SOUND Ch 26 How is sound produced Sound
- Slides: 15
SOUND Ch. 26
How is sound produced? • Sound is produced by the vibration of material objects. (Ex. Guitar strings, saxophone reeds, vocal cords, etc. )
What sounds can humans hear? • Pitch refers to the vibrational frquency. • The human ear can hear frequencies between 20 and 20, 000 Hz. • Bass increases the low frequency sounds and treble are high frequency sounds.
What is infrasonic and ultrasonic? • Infrasonic waves are less than 20 Hz. • Ultrasonic are greater than 20, 000 Hz. • Sound moves in waves of high pressure and low pressure regions called compressions and rarefactions.
How does sound move? • When sound travels through air, it is the pulse or disturbance that moves, not the air itself. • The speed of sound depends on the material through which it travels. • Sound does not travel through a vacuum
• The speed of sound in dry air at 0 degrees Celcius is about 330 m/s. • Water vapor and temperature increase this speed slightly. • The speed of sound through a material depends on elasticity. • Steel is very elastic therefore sound travels about 15 X faster than in air.
Who is the decibel named for? • Loudness of sound is subjective but the intensity can be measured by an oscilloscope. • Sound intensity can be described using a unit called the decibel, d. B (named after Alexander Graham Bell.
How is loudness measured? • On the decibel scale an increase of 10 d. B means that sound intensity increases by a factor of 10. • Going from 20 to 40 d. B means that the sound is 100 X as loud.
How can sound be amplified? • When a vibrating object is placed in contact with another object causing it to vibrate this is called a forced vibration. • A piano as well as most musical instruments use this principal to amplify sound.
• When an elastic object is disturbed it will vibrate at its own natural frequency. • This is determined by factors such as size, shape and the nature of the material. (ex. Bell and tuning forks. • A minimum amount of energy is required to vibrate at this frequency.
What is resonance? • When the frequency of forced vibration matches the natural frequency of an object, a phenomenum called resonance occurs. • Pumping a swing is an example of this.
• For sound waves, the crest corresponds to a compression and the trough to a rarefaction. • Interference affects the loudness of sound. • With two speakers the sound seems louder because the waves constructively interfere
• When destructive interference occurs this creates “dead spots”. • Some loud devices such as jackhammers are equipped with microphones that create the mirror image of the soundwave. • This is sent to earphones thus cancelling the sound for the hearer.
What are beats? • Beats occur when tones of slightly different frequencies are sounded together. • When this occurs, the hearer experiences a pattern of louder then softer sounds. • This is due to periods of constructive and destructive interference.
How are instruments tuned? • Musicians use beats when they tune their instruments. • When the frequencies of the instruments are identical, the beats disappear. •
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