Waves and Sound Unit Overview

 

The Waves and Sound Unit is intended to engage students in many experiences with hands-on and computer-based materials that will help them modify their existing ideas and construct new ideas about waves and sound.  We begin this unit with an activity involving a boat on water, one with which students are familiar but have developed misconceptions.  This first unit is designed to help student see the various properties of waves and their relationship.    The second cycle focuses on the source of sound and properties of standing waves.  The third cycle investigates the effect of various changes in the medium on the speed of a wave.  The final cycle looks at the wave phenomena of interference and diffraction.   This unit was designed so that teachers can work through it during 40 hours of class time. 

The Unit consists of four cycles:

 

Cycle I: Introduction to Wave Properties

Cycle II: Sources of Sound, Harmonics and Quality of Sound

Cycle III: Factors that Affect the Speed of a Wave

Cycle IV: Interference and Diffraction

 

 

Target Ideas for Unit

 

The Waves and Sound Unit was designed to provide the opportunities for students to construct ideas, which are closely aligned with the ones listed below.   At the end of each activity in the development phase, students are asked to add or modify an idea in their Idea Journal, based on evidenced gathered within that activity.   We have found this semi-structured approach for development of a common set of ideas to work well with high school students, prospective and in service elementary teachers.  Naturally, as part of their consensus discussion for each cycle, students will probably develop these ideas in their own words.  However, the conceptual content of their own ideas should be similar to these. In a latter section we include samples of the ideas actually constructed by the students themselves. The Teacher Guide for each cycle provides examples of the kinds of statements students actually develop in the class. After the class agrees on a set of ideas the teacher should introduce appropriate terminology and conventions so that the students' are more closely aligned with the corresponding ideas they would find in textbooks or when they talk with other students.

 

Target Ideas for Cycle I

 

  1. Waves have measurable properties such as speed, amplitude, frequency and wavelength. Speed can be determined by dividing the distance a pulse travels by the time it takes to travel that distance.  Amplitude can be determined by measuring the maximum motion of the medium.  Our ear interprets amplitude as loudness.  Frequency can be found by measuring the number of waves created or passing a marker per time period.  Frequency is interpreted by our ears as pitch.  Wavelength is the distance between two positions on a wave with the same displacement, heading the same way.

 

  1. Speed, frequency and wavelength are independent of amplitude.

 

  1. Changing the frequency of a wave changes its wavelength inversely.  Changing the medium or the tension in the medium changes the speed of the wave.

 

  1. Particles of a medium in which a wave travels may move perpendicular or parallel to the direction of propagation of the wave.

 

  1. Waves can change direction when they hit a boundary.  If the boundary is rigid, the pulse experiences a phase inversion.  If the boundary is flexible, the pulse is reflected with the same phase as the incoming wave.  If a pulse strikes the boundary at an angle, the incoming angle is equal to the reflected angle.
  2. Frequency = velocity/ wavelength

 

  1. The wave properties discovered in this cycle apply to waves on springs, water waves, sound waves.

 

Target Ideas for Cycle II

 

1.      Sound sources are vibrating objects. 

 

2.      The length of the vibrating material determines the wavelength produced.  For straws, chimes and a bottle you blow into, the vibrating material is the air.  For a meter stick, tapped water bottle and a singing goblet, the vibrating material is the solid.

 

3.      Vibrations can be transmitted when a vibrating object touches another object.  The larger the area of vibration the louder the sound. When two objects have close natural frequencies, vibrating one can cause the second to vibrate even when they are not in direct contact. This is known as resonance.

 

4.      For every vibrating object there is a fundamental frequency and other related frequencies (overtones) that will produce standing waves for that length.  In sound waves, we call the frequencies that produce standing waves harmonics.  In open and closed pipes, the fundamental frequency is different due to the different standing waves that are established.  Open pipes the same length as closed pipes have fundamentals with about twice the frequency of the closed pipe.

 

5.      The number of harmonics and their relative intensities affects sound quality.  Open pipes have a richer tone than closed pipes due to the open pipe having all harmonics and the closed pipe having only the odd harmonics.

 

6.      Different musical instruments playing the same note do not sound the same due to the number of harmonics and their relative intensities (quality).  

 

 

                                                Target Ideas for Cycle III

 

  1. Sound waves travel at a measurable speed that is not affected by the amplitude or frequency of the wave.

 

  1. Increasing the tension in a spring causes waves to travel faster, which affects the wavelength.

 

  1. The speed of sound and the wavelength is affected by the type of medium in which it travels.

 

  1. Sounds travel faster at higher temperatures.  Temperature affects the wavelength of the sound.

 

  1. Observers detect an apparent change in frequency when the source of a wave is in motion relative to the observer.

 

  1. When a wave reaches a boundary at an angle and passes into a medium where the speed changes, the wave direction changes or refracts.

 

 

                                                Target Ideas for Cycle IV

 

  1. Wave energy can pass through each other undisturbed.

 

  1. When pulses encounter each other their amplitudes add creating constructive and destructive interference.

 

  1. When waves of different frequencies come together beats occur.  This can be quantified by the equation Beat Frequency = | frequency1 – frequency2|.

 

  1. Every point on a wave front can be viewed as a point source for the next wave front.

 

  1. Waves energy can spread behind a barrier in a medium