Sound Ch. 26 in your text book

Transcription

1 Sound Ch. 26 in your text book Objectives Students will be able to: 1) Explain the relationship between frequency and pitch 2) Explain what the natural frequency of an object is 3) Explain how wind and string instruments work 4) Explain what the Doppler effect is

2 Pitch Sound travels through some medium (for us, it s usually air) Bell in a jar demonstration When there is no air inside the jar, the sound can t travel.

3 Pitch L n = 1 n = 2 n = Reminder the frequency of a wave is how many of them there are in a given time What do you notice about the frequency of the wave as the harmonic number increases? We learned that as the wavelength decreases, the frequency increases n = Speaker demonstration

4 Pitch Higher frequencies in sound are heard as higher pitches The frequencies for like notes of different octaves are double (or half) that of the other octave. They are harmonics of each other.

5 Pitch Tuning forks have a primary frequency at which they vibrate, but listen carefully and you ll hear other pitches also. The other pitches are harmonics of the fundamental frequency. Different sizes are for different frequencies

6 Pitch How is the tuning fork moving? The frequency at which the fork moves back and forth is its natural frequency. Natural frequency frequency at which any object will vibrate (oscillate)

7 Resonance and Natural Frequency Everything has a natural frequency. When something vibrates, it is vibrating at its natural frequency. If a wave interacts with an object at that object s natural frequency, the object will start to vibrate. The extent to which it vibrates depends on the amount of energy in the wave that is hitting it.

8 Resonance and Natural Frequency Crystal glass Tacoma Narrows bridge Waterphone (creepy music instrument)

9 Instruments All instruments work under this same principle. For string instruments, the vibration of the string makes the pitch, the sound, that we hear. The string alone doesn t give a very loud sound. Soundboards amplify the sound.

10 Sound Intensity What is the loudest thing you ve ever heard? How do we measure how loud something is? Intensity of a sound is measured in decibels (db). Decibels is unit that is compared to some initial number. Each level is some amount more or less than that initial number. Each increase of 10 db is 10 times more intense. An increase in intensity of 20 db is 100 times more intense. Check for understanding How much more intense is a 80 db sound than a 40 db sound?

11 Many of you may argue about how dangerous some of these sounds are. These intensity levels are scientifically shown to be bad for your hearing. Sound Intensity How could you decrease the intensity felt by your ears?

12 Sound Intensity Ear plugs get in between the sound and our ear, similar to how the jar was in between the bell and our ear. We reduce the intensity using some medium through which sound doesn t travel well.

13 Doppler Effect Christian Doppler If an object emits sound, the sound waves travel outward, similar to the waves made when a rock hits water. What happens if the sound source is moving?

14 Doppler Effect If the object emitting sound is moving, it is still emitting the same sound waves but now the origin of the waves changes. Each black circle represents the crest of a sound wave. What do you notice happens to the crestss in front of the source (the red dot)?

15 Doppler Effect If the object emitting sound is moving, it is still emitting the same sound waves but now the origin of the waves changes. What do you notice happens to the crests behind the source (the red dot)?

16 Doppler Effect The moving sound waves hit an outside observer at a higher frequency, which makes a higher pitch. On the other side of the object, the sound waves hit the observer at a lower frequency so they hear a lower pitch. This is known as the Doppler Effect. Where do you see examples of this phenomena?