Saturday, April 9, 2011

ACOUSTICS: THE STUDY OF SOUND

AR-461: BUILDING SCIENCE
By:
RAVINDAR KUMAR
Assistant Professor
Department of Architecture and Planning
NED University of Engineering and Technology
Karachi
LECTURE NO. 14
TOPIC:                                     ACOUSTICS: THE STUDY OF SOUND

INTRODUCTION:
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical or audio engineer. The application of acoustics can be seen in almost all aspects of modern society with the most obvious being the audio and noise control industries. 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. So it is no surprise that the science of acoustics spreads across so many facets of our society—music, medicine, architecture, industrial production, warfare and more. Art, craft, science and technology have provoked one another to advance the whole, as in many other fields of knowledge. Lindsay's 'Wheel of Acoustics' is a well accepted overview of the various fields in acoustics. The word "acoustic" is derived from the Greek word ‘akoustikos’, meaning "of or for hearing, ready to hear" and that from ‘akoustos’, "heard, audible", which in turn derives from the verb ‘akouo’, "I hear". The Latin synonym is "sonic", after which the term sonics used to be a synonym for acoustics and later a branch of acoustics.[6] After acousticians had extended their studies to frequencies above and below the audible range, it became conventional to identify these frequency ranges as "ultrasonic" and "infrasonic" respectively, while letting the word "acoustic" refer to the entire frequency range without limit.[1]

ARCHITECTURAL ACOUSTICS:[2]
Architectural acoustics is the science of noise control within buildings. The first application of architectural acoustics was in the design of opera houses and then concert halls. More widely, noise suppression is critical in the design of multi-unit dwellings and business premises that generate significant noise, including music venues like bars. The more mundane design of workplaces has implications for noise health effects. Architectural acoustics includes room acoustics, the design of recording and broadcast studios, home theaters, and listening rooms for media playback.

BUILDING SKIN ENVELOPE:[3]
This science analyzes noise transmission from building exterior envelope to interior and vice versa. The main noise paths are roofs, eaves, walls, windows, door and penetrations. Sufficient control ensures space functionality and is often required based on building use and local municipal codes. An example would be providing a suitable design for a home which is to be constructed close to a high volume roadway, or under the flight path of a major airport, or of the airport itself.
INTER-SPACE NOISE CONTROL:[4]
It is the science of limiting and controlling noise transmission from one building space to another to ensure space functionality and speech privacy. The typical sound paths are room partitions, acoustic ceiling panels (such as wood dropped ceiling panels), doors, windows, flanking ducting and other penetrations. An example would be providing suitable party wall design in an apartment complex to minimize the mutual disturbance due to noise by residents in adjacent apartments.
INTERIOR SPACE ACOUSTICS:[5]
This is the science of controlling a room's surfaces based on sound absorbing and reflecting properties. Excessive reverberation time, which can be calculated, can lead to poor speech intelligibility. Sound reflections create standing waves that produce natural resonances that can be heard as a pleasant sensation or an annoying one.

Reflective surfaces can be angled and coordinated to provide good coverage of sound for a listener in a concert hall or music recital space. To illustrate this concept consider the difference between a modern large office meeting room or Lecture Theater and a traditional classroom with all hard surfaces. Interior building surfaces can be constructed of many different materials and finishes. Ideal acoustical panels are those without a face or finish material that interferes with the acoustical infill or substrate.

Fabric covered panels are one way to heighten acoustical absorption. Finish material is used to cover over the acoustical substrate. Mineral fiber board, or Micore, is a commonly used acoustical substrate. Finish materials often consist of fabric, wood or acoustical tile. Fabric can be wrapped around substrates to create what is referred to as a "pre-fabricated panel" and often provides the good noise absorption if laid onto a wall. Prefabricated panels are limited to the size of the substrate ranging from 2 by 4 feet (0.61 × 1.2 m) to 4 by 10 feet (1.2 × 3.0 m).

Fabric retained in a wall-mounted perimeter track system, is referred to as "on-site acoustical wall panels" This is constructed by framing the perimeter track into shape, infilling the acoustical substrate and then stretching and tucking the fabric into the perimeter frame system. On-site wall panels can be constructed to accommodate door frames, baseboard, or any other intrusion. Large panels (generally, greater than 50 square feet or 4.6 square meters) can be created on walls and ceilings with this method. Wood finishes can consist of punched or routed slots and provide a natural look to the interior space, although acoustical absorption may not be great.

