In addition to wave speed, there are three other important parameters of a
continuous wave that can be measured: amplitude, wavelength and frequency.
Refer to figure 6.5 (page 223):
The maximum displacement of points on a wave measured from the equilibrium position is known as the amplitude. The amplitude is just a distance equal to the height of a peak or the depth of a valley.
The distance between two successive “like” points on a wave is defined as the wavelength. For example, the distance between two adjacent peaks or two adjacent valleys, wavelength is represented by the lowercase Greek letter lambda.
The number of cycles of a wave passing a point per unit time is known as frequency. The number of oscillations per second in the wave.
The speed of a wave, its wavelength and the frequency of a wave are related by
Wave speed = (number of cycles per second)(length of each cycle)
= (frequency) (wavelength)
For a wave moving in three-dimensional space, like the sound traveling outward from you in all directions as you shout or whistle, the wave fronts are spherical shells surrounding the source of the wave.
A wave is reflected whenever it reaches a boundary of its medium or
encounters an abrupt change in the properties of its medium. Similarly, a wave on a surface or a wave in three dimensions is reflected when it encounters a boundary. The wave that “bounces back” is called the REFLECTED WAVE.
Doppler effect:
Can you recall the last time a fast-moving emergency vehicle with its siren blaring passed near you? If so, you may remember that the pitch or tone of its sound dropped suddenly as it went by. This is a manifestation of the DOPPLER EFFECT.
A SIMILAR SHIFT IN FREQUENCY OCCURS IF YOU ARE MOVING TOWARD A STATIONARY SOUND SOURCE.
Echolocation is the process of using the waves reflected from an object to determine its location. You can determine the distance to a cliff (Fig 6.22, page 233) by timing the echo. Incorporating the Doppler effect in echolocation makes it possible to immediately determine the speed of an approaching or departing object.
Doppler radar uses the combination of echolocation and the Doppler effect.
Law-enforcement officers use Doppler radar to check the speeds of vehicles.
Bow Waves and Shock Waves: In previous discussion, we have implicitly assumed that the speed of the wave source is much less that the wave speed itself. However, if you’ve heard a sonic boom or been jostled be the wake of a passing watercraft while floating in the water, you’ve had experience with circumstances where the reverse is true. Fig 6.24 a (page 234) shows another series of wave fronts produced by a moving wave source. This time the speed of the wave source is greater than the wave speed. The wave fronts “pile up” in the forward direction and form a large-amplitude wave pulse called a shock wave.
Diffraction: Think about walking down a street and passing by an open door or window with sound coming from inside. You can hear the sound even before you get to the opening, as well as after you’ve passed it. The sound doesn’t just go straight out of the opening like a beam, it spreads out to the sides. This is diffraction. The extent to which the diffracted wave spreads out depends on the ratio of the size of the opening to the wavelength of the wave. When the opening is much larger than the wavelength, there is little diffraction.
Interference: Interference arises when two continuous waves, usually with the same amplitude and frequency, arrive at the same place. Let us take a look at Figure 6.28 (page 235): Interference pattern of water waves from two nearby sources. The thin lines of calm water indicate destructive interference.
Between these lines are regions of large-amplitude waves caused by destructive interference.
Sound is produced by anything that is vibrating and causing the sir molecule next to it to vibrate. Fig 6.3 (Page 238) shows a representation of a sound wave that was emitted by a vibrating tuning fork.
The main categories that we use to describe sounds subjectively are pitch, loudness and tone quality:
The pitch of a sound is the perception of highness or lowness. The sound of a soprano has a high pitch and that of a bass voice has a low pitch. The pitch of a sound depends primarily on the frequency of the sound wave.
The loudness of a sound is self-descriptive. The loudness of a sound dependsprimarily on the amplitude of the sound wave.
The tone quality of a sound is used to distinguish two different sounds even though they have the same pitch and loudness. A note played on a violin does not sound quite like the same note played on a flute. The tone quality of a sound depends on the waveform of the sound wave.
Sections covered from this chapter: Sec 6.1, 6.2, 6.3 and 6.6.
Homework 5
Due date : 6th June 2020 (6 pm)
A person riding on a bus hears the sound from a horn on a car that is stopped. What is different about the sound when the bus is approaching the car compared to when the bus is moving away from the car?A sonic depth gauge is placed 10 m above the ground. A pulse sent downward reflects off snow and reaches the device 0.03 seconds after it was emitted. The air temperature is -35 Celsius.(a) How far is the surface of the snow from the device?
(b) How deep is the snow?
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