University of Minnesota
School of Physics & Astronomy

Physics Force

Waves and Sound

Audience Wave - Transverse

(Wave Motion)

Transverse waves are waves that move along a medium where the medium moves perpendicular to the direction that the wave moves.

Water waves move in this manner. The wave moves across the water horizontally, but the water (the medium) actually moves up and down, perpendicular to the direction the wave moves.

If you send a wave down a rope suspended between two people, the rope moves up and down, but the wave moves horizontally.

If you lay a slinky on the floor and send a horizontal wave by moving one end of it back and forth horizontally, the wave moves from you to the other end, but the slinky moves back and forth, and returns to the same spot after the wave has passed.

When Crazy George, the Vikings Cheerleader, sends a wave around the Metro dome, the people stand up and sit down as the wave moves horizontally and after the wave has passed they all remain in the same seat.

Materials

15 to 20 students sitting in a row next to each other. There can be many students sitting in many rows just like at the Metro dome which adds a little excitement

Instructions

Start at one end of the row and have the first students stand up and sit down. All other student in the row must wait and respond when the wave gets to them. If students are to quick to respond have them hold hands and close their eyes in a practice run. If they are in a circle, it can continue around several times. If you have a video camera, you may want to record it and show them a recording of the action.

FOSS Connection: Physics of Sound - Activity 2: How Sound Travels

 

Sound And Waves

 

Audience Wave - Compression

(Types of Waves)

A compression is characterized by having a particle in the medium move forward, bump into the next particle in the medium and then bounce back to its original position. This continues with each particle bumping its neighbor and then returning to its original position thereby transmitting the disturbance through the medium.

 

Materials

A group of people seated side by side and shoulder to shoulder in a row. People on bleacher seats or in chairs placed next to each other are in the correct orientation for this demonstration.

Instructions

Instruct a person on one side of the room to bend and gently bump shoulders with their neighbor and return to their normal position. The person bumped responds by bending to bump shoulders with the third person in line and so on. This shows the characteristics of a compression wave, (longitudinal wave). It is useful to illustrate the wave cannot be transmitted without a medium. To do this make a break between a couple of the people and show the wave cannot travel across the break because there is no medium, (people), to transmit the wave. It also shows the medium does not move forward, just the wave. Sound waves are an example of compression or longitudinal waves.

 

FOSS Connection: Physics of Sound - Activity 2: How Sound Travels

 

Bubbles and Bugles

(Wave Motion versus Medium Motion)

 

When sound is sent off from a source the air wiggles a little, but does not go off producing a wind.

Sound can break a glass (it was shown on television in yet another ad for some recording device), but it does it with very small vibrations, not with a straight line wind.

In an explosion there can be a powerful rush of air radiating out from the detonation site. This air motion is produced by the heating effects of the exploding materials and is not due to the sound that is generated.

 

Materials

A plastic bugle, bubble solution, and something to catch any drippings. A metal horn will work, but the soap bubble is harder to keep in tact. Bubble solution formula: Add one cup of Dawn detergent to one gallon of water. Add one tsp. of glycerine.

 

Instructions

Dip the end of the horn into the bubble solution. After you have a bubble across the bell of the horn blow a long and steady note. Start softly with little air to keep from breaking the bubble prematurely. The sound is heard by everyone in the room and yet the bubble is not broken. You can darken the room and shine a light on the bubble which will allow students to see the sound vibrations as they pass from the air in the bubble to the outside. If you can change notes from high to low frequency students can also make the connection between frequency a the pitch they hear.

FOSS Connection: Physics of Sound - Activity 3: Good Vibrations

Science Specialists Connection: Bubbleology

Nodes and Anti-nodes (flexible hose)

(Standing Waves)

Any time you get waves from different sources to cross each other they will add together. This addition will produce places where the waves are large in size as well as places where the addition produces minimums, or even no wave. A simple way to see this is to send a series of waves down a rope, hose, or the like and let them reflect off the other end. The reflected waves that are returning act as a second source that will cross the newly generated incoming waves and wave interference results. If you play with the rhythm (frequency) that you produce, the waves will hit several situations where the quiet spots (nodes) and the large waving spots (anti-nodes) remain in fixed positions. This phenomenon is called a standing wave.

