Acoustics Building Services

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Acousticsis the interdisciplinary science that deals with the study of all mechanical waves in galiquids, and solids including vibration, sound, ultrasoundand infrasound. A scientist who works ifield of acoustics is an acoustician while someone working in the field of acoustics technology mbe called anacoustical engineer. The application of acoustics can be seen in almost all aspects

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 t

most distinctive characteristics of human development and culture. So it is no surprise that thescience of acoustics spreads across so many facets of our societymusic, medicine, architectuindustrial production, warfare and more. Art, craft, science and technology have provoked oneanother to advance the whole, as in many other fields of knowledge. Lindsay's 'Wheel of Acous

is a well accepted overview of the various fields in acoustics.[1]

The word "acoustic" is derived from the Greek word (akoustikos), meaning "of or for

hearing, ready to hear"[2]

and that from (akoustos), "heard, audible",[3]

which in turn

derives from the verb (akouo), "I hear".[4]

The Latin synonym is "sonic", after which the term sonics used to be a synonym for acoustics[5

later a branch of acoustics.[6]

Frequencies above and below the audible range are called "ultras

and "infrasonic", respectively.

Contents

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1 History of acousticso 1.1 Early research in acousticso 1.2 Age of Enlightenment and onward
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2 Fundamental concepts of acousticso 2.1 Wave propagation: pressure levelso 2.2 Wave propagation: frequencyo 2.3 Transduction in acoustics3 Divisions of acoustics

4 See also5 References6 Further reading7 External links

[edit]History of acoustics

[edit]Early research in acoustics

The fundamental and the first 6 overtones of a vibrating string. The earliest records of the study

this phenomenon are attributed to the philosopher Pythagoras in the 6th century BC.

In the 6th century BC, the ancient Greek philosopher Pythagoras wanted to know why some mu

intervals seemed more beautiful than others, and he found answers in terms of numerical ratiosrepresenting the harmonicovertone series on a string. He is reputed to have observed that whe
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the lengths of vibrating strings are expressible as ratios of integers (e.g. 2 to 3, 3 to 4), the toneproduced will be harmonious. If, for example, a string sounds the note C when plucked, a stringtwice as long will sound the same note an octave lower. The tones in between are then given b16:9 for D, 8:5 for E, 3:2 for F, 4:3 for G, 6:5 for A, and 16:15 for B, in ascendingorder.[7]Aristotle (384-322 BC) understood that sound consisted of contractions and expansion

the air "falling upon and striking the air which is next to it...", a very good expression of the natuof wave motion. In about 20 BC, the Roman architect and engineer Vitruvius wrote a treatise onacoustic properties of theatres including discussion of interference, echoes, and reverberationbeginnings of architectural acoustics.

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Principles of acoustics were applied since ancient times : Roman theatre in the city of Amman.

The physical understanding of acoustical processes advanced rapidly during and after the ScieRevolution. Mainly Galileo Galilei (15641642) but also Marin Mersenne (15881648),

