Introduction
ACOUSTICS ( uh-koo-stiks)
Acoustics is the branch of physics concerned with the study of sound (mechanical waves in gases, liquids, and solids) including topics such as vibration, sound, ultrasound and infrasound.
A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering.
Hearing is one of the most crucial means of survival in the animal world, and speech is one of the most distinctive characteristics of human development and culture. Accordingly, the science of acoustics spreads across many facets of human society—music, medicine, architecture, interior, industrial production, warfare and more.
Building acoustics is the science of controlling noise in buildings.
This includes the minimisation of noise transmission from one space to another and the control of the characteristics of sound within spaces themselves.
Building acoustics are an important consideration in the design, operation and construction of most buildings, and can have a significant impact on health and wellbeing, communication and productivity. They can be particularly significant in spaces such as concert halls, recording studios, lecture theatres, and so on, where the quality of sound and its intelligibility are very important.
Building acoustics can be influenced by:
• The geometry and volume of a space.
• The sound absorption, transmission and reflection characteristics of surfaces enclosing the space and within the space.
• The sound absorption, transmission and reflection characteristics of materials separating spaces.
• The generation of sound inside or outside the space.
• Airborne sound transmission.
• Impact noise.
Characteristics of sound
Sound intensity is measured in Decibels (dB). This is a logarithmic scale in which an increase of 10 dB gives an apparent doubling of loudness.
Sound pitch is measured in Hertz (Hz), the standard unit for the measurement for frequency. The audible range of sound for humans is typically from 20 Hz to 20,000 Hz, although, through ageing and exposure to loud sounds the upper limit will generally decrease.
As well as intensity and frequency, sound also transmits information. For example, music or speech, transmit information which people may perceive differently from other sounds.
Reverberation ( is continuance of
sound after the sound has stopped) time
The ‘reverberation time’ of a space changes the way the space ‘sounds’ and can affect the intelligibility acoustic information. A high reverberation time can make a room sound muffled, loud and noisy. Rooms designed for speech typically have a low reverberation time, whereas a higher reverberation time can add depth, richness and warmth to music.
The reverberation time of a room is defined as the time it takes for sound to decay by 60 dB after an abrupt termination. It is linked to the total quantity of soft treatments and the volume of the room.
Sound absorption
Sound absorption is the loss of sound energy when sound waves come into contact with an absorbent material such as ceilings, walls, floors and other objects, as a result of which, the sound is not reflected back into the space. Acoustic absorption can be used to reduce reverberation times.
Absorbent materials are sometimes categorised from A to E, where A is highly absorbent and E is almost fully reflective.
Sound absorbers can be divided into three main categories:
• Porous absorbents, such as fibrous materials or open-celled foam.
• Resonance absorbents, which consist of a mechanical or acoustic oscillation system, such as membrane absorbers.
• Single absorbers such as tables, chairs or other objects.
Sound insulation
Sound transmission paths can be interrupted by sound insulation and by blocking air paths. The sound insulation of a single leaf of a material is governed by its mass, stiffening and damping.
The sound insulation across a good conventional, lightweight, office to office construction is typically in the order of 45 dB. This means that if the sound level in the source room is around 65 dB (a typical level for speech), the sound level in the adjacent room, the receiver room, will be approximately 20 dB (barely audible).
If sound levels are increased in the source room however, to 75 dB (raised voice), sound levels within the adjacent room will also increase to around 30 dB (audible). Sound insulation therefore describes the level of sound lost across a partition and not the level of sound within an adjacent room.
[Dw represents the sound insulation between rooms on-site. Rw represents the lab tested sound insulation of an element making up a partition wall/floor type. Standards achieved in labs may not be possible on site because of the quality of workmanship and due to sound ‘flanking’ acoustic elements, that is, travelling around them through an easier path, rather than only directly through them as under lab conditions.]
The building regulations set minimum standards for design and construction in relation to the resistance to the passage of sound.