The production of standing waves is very important in the operation of musical instruments. The sound played on a string of a violin enters the wooden resonant chamber to establish a standing wave. The odd shape and size of the chamber allows for at least one dimension to establish a standing wave for every note the instrument can play. In just a few vibrations of the string the standing wave becomes large and strong able to produce full rich sound to be heard across the room.

When we sing in the shower and our voice becomes full, powerful and rich on certain notes it's because the dimensions of the small room produces a standing wave. If you have a normal size bathroom this phenomenon is much more likely for deeper voice of men than women.

On occasion you can see a standing wave in high line wires and in the radio antenna of cars. The high line wires establish a standing wave during wind storms. There is usually a node in the middle and two anti nodes nearer the poles. When cars with large engines idle, the vibrations of the motor can produce a standing wave in the radio antenna.

 

Materials

Any rope, hose, or the like will work for this demonstration. However, the slower the waves move down the medium, the lower the frequency needed to produce an observable standing wave. A rubber hose filled with water works well.

Instructions

Pull the hose somewhat tight and send single frequency waves down towards the fixed end. Don't get discouraged in the first few moments because waves must reach the other end and start to return before you can expect a standing wave to set up. You may have to adjust your frequency a little to establish a good standing wave. You can try circular waves as well. They are a little easier to produce at a constant frequency. Once you have a standing wave the energy you need to put in to keep it going is smaller than that needed to get things started.

FOSS Connection: Physics of Sound - Activity 2: How Sound Travels

Hoot Tubes

(Standing Waves)

This demonstration illustrates the preference of a certain length column of air inside a hollow tube to choose a particular frequency of sound to amplify (resonate) by setting up a standing wave. This standing wave will have one half the wavelength of the frequency it is amplifying or a whole number of half wavelengths, depending on the energy of the source. (i.e. 1/2, 2/2, 3/2, etc.) When you light the burner, it heats up the burned gases and the air near the flame. This causes these gases to become less dense and they are pushed up by cooler, heavier gasses near by. As these gases move through the wire mesh turbulence is established producing many different frequencies of sound called white sound. We hear it as a hissing or slight roaring noise. When the tube is placed over this array of many frequencies it will, because of its length, pick out one of the frequencies and set up a standing wave in the tube. This amplifies that frequency dramatically, and we will hear the characteristic hoot.

This is really the principle used by any wind instruments in a band or orchestra. The instrument has a hollow tube of some sort, its length will determine which frequencies it will amplify and its shape will determine the quality or type of sound it will give out. The musician produces a vibration, usually with the lips or a reed, and the air in the hollow tube will vibrate in resonance with whatever frequency it is designed to amplify.

The simplest and easiest to understand is the trombone. Notice that when a trombonist wants a lower note, she will move the slide to make the vibrating air column longer, and a higher note is achieved by moving the slide to make the vibrating air column shorter. There is also an interplay of the tightness of the embouchure to get other notes, but with a given tenseness given by the musician's lips, the length of the tube will then decide which note is sounded.

Materials

A large hollow tube (center core of a carpet roll works well), a Fisher burner (or any other burner that has a wire mess to spread the flame), and a source of propane or natural gas to operate the burner.

Instructions

Light the burner and set it on the floor so that you can take the hollow tube and place its mouth over the burner. Move the tube slowly down toward the base of the burner until the tube begins to vibrate, resonating its characteristic frequency. It should be quite loud and a frequency that depends on the length of the tube. The tube is open and therefore will have an anti node at both ends. This allows only whole-number multiples of half wavelengths within the tube as illustrated below.

Each of the examples above would be consecutively higher notes, going from left to right. The one at the far left is considered the "fundamental" frequency of that tube of that particular length.

 

Use caution with the flame and the cardboard tube. Its a good idea to spray the inside of the tube with a fire retardant.

 

FOSS Connection: Physics of Sound - Activity 2: How Sound Travels

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