A sound is a form of energy, just like electricity, heat, or light. When you strike a bell, it makes a loud ringing noise. Now instead of just listening to the bell, put your finger on the bell after you have struck it. Can you feel it shaking? This movement or shaking, i.e. the to and fro motion of the body is termed as vibration.
The sound moves through a medium by alternately contracting and expanding parts of the medium it is travelling through. This compression and expansion create a minute pressure difference that we perceive as sound. The movement of molecules in a medium is essential for the propagation of sound waves. Hence sound waves cannot travel through the emptiness of a vacuum.
Sound as a Waveform
When sound waves are represented in a waveform, we instantly notice some basic characteristics. The waveform is a pictorial representation of the pressure variation in the air, which travels as sound. These waves are alternately regions of high pressure and low pressure. Thanks to the waveform, sound waves now seem very similar to light and other electromagnetic radiation.
A depiction of Sound Waves in Waveform and representation of Amplitude/ Loudness and Wavelength
Characteristics of Sound Waves
Sound cannot travel through a vacuum. This is very much in contrast with the property of light. Another difference which is above the scope of the syllabus is the fact that sound waves are generally longitudinal waves and light waves are transverse waves. But they’re not very different either. Let’s take a look at the characteristics of sound when propagating through air.
Amplitude
Amplitude in light refers to the amount of energy in an electromagnetic wave and its meaning is the same here. Amplitude refers to the distance of the maximum vertical displacement of the wave from its mean position. Larger the amplitude, the higher the energy. In sound, amplitude refers to the magnitude of compression and expansion experienced by the medium the sound wave is travelling through. This amplitude is perceived by our ears as loudness. High amplitude is equivalent to loud sounds.
Two graphs showing the difference between sound waves with high and low amplitudee
Wavelength
The waveform representation converts the pressure variations of sound waves into a pictorial graph which is easier to understand. A sound wave is made of areas of high pressure alternated by an area of low pressure. The high-pressure areas are represented as the peaks of the graph. The low-pressure areas are represented as troughs on the graph. The physical distance between two consecutive peaks in a sound wave is referred to as the wavelength of the sound wave. It is labelled in the image above.
Frequency/ Pitch of the Sound Waves
Frequency in a sound wave refers to the rate of the vibration of the sound travelling through the air. This parameter decides whether a sound is perceived as high pitched or low pitched. In sound, the frequency is also known as Pitch. The frequency of the vibrating source of sound is calculated in cycles per second.
The SI unit for frequency is hertz and its definition is ‘1/T’ where T refers to the time period of the wave. The time period is the time required for the wave to complete one cycle. The wavelength and frequency of a sound wave are related mathematically as:
The velocity of Sound = Frequency * Wavelength
The below graphs can be used for understanding more about sound. The first graph represents a sound wave from a drum while the second graph represents the sound wave from a whistle. You probably already know the difference in the sounds but have a look at the difference in their frequencies.
Two graphs showing the difference between sound waves with high and low frequencies and their corresponding pitches
Timbre
Imagine a bell and a piano in an orchestra. The same musical notes can be obtained by both instruments but their sounds are very different. The piano produces a distinct note whereas the bell struck to the same pitch and amplitude produces a sound that continues to ring after it has been struck. This difference in the sound is referred to as the Timbre. Timbre is actually defined as the quality of a sound which is used for differentiating two sounds when they are in the same frequency. If two different sounds have the same frequency and amplitude, then by definition they have different timbres.
Reflection of Sound Waves:
This property of sound is responsible for the phenomenon of the Echo. Also, the rolling of thunder is largely due to the repeated reflections from the clouds and land surfaces. The reflection of sound follows the same principle as light waves. The angle of incidence is equal to the angle of reflection. For an appreciable reflection, the reflecting surface should have a large surface area, like a cloud. This principle of reflection is used in a technology known as SONAR (Sound Navigation and Ranging) where the sound waves are used, usually underwater, to navigate and communicate. The sound waves that reflect from objects are used to detect objects on or under the surface of the water.
Refraction of Sound Waves:
Refraction in light occurs when the density of the medium in which light is travelling changes. Similarly, Refraction in Sound occurs when the density of the atmosphere it is travelling through changes. The density of a gas decreases with the rise in temperature, inversely proportional. In fact, it is so similar to light waves that it even undergoes total internal reflection.
Diffraction of Sound Waves:
Think about this for a minute. If you shut the door and shout for your friend outside your room, he can still hear you. Sound waves have the ability to bend around obstacles. If there is a small hole in the door, the small opening itself would act as a localized source of the sound. The diffraction of sound waves is an important part of our experience of the world around us. The lightning strikes close to your sound like a sharp crack and yet the distant strikes sound of deep rumbling thunder. This is because the deeper tones of sound waves can bend across obstacles better than the sharp sounds so you hear only the deep rumbling. Light waves too undergo diffraction but of a significantly lesser magnitude.
We started off saying sound and light waves are not alike at all. But by the end, the sound seemed more like light waves. The understanding of science is very similar to this. In this article, you learn about the different characteristics of sound waves and how two sound waves of even the same amplitude and frequency can produce a completely different sound.
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