Sound waves

Objectives Investigate and analyze characteristics of sound waves: frequency, wavelength, and amplitude. Examine and describe sound wave propagation in a variety of media.

Assessment Based on this graph: What is the frequency of the sound wave? Is this a transverse or longitudinal wave, and why? What can you say about the loudness of this sound? Is this a sound humans can hear? Why or why not?

Assessment Sound waves with a frequency of 172 Hz have a wavelength of 2.0 meters in air. When these waves enter water, their wavelengths change to 8.7 meters. What is the speed of sound in water? A. 19.7 m/s B. 40 m/s C. 1500 m/s D. 2990 m/s

Physics terms pitch speed of sound decibel (dB)

Equations The speed of a sound wave equals the product of its wavelength and frequency.

What is sound? Sound is a longitudinal wave, like this compression wave on a Slinky®. The difference: it is AIR that is being compressed.

What is sound? Sound is a tiny oscillation of air pressure. Imagine this cymbal vibrating up and down when struck.

What is sound? When the surface moves up, the air above it is slightly compressed (slightly higher pressure). When it moves down, the air is drawn out (slightly lower pressure). The result is an oscillation of air pressure – a sound wave!

Why is sound a wave? Sound waves are traveling oscillations of air pressure. Sound waves can interfere. Sound obeys a wavelength and frequency relationship like a wave. Sound doesn’t look like a water wave, or a wave in a stretched string. But it is still a wave.

Describing sound List some properties of sound. If two musical notes sounded different to you, what words could you use to describe that difference?

Describing sound List some properties of sound. Loudness Pitch – the perception of high or low Timbre – the property that makes a piano note sound different from the same note produced by a guitar or a vocalist.

Loudness and amplitude The loudness of a sound wave depends on its amplitude. Louder sounds waves have larger amplitude pressure variations. A stereo’s speakers move back and forth a greater distance when producing a loud sound than when producing a soft sound.

Pitch and frequency The pitch of a sound depends on the frequency of the sound wave: Low-pitched sounds have lower frequencies. High-pitched sounds have higher frequencies. Humans can hear sound frequencies from about 20 Hz to 20,000 Hz. Following the investigation, students will use the interactive sound wave generator pictured here to test their own hearing range. See slide 25.

Timbre and overtones Most sounds contain multiple frequencies at the same time. Musical instruments produce a fundamental frequency and many overtones (additional frequencies). Overtones give the sound its timbre, its “piano-ness” or “guitar-ness”. Different instruments produce overtones of varying amplitudes for the same note.

Investigation In Investigation 16A you will experiment with the wave characteristics of sound. Click to open the simulation on page 441.

Investigation Part 1: Matching the parameters of a sound wave Choose a note and adjust the speakers so you can hear it. Set time and amplitude values on the graph until you can see at least a few cycles of the wave. With time plotted on the horizontal axis, try to make the black wave match the red sound wave by adjusting the frequency and amplitude.

Investigation Part 1: Matching the parameters of a sound wave Switch to distance for the horizontal axis. Match your black wave to the red sound wave by adjusting the wavelength and amplitude. A good match has a score of greater than 95%. You must match both frequency and wavelength in order to score 100%.

Investigation Questions for Part 1 What is the frequency and wavelength of the note “C” on the left-hand side? Describe how the observed wave varies with loudness. Determine the frequency and wavelength for at least 4 different sounds.

Investigation Questions for Part 1 From your data, discuss possible relationships between frequency and wavelength with your lab partners. Propose and test an equation that expresses your hypothetical relationship.

Investigation Part 2: Going further with octaves Devise an experiment to determine what happens to the frequency and wavelength when you set the octave to different values. Record your data and conclusions on your assignment sheet.

Investigation Questions for Part 2 What do you conclude is caused by setting different octaves? How does this fit with your prior knowledge of music?

Visualizing sound waves Sound waves are harder to visualize than waves in a string. Here, a vibrating surface, such as a speaker, produces a pressure wave that travels to the right. How can this be shown on a graph? High pressure Low pressure High pressure

Visualizing sound waves Amplitude on the graph below represents pressure, NOT distance!

Visualizing sound waves It is the wave that travels, not the air molecules. The air moves in a tiny back-and-forth motion as the wave passes by. Point out that the peaks and troughs of these two representations are aligned (peaks below compressions, and troughs below rarefactions).