Noise nuisance
Building acoustics can help to mitigate the effects of noise disturbance which can have negative effects on health, wellbeing and general quality of life.
The Noise Policy Statement for England (NPSE) defines noise pollution as:
• Environmental noise – which includes noise from transportation sources.
• Neighbour noise – which includes noise from inside and outside buildings.
• Neighbourhood noise – which includes noise arising from industrial and entertainment premises, trade and businesses, construction sites and noise in the street.This can be an important consideration for the location, design and construction of new developments.
(CHECK NOISE POLLUTION LAWS OF DELHI)
Designing The Acoustics Of A Room
1. Sound has a strong influence on how you experience a space.
2. A room, be it commercial or domestic, without any acoustic treatment such as acoustic panels in the walls or softer flooring, can be very noisy.
3. Sound waves will reverberate around the room, and without anything soft to absorb them, noise becomes amplified and speech can become difficult to understand.
4. This is particularly distressing in open plan spaces which are typically multifunctional and thus need to be appropriate for socialising, relaxing and everyday ‘living’.
When designing a room there are a few things which should be considered in terms of its acoustics: (1)the size of the finished space, (2)what its functionality is to be, (3)and how it will be laid out. It is both a visual and practical science – and while soft furnishings play a vital role it is not just about cushions and curtains. Here are a few pointers to consider when it comes to controlling sound in an open plan space:
- Consider the overall layout of the room(s), and if there are ‘zones’ for different activities, try to separate noisy activities from quieter ones.
• Avoid giving the whole space an overly high ceiling, especially if the room or part of the room is to be used for dining. A high ceiling can make background noise difficult to control.
• Try to minimise background noise from machinery and air-conditioning with quieter models of appliances.
• Look at the layout of both the room and the furniture. Whilst the preferred focus may be on the TV or feature fireplace, a scheme where the seating is arranged so guests are facing each other may create a more social environment, and is a particularly practical solution for those with hearing issues.
• Create ‘flow’ throughout the room. Spaces with clear sight lines from one area to another assist with communication.
• Many modern extensions are large, open plan, modern spaces and feature hard surfaces such as, glass, concrete and wood which allow noise to echo and reverberate.
As well as considering the above there are some other simple solutions that you can introduce to minimise sound reverberation:
• Carpets, fabric wall hangings, cushions, rugs, curtains and upholstered furniture all help to absorb noise.
• Shaped surfaces, such as curves, can also cleverly diffuse sound.
• On walls, textured acoustic panels made of wool, felt or polyester serve a dual purpose as they can be made to look like pieces of artwork. There are some stunning wallpapers and cork wall finishes being produced which also act as sound absorbers. Many other materials are available to designers where there is a need to minimise ambient noise(is the background sound pressure level at a given location)without compromising on modern decor.
• When it comes to flooring, by far the easiest and simplest solution is to lay down a carpet with a really good underlay. If you’re a timber flooring fan, under-floor products that help acoustics are available and will make a huge difference.
• Cork is also a very popular choice. With over 40 million natural “cushion cells” per cubic centimeter it is a natural sound and thermal insulator and can be used on walls and floors. It is remarkably durable, beautifully quiet and comfortable underfoot. As a natural product, it also warms and enriches an interior.
Building materials have different capacities of absorbing sounds.
Normally sound absorbing materials can be put into one of the following categories:
Porous materials: Porous absorbers are the most commonly used sound absorbing materials. Commonly used porous materials are Wood wools, soft plaster, asbestos fibre, glass wool, Fibre boards carpet ect. Generally, all of these materials allow air to flow into a cellular structure where sound energy is converted to heat. Thickness plays an important role in sound absorption by porous materials. Thin film of Fabric applied directly to a hard, massive substrate, such as plaster or gypsum board does not make an efficient sound absorber. Thicker materials generally provide more bass sound absorption or damping.
