The Science of Sound
Welcome to Billund Builds Music! We are so excited and grateful that so many schools and teachers in Billund are willing to work with us to put on what will be a student-driven and interdisciplinary exploration of sound, music and engineering in Billund.
We expect that not every teacher is an expert in the science of sound and music, and that is okay! Hopefully, this teacher’s guide gives you an overview of what you will need to facilitate meaningful student learning throughout the week. Please also find other available resources on our website at billundbuilds.dk/music where we have compiled a list of suggested materials, possible activities and pictures and videos of students engaging in the activities.
Where does sound come from?
There are three requirements for sound to take place in an environment: (1) a vibrating source to start sound, (2) a medium to transmit sound vibrations throughout the environment, and (3) a receiver to hear or record sound vibrations.
Sound is started in an environment by a vibrating source. There are numerous and varied vibrating sources – pushing air past vocal cords, plucking a stretched string, striking a membrane of animal hide or synthetic material, buzzing lips on an instrument, and many other natural vibrating sources. Sound may also be produced by electronic vibration sources – whirling of machines, playing of electronic keyboards and synthesizers, pulsing speakers, and more.
Not all vibrations create sounds that humans can hear. Human ears sense vibrations only that are moving at speeds between 20 vibrations per second and 20,000 vibrations per second. If a vibration is too fast or too slow, it will not be heard as “sound” by humans. One vibration too fast for human hearing is the ultrasonic signal emitted by bats for echo-locating. Vibrations that are too slow for human hearing include ocean waves and earthquakes. We detect these vibrations by seeing and feeling them, but we do not hear them.
Why are sounds different?
Volume depends on the amount of the force used to start the sound vibrations. The more force used to start the vibrations, the louder the sound produced. For example, the volume of an acoustic guitar is determined by the force with which the string is plucked.
The amplitude (size) of a vibration determines the loudness and softness of sound. The loudness or softness of a sound is commonly known as its volume. A louder sound has a wider vibration than a softer sound. The figure below shows a comparison between a loud and soft sound. Smaller vibrations are equivalent to softer sounds, while larger vibrations are equivalent to louder sounds.
Sound is generally measured by the number of vibrations per second produced by a vibrating body or source. If the vibration occurs at a steady number of vibrations per second, the musical effect produced is known as a pitch, tone, or note. The physical properties of vibrating objects come into play when comparing the pitch of one vibrating object to another. Smaller, thinner, or tighter vibrating objects are higher in pitch, and conversely, larger, thicker, and looser vibrating objects are lower in pitch.
How does sound get transmitted to our ears?
For sound to travel from its source to a receiver, such as ears, a medium must be present. Gases (such as air), liquids (such as water), and solids (such as steel) are all mediums that can transmit sounds. Sound is not capable of being transmitted through empty space.
For a medium to be a good transmitter of sound, it must be elastic. Elastic substances are able to transmit vibrations without losing much energy to friction or absorbing much energy. Imagine a blob of wet clay and a cube of steel. Your initial judgment might be that the wet clay is more elastic than the steel, in the everyday usage of the word “elastic.” However, if you put a finger into the clay, the dent does not bounce back. Instead, the clay absorbs the energy of your finger push. In contrast, the steel cube does not seem to be affected by your finger push at all. The steel, then, is more “elastic” in the scientific usage of the word, and it will transmit sound more quickly and without much loss.
Air is also a good transmitter of sound. However, volume decreases more quickly when sound travels through air than through stiff solids. In air, sound moves outward in all directions, while in solids sound travels only in the direction of the solid object.
Water is a more efficient transmitter of sound than air because sound travels faster in water. The speed of sound in liquids is faster than the speed of sound in gases, and the speed of sound in solids is faster than the speed of sound in liquids.
How do different instruments make different sounds?
When studying sound-making devices, it is important to pay close attention to the source of the vibrations, which is the initially vibrating substance. Correctly determining the source is vital to drawing accurate conclusions about the type of sound that will be made.
The physical properties of vibrating objects determine how the pitch of one object compares to the pitch of another. Smaller, thinner, or tighter vibrating objects are higher in pitch and, conversely, larger, thicker, and looser vibrating objects are lower in pitch. For example, when rubber bands of equal length and thickness, but unequal tension are plucked, the tighter band will produce a sound with a higher pitch. The additional tension in the band causes it to vibrate faster, and these faster vibrations are perceived by our ears and brains as higher-pitched sound. Similarly, when rubber bands of equal length and tension, but unequal thickness are plucked, the thinner band will vibrate faster and thus produce a sound with a higher pitch. Finally, when rubber bands of equal tension and thickness, but unequal length are plucked, the shorter band will vibrate faster and create a higher-pitched sound.
