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 15X 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, dB (named after Alexander Graham Bell.

How is loudness measured? On the decibel scale an increase of 10 dB means that sound intensity increases by a factor of 10. Going from 20 to 40 dB means that the sound is 100X 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 principle 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. \