[Physics Class Notes] on Sound Pdf for Exam

What is Sound?

What we hear is nothing but sound, right? That’s it? Is it that simple? Physicists sometimes tend to complicate things. They define sound as a vibration that propagates the mechanical wave of displacement and pressure, through a medium can be of any matter. Sound just got complicated!

What we hear is the effect produced because of the to-and-fro motion of the particles from any medium. The to-and-fro motion is referred to as vibration. Sound moves through a medium by alternately expanding and contracting the parts of the medium which it is traveling through. This compression and expansion create a minute difference in pressure that we perceive as sound. Thereby, it is a mechanical wave of displacement and pressure. In other words, the sound is the thin line between Music sound and Noise.

In human psychology and physiology, the sound is the reception of such waves and perception by the brain.

Types of Sound

Sounds are of various types, based on the pitch, loudness, frequency, and amplitude of the sound wave, but not all these please our hearing sense. Based on whether we like it or not, the types of sound are widely classified into noise and music. Music is what pleases our hearing sense. It depends on various factors and varies from person to person. On the other side, the definition of noise is simply hazy, which is not clear. The boundary that separates the musical sound from noise is blurry. Music to someone can be noise to someone else.

Acoustics of Sound

Acoustics is referred to as an interdisciplinary science, deals with the study of mechanical waves in solids, liquids, gases, together with vibration, sound, ultrasound, infrasound. One of the scientists who work in the acoustics field is referred to acoustician, whereas someone working in the acoustical engineering field may be called an acoustical engineer. On the other side, an audio engineer is concerned with the recording, mixing, manipulation, and reproduction of sound.

Applications of acoustics are found mostly in every aspect of modern society; subdisciplines include audio signal processing, aeroacoustics, architectural acoustics, electro-acoustics, bioacoustics, environmental noise, musical acoustics, psychoacoustics, noise control, ultrasound, vibration, speech, and underwater acoustics.

Acoustic Foam

An open-celled foam used for acoustic treatment is an Acoustic foam. It attenuates airborne sound waves, reducing their amplitude, for noise reduction or noise control purposes. The energy is dissipated as heat. Acoustic foam panels can be made in several colors, sizes, and thicknesses.

Acoustic foam can be attached to walls, doors (as a soundproof door), ceilings, and other features of a room to control noise levels, echoes, and vibrations, which are called acoustic panels or sound-absorbing panels or soundproof panels.

Many of the acoustic foam panels or products are treated with dyes and/or fire retardants.

Acoustic Treatment

Treating a room acoustically is important for audio production because of the fact that very few “spaces” contain physical qualities that make for desired recording or accurate monitoring. There are various things that are done to space before and during construction to optimize its acoustic behavior. These include the shape of the space, surface materials, and isolation. Once a room is constructed already, Acoustic Treatment mostly tends to consist of treating the surfaces. There are two primary elements to consider, which are diffusion and absorption.

Acoustic foam panels (sound free panels) are well suited to flutter echo and alleviate slap, the most two common problems in rooms not specifically designed for music sound recording and reproduction or performance. In fact, foam even can turn the gymnasium or most cavernous warehouse into a suitable acoustic environment. Diffusion holds wave sounds from grouping, so there are no nulls or hot spots in a room. In conjunction with absorption, diffusion effectively can turn any space virtually into a useful and appropriate for the recording or monitoring sound purpose with a high degree of accuracy.

Sound Pressure Level

Sound Pressure Level (SPL), usually refers to the root mean squared sound pressure of a sound relative to a reference value. The SPL is written as Lp.

A sound pressure level is the logarithmic measure of the root mean squared (RMS) sound pressure of a sound that is relative to a reference value.

A simple sound wave can be represented by a sine wave. A sine wave diagram which would be the typical output signal of a single tone noise level (a calibrator tone), if it is shown on an oscilloscope. The fundamental or basic parameters associated with a sound wave are Peak, Periodic Time, Peak-Peak, RMS (root-mean-square).

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Sound level meters measure the acoustic pressure, and by international agreement, they are calibrated in decibels (dB). The sound pressure level (Lp) in decibels is given as,

Lp = 20log ([frac{P}{P₀}])

Where P is the sound pressure of measured root mean squared (RMS), and P0 is the RMS reference sound pressure (20µPa).

Also, note that the decibel is a ratio of two quantities having dimensions of power and is not a unit.