Explain the term acoustic in detail

Explain the term acoustic in detail

The science of sound acoustics can be broadly divided into two major areas:

1. The handling of wanted sound, i.e. creating of the most favorable conditions for listening to a sound we want to hear: room acoustics.

2. The handling of unwanted sound, i.e. the control of noise.

The former is a rather specialized task. The control of noise is, however, closely related to other factors influencing the design.

In tropical climates, even if today the given noise is much less than in the highly industrialized regions of moderate climates, tomorrow’s problems will be no less severe.

Explain the term acoustic in detail
                              Explain the term acoustic in detail

When this stage is reached, the designer’s task will be much greater in the tropics than in moderate climates, for several reasons:

a. A greater part of life in the tropics goes on out-of-doors, where noise control is not possible, as opposed to the predominantly indoor living of moderate climates.

b. There will be a conflict between thermal and aural requirements, especially in warm-humid climates, where the building is of lightweight construction with large openings, therefore it cannot effectively control noise penetration In tropical climates the design of the building will be strongly influenced by noise considerations.

Noise control performance will depend on planning and basic design decisions, rather than on constructional details. Remedial measures will rarely be possible.

Far greater foresight and skill will be demanded of the designer. He will have to possess a much clearer understanding of noise problems and the means of their control, than his colleagues operating in moderate climates.

What do you mean by term sound? What are the various medium conveying the sound?

Sound is, strictly speaking, the sensation caused by a vibrating medium acting on the ear, but the term is usually applied to the vibration itself.

The source of sound is some vibrating solid body (e.g. a string or a sheet) which in turn generates vibrations in the air, but it may be generated by vibrations of a gaseous medium, such as the air in a whistle or flute.

Sound Waves
                                  Sound Waves

The medium conveying it to the ear can be gas (air) or a liquid, in which the vibrations are transmitted as a longitudinal wave motion, i.e. successive compressions and rarefactions of the molecules. Figure 6.3 shows how these longitudinal waves are represented graphically by a sine curve.

If the conveying medium is a solid body, the vibration may be transmitted as lateral wave motion (an actual sine-curve type movement).

The wavelength (or the frequency, which is the number of waves per unit of time) determines the pitch of the sound, Its strength is indicated by the amplitude of the sine curve.

What do you mean by wave length? What are the various quantities which describes wave motion

The wave motion can be described in terms of the following three quantities:

λ = wavelength (m) see figure 6.3
f = frequency (Hz) number of vibrations per second
v = velocity (m/s)

The relationship between these quantities is v= fxλ:

Therefore if any two are known, the third one can be calculated/found.

The value of v is constant for a conveying medium of a given density. As the density of air changes rapidly with temperature, the velocity of sound also varies with air temperature. Some typical velocities in various media (in m/s) are:
air at -20°c  319.3
0°c 331.8

Explain the term power intensity related to the sound?

The output of a source is measured as the rate of energy flow (i.e. power) in units of watt (w)

The average output of some sources (in watts) is:

In a carrying medium (e.g. in the air) the ‘strength’ of sound is usually measured as intensity, that is the density of energy flow rate through the unit area, in w/m.

When a point source emits a sound (or any other form of energy) uniformly in all directions in a free field, it is spreading over the surface of a sphere of increasing radius.

The same amount of energy is distributed over a larger and larger area, therefore the intensity will decrease. At a distance of d meters from the source it will be:

(as the surface of the sphere is 4
I=W_(as the surface of the sphere is 4zd²) m/w²
4td²
where/is in w/m²

What is the effect of sound in a room? How absorption and transmission coefficients are taken care in room?

Sound incident on the surface of a solid body (e.g. a wall) is partly reflected, partly absorbed (converted into heat), and partly transmitted to air on the opposite side (figure 6.4).

The term ‘absorption coefficient is used normally to indicate all the sound that is not reflected (that is, it includes the part actually absorbed a that which is transmitted). The absorption coefficient is denoted by a: it is a decimal fraction-a non dimensional quantity.

Airborn sound transmissions
                               Airborn sound transmissions

r = reflected
a = absorbed
Figure 6.4 Airborne sound transmissions
if source l = 1
r+a+t=1
t=transmitted
For room 1: ‘absorption coefficient = a +t
(all that is not reflected: 1-r)
For room 2: transmission coefficient = t
(r+a is not transmitted)

When the sound is in an enclosed space, reflection will occur from the bounding remainder will be lost for the system. surfaces: the reflected part will reinforce the sound within the space and the given surface (s):

Absorption (A) is the product of the absorption coefficient and of the area of a A = axs

It is measured by the ‘open window unit’, which is the absorption of a I m² opening having an absorption coefficient of 1. (i.e. zero reflectance).

In an enclosed space, even from a single source, there will be a complex pattern of interreflected sound, which is usually referred to as ‘reverberant sound’.

Thus at any point in the space the total sound received will consist of two parts: a the direct component

What is the role of absorption in a building? How they are used in ventilators?

In many cases, opening must be left for some reason, such as for ventilation. A ventilating duct may also pierce the noise-insulating airtight envelope.

These present a special problem which cannot be solved in a positive way, but can only be indirectly ameliorated by the use of absorption.

Explain the term acoustic in detail

The method is based on the follow principles:

1. The air is passed through not only an opening, but a length of duct (minimum length 1 m)

2. The duct is curved or shaped in such a way that there is no direct straight line path left for the sound.

3. As the shape induces multiple sound reflections within this duct, all internal surfaces are lined with a highly absorbent material

4. To further increase the number of reflections and the total absorbent surface available, absorbent baffles can be placed inside the duct.

 

What is Sabine’s formula for reverberation time? Explain.

Sabine’s formula for reverberation time. Prof. W.C. Sabine on the basis of his experiments found that reverberation time of a room or hall depends upon the volume of the hall and absorbing powers of various surfaces in the hall.

Sabine and others derived the relation for the standard period of reverberation i.e. the time that the sound takes to fall in intensity by 60 decibels or to one millionthof its original intensity after the source is stopped, is given by

t= 0.16 V/εas seconds

t = Reverberation time in seconds
V = The volume of the room in cubic metres
S= The area of the absorbing surfaces in square metres
a = The co-efficient of absorption
and,
caS = Total surface absorption.

= Sum of the products of the co-efficient of absorption and areas of the corresponding surfaces.
where, Thus if A1, A2…….An are the co- efficient absorption of the various surfaces inside the hall and S1, S2…..Sn are the respective areas of the surfaces, then ɛaS= a1S1 + a2S2+. .anSn.

A hall may contain several types of surfaces with different absorption co-efficient.

Also Read This.

Give the different uses of sound waves

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