Acoustics and Sound Insulation Methods

Acoustics and Sound Insulation

ABSTRACT

Acoustics is the study of sound. The knowledge of this field is necessary as it helps in creating peaceful surroundings. The technical knowhow of sound insulation has been helpful in constructing functional theatres, hospitals, Auditoriums, jam pads, etc. Sound is created when a surface vibrates. When this vibration is periodic or rhythmic then the sound produced is pleasing to the ears eg. Musical Notes. On the contrary when the vibration is non-periodic and irregular then it is termed as noise. eg. Infants crying.

INTRODUCTION

Sound travels in the form of longitudinal waves. Each wave moves backward and forward passing on the vibration to the next. For us to be able to hear sound clearly it is very important for the waves to travel uninterrupted. It is proven that sound travels the fastest in solids, then in liquids and the least in air. This is because in solid the molecules are so tightly packed that the transmission is the easiest. In liquids too the molecules are fairly close to each other where as in air it becomes a little tough for sound to travel. Sound cannot travel in vacuum as it needs a medium to travel. The speed of sound in air depends upon the moisture or dryness in air. The more the moisture the faster sound will travel.

CHARACTERISTICS OF SOUND

1. Intensity and Loudness of sound:

Loudness is the response of humans to the impact of sound. Whereas intensity is the amount of sound waves passing through per unit area per unit time.

2. Frequency and Pitch

Sounds can be of different types and wavelengths. Pitch is What helps us distinguish between sounds of same loudness but of different frequencies. This is the characteristic which helps us recognise a man’s voice and women’s Voice.

3. Quality

The quality of sound is what enables us to differentiate between two musical tones played on different instruments. A study has shown that the notes produced by instruments are at times pure notes. They have some fundamental tones of frequency fo and additional tones of frequency 2fo, 4fo, etc. called over tones. The fundamental is heard very clearly because it has greater impact human ear.

TABLE 1.1, ACCEPTABLE NOISE LEVEL

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REFLECTION OF SOUND

Sound waves from a plain surface reflect in the same way as that of light. The angel of incidence is equal to the angel of reflection.

Following are the characteristics of reflection of sound:

1. When sound waves reflect from a flat surface the wave fronts form a spherical shape and the centre of curvature is the source of sound

2. When sound waves reflect from a convex surface they get magnified and therefore become thinner and weaker. Convex surfaces can be used in interior spaces to reduce the impact of sound.

3. Sound waves that reflect from a concave surface are very dense and compressed therefore the sound is amplified. Concave surfaces should be avoided in interior spaces as the result is undesirable.

Reflection on flat surface Reflection on convex surface Reflection on concave surface 

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ABSORPTION

When sound waves strike a surface some of the sound is absorbed by friction. The materials that absorb are known as absorbants. Absorption in auditoriums or theatre the takes place in three ways –

1. In Air – This absorption happens due to friction between two molecules though this is very small

2. By Audience — Cloth is an absorbant. So in theatres and auditoriums the sound is absorbed by the clothes o f the audience. The more the audience the more the absorption takes place.

3. By furniture and furnishing – Materials such as curtains, carpets absorb sound.

The following materials are commonly used for absorption of sound –

1. Acoustic Plaster – This contains shredded insulation material with cement
2. Perforated and Unperforated compressed cane or wood fibre board.
3. Wood Particle board
4. Compressed wood wool
5. Mineral/ glass wool mats
6. Mineral/ glass wool tiles
7. Composite units of perforated hard board backed with perforated fibreboard

Requirements of a good acoustic material-

1. Should have high absorption power
2. It should be able to absorb a wide range of frequencies
3. Should be cheap and easily available
4. Should look attractive after fixing it
5. It should be fire resistant
6. Should have adequate structural strength
7. Should be non- hygroscopic.
8. It should be insects and termites free

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SOUND INSULATION

Sound insulation/sound proofing is a method used to subdue the level of sound passing through the insulating building component. People generally confuse sound absorption and sound insulation to be the same. The two are very different from each other. Sound absorbents which are mostly pours materials absorb thus reduce the sound which is reflected from surfaces. On the contrary sound insulating construction reduces sound passing through it. Sound absorbers, are poor sound insulators. While hard material used for sound insulation areinferior sound absorbers.

