SUBJECT: Physics TEACHER: Mr. S. Campbell DATE: 15/1/2017 GRADE: DURATION: 1 wk GENERAL TOPIC: The Physics Of Hearing

Transcription

1 SUBJECT: Physics TEACHER: Mr. S. Campbell DATE: 15/1/2017 GRADE: DURATION: 1 wk GENERAL TOPIC: The Physics Of Hearing

2 The Physics Of Hearing

3 On completion of this section, you should be able to: Describe how the ear responds to an incoming sound wave Give reasons for the significance of the terms sensitivity and frequency response State the magnitude of the threshold of hearing State the intensity at which discomfort is experienced Use the equation for intensity level Understand the terms noise and loudness

4 The Human Ear

5 The Hearing Process The outer ear collects and directs sound waves into the auditory canal. At the end of the auditory canal is the ear drum that vibrates in response to sound waves. The middle ear consists of three tiny bones called the hammer, anvil and stirrup, which act like a lever system. They help to reduce the amplitude of vibration produced on the tympanic membrane, while increasing vibrational pressure on the oval window. The middle ear is connected to back of the throat via the eustachian tube.

6 The inner ear is filled with liquid. It contains the cochlea, which is attached to the auditory nerve. Inside the cochlea, there are many tiny hairs, which runs its entire length. When the oval window vibrates, the liquid inside the inner ear vibrates, causing the tiny hairs to resonate. The hairs then produce electrical signals which are then transmitted via the auditory nerve to the brain, where they are then interpreted.

7 Frequency response and intensity The Frequency response of the ear is the range of frequencies that the human ear is able to detect, which is from 20 Hz to 20 khz. Sound within this frequency range is called audible sound or just 'sound'. Sounds of frequenceies less than 20 Hz are called infrasound (or subsound); sounds of frequencies higher than 20 khz are called ultra sound. TONE GENERATOR:

8 Approximate range of frequencies heard by various animals Animal Bat Cat Dog Dolphin Elephant Mouse Approximate hearing range 20 Hz to 110,000 Hz 30 Hz to 64,000 Hz 40 Hz to 50,000 Hz 100 Hz to 150,000 Hz 15 Hz to 12,000 Hz 800 Hz to 75,000 Hz

9 We use three different terms to describe the 'loudness' of a sound. They are: Intensity (measured in watts per square metre) Sound level (measured in decibels) and Loudness level (measured in phons)

10 Sound intensity, I, is defined as the sound energy crossing perpendicularly across unit area per unit time. Sound Intensity/ W m -2 Energy/ J Area/ m 2 I = E/A t time/ s

11 Since energy per unit time is power, P, then sound intensity is the sound power per unit area. Sound Intensity/ W m -2 Power/ W I = P A Area/ m 2

12 The Threshold intensity of hearing, I 0, is the smallest intensity of a 1 khz sound that the human ear can typically detect. This intensity, I 0, has a value of approximately 1.0 X W m -2 (see graphs below). From the graphs, why is it that is that 1.0 X W m -2 always correspond to 0 db? The Threshold intensity of discomfort, I p, is the largest intensity of a 1 khz sound that the human ear can typically detect without the discomfort of pain. The value of this intensity is approximately 1.0 W m -2.

13 Sound level Sound level, β, measured in bels (B), corresponding to a sound of intensity I is defined by the logarithmic ratio: β (bel) = log 10 I 0 I Threshold intensity of hearing, W m -2

14 Sound level, β, measured in decibels (db), corresponding to a sound of intensity I is defined as 10 times the logarithmic ratio of the intensity, I, of the sound to the threshold intensity of hearing, I 0. β (db) = 10 X log 10 I 0 I Sound level, β, measured in db Threshold intensity of hearing, W m -2

15 } The sensitivity of the human ear is the ability to detect the smallest fractional change, I, of intensity, I.

16 Loudness level, phons, is the subjective response of a person to a given intensity. Loudness level is numerically equal to sound level in db of a 1 khz sound with the same apparent loudness.

17 The figure above (previous slide) shows curves of equal loudness as perceived by the human ear, based on frequencies and sound levels. Note that in order to be able to display the full audio range on a graph that is of reasonable size, both the intensity and frequency scales used are logarithmic and not linear. Hence the ear's response to sound intensities and frequencies is best displayed on a logarithmic scale.

18 Yesssssss!!!!!Assignment!!

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