PROPERTIES OF SOUND WAVES

Section 8.5

Key Terms

Audible Sound Waves Infrasonic Wave Ultrasonic Wave Echo Mach Number (M) Pressure (p) Sound Intensity Decibel (dB)

Categories of Sound Waves

Sound waves fall into three categories covering different ranges of frequencies. Audible sound waves

In the range of human hearing (20 Hz – 20 kHz) Infrasonic waves

Frequencies below the audible range (< 20 Hz) Ultrasonic waves

Frequencies above the audible range (> 20 kHz)

Applications of Ultrasonic Waves Widely used in medical applications

Diagnostic tool AND treatment Ultrasound Imaging

Transducer placed on mother’s abdomen Emits ultrasonic waves Reflected waves are picked up by transducer and

converted into an electric signal that forms an image

Ultrasound Treatment Ultrasonic waves used to break up kidney

stones or promote healing in biological tissues.

Speed of Sound

Depends on the density of the air and its temperature. Value increases by 0.606 m/s for every

increase of 1oC.

T = temperature in oC

Sample Problem 1

The temperature outside is 23oC. What is the speed of sound in air at this temperature?G: T = 23oCR: v = ?A: v = 331.4 m/s + (0.606 m/s/oC) TS: v = 331.4 m/s + (0.606 m/s/oC)(23oC) = 345 m/sS: The speed of sound in air at 23oC is 345 m/s

Sample Problem 2

If the speed of sound is measured to be 318 m/s, what is the current air temperature?G: v = 318 m/sR: T = ?A: v = 331.4 m/s + (0.606 m/s/oC) TS: T = -22.1 oCS: The temperature of the air is -22.1 oC

Practice Questions

Page 393 1. 351 m/s 2. 2.64 oC 3. 31 oC

Mach Number

Ernst Mach researched sound waves and devised a way to describe air speeds of objects in terms of the speed of sound. Ratio of airspeed to the local speed of

sound

No units for M Mach number is not fixed – depends on

speed of sound in its vicinity

Sample Problem

Practice Problems

Page 394 1. 0.73 2. 3.0 x 102 m/s = 1100 km/h 3. 290 m/s = 1.0 x 103 km/h

Sound Intensity

Loudness describes how humans perceive sound energy. Depends on a quantity called sound intensity.

A sound wave is a longitudinal wave Amplitude of a longitudinal wave is a

difference in pressure

The larger the amplitude, the louder the sound that is perceived. The amount of sound energy being transferred

per unit area is called sound intensity Measured in W/m2

Human Perceptions of Sound Intensity

The threshold of human hearing ranges from about 1x10-12 W/m2 to about 1 W/m2

Easier to use decibels The unit of sound level used to describe sound

intensity level 1/10 of a bel (B)

The decibel commonly gives measurements on a scale of 0 to 100, sometimes exceeding 200 Decibels refer to sound level, not intensity,

so an order of magnitude is an increase of 10 decibels.

Typical Sound Levels

Loudness and Distance

The farther you are from a sound, the quieter it becomes. As the sound wave expands from

the source, the total energy stays the same, but the area of air it acts on is greatly increased.

Loudness drops off quickly, but audible levels persist for quite a distance

Sound Safety

Any sound levels greater than 100 dB that persist for more than a few minutes will damage hearing. The louder a sound, the

less time that can be spent near it without damaging hearing.

Summary

Audible sound waves range from 20 Hz to 20 kHz. Infrasonic waves have frequencies below 20 Hz. Ultrasonic waves have frequencies above 20 kHz.

We can apply our understanding of the properties of sound to technologies that benefit society.

The speed of sound through the atmosphere, in metres per second, is given by the relationship v = 331.4 m/s + (0.606 m/s/°C) T, where T is the temperature in degrees Celsius.

Sound intensity is a measure of the energy flowing through the unit area due to a sound wave.

Human hearing can detect a range of sound intensities over many magnitudes in intensity.

Loudness levels are usually described on the decibel scale, which is more convenient than the range of values for sound intensity. Loudness levels are dependent on the distance from the source of the sound.

Sound levels in industry and recreation must be kept to a reasonable level to avoid hearing damage.

Homework

Page 397 Questions 1-7, 9, 10