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.
In real life, we hear all sorts of noises, screaming, shouting, laughing and this is not just restricted to humans. Animals also make noises and these are distinctly different from the human voice. Does a drum make the same sound like a flute? So what’s the difference?
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.
This 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.
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 depicted in the Valleys. The physical distance between two consecutive peaks or valleys in a sound wave is referred to as the wavelength of the sound wave. It is labelled in the image above.
Frequency in a sound wave refers to the rate of the vibration of the sound traveling 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 being hertz and its definition being ‘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. 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.
Imagine a bell and a piano in an orchestra. The same musical notes can be obtained by both the 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; if two different sounds have the same frequency and amplitude, then by definition they have different timbres.
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 is a vibration that moves as an audible form of energy through a medium. The sound moves through a medium by alternately contracting and expanding parts of the medium it is traveling through. The movement of molecules of a medium is essential for the propagation of sound waves. Hence sound waves cannot travel through the emptiness of vacuum.
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.
This property of sound is responsible for the phenomenon of 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 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 traveling 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.
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. 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.