4. SOUND WAVES

Sound Waves

Sound can travel through any kind of matter, but not through a vacuum.

Using a Tuning Fork to Produce a Sound Wave

The speed of sound is different in different materials; in general, it is slowest in gases, faster in liquids, and fastest in solids. As the molecular mass increases, the speed of sound decreases. At room temperature, the speed of sound in helium is greater than that in air.

The speed depends on temperature, especially for gases. As the temperature of air increases, the speed of sound also increases.

Characteristics of Sound

Loudness: related to intensity of the sound wave

Pitch: related to frequency.

Audible range: about 20 Hz to 20,000 Hz; upper limit decreases with age

Ultrasound: above 20,000 Hz; see ultrasonic camera focusing below

Infrasound: below 20 Hz

Forced Oscillations; Resonance

Forced vibrations occur when there is a periodic driving force. This force may or may not have the same period as the natural frequency of the system.

If the frequency is the same as the natural frequency, the amplitude becomes quite large. This is called resonance.

Other Examples of Resonance

• Child being pushed on a swing

• Shattering glasses

• Tacoma Narrows Bridge collapse due to oscillations caused by the wind

What is the Echo?

The echo is a distinct, reflected sound wave from a surface.

• A reflected sound can be heard separately from the original sound if the sound source is closer to the receiver while the reflecting hard surface is sufficiently far from the receiver. Such reflected sound is called an echo.
• Generally, the reflected sound is not distinctly heard, as it follows so closely behind the original sound and prolongs the sensation of the original sound. This effect is called reverberation.
• If the surface is rough, the incident sound waves are broken up and the original waveform is lost, thus no reflected sounds are heard. To reduce the effects of echo, walls can be roughened or “softened” (with padding) or covered with curtains and floors covered with carpets.
• The principle of echo is used in echo sounder to find the depth of a sea or the location of shoals of fish. Echoes can be used to measure the speed of sound.

Note: Remember that the distance traveled by the sound is doubled for an echo. (The sound “go there and come back”) For instance, if a sound wave takes 10 seconds to travel to the bottom of the sea and back, the total distance traveled is 2d, where d is the depth of the sea.

Hence, the velocity of the sound for echoes can be calculated by:

$$\begin{array}{cc}v& =\frac{\text{Total distance travelled by sound /}}{\text{Time taken}}\\ & =\frac{2\mathrm{d\; /}}{t}\end{array}$$

Applications: Sonar, Ultrasound, and Medical Imaging

• Sonar is used to locate objects underwater by measuring the time it takes a sound pulse to reflect back to the receiver.

https://oceanservice.noaa.gov/facts/sonar.html

• Similar techniques can be used to learn about the internal structure of the Earth.
• Sonar usually uses ultrasound waves, as the shorter wavelengths are less likely to be diffracted by obstacles.
• Ultrasound is also used for medical imaging.

• Modern high-resolution ultrasound can produce very detailed images.