Final Acoustic Design Ppt

7/28/2019 Final Acoustic Design Ppt

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INTRODUCTION

Acoustics

a science that deals with the production, control,transmission, reception, and effects of sound.

acoustics is addressed in the text InteriorConstruction & Detailing

Sound is reflected, transmitted, or absorbed by thematerials it encounters.


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Soft surfaces, such as textiles, and batt insulation,tend to absorb sound waves, preventing them fromfurther motion.

Hard surfaces, such as ceramic tile, gypsum board, orwood, tend to reflect sound waves, causing echo.Reverberation is the term used to describe sound

waves that are reflected off of surfaces.

Dense, massive, materials, such as concrete or brick,tend to transmit sound waves through the material.


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Learning Outcomes Explain Sound behaviour including reflection,

absorption, energy density, sound decay andreverberation.

Learn how to design an acoustic room and itsdesign considerations.

Learn what is Electroacoustics.

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History of Acoustical Design

The history of the acoustic design of buildings begin

with the construction of amphitheatres by the ancient

Greeks .

These were open air amphitheatres that housed up

to 2000 people, all listening to a single orator or

small group of actors.


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Sound behavior

Sound waves propagate away from the source until theyencounter one of the room's boundaries - some of the energywill be absorbed, some transmitted and the rest reflected backinto the room.

Sound arriving at a particular receiving point within a room canbe considered in two distinct parts.

1. Sound that travels directly from the sound source tothe receiving point itself. This is known as the direct soundfieldand is independent of room shape and materials, butdependant upon the distance between source and receiver.

2. After the arrival of the direct sound, reflections fromroom surfaces begin to arrive. These form the indirect soundfieldthat is independent of the source/receiver distance butgreatly dependant on room properties


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The Growth and Decay of Sound

The sound intensity measured at a particular pointincreases suddenly with the arrival of the directsound and will continue to increase in a series ofsmall increments as indirect reflections begin tocontribute to the total sound level.

Eventually an equilibrium will be reached where thesound energy absorbed by the room surfaces isequal to the energy being radiated by the source.

This is because the absorption of most buildingmaterials is proportional to sound intensity, as thesound level increases, so too does the absorption.


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The Growth and Decay of Sound

If the sound source is abruptly switched off, the

sound intensity at any point will not suddenlydisappear, but will fade away gradually as the

indirect sound field begins to die off and reflections

get weaker.

The rate of this decay is a function of room shape

and the amount/position of absorbent material.

The decay in absorbent rooms will not take very long at all,whilst in large reflective rooms, this can take quite a long

time.


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Reverberant Decay of sound in a small absorbent

enclosure.

This gradual decay of sound energy is known as reverberation and,as a result of this proportional relationship between absorption and

sound intensity, it is exponential as a function of time.

Reverberation Time refers to the amount of time it takes for soundenergy to bounce around a room before being absorbed by the materialsand air.


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Optimum Reverberation Times


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Geometric Acoustics

Wave Theory and Normal Modes The concept of a sound ray and the geometrical study

of sound ray paths play an important role in the designof large rooms and auditorium.

A limitation of the geometrical approach is thatusually only primary and possible secondaryreflections can be studied before the sound ray being

followed becomes 'lost' in the reverberant sound fieldand, in most enclosures, it is restricted to frequenciesof 500 Hz and above.


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Using Geometric Acoustics

As enginneers, we need to be able to determine notonly how much absorber to use, but what type ofabsorber and where to put it. This is where theconsideration of reflected sound rays can be quiteuseful.


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The Placement of Reflectors and Absorbers

By analyzing the paths of sound rays, it is easy to

determine which areas require reinforcement (in theform of a reflector) and which require damping (in

the form of absorber).

Consider someone speaking at the rate of up to 8

syllables per second. Each syllable takes about 125

ms. Therefore, if clear reflections of the first syllable

arrive mid-way through the second (or even the third)

the speech may not be easily discernible by thelistener.


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Objective Measures

For many years the reverberation timewas the onlyreal objective measure of the acoustic performance of anauditorium. However, there are many more aspects tosound behavior in rooms.

Early Decay Time

Clarity and Definition

Spatial Impression Speech Intelligibility


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Objective Measures

Early Decay Time The reverberation time, as discussed earlier, refers to the time taken for

the reverberant component of an enclosure to fall by 60 dB after thesource is abruptly switched off. In an ideal enclosure this decay isexponential, resulting in a straight line when graphed against SoundLevel. Studies of actual auditoria, however, show that this is not alwaysthe case.

