Acoustical properties of building materials

Sound: Any thing that can be heard. It is the sensation caused by a vibrating medium acting on the air. Source of sound is most often vibrating solid body . The medium conveying sound to hear can be gas, liquid or solid . It is transmitted as the longitudinal wave motion, I.e. successive compression of molecules. In solid body, the transmission is by lateral wave motion. Wave length determines pitch of sound . Higher the frequency higher would be the pitch (frequency is the waves per unit time. Loudness depends on distance from / to vibrating body. Unit of loudness is dB.

• Range of hearing loudness

– Rustle of leaves, whisper 10dB. 

– Airplane, pneumatic drill 130dB.

• Ranges of hearing frequency 

– Low 20 Hz/s.

– High 20000 Hz/s.

This range changes with age and other subjective factors.

Reaction produced by sound 

1. Reflection- from walls, floors, ceiling etc.

2. Absorption- by walls, by floors, by ceiling, by furniture etc. 

3. Transmission- to adjacent rooms.

Sound classification

1. Air borne sound- sound through air to air

2. Impact or structure borne sound- sound through direct contact; such as footsteps, hammering or vibration etc. it is very sharp and troublesome.

Acoustics

Acoustic is the science of sound. It assures the optimum conditions for producing and listening to speech and music. The panning of acoustical design has to provide for dissipation of noise and insulation against sound.

Noise and its effect: Noise is the undesired or unwanted sound. It has following effects;

1.      Annoyance- irritation 

2.      Disturbance of sleep

3. Interface or disturbing conversation 

4. Damage of ear

Measurement of annoyance is subjective attitude and depends on mental and physical well being of listeners with their experience.

Magnitude of Noise level

Types of sound Noise level (dB) Effects

Light road traffics         60-70 Physiological effect (annoyance)

Medium road traffics 70-80 Physiological effect (annoyance)

Heavy road traffics         80-90 Prolonged exposure cause permanent hearing loss

Rail traffics         90-100 Prolonged exposure cause damage to auditory organ

Air traffics         100-130 Causes pain

                >130 Instantaneous loss of hearing

Defects due to reflected sound

1. Echoes is the reflected sound and heard just after the produced sound as a repetition.

2. Reverberation is continuous reflection of produced sound waves 9reflection, inter-reflection etc.) until they are gradually faded out. Certain amount of reverberation is necessary to enhance the sound, but excessive is damaging to clarity. Reverberation time: It is the time taken for sound to decay by below annoyance level (60dB) after the sound source has stopped. It depends on; volume of room, absorption in walls, roofs, and floors etc. it has to be minimized using sound absorbing materials.

Sound insulation

1.Sound absorption (prevention of reflection) 

2.Sound insulation (prevention of transmission)

Sound absorbents 

1. Porous materials

2. Resonant panels 

3. Cavity resonators 

4. Composite types

• In porous materials, the sound waves on striking its surface enter to the pores, vibrate inside and die-out there . Normally, these materials are soft and have large pores with interconnected channels. Porous materials may also be put in the gap between boards . It is suitable for low frequency waves.
• Resonant panels are semi-hard in the form of porous fiber boards than acts as sound absorbent. These boards are fixed on timber frame with air gap between and also with wall backing . In the resonant panels, the sound pressure waves cause vibration and this vibration is absorbed by air gap (space) called damping.
• Cavity resonators are the chambers with the narrow openings. The absorption of sound absorption of sound takes place in this case by the resonance of air.
• Composite types are the perforated panels fixed with air space containing porous absorbents. The panels may be of metal, plywood, hard board, plaster boards etc. The perforation should be at least 10 percent of area high frequency sounds are absorbed in this perforated panel.

Noise control and constructional precautions to reduce noise

General consideration

1.Isolate sound source

2.Proper orientation of building, I.e. no opening towards noise 

3.Properly planed rooms in building

4.Furnishing materials in room helps sound absorption 

5.Partitions- rigid and movable

6.Control of impact sound, I.e. use of resilient materials as carpets in floor

7.Discontinuing the path of vibration by using sound absorbing materials

8.Use of headphones and air plugs in case of high sound

Constructional measures

1.Walls/partitions: Absorbents in the walls/partitions act as the barriers to air borne sound transmission.

Types:

•Rigid and homogeneous partitions: Insulation in this case depends on the weight of thee partition per unit area and increases with thickness

• Partition of porous material Insulation increases to 10% or higher. Material may be rigid of flexible.

• Hollow and composite partition Cavity is better: Filling of cavity with resilient material is preferred.

Following table illustrates the insulation properties for different walls;

S.No. Types of wall     Approx. wt. of wall kg/m2     Average sound reduction, dB

1 One brick wall, plastered in both side         490             50

2 One and half brick wall plastered in both side 710             53

3 Cavity (50mm) with half brick in both leaves 490             50-53

4 Half brick or concrete with plaster both side     170             45

5 200mm concrete wall                 185             45

6 Gypsum board partition on timber frame 70             45

7 75mm hallow clay block with plaster both side 110             36

Hard reflecting surface outside partition increases insulation

 Constructional measures

1. Floors/Ceilings

These are the horizontal barrier to noise. They act as barrier to air borne and impact sounds, but offer poor insulation for insulation for structure borne or impact sounds. Insulation in floor; resilient surface materials, floating floors. 

• Resilient surface materials on floors: Cotton/wooden carpets, asphalt/mastics, pvc carpets, corks etc. Softer the material used greater would be the insulation value.

• Floating floor construction: Provides insulation from any other parts of structure. It is made to rest or float over existing floor by means of resilient materials such as glass wool, quilt, hair felt, cork rubber etc. Impact sound do not transmits. It can be adopted in all floors. On concrete floor, partition is constructed off the structural floor and it is independent

Types of floating floors:

• Concrete floor with floating concrete screed: It is the PCC of 1:1.5:3 on resilient materials above concrete floor.

• Concrete floor with floating wooden raft: It is wooden boarding nailed to battens forming raft on resilient quilt (20mm).

• Heavy concrete floor with soft floor (resilient) finish or covering. 

Wooden floors: It has the problem of impact sound.

2. Windows and doors: 

It should be,

•Air tight

•Double glazed

•Thickness of glass to be increased 

•Increased weight of shutter

3. Insulating sanitary fitting

•WC be insulated, pan to rest upon thin pad of felt, PVC, corks, rubber etc.

•Cisterns not on wall of bed rooms, brackets be fixed with insulating materials (clips)

4. Machine mounting and insulation of machinery

Machines resting on the resilient materials as steel spring, rubber, corks etc.