There are three ways to improve workplace acoustics and solve workplace sound problems – the ABCs.

A = Absorb {via drapes, carpets, ceiling tiles, etc.)
B = Block (via panels, walls, floors, ceilings and layout)
C = Cover-up (via sound masking)

While all three of these are recommended to achieve optimal results, C = Cover-up by increasing background sound produces the most dramatic improvement in speech privacy – with the least disruption and typically the lowest cost.

MECHANICAL EQUIPMENT NOISE:[6]
Building services noise control is the science of controlling noise produced by:

  • ACMV (air conditioning and mechanical ventilation) systems in buildings, termed HVAC in North America
  • Elevators
  • Electrical generators positioned within or attached to a building
  • Any other building service infrastructure component that emits sound

Inadequate control may lead to elevated sound levels within the space which can be annoying and reduce speech intelligibility. Typical improvements are vibration isolation of mechanical equipment, and sound traps in ductwork. Sound masking can also be created by adjusting HVAC noise to a predetermined level.

THE NATURE OF SOUND:
"Consider the air close to the surface of some vibrating object. As the surface moves outwards the air molecules next to the surface are pushed closer together (the air is compressed). The air cannot move back into its original position for the moment as the space is occupied by the advanced surface of the vibrating object and therefore a movement of air occurs away from the object."[7] A vibrating object will produce a sequence of compressions and rarefactions in the air surrounding it. These small fluctuations in air pressure travel away from the source at relatively high speed, gradually dying off as their energy is absorbed by the medium. What we call sound is simply the sensation produced by the ear when stimulated by these vibrations.

If you were to graph the pressure maxima and minima at a given instant, what you get is a sound wave. It should be noted that air cannot sustain any form of shear stress so sound can only be transmitted as a longitudinal wave. Thus the graph showing a sine wave refers only to variations in pressure or compression, not to the actual displacement of air.
     
PROPERTIES OF SOUND
The wave motion of sound can be described in terms of Amplitude, Frequency, Velocity and Wavelength

Amplitude
Refers to the difference between maximum and minimum pressure.
WavelengthRefers to the physical distance between successive compressions and is thus dependant on the speed of sound in the medium divided by its frequency:
V = l * f

[Velocity = Wavelength * Frequency]

Frequency
Refers to the number of peak-to-peak fluctuations in pressure that pass a particular point in space in one second.
Velocity
Refers to the speed of travel of the sound wave. This varies between mediums and is also dependant on temperature. Assuming air acts as an ideal gas, its velocity (V in m/s) relates to temperature (T in °C) as follows: 
V = 331.5 + (0.6 T)

(m/s)

In other materials, the speed of sound can vary quite substantially. The following table shows the speed of sound in a number of different materials.
Material
Speed of Sound (m/s)
Air
343
Steel
6100
Timber
5260
Brick
3650
Speed of sound through materials at 20 °C.


REFERENCES AND ACKNOWLEDGEMENTS:
  1. This lecture is not possible without the support and data taken from: http://www.kemt.fei.tuke.sk/Predmety/KEMT320_EA/_web/Online_Course_on_Acoustics/index_acoustics.html (Retrieved April 9, 2011) Copyright © Andrew Marsh, UWA, 1999. The School of Architecture and Fine Arts The University of Western Australia
  2. Glen Ballou & Howards Sams, editors. "Handbook for Sound Engineers", page 56.

[1] Acoustics; From: http://en.wikipedia.org/wiki/Acoustics (Retrieved April 08, 2011)
[2] Architectural acoustics; From: http://en.wikipedia.org/wiki/Architectural_acoustics (Retrieved April 08, 2011)
[3] Ibid
[4] Ibid
[5] Ibid
[6] Ibid
[7] Parkin, P. H., and Humphreys, H.R., "Acoustics, Noise, and Buildings", 3rd edition (Book Review) Town Planning Review Publisher Liverpool University Press ISSN: 0041-0020 (Print) 1478-341X (Online) Issue Volume 40, Number 1 / 1969:Apr.

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