We can also use the example of “musical cups” to consider how physical properties affect pitch. When glasses are filled with water, they can become musical sound sources. The pitch of the sounds can be controlled by the amount of water in the cups. Consider the four cups below.
A B C D
When someone blows over the top of the cups, cup A will have the highest pitch. Blowing over the top of the cups causes the air column at the top of the cup to begin to vibrate. With the shortest air column, cup A will produce the sound with the highest pitch. Since cup D has the longest air column, it will produce the lowest-pitched sound.
The cups also produce sound when someone strikes them with a spoon. The glass or material out of which the cup is made is the source of vibrations. The more water in the cup, the more difficulty the glass will have vibrating, and the slower its vibrations will be. Cup D, with the least water, will vibrate the fastest and produce the highest-pitched sound. Cup A, with the most water, will vibrate the slowest and produce the lowest-pitched sound.
What happens to sound before it reaches our ears?
One way to define sound is as a form of energy produced by a vibrating object. This sound energy is transferred from a vibrating source to a sound receiver. It can be transferred through air, water, or solid material, via vibrations. All sounds have energy, but different sounds have different amounts of energy. In general, loud sounds always have more energy than quiet sounds.
When sound is effectively transferred from source to receiver, we say that the sound is being transmitted. Sound energy may also be reflected or absorbed. To determine which of these three actions occur, we must consider the materials in the vicinity of the sound source. When there are large, flat surfaces near the sound source, the sound is often reflected, and echoes (reflections) of the sound can be heard. Smooth and shiny surfaces reflect more sound than rough and dull surfaces.
As sound travels though any medium, there is some loss due to absorption. Sound absorption occurs when the particles in a medium take on the energy of the sound vibration instead of passing the vibration along to the next particles. Soft, airy materials are particularly prone to sound absorption, including drapes, curtains, foam ceiling tiles, soft furnishings, rugs, and most concert hall wall panels. When these materials are nearby a sound source, some of the sound will likely be absorbed.
How does sound travel through a medium?
Sound travels in patterns called waves. Waves are disturbances that propagate energy.
A transverse wave is a wave where the deviations from the resting position occur perpendicular to the direction of the motion of the wave. You can think of the way a wave in the ocean moves the water up and down as the wave travels forward. The motion of a wave through a string would also be transverse. In a transverse wave, the highest point in the wave is called the peak or crest and the lowest point is the trough. Wavelength of a transverse wave is measured from peak to peak or trough to trough.
Sound waves are longitudinal compressions of a medium, such as the molecules of air, water or a solid. A longitudinal wave is one where the deviations from the resting position occur in the same direction as the motion of the wave. In sound waves, the traveling wave causes compressions where there is a higher pressure than when at rest. Behind the compressions are voids where the pressure is lower than when at rest.
In this picture, the top wave is a transverse wave. The bottom wave is a longitudinal wave, where the darker regions indicate a compression.
It is important to keep in mind that waves transmit energy, not particles. The particles that are disturbed by a wave will return to their resting positions after the energy of the wave has passed by.
Sound waves travel at different speeds through different mediums. Sound travels through air at approximately 343 m/s and through water at approximately 1482 m/s. Sounds can travel even more quickly through solids because they are more dense.
Sound or Sound Energy – Energy transported when an object vibrates in a medium.
Tension – The condition of being stretched tightly.
Pitch – How high or low a sound is.
Medium – A substance which carries energy or another substance.
Percussion Instrument – Any musical instrument whose vibrations are caused by tapping or hitting.
Wind Instrument – Any musical instrument whose vibrations are caused by moving air
Vibration: oscillatory, or back and forth, motion through a medium- in the case of sound waves we hear, the medium is air, but sound can travel faster in water and even faster through solid materials.
Absorb – To take in or soak up.
Reflect – To bounce back light, heat, or sound.
Transmit – To pass through a space or material, or to carry through a space or material
Elasticity – in physics,ability
Frequency – how many waves will pass by a given point in one second- the faster the wave oscillates, the higher the frequency and the higher the pitch. The slower the oscillation, the lower the frequency and the lower the pitch. Measured in Hertz (Hz).
Wavelength – the distance from the peak of one wave to the next
shorter wavelength= higher pitch
longer wavelength= lower pitch
Amplitude/Volume – the displacement of a wave from its rest position
high amplitude= high volume
low amplitude= low volume
Timbre – the unique combination of harmonics that make every sound and instrument distinguishable from each other. You can play the same note on different instruments, but you would be able to tell the sound of a violin from the sound of a piano, for example. This is because when the instrument is vibrating it not only vibrates at the frequency of the note being played but also a little bit at other frequencies. These waves combine to produce unique patterns that vary from instrument to instrument.