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Sound

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**Production of Sound Waves**

Sound waves are Longitudinal Longitudinal means that the particles in the medium vibrate how compared to the wave motion? Parallel Perpendicular Circular

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**Compression/Rarefaction**

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**Characteristics of Sound Waves**

Audible Range: Range of sound the average human can hear. Low Frequency: 20 Hz High Frequency: 20,000 Hz

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**What is the range of sound called that falls below the frequency of 20 Hz?**

non-sonic infrasonic unsonic Antisonic

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**Infrasonic is the term used for a sound wave that falls below the 20Hz frequency.**

Humpback whales communicate on infrasonic frequencies. The term that describes sound waves above the 20,000 Hz frequency is called ultrasonic. Dogs can hear ultrasonic frequencies. This is why a good dog whistle cannot be heard by humans.

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If the speed of sound is 343 m/s, what is the wavelength of an ultrasonic wave with a frequency of 35,000 Hz? .0098 0.01

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**Frequency determines the pitch of the sound**

Frequency determines the pitch of the sound. Whether the sound is a high or low pitch. Ultrasonic waves produce ultrasonic images.

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**Speed of Sound Depends on Medium**

Sound travels fastest through medium which are more dense. Why? Sound is the vibration of particles in a substance. The closer the particles, the faster the transfer of energy. Iron: 5130 m/s Water: 1490 m/s Air: 331 0° C

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**Why would the temperature of air effect the speed of sound in air?**

because hot air rises. because the temperature adds energy to the wave. because the particles in the air move faster.

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In Fact….. The speed of sound in air increases by .6 m/s every °C the air temperature goes up. So at room temperature, 20 °C, the speed of sound in that room is m/s.

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**The temperature on a hot sunny day is around 30° C, what is the speed of sound in m/s?**

349 0.1

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**Sound waves propagate in 3 dimensions.**

Spherical wave fronts that are large distances away from the source appear to be plane waves.

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The Doppler Effect Relative motion creates an Apparent change in frequency.

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The Doppler Equation Vdetector: Positive if listener is moving toward source Vdetector: Negative if listener is moving away from source Vsource: Negative if source is moving toward listener Vsource: Positive if source is moving away from listener

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**A siren produces a frequency of 1500Hz**

A siren produces a frequency of 1500Hz. A runner who is passing the siren at 25 m/s hears the siren. What frequency is heard when the runner is going toward the siren? (Use 343 m/s for Vsound) 1390 Hz 1398 Hz 1617 Hz 1609 Hz

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**Loudness (Sound Intensity)**

The intensity or loudness of a sound is perceived by a person’s sense of hearing. Each person perceives sound differently. The intensity scale numerically determines the “strength” of the sound.

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**Intensity Scale Units: Watts/m2 2000 Watt speaker 1 meter away**

= W/m2

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**A speaker has a intensity of. 50 W/m2 at a distance of 6 meters away**

A speaker has a intensity of .50 W/m2 at a distance of 6 meters away. What is the power output of the speaker? 226.2 0.5

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**Examples from the Intensity Scale**

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**Decibel vs. Intensity Decibel scale is a Relative Loudness scale.**

It is based off of Created by A. G. Bell

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Decibels For every power of 10 increase on the sound intensity scale, the decibels increase by 10. Scale starts at 0dB = W/m2

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**Examples Decibel Scale**

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Decibels As stated earlier, decibels measure the relative loudness of sound. For every 10 dB, the sound is 2 times as loud. Therefore the difference between 0 dB and 120 dB is 4096 x louder.

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Decibel If we have an object that is 20 dB and want to make a 50 dB sound, how many more objects do we need? Rule: for every 10 dB added, we increase the # of objects by a power of 10.

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Decibel So for our last example we went from 20 dB to 50 dB which is a difference of 30 dB. 30 dB is 3 sets of 10 Therefore for every set of 10 we increase by a power of 10 1 = 20 dB, then 1000 = 50dB

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**Decibel What is we want to double the number of objects?**

Rule: if the number of objects are doubled, then the dB increase by 3. Example: if there are 200 goats that make 70 dB, then 400 goats make 73 dB.

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**IF one horn produces 50 dB how much louder is it if it produces 80 dB?**

2 4 6 8

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**If there are 30 ants making a noise of 20 dB, how many ants would make a 60dB noise?**

300000 30000 3000 300

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**40 bells make 70 dB. How many Bells are needed to make 96 dB?**

16000 0.0

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