Panel absorber: Panel absorbers are non-rigid, non-porous materials which are placed over an airspace that vibrates in a flexural mode in response to sound pressure exerted by adjacent air molecules. When the sound wave strikes the pannel due to vibration of the panel the energy is absorbed and finally converted into heat. Common panel (membrane) absorbers include thin wood paneling over framing, lightweight impervious ceilings and floors, glazing and other large surfaces capable of resonating in response to sound. Panel absorbers are usually most efficient at absorbing low frequencies.
Resonators: Resonators typically act to absorb sound in a narrow frequency range. Resonators include some perforated materials and materials that have openings (holes and slots). When sound waves enters into the resonator, due to multiple reflection insde the resonators waves are absorbed. The classic example of a resonator is the Helmholtz resonator, which has the shape of a bottle. The size of the opening, the length of the neck and the volume of air trapped in the chamber govern the resonant frequency . Typically, perforated materials only absorb the mid-frequency range unless special care is taken in designing the facing to be as acoustically transparent as possible. Slots usually have a similar acoustic response. Long narrow slots can be used to absorb low frequencies. The resonators are suitable for certain frequencies for which they are designed. Therefore resonators have to be designed for specific purpose for example to absorb noise from air conditioner or from pumps etc.
Types of Soundproofing Materials
These are the most used soundproofing materials; each category has different best use scenarios. Each of these acoustic materials falls into one of these categories: Sound Absorbing, Sound Insulation, Sound Dampening, and Decoupling.
Acoustic Foam – This material, commonly called Studio Foam, has a distinctive wedge or pyramid shape that is highly effective at absorbing sound. They attach to walls as panels, hang from ceilings as baffles, or sit in corners as bass traps.
Sound Insulators – Sound insulators are batts made of mineral wool, rock wool, and fiberglass, designed to fit in between the studs of walls. The batts fit snugly between studs to take up airspace that can transmit sound.
Acoustic Panels/Boards – These are decorative versions of sound insulation and sound absorbing foam. They can come in many appealing colors, patterns, and fabrics to serve a dual purpose in the home and workplace.
Acoustic Fabrics – Acoustical fabrics are thicker and heavier than other fabrics and used in theater curtains, blackout curtains, and studio blankets.
Acoustic Coatings – Materials like Mass Loaded Vinyl (MLV) is a dense rubber like material, used in many different situations such as car soundproofing, machinery, appliances, and as an underlayment. The mass of the material acts as a sound barrier.
Floor Underlayment – Soundproofing a hardwood or tile floor requires the decoupling of the flooring surface and the subfloor to reduce the noise transmission. Cork, felt, and polymers are commonly used as underlayment materials.
Architectural Soundproofing– This group includes anything used in the structure of a building, such as soundproof windows, soundproof walls, doors, and decoupling products used to install them.
Guidelines For Good Acoustical Design
According to classic acoustics theory there are five requirements which, when met, result in good acoustics:
Appropriate reverberation time depends on the size of the room. W. Furrer’s recommendations can be used in rooms which are between 200 and 20,000 cubic metres. Unless it concerns a concert hall for classical music, the reverberation time must in so as far as possible be the same throughout the entire frequency range.
Uniform sound distribution is important in large rooms and halls, where the sound must be able to be heard equally well everywhere. It is important to take sound distribution into account in the architecture. A variation of max. ±5 dB anywhere in the room is an appropriate requirement.
Appropriate sound level for normal conversation is 60-65 dB, and in a busy street 70-85 dB. In large gatherings, a public address system in a dampened room can be used to ensure an appropriate sound level.
Appropriate, low background noise is one of the most important acoustic criteria – especially in concert halls and theatres. In a room, the background noise may come from technical installations or ventilation systems.
No echo or flutter echoes must occur for the acoustics to be good. It is easy to prevent echo by installing a little sound-absorbing material on the wall.
Submissions required:
Research ppt on Interior Acoustics with acoustic materials available in the market ( Company name, cost etc), Acoustics materials used in auditoriums, residential areas near highways, aerodromes , classrooms etc.