independently, discovered the complete laws of vibrating strings (completing what Pythagoras aPythagoreans had started 2000 years earlier). Galileo wrote "Waves are produced bythe vibrations of a sonorous body, which spread through the air, bringing to the tympanum ofthe ear a stimulus which the mind interprets as sound", a remarkable statement that points to th
http://en.wikipedia.org/wiki/Acoustics#cite_note-6http://en.wikipedia.org/wiki/Aristotlehttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Vitruviushttp://en.wikipedia.org/wiki/Architectural_acousticshttp://en.wikipedia.org/wiki/Acoustics#cite_note-7http://en.wikipedia.org/wiki/Roman_theatre_(structure)http://en.wikipedia.org/wiki/Ammanhttp://en.wikipedia.org/wiki/Scientific_Revolutionhttp://en.wikipedia.org/wiki/Scientific_Revolutionhttp://en.wikipedia.org/wiki/Galileo_Galileihttp://en.wikipedia.org/wiki/Marin_Mersennehttp://en.wikipedia.org/wiki/Vibrationhttp://en.wikipedia.org/wiki/Earhttp://en.wikipedia.org/wiki/File:Amman_Roman_theatre.jpghttp://en.wikipedia.org/wiki/File:Amman_Roman_theatre.jpghttp://en.wikipedia.org/wiki/File:Amman_Roman_theatre.jpghttp://en.wikipedia.org/wiki/File:Amman_Roman_theatre.jpghttp://en.wikipedia.org/wiki/Earhttp://en.wikipedia.org/wiki/Vibrationhttp://en.wikipedia.org/wiki/Marin_Mersennehttp://en.wikipedia.org/wiki/Galileo_Galileihttp://en.wikipedia.org/wiki/Scientific_Revolutionhttp://en.wikipedia.org/wiki/Scientific_Revolutionhttp://en.wikipedia.org/wiki/Ammanhttp://en.wikipedia.org/wiki/Roman_theatre_(structure)http://en.wikipedia.org/wiki/Acoustics#cite_note-7http://en.wikipedia.org/wiki/Architectural_acousticshttp://en.wikipedia.org/wiki/Vitruviushttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Aristotlehttp://en.wikipedia.org/wiki/Acoustics#cite_note-6


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beginnings of physiological and psychological acoustics. Experimental measurements of the sp

of sound in air were carried out successfully between 1630 and 1680 by a number of investigatprominently Mersenne. Meanwhile Newton (16421727) derived the relationship for wave velocsolids, a cornerstone of physical acoustics (Principia, 1687).

[edit]Age of Enlightenment and onward

The eighteenth century saw major advances in acoustics as mathematicians applied the newtechniques of calculus to elaborate theories of sound wave propagation. In the nineteenth centuthe major figures of mathematical acoustics were Helmholtz in Germany, who consolidated the of physiological acoustics, and Lord Rayleigh in England, who combined the previous knowledg

with his own copious contributions to the field in his monumental work "The Theory of Sound". Ain the 19th century, Wheatstone, Ohm, and Henry developed the analogy between electricity anacoustics.

The twentieth century saw a burgeoning of technological applications of the large body of scienknowledge that was by then in place. The first such application was Sabines groundbreaking win architectural acoustics, and many others followed. Underwater acoustics was used for detectsubmarines in the first World War. Sound recording and the telephone played important roles in

global transformation of society. Sound measurement and analysis reached new levels of accuand sophistication through the use of electronics and computing. The ultrasonic frequency rangenabled wholly new kinds of application in medicine and industry. New kinds of transducers

(generators and receivers of acoustic energy) were invented and put to use.[edit]Fundamental concepts of acoustics

Jay Pritzker Pavilion
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At Jay Pritzker Pavilion, a LARES system is combined with a zoned sound reinforcement systeboth suspended on an overhead steel trellis, to synthesize an indoor acoustic environmentoutdoors.

The study of acoustics revolves around the generation, propagation and reception of mechanicwaves and vibrations.

The steps shown in the above diagram can be found in any acoustical event or process. Thermany kinds of cause, both natural and volitional. There are many kinds of transduction procesthat convert energy from some other form into sonic energy, producing a sound wave. There one fundamental equation that describes sound wave propagation, but the phenomena thatemerge from it are varied and often complex. The wave carries energy throughout the propag

medium. Eventually this energy is transduced again into other forms, in ways that again may natural and/or volitionally contrived. The final effect may be purely physical or it may reach farthe biological or volitional domains. The five basic steps are found equally well whether we ar
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talking about an earthquake, a submarine using sonar to locate its foe, or a band playing in a concert.

The central stage in the acoustical process is wave propagation. This falls within the domain ophysical acoustics. In fluids, sound propagates primarily as a pressure wave. In solids, mechawaves can take many forms including longitudinal waves, transverse waves and surface wave

Acoustics looks first at the pressure levels and frequencies in the sound wave. Transductionprocesses are also of special importance.