Characteristics of sound waves There are three key characteristics of sound waves: frequency speed amplitude These characteristics are each examined in more detail in the next series of slides.

Frequency Sound has a huge frequency range. Humans can hear sounds in this frequency range: 20 Hz < f < 20,000 Hz. By middle age, most people can only hear sounds less than about 12,000 Hz. Click on this Sound wave generator on page 440 to test your own hearing range.

Audible frequencies Some animals can hear higher and lower frequencies than humans:

Ultrasound technology Medical ultrasound technology uses very high frequency sound waves. Differences in tissue density reflect ultrasound waves back to a detector and allow sophisticated imaging without harm to the patient. Ultrasound is also used in welding, and for non-medical imaging purposes.

Speed Sound waves are fast. The speed of sound in air is 343 m/s (767 mph!) Many military jets are capable of supersonic flights. This (343 m/s) is the speed of sound at room temperature (21° C), at a pressure of one atmosphere. Chuck Yeager was the first person to break the sound barrier. He flew a X-1 rocket plane to a maximum speed of 361 m/s (807 mph).

Speed in various materials Sound travels even faster in water, or ice, or steel. The stiffer the medium, the faster the sound speed tends to be. When sound passes from one medium to another . . . speed and wavelength change frequency stays the same

Speed in various materials A 1000 Hz sound in … AIR has a speed of 343 m/s and a wavelength of 34 cm. WATER has a speed of 1480 m/s and a wavelength of 1.5 m. ICE has a speed of 3500 m/s and a wavelength of 3.5 m.

No sound in a vacuum Sound can’t travel in a vacuum. The loud explosions from space battles in science fiction movies are not realistic. If you were actually watching a space battle from a distant space ship, you would hear total silence.

Amplitude Sound waves have small amplitudes. Typically the variation in pressure is about 0.0001 atmospheres, far below our ability to detect through our sense of touch. BUT our ears are extremely sensitive and can easily detect these tiny pressure oscillations.

Amplitude and loudness The amplitude of a sound wave determines its loudness. Larger amplitude means louder sound. BUT, to a human ear, frequency also matters. A high amplitude sound at a frequency of 40,000 Hz is silent to a human ear but quite loud to a bat!

Loudness and frequency The Equal Loudness Curve shows how sounds of different frequencies compare in perceived loudness to an average human ear. Examine the graph. Which frequencies do we hear the best? Human speech frequencies tend to range between 100 Hz to 2,000 Hz, where our ears are most sensitive. The frequency of a baby’s cry ranges around 3000 Hz (above the range of typical speech) where our ear is very sensitive.

The decibel scale Our ears can detect an enormous range of pressures. For this reason, the logarithmic decibel (dB) scale is used to measure loudness. On the decibel scale, an increase of 20 dB means the wave has 10 times greater amplitude (and 100 times greater power). Power and energy are related to the square of the amplitude of a wave. Point out that most of the sounds and environments we encounter are in the 30 to 100 dB range.

Assessment Based on this graph: What is the frequency of the sound wave? Is this a transverse or longitudinal wave, and why? What can you say about the loudness of this sound?

Assessment Based on this graph: What is the frequency of the sound wave? 400 Hz Is this a transverse or longitudinal wave, and why? longitudinal (sound) What can you say about the loudness of this sound? It is constant. Is this a sound humans can hear? Why or why not? Point out that

Assessment Based on this graph: Is this a sound humans can hear? Why or why not? Maybe. At 400 Hz, it is within our frequency range—but we don’t know if it is loud enough to hear. The pressure axis has no numbers or units. Point out that

Assessment Sound waves with a frequency of 172 Hz have a wavelength of 2.0 meters in air. When these waves enter water, their wavelengths change to 8.7 meters. What is the speed of sound in water? A. 19.7 m/s B. 40 m/s C. 1500 m/s D. 2990 m/s

Assessment Sound waves with a frequency of 172 Hz have a wavelength of 2.0 meters in air. When these waves enter water, their wavelengths change to 8.7 meters. What is the speed of sound in water? A. 19.7 m/s B. 40 m/s C. 1500 m/s D. 2990 m/s The frequency stays the same.