INTRODUCTION TO SOUND Longitudinal Waves
S OUND W AVES A sound wave is a travelling disturbance of compressions — regions in which air pressure rises followed by rarefactions regions where air pressure drops compared to quiet or still air.
S OUND W AVES Sound is an example of a longitudinal wave the wave moves in a direction parallel to the direction in which the medium moves. The wavelength of a sound wave is the distance between successive compressions or rarefactions. The amplitude of the sound wave is measured by how much the medium moves from its equilibrium state..
S OUND S PECTRUM Sounds come to us in an entire spectrum of frequencies. Conversation with your friends is in the range of 80 Hz to about 300 Hz. A opera singer can reach frequencies as high as 1100 Hz Musical instruments extend this range from as low as 20 Hz to as high as 5000 Hz Bird songs can reach higher than Hz. I
S OUND S PECTRUM In general, your ears are able to respond to sounds in the range of 20 Hz to Hz, But they are most sensitive to sounds in the range of about 2000 Hz to 5000 Hz.
S OUND S PECTRUM 1. Infrasonic sounds have frequencies of less than 20 Hz. Rather than being able to hear sounds in this range, you may “feel” them as a rumble that passes through your body. 2. Audible sounds are in the range of 20 Hz to 20 kHz 3. Ultrasonic sounds have frequencies greater than 20 kHz
S PEED OF S OUND The speed of sound through a medium depends on the physical characteristics of the medium. If you compare sound waves in a solid medium (such as steel) with those in a gaseous medium (air), you will find that they have very different speeds. The speed of sound in steel is almost 5800 m/s, whereas in air it is only about 350 m/s.
S PEED OF S OUND There are 3 major factors that effect the speed of sound through a medium: 1. The distance between particles in the medium 2. The stiffness of the particles Stiffness is a measure of how much force is required to create a compression in a given substance 3. Temperature
S PEED OF S OUND
If an object is 6.56 m from the camera and sound travels at 344 m/s, determine the length of time it takes the emitted sound pulse to return to the camera.
S PEED OF S OUND
What is the wavelength of a sound of frequency 225 Hz that is produced in air at a temperature of 20.0°C?
I NTENSITY Our ears are marvellous organs. They can respond to the faintest of whispers or the roar of a jet engine. The ability to distinguish variations in loudness is an important environmental cue that humans and other animals use to navigate in their surroundings.
I NTENSITY The intensity of a sound is the energy per unit area that passes a point each second. It has units of (J/m 2 )/s or J/s·m 2, which is the same as W/m 2. Our ears can respond to sounds as faint as one-trillionth of a watt per square metre
D ECIBEL S CALE
The Greek letter beta, , is commonly used to represent sound intensity expressed in dB. The faintest sound that humans can hear represents the start of the decibel scale and is given a value of 0 dB. The decibel scale is a logarithmic scale that corresponds to how our ears perceive loudness.
D ECIBEL S CALE
In general, the smallest difference in loudness that can be detected by the human ear is 1 dB. When using the decibel scale, every 3-dB increase in SIL is a doubling in intensity. A 10-dB increase increases the intensity by 10 times.
D ECIBEL S CALE
1. By what factor would the sound intensity increase if the sound intensity level in an office increased from 68 dB to 77 dB? 2. A sound changes from an intensity of 5 x10 -6 W/m 2 to 5 x10 -7 W/m2. Has the sound intensity level increased or decreased? 3. By how much has the sound intensity level changed in question 2? Express your answer in decibels.
I NTENSITY L OUDNESS The terms “ loudness ” and “ intensity ” do not have the same meaning. Loudness is a measure of the ear’s response to sound. Two sounds can have equal intensity, but you may hear one sound as louder than another because your ears can detect it better.