3&4: Properties of Sound Waves and Wave Interference

Audible Sound Waves

Sound wave in range of human hearing, 20 Hz to 20,000 Hz.

Infrasonic Waves

Sound wave with a frequency below 20 Hz.

Ultrasonic Waves

Sound wave with a frequency above 20,000 Hz (20 kHz); used for ultrasounds in medical applications.

Speed of Sound

The speed of sound through air depends on the density of the air and its temperature.

Speed Increase Rate

The speed increases by 0.606 m/s for every increase of 1 °C.

Mach Number

The ratio of the airspeed of an object to the local speed of sound.

Mach 1

Means the object is travelling at the speed of sound.

Mach 2

Is twice the speed of sound.

Sound Barrier

When an object moves at the speed of sound, the sound waves emitted build up producing very dense air, or intense compressions, called the sound barrier.

Sonic Boom

Produced when an aircraft breaks the sound barrier.

Speed of Sound in Glass (Pyrex)

5170 m/s.

Speed of Sound in Steel

5000 m/s.

Speed of Sound in Water

1496 m/s.

Speed of Sound in Wood (maple)

4110 m/s.

Sound Intensity

The amount of sound energy being transferred per unit area; unit W/m².

Loudness

Describes how humans perceive sound energy; depends on sound intensity.

Threshold of Hearing

Ranges from 1 x 10^-12 W/m² to about 1 W/m².

Decibel (dB)

Unit used to deal with the large range of sound intensity.

Sound Intensity Increase

Each increase of 10 dB is a 10 times increase in sound intensity.

Sound Intensity at Threshold of Human Hearing

1 x 10^-12 W/m² corresponds to 0 dB.

Sound Intensity of Library

1 x 10^-7 W/m² corresponds to 50 dB.

Sound Intensity of Alarm Clock

1 x 10^-6 W/m² corresponds to 60 dB.

Sound Intensity of Rock Band

0.1 W/m² corresponds to 110 dB.

Sound Intensity of Power Saw

1.0 W/m² corresponds to 120 dB.

Sound Intensity at Threshold of Pain

10 W/m² corresponds to 130 dB.

Distance and Sound Level

As a sound wave expands from its source, the energy per unit area decreases and it sounds quieter.

Sound Safety Guidelines

Sound levels greater than 100 dB that persist for more than a few minutes will harm your hearing.

Interference

the process of generating a new wave when two or more waves meet

Constructive Interference

the process of forming a wave with a larger amplitude when two or more waves combine

Destructive Interference

the process of forming a wave with a smaller amplitude when two or more waves combine

Principle of Superposition

at any point the amplitude of two interfering waves is the sum of the amplitudes of the individual waves

Free-End Reflection

a reflection that occurs at a media boundary where the second medium is less dense than the first medium; reflections have an amplitude with the same orientation as the original wave

Fixed-End Reflection

a reflection that occurs at a media boundary where one end of the medium is unable to vibrate; reflections are inverted

Wave Transmission

When waves travel from one medium into another, some of the energy will transmit through, while some will be reflected

Standing Wave

an interference pattern produced when incoming and reflected waves interfere with each other; the effect is a wave that appears to be stationary

Nodes

in a standing wave, the location where the particles of the medium are at rest - constant total destructive interference

Antinodes

in a standing wave, the location where the particles of the medium are moving with the greatest speed; the amplitude will be twice the amplitude of the original wave

Fundamental frequency

the lowest frequency that can produce a standing wave in a given medium

Harmonics

whole number multiples of the fundamental frequency

Overtone

a sound resulting from a string that vibrates with more than one frequency

Standing Waves in Fixed Ends

To produce standing waves in media with two fixed ends or two free ends, the length of the medium, L, is equal to the number of the harmonic, n, times half the standing wave's wavelength, λ/2.

Standing Waves in Fixed-Free Ends

To produce standing waves in media with one fixed end and one free end, the shortest possible length is λ/4. The next possible length is λ/2 more than this.

Resonance Tube Activity

Using the resonance tube, calculate the temperature of the air in the room.

Frequency of tuning fork

Data point for resonance tube activity.

Length of air column

Data point for resonance tube activity.

Wavelength

Data point for resonance tube activity.

Temperature

Data point for resonance tube activity.