Architectural Acoustics

           Important  Questions 

   

Ø  Standard intensity of sound:

The minimum sound intensity that could be heard by humans is known as the

standard intensity and its value is 10^–12 W/m².

Ø  Reverberation Time :

Reverberation is defined as the persistence of sound even after the source is cut off.

Ø  Classification of Souns based on frequency :

Sound is classified according to its ability of being heard by humans as audible sound and inaudible sound. Audible sound has a frequency range that lies between 20 and 20,000 Hz. The sound frequencies that lie outside of this upper limit are known as ultrasonic frequencies (>20,000 Hz) and those that lie outside of the lower limit are known as infrasonic frequencies (<20 Hz).

Ø  Musical sound is characterized by three distinct qualities :

(i) Pitch

(ii) Intensity or loudness

(iii) Quality or timbre

Ø  Acoustics of a building are affected by various factors :

Reverberation time

Loudness

Focussing

Echo

Echelon effect

Resonance

Noise

Ø  Echo :

An echo is the repetition of the original sound by a reflecting surface. An echo

is produced when the reflecting surface is more than 17 m away from the sound

source or the time interval between the two sounds is about 1/7th of a second. In

both the cases, the reflected sound is heard little later as a distinct repetition of the

original sound causing confusion to the listener (The reflected sound is heard along

with the original sound.) This effect is more common when the reflecting surface

is curved.

Ø  Absorption Coefficient :

Absorption coefficient is an important quantity that is essential in designing an acoustically good building. The value of the absorption coefficient affects reverberation and reverberation time. If the absorption value is higher, then more sound energy is absorbed and the sound dies quickly. If the value is lower, then there is less absorption and the reverberation time increases. For example, if an auditorium’s intended use is for musical concert, then the value of the absorbent coefficient for the auditorium should be lower

If the auditorium is designed for public speaking, then the value of absorption coefficient should be higher (but not very high). This absorbs a large portion of the sound energy and thus there is no overlapping of individual words. Typically for a musical hall, the absorption coefficient value is such that the reverberation time is of the order of 1.5–2 s. In the case of the auditorium being used for the purpose of speech, the reverberation time should be between 0.5 and 1 s. The absorption coefficient is defined as the ratio of the sound energy absorbed by the surface to that of the total sound energy incident on the surface.

Absorption Coefficient ‘a’ = Sound energy absorbed by the surface / Total sound energy incident on the surface

Alternatively, the absorption coefficient is defined as the reciprocal of an area which absorbs the same amount of sound energy as absorbed by a unit area of an open window

The unit for absorption coefficient is W/m2 or open window units (OWU). For example, if a wall of 4 m2 area absorbs the same sound energy as that of a 1 m2 area of an openwindow, then the absorption coefficient of the wall is = 1/4 = 0.25 OWU.

Important Formulas

             * IL = 10 log10(I/Io)

             * T = 0.165V/aS

             * Intensity of Sound = Radiating Power/ Area = P/ 4pi(r*r)