1) Tell students that they have been examining how our experience of sound starts as the physical vibrations of energy pulsing through a medium. Ask students if they can think of times that they have actually felt sound vibrations with their bodies. (Answers may include the rumbling of distant thunder or the thump of loud music coming through the floors, ceilings, or walls of their homes.) Provide a FOCUS question for the next video clip by asking students how deaf percussionist Evelyn Glennie “hears” music. PLAY Clip 2: “An Ocean of Vibration.”
2) Review the focus question: how does Evelyn Glennie experience music? (She feels it through the sounds vibrations in her body.) Explain that the human body itself is a medium through which sound travels as described in the introductory activity. Log on to the “Introduction To What Sound Is” website (www.fearofphysics.com/Sound/dist.html). Have the students click “Play” for the animation and explain that the first page of this website depicts, like the Slinky demonstration, a sound wave in a single, straight-line pulse. Have students click on the blue link at the bottom of the yellow box (“The wave is able to spread out more…”) and explain this second page of the website (“Sounds: Allow the Wave to Expand”) illustrates a sound wave in two dimensions. Ask students to review what the red dots in each illustration represent. (The molecules of the medium.) Minimize the browser window with this website, as you’ll be returning to it later in the lesson.
3) Divide the class into groups of 3-5, and explain that each group will be conducting a series of experiments to determine how sound travels though different mediums. Distribute a wire hanger, a metal spoon, some yarn, some string, some metal wire, a ruler and the “Sound Mediums” Student Organizer (PDF) (RTF) to each group. Each member of the group should hold the wire hanger and tap it with the ruler. The group should collectively agree on how to describe the sound, and enter this description on the student organizer.
4) Next, tie a four foot length of string onto the hook of the hanger. Have each group member in turn wrap a few inches of each string around a finger on each hand. Gently plug the string-wrapped fingers into each ear. Have another person tap the hanger with the ruler. How is the sound different than before? Enter a collective description on the student organizer.
5) Replace the string with a four foot length of yarn. How do the sounds compare to those when string was used? Replace the yarn with a four foot length wire. How does that compare? Replace the four foot wire with a six foot wire. Does length affect loudness? Why? Have the group note all their collective responses on their student organizer.
6) Have each group take turns reporting their findings as recorded on their student organizers. Explain that the speed at which sound travels depends on the medium in which it is traveling and the molecular packing of that material. Molecules are packed most densely in solids and least densely in gases. The closer the molecules are together, the better sound travels. In this activity, when the string and fingers were plugged into the ears, the sound traveled better than just through the string and air. Solid wire is a denser, better conductor of sound than tightly woven string, which is in turn a better conductor of sound than loosely woven yarn.
7) Tell students that so far they’ve been learning about how sound moves from an initial source vibration through a medium. But what happens when a sound wave actually reaches our ear? What enables us to hear sound, and distinguish between different sounds? (Accept all answers.) Provide a FOCUS question for the next video clip by asking students what the cochlea is and how it works. PLAY Clip 3, “How We Hear”
Review the focus question: what is the cochlea and how does it work? (The cochlea is a snail-like part of the ear filled with tiny hair cells which are tuned to vibrate at different frequencies. The cochlea converts these physical vibrations into electromagnetic signals to the brain’s auditory cortex.) Explain that the pitch we hear is directly related to the speed, or frequency, of the sound wave.
9) Log on to the “Pitch, Notes, and Duration” activity page (http://www.bbc.co.uk/music/parents/activities/musicsense/ms2.swf). Ask students to compare the speeds of the two pitches they hear—what can they conclude? (That higher pitches have faster speeds of vibration.) Now restore the “Introduction to What Sound Is” website and click to the next (third) page of the site (also directly accessible at http://www.fearofphysics.com/Sound/dist_hear.html). Ask what a Hertz measures (The number of vibrations per second).
10) Explain that the range of sound frequencies generally audible to humans varies among individuals, and particularly with age. Allow students to discover the limits of their own hearing by directing them to the “Hearing Test” available at http://www.freemosquitoringtones.org/. Invite one student to sit at the computer hidden from view. Tell the student that he or she should activate the tones at irregular intervals, beginning with the lowest frequencies and moving progressively higher. Ask everyone else in the room to stand up, close their eyes, and only sit down when they are sure they can no longer hear the activated pitch. You play too!
11) Explain that frequencies higher than humans can hear are referred to as ultrasonic, while those too low for humans to hear are called infrasonic. Provide a FOCUS question for the next video clip by asking students what infrasonic pitch the black hole described in the clip makes. PLAY Clip 4: “Cosmic Music.”
12) Review the focus question: What pitch does the black hole described in the clip make? (B flat—but 57 octaves too low for us to hear.) The video clip referred to sound as a “unifying thread” connecting everything in the universe. Explain that the final clip will approach that same theme from a different angle. Provide a FOCUS question by asking students what similarities may be found between the sounds made by whales and those made by catbirds. PLAY Clip 5: “Whale Song.”
13) Review the focus question: what similarities may be found between the sounds made by whales and those made by catbirds? (The whale song, when sped up, has a similar pattern and form to the birdsong). Ask students why they think this might be? (Accept all answers, but suggest that the longer, slower, lower whale song may reflect whales’ larger size and slower metabolisms compared with the higher speed, more tightly wound metabolisms of small birds, and that the lower, slower whale song may be better suited to travel through the denser medium of water.)