Sound

Speed of sound in solids, liquids,and gases

• Speed of sound in gas (air): 344 m/sec.

• Speed of sound in liquid (water): 1100 m/sec

• Speed of sound in solid (steel): 5000 m/sec

• Speed of sound wave dependenton strength of bonds between atoms.

• Why is helium so high and what is an example of this high speed

Temperature dependence of sound speed

• Speed dependent on temperature

• For liquids and gases speed increases for

increasing temperature.

• One experiment to measure global warming is to measure speed of sound in oceans.

• Warmer oceans will result in slightly faster speed.

Speed of sound and sonic boom

• Plane moves through sound waves and creates a shock wave or “sonicboom”.

Intensity of sound

• An explosion or other sound producer such as a rock produces power. Units are Watts (W).

• Intensity (I) is a measure of the loudness of the sound a distance from the source. Units are W/m2

• The intensity decreases with the square of the distance from the source.

Intensity

• Intensity and Distance

• Example: At a distance of 10 m from source a sound has an intensity of 100 W/m2

• What is intensity at a distance of 50 m?

• (10/50)2 = 1/25 = 0.04

• I = 100 *0.04 = 4 W/m2

Examples of sound intensity

For each example a person is 1 m from source.

• A Saturn rocket has a power 50,000,000 W

• compared to: 0.0000000000001 W for the threshold of hearing.

• Decibel scale- measures sound intensity, based on log scale (powers of 10)

Changes in intensity and dB.

• Question. How does a 80 dB compare to 70 db sound?• 10 dB means 10 times louder.• Question. How does a 80 dB compare to 60 db sound?• 20 dB means 10 x 10 = 100 times louder.• Question. How does a 80 dB compare to 50 db sound?• 30 dB means 10 x 10 x 10 = 1000 times louder.• Question: If a sound intensity is doubled from• 1 W (120 dB) to 2 W what is the new dB?• 123 dB (change in 3 dB).

Frequency and pitch

• Pitch is how high or low the sound is(not loud or soft)Pitch is dependent on vibrations per second

or frequencyHumans can hear a range between 20-

20000HzAbove 20000Hz is called ultrasonic – above

our abilty to hear Timbre – refers to the quality of sound

Hearing

• Infrasonic –frequencies below 20Hz

• Ultrasonic –frequencies above 20000Hz

• So intensity is Amplitude

• Pitch is Frequency

Echoes and Reverberation

• Echoes and Reverberation

• Echo – reflection of sound.

• Reverberation – multiple reflection in a small area

• SONAR – SOund NAvigation and Ranging

• Uses an echo to determine distance

Doppler effect

• A change in frequency and pitch due to either motion of the observer or object

Resonance

• Adding energy at just the right frequency to get the amplitude to increase.

• Example: breaking a crystal glass