Research (Kuttruff 1973) has shown that it is the initial portion of thesound decay curve process, which is responsible for our subjectiveimpression of reverberation as the later portion is usually masked bynew sounds. To account for this, the Early Decay Time (EDT) is used.This is measured in the same way as the normal reverberation time butover only the first 10 - 15 dB of decay, depending on the work beingreferenced.


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Clarity and Definition

Clarity and Definition - Refers to the ease with which

individual sounds can be distinguished from within a

general audible stream.

This stream of sound may take many forms; a

conversation, a passage of music, a shouted

warning, the whirring of machinery, whatever.

The degree of clarity is, of course, greatly dependanton the particular sounds involved.


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Spatial Impression

Spatial Impression refers to a feeling of being envelopedwithin the music, surrounded by it not just 'looking in at it'.

Simply put, spatial impression is determined by the subtle

differences in signal received by each ear. If all of thesound energy comes from straight in front of or behind

you, the signal at each ear will be the same.

If the sound bounces around the auditorium andapproaches from the sides, the signals at each ear will be

quite different due to diffraction around the head and

slight time delays.


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Speech Intelligibility

In terms of individual communication, speech isprobably the most important and efficient means,even in today's multi-media society.

TheIntelligibility of Speech refers to the accuracy

with which a normal listener can understand aspoken word or phrase.

Given the fact that some of the informationcommunicated through speech is contained within

contextual, visual and gestural cues, it is stillpossible to understand meaning even if only afraction of the discrete speech units are heardcorrectly.


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Designing Auditoria

These are serious requirements and it must be

remembered that, when an audience enters anauditorium, they have every right to expect comfort,safety, pleasant surroundings, good illumination,proper viewing and good sound." L.L. Doelle,

Environmental Acoustics


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Outline of Acoustic Requirements for Good Sound

There should be adequate loudness in every part of the

auditorium, especially in remote seats.

The sound energy should be uniformly distributed within

the room.

Optimum reverberation characteristics should be

provided in the auditorium to facilitate whatever functionis required.

The room should be free from acoustical defects

(distinct echoes, flutter echoes, picket fence echo,

sound shadowing, room resonance, sound

concentrations and excessive reverberation).

Background noise and vibration should be sufficiently

excluded in order not to interfere in any way with the

function of the enclosure.


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Adequate Loudness

The auditorium should be shaped so that the audience is as close to

the sound source as possible. In larger auditoria the use of a balcony

brings more seats closer to the sound source.

The sound source should be raised as much as is feasible in order to

secure a free flow of direct sound to every listener.

The floor on which the audience sits should be properly raked as

sound is more readily absorbed when it travels at grazing incidence

over the audience.

As a general rule, however, the gradient along aisles of sloped

auditoria should not be more than 1:8 in the interests of safety. The

audience floor of theatres for live performance, especially open or

arena stages should be stepped.


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Adequate Loudness

The sound source should be closely and abundantly

surrounded by large sound-reflective surfaces in order toincrease the sound energy received by the audience.

It must be remembered that the dimensions of thereflecting surfaces must be comparable with the soundwaves to be reflected.

In addition, the reflectors should be positioned in such away that the time-delay between the direct and reflectedsound is as short as possible, preferably not exceeding30 msec and definitely not more that 80 msec.

The floor area and volume of the auditorium should bekept at a reasonable minimum, thus shortening the soundpaths.


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Recommended Volume-per-seat values for various auditoria

Type of

Aud i to r iumMin imum Opt imum Maximum

Rooms for

Speech2.3 3.1 4.3

Concert Halls 6.2 7.8 10.8

Opera Houses 4.5 5.7 7.4

Catholic

Churches5.7 8.5 12.0

Other

Churches5.1 7.2 9.1

Multipurpose

Halls5.1 7.1 8.5

Cinemas 2.8 3.5 5.6


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Elimination of Defects

1. ECHOESThese are probably the most serious and most common defect. They occur

when sound is reflected off a boundary with sufficient magnitude and delay

to be perceived as another sound, distinct from the direct sound. As a rule, if

the delay is greater than 1/25 sec (14m) for speech and 1/12 sec (34m) for

music then that reflection will be a problem.Solut io n: Either alter the geometry of the offendin g su rface or app ly

absorber or di f fus ion.

2. SOUND CONCENTRATION

Sometime referred to as 'hot-spots', these are caused by focused reflectionsoff concave surfaces. The intensity of the sound at the focus point is

unnaturally high and always occurs at the expense of other listening areas.

Solut io n: Treat with absorb er or dif fus ers, better st i l l , redesign it to focus

the sound outs ide or above the enclosure.


