Sound – visualising sound waves
Sound is a form of energy that is caused by the vibration of matter. Sound is transmitted through waves, which travel through solids, liquids and gases. We are most used to the sound travelling through air, but sound is able to travel faster and further in solids and liquids.
Sound waves in air and water
The molecules of air are much further apart than the molecules in a liquid. A sound wave therefore travels more slowly in the loosely packed air than it does in a much more tightly packed liquid. Sound waves also travel further in liquids and solids than they do in air.
The nature of the medium is a major factor in the speed of a wave. For example, if you make a wave on a string stretched loosely across a classroom, you will see the wave travel down the string. If you tighten the string the wave will move down the string faster. Tightness or stiffness of the string influences the speed.
Note: There is no sound on this video.
‘Seeing’ sound
If we could see the molecules that make up the air around us, we would see sound as a series of more and less dense areas of air that are moving away from the source of the sound at about 340 metres per second. We say sound is a wave because the air molecules move back and forth while the sound travels along. The air behaves much like a longitudinal or compression wave on a spring.
Demonstrating longitudinal and transverse waves
The video shows a hand making longitudinal and transverse waves on a slinky. The audio describes longitudinal and transverse wave characteristics.
It is difficult to draw compression waves , so waves are generally represented as transverse waves for simplicity. The dense areas of the compression wave are the peaks of the transverse wave and the sparse areas are the troughs.
Longitudinal and transverse waves
Diagram illustrating longitudinal and transverse waves. The high points of the transverse waves (peaks) represent more-dense areas of the longitudinal waves, and the low points (troughs) represent less-dense areas. The arrows show the directions of wave material movement.
Microphone – transforming sound energy into electrical energy
In order to visualise a sound wave, we can use a microphone to transform sound energy into electrical energy. A simple microphone is made up of a very thin membrane with a coil of very fine wire attached. A magnet is positioned so that it is just inside the coil of wire but not touching it. When a sound wave strikes the membrane , it jiggles (vibrates) back and forth because of the high and low pressure areas of the wave. This causes the coil to jiggle, and when a coil moves in a magnetic field, an electrical current is produced. If we look at the electrical current using an oscilloscope, we can see the sound as a series of peaks and troughs.
Dynamic microphone
Diagram of a dynamic microphone showing sound wave input and electrical output.
The sound from single pitch or note will make a simple sine wave on the screen. The wave will change as you change the volume or pitch of the note.
Graphs of sound waves
Sound has both volume and pitch. Volume is seen as an increase in amplitude of the sound wave. Pitch is seen as a change in the frequency of the sound wave.
Related content
This article is part of an article series :
with the accompanying investigations:
Additional articles and activity ideas
Find out more about studying sound under water and read about what is needed for sound to be heard, and how sound travels through water to understand some of the key science concepts.
Investigating waves and energy uses slinkies to explore longitudinal and transverse waves.
Make and use a hydrophone explains how to construct an underwater microphone.
Sound on an oscilloscope uses a computer’s microphone to create a visual display.
The PLD article Physical World – Sound curates Hub resources for the early years through to year 10.
Visit the sound topic for additional resources.
