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

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!

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

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

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

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.

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

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.

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.

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 9613-1 (1993). The table gives values of attenuation in dB km 1 for a temperature of 20C and a pressure of 101.325 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. http://www.resonancepub.com/unwateracou.htm

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