[edit]Wave propagation: pressure levels

Main article:Sound pressure

Spectrogram of a young girl saying "oh, no"

In fluids such as air and water, sound waves propagate as disturbances in the ambient pressulevel. While this disturbance is usually small, it is still noticeable to the human ear. The smallesound that a person can hear, known as the threshold of hearing, is nine orders of magnitudesmaller than the ambient pressure. The loudness of these disturbances is called the soundpressure level (SPL), and is measured on a logarithmic scale in decibels.

[edit]Wave propagation: frequency

Physicists and acoustic engineers tend to discuss sound pressure levels in terms of frequencpartly because this is how our ears interpret sound. What we experience as "higher pitched" o
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"lower pitched" sounds are pressure vibrations having a higher or lower number of cycles persecond. In a common technique of acoustic measurement, acoustic signals are sampled in timand then presented in more meaningful forms such as octave bands or time frequency plots. these popular methods are used to analyze sound and better understand the acousticphenomenon.

The entire spectrum can be divided into three sections: audio, ultrasonic, and infrasonic. Theaudio range falls between 20 Hz and 20,000 Hz. This range is important because its frequenccan be detected by the human ear. This range has a number of applications, including speechcommunication and music. The ultrasonic range refers to the very high frequencies: 20,000 Hand higher. This range has shorter wavelengths which allows better resolution in imagingtechnologies. Medical applications such as ultrasonography and elastography rely on theultrasonic frequency range. On the other end of the spectrum, the lowest frequencies are knowas the infrasonic range. These frequencies can be used to study geological phenomena suchearthquakes.

Analytic instruments such as the Spectrum analyzer facilitate visualization and measurement acoustic signals and their properties. The Spectrogram produced by such an instrument is agraphical display of the time varying pressure level and frequency profiles which give a specifacoustic signal its defining character.

[edit]Transduction in acoustics
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An inexpensive low fidelity 3.5 inchdriver, typically found in small radios

A transducer is a device for converting one form of energy into another. In an acoustical contethis usually means converting sound energy into electrical energy (or vice versa). For nearly aacoustic applications, some type of acoustic transducer is necessary. Acoustic transducersinclude loudspeakers,microphones, hydrophones and sonar projectors. These devices conve

electric signal to or from a sound pressure wave. The most widely used transduction principleare electromagnetism, electrostatics and piezoelectricity.

The transducers in most common loudspeakers (e.g. woofers and tweeters), are electromagndevices that generate waves using a suspended diaphragm driven by an electromagnetic voiccoil, sending off pressure waves. Electret microphones and condenser microphones employelectrostatics. As the sound wave strikes the microphone's diaphragm, it moves and induces voltage change. The ultrasonic systems used in medical ultrasonography employ piezoelectri

transducers. These are made from special ceramics in which elastic vibrations and electrical fare interlinked through a property of the material itself.

[edit]Divisions of acoustics

The table below shows seventeen major subfields of acoustics established in the PACSclassification system. These have been grouped into three domains: physical acoustics, bioloacoustics and acoustical engineering.

Physical acoustics

Biological acoustics

Acoustical engineeri Aeroacoustics General linear acoustics

Bioacoustics Musical acoustics

Acoustic measurementand instrumentation
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Nonlinear acoustics Structural

acoustics and vibration Underwater sound

Physiological acoustics Psychoacoustics Speech communication (production;

perception; processing andcommunication systems)

Acoustic signal process Architectural acoustics Environmental acoustic Transduction Ultrasonics

Room acoustics

Auditorium Acoustics

The room in which we listen to sounds has an important influence on what we hear. This section will identify somthe principal means currently available for judging the quality of an auditorium. However, the design of such spac

still considered an inexact science.

Sound Propagation in an Auditorium

As sound waves travel at about 345 meters/second, the sound coming directly from a source within an auditori

will generally reach a listener after a time of anywhere from 0.01 to 0.2 seconds.

