Sound waves Pg. 65 Sound waves Pg. 65

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

Sound is a longitudinal wave, AKA a compression wave where air is compressed. What is sound? Sound is a tiny oscillation of air pressure like what happens when this cymbal is struck.

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! What is sound?

Sound waves are harder to visualize than waves in a string. Below, a vibrating surface, such as a speaker, produces a pressure wave that travels to the right. 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.

In Investigation 16A we will experiment with the wave characteristics of sound. We will do this as a class & you will record your answers in your ppt. Investigation

1.As we change notes going from the left to the right side of the keyboard: What happens to the pitch of the sound you hear for each note? What happens to the frequency of the wave? What happens to the wavelength of the wave? 2.As the volume of the sound is changed, what happens to the wave? Investigation Q & A

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

There are three key characteristics of sound waves that we’ll be discussing: amplitude frequency speed Characteristics of sound waves

Sound waves have small amplitudes. Typically the variation in pressure is about 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

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. Loudness and amplitude 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!

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. The pitch of a sound depends on the frequency of the sound wave: o Low-pitched sounds have lower frequencies. o High-pitched sounds have higher frequencies. Frequency

Audible frequencies Some animals can hear higher and lower frequencies than humans: Differences in tissue density reflect ultrasound waves back to a detector and allow sophisticated imaging without harm to the patient.

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”. Timbre and overtones

Sound waves are fast. The speed of sound in air is 343 m/s (767 mph!) Many military jets are capable of supersonic flights. Speed

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 In air  340 m/s λ : 34cm & f = 1000Hz In water  1480 m/s λ: 1.5m & f = 1000Hz In ice  3500 m/s λ: 3.5m & f=1000Hz

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. Reminder  No sound in a vacuum

The siren on a fire engine operates at a frequency of about 2000 Hz. How long are these wavelengths? Finding the wavelength Givens: Unknown: Equation: Substitute: Solution: wavelength ( λ) v = 343 m/s, f = 2000 Hz

If the siren on a police car has a wavelength of 10 centimeters, what is the frequency of the sound wave? Finding the frequency Givens: Unknown: Equation: Substitute: Solution: v = 343 m/s, wavelength λ = 0.10 m frequency ( f)

1.Based on this graph: Homework a)What is the frequency of the sound wave? b)Is this a transverse or longitudinal wave, and why? c)What can you say about the loudness of this sound? d)Is this a sound humans can hear? Why or why not?

2.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 m/s B. 40 m/s C m/s D m/s Homework