TABLE 4.1. SOUND INSULATION BETWEEN ROOMS

Impact Insulation- is a rating of how well a building floor reduces the impact of sounds, such as footsteps.

Sound Insulating Materials –

1. Non porous rigid partitions- The sound insulation of non-porous rigid constructions such as plastered solid brick masonry walls varies. It depends upon the weight per unit area. There is a point when it requires major increase in thickness to provide small increase in sound insulation.

2. Porous Rigid Materials — Porous concrete masonry and cinder concrete are some examples of porous rigid materials. They provide 10 percent higher insulation as compared to non-porous rigid partitions due to their sound absorptive quality. To enhance the results of insulation it is recommended that porous partitions should be plastered on at least one side and if possible then on both sides.

3. Flexible Porous Material — These materials provide low insulation. Even lower than rigid materials. Flexible porous materials consists of mineral wool, quilt etc. To increase insulation rigid materials and porous absorbers can be combined together and then applied. This will produce better insulation per unit area.

4.1

WALL INSULATION

Walls are a vertical barrier of sound. Proper construction of walls can increase the level of sound insulation. Construction of walls for sound insulation can be of four types –

1. Rigid Homogeneous Walls- Stone, brick or concrete masonry constructions come under this section. The sound insulation in these walls depends upon their weight per unit area. Sound insulation in these increases if the thickness of wall increases. Due to this these walls become uneconomical and bulky after a certain limit.

2. Partition Walls of porous materials- These can be rigid or non-rigid. Rigid porous materials such as porous concrete masonry, cinder concrete etc. increase insulation about 10.%. While partition walls of non rigid porous materials provide very low sound insulation. However they can be used if combined with rigid materials.

3. Double wall partition – A double wall partition comprises of Plaster boards Or fibre boards or plaster on laths on both sides. With sound absorbing cushion in between. Rough wooden blocks are provided to support the cushion. Double wall partition is a wall of rigid as well as non–rigid porous materials.

4. Cavity Wall Construction- This is said to be the most adequate wall construction from the sound proofing point of view. In this two walls are made with a gap of minimum 5cm between them. This gab can be left air filled as we know sound travels the slowest in air or can be filled with some flexible material, like quilt etc. On the surface of the wall celotex or other insulating board may be fixed.

4.2

FLOOR AND CEILING INSULATION

Like walls are vertical sound barriers similarly floor and ceiling are horizontal sound barriers. The materials used for the construction of floors and ceiling i.e. R.C.C, stone etc. provide great insulation again air borne noise but do not function well for structure and impact borne noise. The target of sound proofed floors is to provide insulation against impact and structure borne noise.

This can be achieved by the following ways-

1. APPLING RESILIANT SURFACE MATERIALS ON FLOOR – In this a thin concrete layer is provided as R.C.C floor slab. On top of this then a soft floor finish or covering is applied. This finish or covering can be of linoleum, insulation board, cork, carpet, etc. this helps in reducing impact noises.

2. CONCRETE FLOOR FLOATING CONSTRUCTION – This is similar to Cavity wall construction as discussed earlier. In this method we construct an isolated floor from the existing concrete floor. Then a resilient or porous material like glass wool is laid on the R.C.C flooring. On top of this a water proof sheet is put and then a 5cm thick layer of concrete is given. This kind of construction provides complete insulation against impact sounds.

3. TIMBER FLOOR FLOATING CONSTRUCTION – In case of wooden flooring the problem of sound insulation is even more. This process is similar to concrete floor floating construction. The only difference is that mineral or glass wool quilts are used for isolation purpose. At times sand or ashes are also used for isolation.

CONCLUSION

Sound is mechanical wave which travels through a medium. Sound can be pleasing as well as disturbing. It’s necessary to emit disturbing sounds to have the perfect required atmosphere. To make an area sound proof its walls/partitions, floor and ceilings need to be treated and insulated. The methods of insulation as discussed above are some ways through which sound proofing can be done. Sound proofing or insulation is the method by which transmission of sound waves can be bard from passing from one area to another.

References:

1. Building Constructions by B.C Punamia.: Sound Insulation
2. http://www.meritnation.com/ask-answer/question/how-many-characteristics-a-sound-wave-have.
3. www.scribd.com/doc/102689279/acoustics-ppt