end.
Figure 2: A set of equal-loudness curves for a typical human (OSHA, 2014).
Citation: Culjat, Martin O.; Goldenberg, David; Tewari, Priyamvada; Singh, Rahul S. (2010). A Review of Tissue Substitutes for Ultrasound Imaging. Ultrasound in Medicine & Biology 36 (6): 861–873.
[post_title] => The effects of ear canal acoustics on hearing ability
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[question] => Which of the following properties describes the wave patterns most strongly amplified by the ear canal?
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[answer] => 2
[description] => Reason for the Correct Answer:
The positions of nodes and antinodes determine the constraints on wave frequencies in most harmonic systems
Waves with maxima/minima at the closed end would be rapidly absorbed
Waves with minima at the open end would fail to fully exploit the resonance of the cavity
Standing waves in the ear canal should have a minimum at the sealed end and a maximum at the open end.
Waves are most strongly amplified when they have a node at the end of the ear and an antinode at the opening.
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[answers] => Array
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[0] => Array
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[each_answer] => A. A node at the opening of the ear canal, and a node at the end of the ear canal
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[each_answer] => B. An antinode at the opening of the ear canal, and a node at the end of the ear canal
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[2] => Array
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[each_answer] => C. An antinode at the opening of the ear canal, and an antinode at the end of the ear canal
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[each_answer] => D. A node at the opening of the ear canal, and an antinode at the end of the ear canal.
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Which of the following gives a frequency for an ear canal of length L would be most sensitive (𝒗 denotes the speed of sound in air)?
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[answer] => 3
[description] => Reason for the Correct Answer:
The wavelength of a sound wave times its frequency gives its speed.
For a tube sealed at one end, the allowed frequencies are those corresponding to a sine wave with a node at the sealed end and a peak at the open end
Only one answer choice will satisfy these boundary conditions
The longest-wavelength sine wave that satisfies the boundary conditions has frequency 𝒗/4L
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[answers] => Array
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[0] => Array
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[each_answer] => A. 𝒗/3L
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[1] => Array
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[each_answer] => B. 𝒗/5L
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[2] => Array
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[each_answer] => C. 𝒗/4L
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[3] => Array
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[each_answer] => D. 𝒗/7L
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Which of the following statements is generally true for hearing in a typical human?
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[answer] => 1
[description] => Reason for the Correct Answer:
As noted in the passage, a peak on the graph for a given loudness (in phons) corresponds to reduced sensitivity to that frequency.
A minimum on the graph corresponds to increased sensitivity to sounds of a given intensity.
The 2000-5000 Hz portion of the graph is generally a minimum region for all perceived loudnesses plotted.
The minimum region means that, for point on the graph defined by a given sound level (in dB) and a given frequency, a higher-loudness curve will pass through the point than would in the absence of a minimum.
Patients perceive sounds as louder when they are in the 2000-5000 Hz range.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Patients perceive sounds as louder when they are in the 2000-5000 Hz range.
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[1] => Array
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[each_answer] => B. Patients perceive sounds as less loud when they are in the 100-1000 Hz range than when they are in the 20-100 Hz range.
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[2] => Array
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[each_answer] => C. Patients generally perceive sounds in the 1000-5000 Hz range as less loud than those in other frequency ranges.
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[each_answer] => D. Patients perceive sounds with frequencies greater than 5000 Hz as louder than those with frequencies below 5000 Hz.
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[quiz_unique_key] => 2261298308
[question] => Suppose a patient has a novel condition that causes the ear canal to be less stiff and thus more easily deformed by pressure fluctuations from sound waves. Which of the following symptoms may result?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
The amplitude versus displacement graph for a sound wave is a sine wave, but the waves themselves are radially symmetric as they pass into the ear—-so there should not be any asymmetric tissue fatigue.
While a softer ear canal could affect resonance frequencies, this is not a large effect and it should not greatly change the number of resonance frequencies in the frequency range of interest.
If passing waves are more easily able to deform the ear canal, it will absorb more of their energy and dissipate it
A softer ear canal would reduce hearing sensitivity due to absorption of the sound waves.
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[0] => Array
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[each_answer] => A. An increase in hearing sensitivity due to additional resonant frequencies in the ear canal
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[1] => Array
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[each_answer] => B. A decrease in hearing sensitivity due to absorption of sound waves as they pass through the ear canal.
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[2] => Array
(
[each_answer] => C. An increase in hearing sensitivity due to expansion of the ear canal as sound waves enter.
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[3] => Array
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[each_answer] => D. The cold gel would make for less friction between the skin and probe, making the imaging more comfortable
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[4] => Array
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[quiz_unique_key] => 2377279144
[question] => Based on the capped tube model of the ear, which of the following features would one expect to observe 12000 Hz if the equal-loudness chart was extended to that point?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
Capped tubes have multiple resonance frequencies.
On the graph, a resonance is responsible for the minimum in the 1000-5000 Hz region on each of the equal-loudness curves
The higher-order resonant frequencies are given by integer multiples of the fundamental frequency
Thus 4000 x 3 Hz = 12000 Hz should represent a resonant frequency for the tube
The capped tube model would predict an additional loudness-curve minimum at 12000 Hz.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The model does not predict any additional features in the graph.
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[1] => Array
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[each_answer] => B. A minimum
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[2] => Array
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[each_answer] => C. A peak
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[each_answer] => D. An asymptote
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