Vibrations through a medium Sound
oAll sounds are produced by the vibrations of material objects. PITCH = The impression about the frequency of a sound high pitched – high frequency (ex: piccolo) low pitched – low frequency (ex: fog horn)
The average frequency range for normal human hearing is between 20 Hz and 20,000 Hz (for younger people; older people lose the higher frequencies) Infrasonic or subsonic = Sounds below 20 Hz Ultrasonic = frequencies above 20,000 Hz Frequency
Sound in Air Compression waves travel through air or along springs. These waves travel with areas of compression and rarefaction The medium does not travel from one place to another, but the pulse does.
Speed of Sound The speed of sound in dry air at 0 C is about 330 m/s. For each degree above 0 C the speed of sound increases by 0.60 m/s. Sound travels 15 times faster in steel than air and about 4 times faster in water. Any matter will transmit sound, whether it is a solid, liquid or a gas. Sound cannot travel through a vacuum.
Intensity Intensity is the rate of energy flow through an area. Sound intensity is objective and is measured by instruments such as an oscilloscope. Loudness is a physiological sensation sensed by the brain. It is subjective but related to sound intensity. The unit of intensity for sound in the decibel (dB).
Common Levels of Sound SOURCE OF SOUND LEVEL (dB) Jet Engine, at 30 m140 Threshold of pain 120 Loud rock music115 Old subway train100 Average factory90 Busy street traffic70 Normal speech60 Library40 Close Whisper20 Normal breathing10 Hearing threshold0
Forced Vibration The vibration of one object causes another object to vibrate. Sounding boards are used to increase the volume (amplitude) of a vibrating object (like a string). STRINGS SOUNDING BOARD
Natural Frequency Everything vibrates, from planets and stars to atoms and almost everything in between. A natural frequency is one at which minimum energy is required to produce forced vibrations Also requires the least amount of energy to continue this vibration
Resonance Resonance – when the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude of the vibrations occurs. For example, a swing, or the hollow box parts of musical instruments are designed to work best with resonance. In order to resonate, an object must be elastic enough to return to its original position and have enough force applied to keep it moving (vibrating)
Interference Sound waves interfere with each other in the same way as all waves. Constructive interference - augmentation Destructive interference - cancellation
The Doppler Effect v=f so a smaller wavelength means a higher frequency. Animation courtesy Dan Russell, Kettering University
The Doppler Effect Motion of either the source or the observer of a wave causes the frequency to shift. If the relative motion results in more wave crests reaching the observer per second, the frequency increases. If the relative motion results in fewer wave crests reaching the observer per second, the frequency decreases.
The Doppler Effect
Standing Waves Although waves usually travel, it is possible to make a wave stay in one place. A wave that is trapped in one spot is called a standing wave. Standing waves are the result of interference. The resultant wave is created by the interference of two waves traveling at the same frequency, amplitude and wavelength but in opposite directions.
In a standing wave, the nodes remain stationary. This is where you can touch a standing wave on a rope without disturbing the wave. The positions on a standing wave with the largest amplitudes are know as antinodes. They occur halfway between nodes. Standing waves can be set up on the strings of musical instruments, in organ pipes, and by blowing across the top of a soda bottle. Standing Waves
Only certain frequencies of vibration produce standing waves for a given string length. The wavelength of each of the standing waves depends on the string length, L, and the number of nodes, n. λ n = 2L/n Standing Waves