1. Helmholtz Resonator
AIR
Pressure 1
Pressure 2
Column of air in neck of bottle moves up and down (oscillates) as you blow across it
Changes pressure inside the bottle cavity
Makes sound waves that move out from the bottle
Twice the volume, half the frequency (down one octave)
Longer neck, lower frequency

2. How to Blow Across a Bottle
Hold to lower lip vertically
Pull bottom lip tight
Make rectangle with mouth
Direct air horizontally across bottle top
Breath out!

3. Boomwackers and Singing Tubes
Hit Boomwackers to create standing pressure wave
Twirl Singing tube to create standing pressure wave
These pressure differences create sound waves that ripple out (propagate).
Boomwackers: Open tube will have frequency f, closed tube will have lower frequency of ½ f (down an octave)
Boomwackers: Twice as long of tube, frequency drops ½ (down an octave)
Singing Tubes: Spin faster, higher frequency
Open Tubes
One End Closed
Boomwackers and Singing Tubes
Diagrams depict air vibration

4. Tuning Forks in Water Tuning forks make sound waves we can hear
If placed in water, these waves are visible – just like waves from dropping pebbles in water
Longer tuning forks give lower frequency waves

5. Sympathetic Vibrations
Sympathetic vibrations mean that sometimes, when one thing vibrates, another will start vibrating along with it
Need to be same (have same natural frequencies)
Small wires vibrate faster (high frequency) than long ones
Pluck the long wire and only other long wire will vibrate
Pluck small wire and only other small wire will vibrate
Stationary corks on wires
Plucked long wire

6. Transverse Waves in a Spring
Each person hold an end of the Slinkytm.
One person shake the end back and forth slow.
Try to get the Slinkytm to make this shape:
Now try to shake faster.
If you shake twice as fast, you get this shape:
Shake three times as fast, and you’ll get:
See how many bumps you can make.
Switch off so the person on the other end can shake too.

7. About Waves There are two types: Pulses: Waves have different speeds
Transverse
Like regular water waves.
Compression (Longitudinal)
Material is scrunched at parts
and stretched at others.
Pulses:
Like a single wave
moving in a direction.
Waves have different speeds
Sound waves travel faster in water than air.
Sound travels faster in Helium than air.
This can cause squeaky voices!
Sound can travel much faster in a solid.
Scrunched
Stretched
Pulse

8. Attenuation and Channeling
Sound travels outwards in all directions, like a ball getting bigger.
Because of this, sound gets quieter the farther it travels.
Megaphones help direct the sound so more of it gets where you want it to go.

9. Attenuation and Channeling
We can make sound travel farther if we talk down a tube. The sound has no where to spread, so it sounds the same at both ends. (Talk quietly, it is like you are talking into their ear. )
When sound meets a change in temperature, it can curve. One example is warmer air or water. This effect is called channeling.

10. Sound Propagation http://www.resonancepub.com/unwateracou.htm
At sea level, at a temperature of 15 °C (59 °F) and under normal atmospheric conditions, the speed of sound is 340 m.s-1 (1225 km.h-1 or 761 mph).
Attenuation of sound in air The attenuation of sound in air due to viscous, thermal and rotational loss mechanisms is simply proportional to f 2. However, losses due to vibrational relaxation of oxygen molecules are generally much greater than those due to the classical processes, and the attenuation of sound varies significantly with temperature, water-vapour content and frequency. A method for calculating the absorption at a given temperature, humidity, and pressure can be found in ISO (1993). The table gives values of attenuation in dB km 1 for a temperature of 20C and a pressure of kPa. The uncertainty is estimated to be 10%. And there is a TABLE of Attenuation vs Frequency for air water etc. At 1 kHz the attenuation is 14 dB/km at 10% Humidity. At 0.7 km Intensity is 10%
Whereas in salt water it is 0.06 dB/km at 20 C At 167 km Intensity is 10% (233 times the distance in air!)
Note the difference is quite large.

11. Column of air in neck of bottle moves up and down as you blow across it
Changes pressure inside the bottle cavity
Makes sound waves