Lecture 2 Acoustics PDF

Acoustic at outdoor spaces Lecture-02 Topic - Acoustics  LEARNT TILL NOW Lecture 1: 1. What is Wave and its type (mechanical wave and non mechanical wave) 2. What is sound 3. What is acoustics 4. Objectives of the acoustic 5....

Acoustic at outdoor spaces Lecture-02 Topic - Acoustics  LEARNT TILL NOW Lecture 1: 1. What is Wave and its type (mechanical wave and non mechanical wave) 2. What is sound 3. What is acoustics 4. Objectives of the acoustic 5. Behavior of sound speed of sound in different medium Reflection Absorption Refraction Diffusion Diffraction Transmission Interference Wavelength Frequency Loudness Pitch of sound Sound is caused by vibrations which transmit through a medium and reach the ear or some other form of detecting device. Sound is measured in loudness (decibels (dB)) and frequency(Hertz (Hz)). Objective of acoustics Noise Sound Privacy reduction intelligibility Noise: Unpleasant, unwanted, disturbing sound is generally treated as Noise and is a highly subjective feeling.  Airborne sound (or airborne noise) is sound that is transmitted through the air. Typically, airborne NOISE sound might be generated by: Speech Television and radio Animal sounds such as dogs barking AIRBORNE STRUCTURE Transport NOISE BORNE NOISE Structure borne noise that is carried via the structure of a building’. Structure-borne sound results from an impact on, or a vibration against, a part of a building, resulting in sound being radiated from an adjacent vibrating surface. A typical example of structure-borne sound is footsteps on a floor which can be heard in a room below. Structure-borne sound comprises five processes: Generation – the source of an oscillation. Transmission – the transfer of oscillatory energy from the source to the structure Propagation – the distribution of energy throughout the structural system. Attenuation - when waves moving through structures encounter structural or material changes they can be partially reflected which reduces the energy transmitted, and so attenuates the sound. Radiation – the emission of sound from an exposed surface. Structure-borne sound and airborne sound are sometimes considered to be separate entities but they are closely related. Structural vibrations may radiate from surfaces, creating airborne sound, and airborne sound may cause an element of the building fabric to vibrate when it encounters a surface. Point to be noted The human ear can detect sounds between 20 HZ and 20,000 HZ or 0dB to 140 dB From very beginning of life (womb), humans have been used to hear different kind of sound. One can become mentally ill, if facing the Zero sound condition. Even ear plugs cant reduces the sound to zero dB Perception of noise is highly subjective The aim of acoustical design should be “to minimize the noise level ” Sound Isolation The control of intruding sound ideally begins with the initial building concept and continues to be a consideration through the life of the building. Total sound conditioning affects 1. site selection 2. building orientation on the site 3. room orientation within the building 4. design, detailing, specification 5. construction 6. inspection Predictable sound attenuation can be achieved by careful attention to detail during all phases of planning and construction. Site Selection for Sound Control Orientation There are other methods of reducing the amount of road noise reaching the listener: Planning can in some instances direct major highways away from residential areas, thereby attenuating the noise by distance, within a noise catchment. However the distances needed to provide reasonable attenuation are so great that this is generally not an option in urban areas where new highways are located within the existing urban areas. The noise can be reduced at source by quietening the engine, exhaust or anxillary brake noise or by reducing the level of tyre noise by utilising a quieter road surface. The sound can be attenuated at the receiver by using insulation (double glazing and the like) but outdoor areas will not benefit from building facade treatments. While all of these methods are undoubtedly important but not useful in all situations. If the noise source is intense and no natural sound barrier exists, a man-made sound barrier should be considered as part of the design. A solid fence-type barrier may remove from 10 to 20 db from the noise level High-frequency sounds will be reduce more than low frequency sounds. The cost of an outside barrier may be less than the cost of reducing the sound transmission in the construction. This type of sound barrier must completely shield the building from the noise source. It should be placed as close to the sound source as possible to obtain the greatest sound-shadow angle. If a fence or wall is used, no louvers or openings should be permitted.  The barrier must break the line-of-sight path between the noise source and receiver. Open areas such as maintenance access ports should be kept as small as possible. Barrier should be sufficiently strong to withstand wind loads. It should be sufficiently massive to control sound transmission through the barrier. Barriers must also block any reflected paths between the source and the receiver. The most effective location for barriers is either close to noise source or receiver Vegetation also acts as noise control buffer or barrier Tilted noise barriers can direct noise away from the receiver Tilted noise barriers can direct noise away from the receiver Noise barriers must be continuous to have a mitigating effect. This presents a challenge when providing access, that requires careful design resolution. Noise mounds (Berms) - Where space is available, noise mounds (which are, in effect, simply barriers) are generally a more attractive solution, either on their own or with a low wall type barrier on top of the mound. This is generally only an option in outer suburban or rural projects where wide corridor widths are possible. Berms are an attractive alternative to sound barrier walls. Berms provide additional sound absorption over the walls of equivalent height. The sound absorption is due to ground cover, usually grass, ivy, or some other foliage providing resistance to erosion. Berm provides an additional 3 dB to the noise reduction afforded by an equivalent height wall. Berms require greater footprints, & hence are unpractical alternative than walls Acoustical Zoning building Acoustic site planning involves careful arrangements of buildings on site so as to minimize the effect of traffic noise. Best used for sites such as cluster subdivision. Place as much distance as possible between the noise source such as road and noise sensitive areas. Place noise compatible areas such as parking lots, open spaces and garages between the noise source and noise sensitive areas Use the house and other buildings as barriers to protect or shield outdoor living areas. It involves aspect of building height, room arrangement ,placement of windows and courtyard design. Bedrooms and living rooms , for example , are best placed in that part of the building which is furthest from noise source. Bathrooms , laundries and toilets are areas where activities are more tolerant of noise, and these areas can act as a buffer for ,more sensitive areas. Noise entering a house can be reduced by eliminating or reducing the size of windows in the walls which face the roadway. Single storey house designs generally have greater potential for protection from traffic noise than do to two storey dwellings. Two storey houses generally require a higher level of acoustic construction in upper storey because of reduced shielding from noise barriers and adjacent buildings.  Masking: Creating Background "Noise“ When an undesirable background sound can't be reduced or eliminated, it can sometimes be masked (made less objectionable by introducing a different sound). For example, music in restaurants can mask the din of dish clatter and multiple conversation. At the other extreme, a masking sound may be introduced to correct an oppressively quiet room. For example, a telephone ring or a slight cough can be distracting in a very "dead" room, and speech privacy would be impossible. In many cases the heating and air conditioning systems will provide a sufficient amount of masking noise.