Jan. 17, 2017
Bellwork
A standing wave consists of ___________ and ______________.
Sound
All sounds are produced by vibrations of material objects. The vibrating source then stimulates something larger causing the air to vibrate and producing the sound we hear.
Our subjective impression of a sound is the pitch. A high pitch is a result of a source producing a sound at a high vibration frequency
(i.e. a piccolo).
Whereas a low pitch is a result of a source with a low vibration frequency (i.e. a bass).
Sound
A normal person can hear in the range of frequencies from about 20Hz to 20,000 Hz. As we grow older our hearing range shrinks (especially the high­frequency end).
Infrasonic are sound waves below 20 Hz.
Ultrasonic are sound waves above 20,000 Hz.
We cannot hear infrasonic or ultrasonic sound waves but they are there.
Sound in Air
As a source creates a vibration, the air around the sounds also begins to vibrate at the same frequency. Since sound is a longitudinal wave we don't have "crests" and "troughs" like a transverse wave. Instead sound waves contain compressions (areas of high pressure where the molecules are being pushed together) and rarefactions (areas of low pressure due to the "backward" movement of the source).
Remember, the medium doesn't travel with the wave.
Media that transmit sound
Sound in a bell jar
Sound waves require a medium to travel through. This could be a solid, liquid, or gas. Since sound requires a medium to travel through sound cannot travel in a vacuum. Even though a source may be vibrating, there will be no sound produced (in a vacuum). Mechanical waves require a medium in order to propagate, therefore sound is also a mechanical wave.
Speed of Sound
Since sound requires a medium, the speed of sound therefore is dependent on certain properties of the medium it is traveling through, i.e.
density (matter per volume), and
elasticity (ability of a material to change shape in response to an applied force and then resume its initial shape)
Material
Speed of Sound (m/s)
Rubber 60 Air (40o C)
355 Air (20o C) 343
Fresh water
1497
Sea water 1560 Lead 1210
Gold 3240
Glass 4540 Copper 4600 Aluminum 6320 Forced Vibrations
Forced vibrations are the result when the vibrations in the air (as a result of some source) in turn cause another object to vibrate.
This is how all stringed musical instruments make sound.
(string vibrates ⇒ causes air to vibrate ⇒ causes sounding board to vibrate⇒ creates sound we hear) Resonance ­ Forced Vibrations
Natural Frequency & Resonance
Resonance cart
When an object (made of an elastic material) is disturbed it begins to vibrate at is own special natural frequency, which depends on several factors like the elasticity and shape of the object. This natural frequency is the one that requires minimal amount of energy to continue its vibration. If this continues, the object's vibration amplitudes will continue to increase rapidly. This is called resonance.
An example experienced by children everywhere is swinging on a swing. As you "kick back and forth" you and achieve the resonance of the swing, your amplitude (height) starts to increase.
This is also illustrated by "breaking glass with your voice" and also the "Galloping Gertie" bridge.
Tacoma Narrows Bridge
"Galloping Gertie" (10:01)
http://www.youtube.com/watch?v=ASd0t3n8Bnc
Tacoma Narrows Bridge
"Galloping Gertie" (5:56)
http://www.youtube.com/watch?v=j­zczJXSxnw