Chapter 24 –Sound 24.2 –Sound Waves pp. 584-589

24.2 What is a sound wave? Sound waves are pressure waves with alternating high and low pressure regions. When they are pushed by the vibrations, it creates a layer of higher pressure which results in a traveling vibration of pressure.

24.2 What is a sound wave? At the same temperature and volume, higher pressure contains more molecules than lower pressure.

24.2 The wavelength of sound The wavelength of sound in air is similar to the size of everyday objects.

24.2 The wavelength of sound Wavelength is also important to sound. Musical instruments use the wavelength of a sound to create different frequencies.

24.2 Standing waves A wave that is confined in a space is called a standing wave. A string with a standing wave is a kind of oscillator.

24.2 Standing waves The lowest natural frequency is called the fundamental. A vibrating string also has other natural frequencies called harmonics.

24.2 Standing waves The place on a harmonic with the greatest amplitude is the antinode. The place where the string does not move (least amplitude) is called a node.

24.2 Standing waves It is easy to measure the wavelength of a standing wave on a string. Two harmonics equals one wave!

24.2 Standing waves in pipes A panpipe makes music as sound resonates in tubes of different lengths. The natural frequency of a pipe is proportional to its length.

24.2 Standing waves in pipes Because frequency and wavelength are inversely related, longer pipes have lower natural frequencies because they resonate at longer wavelengths. A pipe that must vibrate at a frequency 2 times higher than another pipe must be 1/2 as long. If the long pipe has a frequency of 528 Hz, what is the frequency of the short pipe?

24.2 Standing waves in pipes Blowing across the open end of a tube creates a standing wave inside the tube. If we blow at just the right angle and we match the natural frequency of the material and the sound resonates (spreads).

24.2 Standing waves in pipes The open end of a pipe is an open boundary to a standing wave and makes an antinode. The pipe resonates to a certain frequency when its length is one-fourth the wavelength of that frequency.

24.2 Sound wave interactions Like other waves, sound waves can be reflected by hard surfaces and refracted as they pass from one material to another. Diffraction causes sound waves to spread out through small openings. Carpet and soft materials can absorb sound waves.

REFLECTION The amount a sound waves reflects depends on the reflecting surface. Sound waves reflect best off smooth, hard surfaces.

ECHOLOCATION Bats, dolphins, whales and some birds use this technique to hunt for food and/or for navigation.

TYPES OF ECHOLOCATION Sonar Stands for sound navigation and ranging Ultrasonic waves are sent down into water. The time it takes for the echo to return helps fishermen determine the location of fish.

TYPES OF ECHOLOCATION Ultrasonography Uses echoes to “see” inside a patient’s body without using surgery

INTERFERENCE OF SOUND WAVES Happens when two or more waves interact Can be constructive or destructive Constructive – when waves combine, add the amplitudes together Destructive – when sound waves combine, subtract the smaller amplitude from the larger amplitude. “Dead spots” in an auditorium are a result of destructive interference

24.2 Reverberation The reflected sound and direct sound from the musicians together create a multiple echo called reverberation. The right amount of reverberation makes the sound seem livelier and richer.