Basic Terminology and definitions


Sound is a vibration in an elastic medium  such as air, water, most building material and earth 


Hearing, or auditory perception, is the ability to perceive sound by detecting vibrations changes in the pressure of the surrounding medium through time

logarithmic dependence of hearing on stimulus


Amplitude is the size of the vibration, and this determines how loud the sound is. larger vibrations make a louder sound.

Amplitude is important when balancing and controlling the loudness of sounds, such as with the volume control on your music player.


Frequency is the speed of the vibration, and this determines the pitch of the sound. 

Frequency is measured as the number of wave cycles that occur in one second.  The unit of frequency measurement is Hertz (Hz for short).

Particle Velocity

Velocity of a particle in a medium as it transmits a wave, in sound it is longitudinal wave of pressure disturbance that travels through a medium

Sound Power

Sound power is the energy rate – the energy of sound per unit of time measured in Watts (W )

I = p \ v

Intensity (I) in  W/m2

Pressure (p) in Pascal Pa

Particle Velocity (v) in m/s

Sound Pressure

Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average, or equilibrium) atmospheric pressure, caused by a sound wave.

Sound Intensity

Sound intensity also known as acoustic intensity is defined as the power carried by sound waves per unit area

P = AI

Power (P) in  W

Area (A) in m2

Intensity (I) in  W/m2


In order to express levels of sound meaningfully in numbers that are more manageable, a logarithmic scale is used, rather than a linear one. This scale is the decibel scale.

Zero decibels (0 dB) is the quietest sound audible to a healthy human ear.

L = Sound intensity level (dB);
I = Measured sound intensity (Wm-2); and
I0 = Standard sound intensity (Wm-2), the softest sound intensity that human ear can hear.

The standard reference of I0 is 10-12 Wm-2.

Diagram taken from Architectural Acoustics Workbook by M. David Egan


Loudness is the way in which we perceive amplitude. A particular change in amplitude is not necessarily perceived as being a proportionate change in loudness. That is because our perception of loudness is influenced by both the frequency and quality of a sound(timbre,लय) .

loundness is measured in unit of Phon

Phon is loundness experienced by listener

Taken from :

Note (from “notation”)

स्वर Note is a certain frequency that you write or read.


सुर  Tone is a certain frequency that you hear


स्वरमान Pitch is a certain frequency that you sing or play , The sensation of a frequency is commonly referred to as the pitch of a sound. A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave


लय Timbre is a certain quality (or color) of the tone, depending on the instrument and how you play/sing the tone.

Sound propagation

Sound propagates, or travels through air, in waves. The waves are formed when the air molecules closest to the vibration source are pushed into their neighbors, and those neighboring molecules push against their neighbors, and so on.

Each individual air molecule barely moves. Each molecule is pressed forward, and compressed, just enough to bump its neighbor and start, or continue, a chain reaction. There is just enough to transfer its energy and momentum to the next molecule.

Behavior of sound in enclosed spaces

Architectural Acoustics (AA)

Architectural Acoustics (AA) can be defined as the science, study and application of acoustic principles as they are implemented inside a building or structure. Architectural Acoustics correlates the countless frequencies and wavelengths of audible sound with the complexities of architectural geometry, and the influences of multiple finish materials. Together, these factors are nearly impossible to compute, using manual methods

Sound wave modeling

Sound travels through air in longitudinal waves. While these waves may not be too difficult to visualize at the most basic level, their behavior can be quite difficult to accurately calculate and predict in three-dimensional space.

An enclosed space is a room or area bounded on every of its sides. The materials for enclosure may be classified into two:

  • Those that allow sound rays to pass through and   
  • Those that do not allow sound rays to pass through

1.Sound absorbed in the air, also applicable to reflected sound.

2.Sound reflected from the wall surface.

3.Sound absorbed at the wall surface or its surface finish.

4.Sound conducted by the wall to other parts of the structure.

5.Sound emitted by resonance of the wall in both directions.

6.Sound inter-reflected between bounding surfaces, setting up reverberation.

7.Resonance of the enclosed volume of air by direct cross reflection

Enclosed Space

On encountering barriers posed by the enclosure, sound waves are likely to behave in the following ways:

  • Reflection
  • Absorption
  • Refraction
  • Diffusion
  • Diffraction
  • Transmission
  • Reverberation and Echo

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