Shortly after the arrival of the direct sound, a series of semi-distinct reflections from various reflecting surface

(walls and ceiling) will reach the listener. These early reflections typically will occur within about 50 millisec

The reflections which reach the listener after the early reflections are typically of lower amplitude and very cl

spaced in time. These reflections merge into what is called the reverberant soundor late reflections.

If the source emits a continuous sound, the reverberant sound builds up until it reaches an equilibrium level. Wthe sound stops, the sound level decreases at amore or less constant rate until it reaches inaudibility.

For impulsive sounds, the reverberant sound begins to decay immediately.
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Direct Sound and Early Reflections

Direct sound will decrease by 6 dB for each doubling of distance propagated.

Our auditory system will determine the direction of a sound source from the direct sounds reaching the ear.

Early reflections which arrive within about 35 milliseconds are not heard as separate from the direct sounds. Rthey tend to reinforce the direct sound.

The source is perceived to be in the direction from which the first sound arrives provided that (1) successive so

arrive within about 35 milliseconds, (2) the successive sounds have spectra and time envelopes reasonably simto the first sound, and (3) the successive sounds are not too much louder than the first. This is referred to as

theprecedence effect.

From a study by Leo Beranek (1962), a concert hall is considered ``intimate'' if the delay time between the dir

and first reflected sound is less than 20 milliseconds.

First reflections usually arrive from the nearest side wall or from the ceiling for those seated in the center.

Reflections from the ceiling or overhead reflectors are not as perceptually desireable as those from side walls.

Late Reflections

During a continuous sound, the reverberant sound level is reached when the rate at which energy is supplied b

source is equal to the rate at which sound is absorbed by the room and its contents.

Too much reverberant sound will result in loss of clarity.

In a bare room, where all surfaces absorb the same fraction of the sound that reaches them, the theoretical

reverberation time is proportional to the ratio of volume to surface area.

Reverberation time is typically defined as the time required for the sound level to decrease by 60 dB (or ).

Calculating Reverberation Time

When expressed in units of cubic and square meters, the reverberation time is given by RT = , where i

volume of the room and is the effective ``total absorption'' area.


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The ``total absorption'' area is calculated as the sum of all surface areas in the room, each multiplied by itsrespective absorption coefficient.

Air Absorption

Air contributes a substantial amount to the absorption of high frequency sound.

Taking account of air absorption, RT = 0.161 , where is a constant which varies with air temperature

humidity, and frequency.

Criteria for Good Acoustics

Optimum reverberation time is a compromise between clarity (requiring short reverberation time), sound inten(requiring a high reverberant level), and liveness (requiring a long reverberation time).

The optimum reverberation time of an auditorium is dependent on the use for which it is designed.

Reflected sound arriving from the sides seems to be important to the overall reverberance of the room.

Important subjective attributes of concert hall acoustics include intimacy, liveness, warmth, loudness of direct

sound, reverberant sound level, definition or clarity, diffusion or uniformity, balance and blend, ensemble,andfreedom from noise.

In addition to the attributes above, spatial impression and early decay time are important. The spatial impressidependent on contributions to the early reflections from above and especially from the sides. The initial rate of

decay of reverberation is apparently more perceptually important than the total reverberation time.

Echoes, flutter echoes, sound focusing, sound shadows, and background noise should be avoided in an auditordesign.

The greater the early decay time (up to two seconds), the greater the preference for the concert hall. Above two

seconds, the trend it reversed.

Narrow halls are generally preferred to wide ones.

Preference is shown for halls having a high ``binaural dissimilarity''.


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Less ``definition'' is preferred. Definition represents the ratio of energy in the first 50 milliseconds to the totalenergy.

Background Noise

Concert halls should meet at least the NC-20 curve and preferably the NC-15 curve (see Figure 23.9 on pg. 47

Rossing).

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