Chapter 26Sound

Sound

• All sounds are produced by the vibrations of material objects.

• These vibrations produce ripples within the air molecules just like a rock thrown in a pond produces ripples in the water. In the case of sound, the medium through which the waves travel is air instead of water.

Can you identify the source of the sound in these objects?

Sound

• The frequence of a sound that we can hear is called its pitch.

• A low note (like a fog horn) would have a low frequency

• A high note (like a piccolo) would have a high frequency

• A young person can hear frequencies (pitches) within a range of about 20 hertz to 20,000 hertz

Ultrasonic and Infrasonic Sounds

• Sound waves with frequencies below 20 hertz are called infrasonic

• Sound waves with frequencies above 20,000 hertz are ultrasonic

• Some animals can hear ultrasonic and infrasonic sounds.

• Can you think of some examples?

Ultrasonic Sounds

• Bats are able navigate in complete darkness using echo location. These are sounds in the form of clicks and pulses that bats use which have frequencies above 20 kilohertz.

• Ultrasonic sounds can also be used in medical technology. A developing fetus is imaged inside the mother’s womb ultrasound.

Infrasound

• Elephants use infrasonic sounds inthe range of 20 hertz or less to communicate. Female african elephantsuse “contact calls” to indicate their location. The advantage to these low frequency sounds is their ability to travel over long distances.

Sound Transmission

• Sound travels at different speeds depending upon the media through which it travels and the temperature of the media.

• For example, the speed of sound in air at 0 celcius is 331 m/s.

• Speed in air varies with temperature:

v = 331 m/s + (0.6 m/s/C)•T

• Here is the speed of sound in a few common materials:

Material Speed

Air 330 m/s

Water 1400 m/s

Concrete 5000 m/s

Steel 6000 m/s

Resonance

• A tuning fork that is struck creates sound by creating compression and rarefaction of the air molecules around it as it vibrates. A tuning fork that is placed on a tabletop sounds louder. This is because the table is forced into vibration. The larger surface sets more air molecules in motion and becomes a sounding board. This is a case of forced vibration.

• “When any object composed of an elastic material is disturbed, it vibrates at its own special set of frequencies, which together form its own special sound. We speak of an object’s natural frequency, which depends on factors such as the elasticity and shape of the object.”

• When the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude occurs. This phenomenon is called resonance.

Nikola Tesla (1856 - 1943) - Master of ResonanceIt was an innocent experiment. Tesla had attached a small vibrator to an iron column in his New York City laboratory and started it vibrating. At certain frequencies specific pieces of equipment in the room would jiggle. Change the frequency and the jiggle would move to another part of the room. Unfortunately, he hadn't accounted for the fact that the column ran downward into the foundation beneath the building. His vibrations were being transmitted all over Manhattan.For Tesla, the first hint of trouble came when the walls and floor began to heave (ref 1). He stopped the experiment just as the police crashed through the door. It seems he'd started a small earthquake in his neighborhood smashing windows, swayed buildings, and sending panicked neighbors rushing into the streets. The police had frequently responded to complaints about Tesla's unusual activities.

http://www.intuitor.com/resonance/tesla.php

Wind blowing across the Tacoma Narrows bridge causes the bridge to sway at a frequency equal to its natural frequency. This caused an amplification of the bridge sway resulting eventually in a failure of the bridge.

Interference

Hewitt p. 397

Beats

• “An interesting and special case of interference occurs when two tones of slightly different frequency are sounded together. A fluctuation of loudness of the combined sounds is heard; the sound is loud, then faint, then loud, then faint, and so on. This periodic wariation in the loudness of sound is called beats.

Hewitt p. 398

Two overlapping wave forms of slightly different frequency: