Phy 103: Fundamentals of Physics III Chapter 20: Sound Lecture Notes

Origin of Sound Sound is a longitudinal wave Sound is produced by the vibration of material objects The frequency range of sound we can hear is about 20 to 20,000 Hz –Sound waves below 20 Hz are called infrasonic –Sound waves above 20,000 Hz are called ultrasonic –We cannot hear infrasonic or ultrasonic sound waves Sound is transmitted as a series of high pressure/low pressure vibrations –The high pressure region is called a compression –The low pressure region is called a rarefaction –The condensations and rarefactions travel (through the air) along the direction of the sound wave

Media that Transmit Sound Any elastic material can transmit sound including: –Solids (fastest) In iron/steel: the speed of sound is ~5000 m/s –Liquids In water: the speed of sound is ~1400 m/s –Gases (slowest) In air: the speed of sound is ~ 340 m/s (at 20 o C) The stiffer (more elastic) the substance the better it will transmit sound

Speed of Sound in Air The speed of sound in air depends on temperature –The colder the air the slower the sound wave –The warmer the air the faster the sound wave At room temperature, the speed of sound is about 340 m/s To calculate the speed of sound (in air) at a particular temperature (T in o C): v sound  ( T) m/s Example: At 20 o C, the speed of sound is v sound = [331 + (0.60)(20)] m/s = 343 m/s What is the speed of sound at 30 o C?

Reflection of Sound A reflected sound wave is called an echo Sound reflects from the surface of objects the same way light does –The smoother & flatter the surface, the greater the reflection –When sound reflects off a smooth surface, the angle of incidence is equal to the angle of reflection This called the law of reflection When presenting sound (for instance, during a concert), special care is made to control the reflective movement of sound waves Acoustics is the study of sound properties

Refraction of Sound When sound travels through air of uneven temperature (or in uneven winds) parts of the wavefront will travel at different speeds The sound waves will tend to bend –Sound waves bend away from warm air –Sound waves bend toward cold air The bending of sound waves is called refraction

Interference & Beats Sound waves exhibit interference like other waves When 2 sound waves (with different frequencies) combine, the resultant disturbance (wave pattern) exhibits both constructive and destructive interference The combined effect of interference produces periodic rises and drops in loudness called beats The frequency of the beats(f beat ) is equal to the difference between the 2 sound frequencies: f beat = f 1 - f 2 where f 1 >f 2 Musicians often tune their musical instruments by listening to beat frequency

Energy in Sound Waves All waves carry (transmit energy) Hearing is possible because energy from sound waves is transferred to the ear (in the form of vibrations) Sound waves carry a very small amount of energy: Example: Consider a 100 W stereo, 100 J of energy are transmitted per second from the speakers (50 J per speaker). At a distance of 2 meters from the speakers, this is only 2 J of energy per 1 m 2 area (your eardrum has an effective area of roughly 3x10 -5 m 2 ) Energy at Ear = (2 J/m 2 ) (3x10 -5 m 2 ) = J So your ear on receives J (or 6x10 -5 J) during 1 second Your ear is a very sensitive instrument!