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3. SOUND SHADOWINGMost noticeable under a balcony, it is basically the situation where a

significant portion of the reflected sound is blocked by a protrusion

that itself doesn't contribute to the reflected component. In general,

avoid balconies with a depth exceeding twice their height as they will

cause problems for the rear-most seats beneath them.Solut ion: Redesign the protru ding surface to prov ide ref lected

sound to the affected seats or g et r id of the protru sion.

4. DISTORTIONS

These occur as a result of wildly varying absorption coefficients at

different frequencies. This applies an undesirable change in thequality and tone coloration (of frequency distortions) to sound within

the enclosure.

Solut ion: Balance the absorpt io n coeff ic ients of acoust ical

f in ishes over the whole audib le range.


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5. COUPLED SPACES

When an auditorium is connected to an adjacent space, which has a

substantially different RT, the two rooms will form a coupled space. Aslong as the airflow is unrestricted between the two spaces, the decay

of the most reverberant space will be noticeable within the least

reverberant. This will be particularly disturbing to those closest to the

interconnection.

Solut ion: Add s ome form o f acoust ic s eparat ion (a screen or adoor) or match the RT of both rooms.

6. ROOM RESONANCE

Room resonance is similar to distortions in that it causes an undesirable

tone coloration, however, room resonance results from particularly

emphasized standing waves, usually within smaller rooms. This is asignificant concern when designing control rooms and recording

studios.

Solut ion: Apply subt le changes in overal l shape of the room or f ind

out w hich sur faces are contr ibut ing and use large sound

dif fusers.


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Electro-acousticsThe reasons for using sound amplification equipment within an

architectural context

To increase the sound level when a sound source is tooweak to be heard.

To provide additional sound to audiences beyond the

intended range of the source. To project sound back to the stage for the benefit of the

performers.

To alter the Reverberation Time or other impression of anauditoria.

To reduce the relative effects of background noise.

To provide paging, information or warning facilities.

To reproduce electronic or recorded material.


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Speaker Placement

There are essentially three types of loudspeaker system;1. A centrally located system

.


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A centrally located system.

Also known as a high level system, this is essentially asingle cluster of loudspeakers located near the source.Such a system gives maximum realism as the amplifiedsound, whilst increasing loudness and clarity, is still

associated with the original source


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2. A distributed systemBasically a number of loudspeakers spaced throughout the auditorium. This is also

known as a low level system as each individual speaker operates at a low amplificationlevel to service only a small part of the whole audience


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Whilst it is preferable to use a centrally located system, there are

many situations in which it must be used, for example;

Where the ceiling height is too low for the installation of a centralsystem.

Where not all of the audience have a direct sightline with the

central loudspeaker.

When the amplified sound is used to overcome high background

noise levels.

Where the serviced space may be divided into several smaller

spaces.

In large halls where the source position may vary significantly.

Whilst realism cannot be expected from a distributed loudspeakersystem, it does provide high intelligibility where the room is not

too reverberant.


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3. A stereophonic systemTwo or more loudspeaker clusters at strategic positions within theauditorium. Such systems are used when there are a number ofdifferent sources to be amplified or the source is quite mobile


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One of the main points to consider when placing speakers is the fact

that their directionality is frequency dependant.

As discussed in previous lectures, low frequency sounds are pretty

much omni-directional, being able to diffract around obstacles

(including the speaker cabinet) quite readily.

High frequencies, however, are highly directional with only limited

diffraction capacity. The speech band (the frequencies in which we are most often

interested) occupies the mid-frequencies.

This means that they only partially diffract around the speaker

cabinet.

As a result, no matter where the speakers are placed, somemembers of the audience will receive significant low frequency

energy but little higher frequency energy. This can make the speech

sound muddy and even more difficult to understand.


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As a result of these problems, line orcolumn speakersare often preferred over conventional radial or

multicellular horn speakers.

These consist of 6-10 loudspeakers mounted next to

each other to form a column. Such loudspeakers act to

concentrate the sound energy into a beam, which has a

wide angular spread in the horizontal plane and a narrow

spread in the vertical plane .

This minimizes the amount of sound energy radiated

away from the audience, which often causes furtherreflection problems.


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THE HAAS EFFECT

The Haas effect refers to a phenomenon where the sound that

arrives at a listener first determines the perceived direction of the

source.

This is pretty reasonable if we consider the normal physical situation

wherein the direct sound travels in a straight line between source

and receiver whilst reflected sound must take a more complex route.

To accommodate this, we need to place a delay on loudspeakers close to

the audience. This has to be such that the direct sound arrives first, very

closely followed by the loudspeaker output.


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THANK YOU FOR YOUR KIND ATTENTION

Final Acoustic Design Ppt

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