Main Ideas
- The sounds we hear travel in patterns called waves
- Wave features are related to audio and music concepts
- Sound requires matter to move, and materials transfer sound differently Background
Sound can be modeled in multiple ways, but in simplest form, we can describe sound as a wave, a dynamic disturbance, or displacement from a reference state (Fig 1.). Think about the water line in a fish tank, before (0) and after (A) a fish splashes the surface. Amplitude (A) is the distance from where the water line was initially (0) to either the top or the bottom of the wave. We often associate amplitude with volume, but volume is calculated using a logarithm of amplitude so while amplitude can be zero, the lowest volume value is negative infinity. The distance from one point on a wave to the same point on the next wave, is called the wavelength, however sound and music are usually measured by frequency, the number of times the entire wavelength passes in one second. If the graph in Fig. 1. is of 1 second, then this wave is a 1Hz wave. If it’s 100 seconds, then this is a 0.01Hz wave. We can see now that higher frequencies complete more cycles per second than lower ones, so how many seconds would this graph have to be to be 10Hz? 1000Hz? 10,000Hz?? How would the two sounds differ? Does frequency remind you of another way we describe sounds, like the ones instruments make?
Frequency is typically associated with pitch however pitch is more about interpretation rather than the sound itself. Frequency and pitch have harmonics, higher- and lower-sounding waves that are whole integer multiples of any single wave (i.e.,1x= 50Hz,2x= 100Hz,3x= 150Hz, etc.). In music, octaves are a form of harmonic that repeat every eight notes in a scale and are twice the frequency of the same note one octave below. Instruments produce large and small vibrations producing low- and high-sounding waves, respectively. One example, string instruments, use different size strings to make different notes.
Violins and other string instruments have thick and narrow strings to produce low and high notes, respectively. Violin and orchestra strings are particularly narrow compared to guitars or bass. This helps orchestral instruments produce such high frequency sounds, although a talented guitar player can perform a large range! Both players manipulate the length of their strings as they vibrate to produce higher and lower notes. The octave on a string instrument is at about the midpoint, and when a player places their finger here it creates a node, a place of no vibration. So we see that half of a wavelength produces a frequency twice as high or vice versa, and a higher pitch or note. This relationship holds for most sounds, large profiles are best for lower frequencies, and small profiles are best for the higher ones. So, if you want to get the most of our sound, you need multiple speakers! So how can they get all that bass in headphones?
Head- and earphone companies are known for sound quality, but if small speakers aren’t great for bass, where does the low end in my music come from? Some companies already put multiple speakers in their headphones. Firstly, it helps that the phones are so close to your ear, making everything more direct. To fit in or over the ear, the scale of these speakers must be quite small, but together they reproduce sounds that a single speaker couldn’t. There are also phones that don’t go over or in your ear at all, but instead rest on a bone just behind them. Isn’t that odd?? Bones can’t hear! Actually, sound travels through your bones much better than it does through the air! Have you ever been told to touch or listen to the ground to hear if a train is coming?
An important feature of sound is that it needs a medium. A medium is another word for material, particularly in which something moves or lives. Soil is a medium for plants, water, a medium for fish, etc. Because sound is made and travels by vibration, there must be something to vibrate for sound to get from its source to our ears. We’re used to hearing things through air, but it’s not as efficient as a solid object because its molecules are further apart. The more firmly the molecules in a medium are packed together, the better it transfers vibration if all other conditions are the same. For example, the notes we hear from a tuning fork, are only a few of the frequencies, or notes, it produces, and the bones in your head conduct sound well enough for you to hear them and more. In the right conditions, placing a
It’s odd that we can perceive some sounds better through parts of the body other than our ears, but this is exactly how many animals without visible ears hear. Snakes and fish are able to detect vibrations around them, especially large objects that could be predators or food! Remember that sound is a form of vibration, regardless of the source or medium. The molecules in the medium exchange energy as the vibration passes through them. If you think about how a slinky behaves when you stretch and compress it, then you have an idea of what this looks like.
Activity Materials
- Plastic and Styrofoam cups w/ holes in the bottom
- String, yarn, and/or plastic line
- Scissors (if necessary)
- Cups for water
- Soap
- Tuning Fork
- Slinky Activity Directions
- Ask the students what they remember about the previous week’s activity and introduce today’s activity.
- This week we’re going to learn how sounds are formed, what they need to travel, and how different materials respond to sound.
- Briefly discuss with the class different types of musical instruments and how they work
- Asking questions can help prompt discussion: “How does a guitar work, are there different kinds? What about singing? What makes all instruments similar?”
- Point out some of the different ways sound is generated by groups of instruments (e.g., drums are hit or shaken, trumpets & flutes use air, guitars have strings.)
- Let the students know that they’re going to begin by getting into groups and building a telephone with easy to find items.
- How do telephones work?
- How are telephones like/unlike instruments and other sound devices?
Activity Directions
- Have the students start in pairs with two cups w/ holes and a length of string.
- For each pair, instruct them to thread the string through the cups and put a knot in it, inside the cup. The String Telephone is complete! Have the pairs spread out and give them a test.
- Start with a quiet voice that cannot be heard at a distance without the telephone. Then use the telephone with a loose line, speaking at the same volume. Tighten the line and try speaking again.
- Can the students hear their partner when the string is loose? What about when it’s tight? Can you see the line vibrate? Are voices louder, or clearer through the phone? What happens if someone touches or holds the string? Is there a distance where the phone doesn’t work?
- Give the group a chance to try a different string or cup material, or a different length. Does sound travel any differently?
- Using the slinky, have one student hold one end still, or tape it down. Stretch out the slinky a bit and give it a quick compression parallel to the direction of the stretch. What do you see?
- You should see alternating areas of tight slinky, called compression, and loose slinky, called rarefaction.
- As sound moves, it pushes air molecules which bump into each other, like marbles, or bumper cars. This bumping moves from the sound source to your ear where it bumps into your eardrum, causing small bones to vibrate, which your brain interprets as sound.
- To show how the material sound moves through affects how we hear it, use the tuning fork to have the group “listen” through their heads instead of their ears
- Strike the tuning fork across something firm, but not hard enough to make anything but the fork vibrate. You should be able to hear at least two notes. Now place the bottom of the fork on a students’ head as it vibrates and ask what they hear.
- Because bone conducts sound so well, the vibrations travel through your skull to the inner ear. Because bone conducts sound better than air, the group might report hearing a third note, perhaps between the two they hear with their ears.
- Bone conduction is good enough to aid people with hearing deficits. In some cases, it’s possible to restore lost hearing!
- Squawking Cup (Optional if time)
- We’re only able to hear sounds between 20-20,000 Hz, usually when we’re young or if our hearing is very good, but most of us use some type of amplifier to better project sounds in our daily lives. You can make a simple amplifier with just a single cup, string, and water.
- String a piece of thread through a cup with a hole in the bottom, and tie off the end inside the cup.
- Holding the cup right-side up in one and, hold the exposed string tightly between your thumb and first finger and give it a short quick tug, so the string slides through your fingers. What do you hear? What does it sound like without a cup?
- While the group builds their speakers, add some water to two cups and a bit of soap to the other. What happens if your fingers are wet? What happens if your fingers are soapy?
- We don’t always think of sound when we think of friction, but in some cases the relationship is more apparent. When a drag car speeds off the starting line and the wheels build up friction on the track, it makes a good bit of sound. The cup lets you hear the small friction sound in the string through the bottom of the cup, like a speaker.