SOUND 24.2

Chapter Twenty-Four: Sound  24.1 Properties of Sound  24.2 Sound Waves  24.3 Sound Perception and Music

Chapter 24.2 Learning Goals  Justify the classification of sound as a wave.  Analyze sound interactions at boundaries.  Explain how factors like temperature and pressure affect the behavior of sound waves.

Investigation 24B  Key Question:  How can resonance be controled to make the sounds we want? Resonance in Other Systems

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.

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.