acoustic design ppt

34

Upload: kunal-mangal

Post on 13-Apr-2015

90 views

Category:

Documents


2 download

DESCRIPTION

Acoustical design

TRANSCRIPT

Page 1: Acoustic Design Ppt
Page 2: Acoustic Design Ppt

Learning Outcomes• Explain Sound behaviour including

reflection, absorption, energy density, sound decay and reverberation.

• Learn how to design an acoustic room and its design considerations.

• Learn what is Electroacoustics.

2

Page 3: Acoustic Design Ppt

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.

Page 4: Acoustic Design Ppt

Sound behaviorSound waves propagate away from the source until they

encounter one of the room's boundaries - some of the energy will be absorbed, some transmitted and the rest reflected back into the room.

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

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

2. After the arrival of the direct sound, reflections from room surfaces begin to arrive. These form the indirect sound field that is independent of the source/receiver distance but greatly dependant on room properties

Page 5: Acoustic Design Ppt

The Growth and Decay of SoundThe sound intensity measured at a particular point

increases suddenly with the arrival of the direct sound and will continue to increase in a series of small increments as indirect reflections begin to contribute to the total sound level.

Eventually an equilibrium will be reached where the sound energy absorbed by the room surfaces is equal to the energy being radiated by the source.

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

Page 6: Acoustic Design Ppt

The Growth and Decay of Sound

If the sound source is abruptly switched off, the sound intensity at any point will not suddenly disappear, 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.

Page 7: Acoustic Design Ppt

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 sound energy to bounce around a room before being absorbed by the materials and air.

Page 8: Acoustic Design Ppt

Optimum Reverberation Times

Page 9: Acoustic Design Ppt

Geometric AcousticsWave Theory and Normal ModesThe concept of a sound ray and the geometrical

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

A limitation of the geometrical approach is that usually only primary and possible secondary reflections can be studied before the sound ray being followed becomes 'lost' in the reverberant sound field and, in most enclosures, it is restricted to frequencies of 500 Hz and above.

Page 10: Acoustic Design Ppt

Using Geometric Acoustics

As enginneers, we need to be able to determine not only how much absorber to use, but what type of absorber and where to put it. This is where the consideration of reflected sound rays can be quite useful.

Page 11: Acoustic Design Ppt

The Placement of Reflectors and AbsorbersBy analyzing the paths of sound rays, it is easy to

determine which areas require reinforcement (in the form 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 the listener.

Page 12: Acoustic Design Ppt

Objective MeasuresFor many years the reverberation time was the only real objective measure of the acoustic performance of an auditorium. However, there are many more aspects to sound behavior in rooms.

Early Decay TimeClarity and DefinitionSpatial ImpressionSpeech Intelligibility

Page 13: Acoustic Design Ppt

Objective MeasuresEarly Decay TimeThe reverberation time, as discussed earlier, refers to the

time taken for the reverberant component of an enclosure to fall by 60 dB after the source is abruptly switched off. In an ideal enclosure this decay is exponential, resulting in a straight line when graphed against Sound Level. Studies of actual auditoria, however, show that this is not always the case.

Research (Kuttruff 1973) has shown that it is the initial portion of the sound decay curve process, which is responsible for our subjective impression of reverberation as the later portion is usually masked by new sounds. To account for this, the Early Decay Time (EDT) is used. This is measured in the same way as the normal reverberation time but over only the first 10 - 15 dB of decay, depending on the work being referenced.

Page 14: Acoustic Design Ppt

Clarity and DefinitionClarity 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 dependant on the particular sounds involved.

Page 15: Acoustic Design Ppt

Spatial ImpressionSpatial Impression refers to a feeling of being enveloped

within 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 the sound 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 and approaches from the sides, the signals at each ear will be quite different due to diffraction around the head and slight time delays.

Page 16: Acoustic Design Ppt

Speech IntelligibilityIn terms of individual communication, speech is

probably the most important and efficient means, even in today's multi-media society.

The Intelligibility of Speech refers to the accuracy with which a normal listener can understand a spoken word or phrase.

Given the fact that some of the information communicated through speech is contained within contextual, visual and gestural cues, it is still possible to understand meaning even if only a fraction of the discrete speech units are heard correctly.

Page 17: Acoustic Design Ppt

Designing Auditoria

These are serious requirements and it must be remembered that, when an audience enters an auditorium, they have every right to expect comfort, safety, pleasant surroundings, good illumination, proper viewing and good sound." L.L. Doelle, Environmental Acoustics

Page 18: Acoustic Design Ppt

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 function is 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.

Page 19: Acoustic Design Ppt

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.

Page 20: Acoustic Design Ppt

Adequate LoudnessThe sound source should be closely and abundantly

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

It must be remembered that the dimensions of the reflecting surfaces must be comparable with the sound waves to be reflected.

In addition, the reflectors should be positioned in such a way that the time-delay between the direct and reflected sound is as short as possible, preferably not exceeding 30 msec and definitely not more that 80 msec.

The floor area and volume of the auditorium should be kept at a reasonable minimum, thus shortening the sound paths.

Page 21: Acoustic Design Ppt

Recommended Volume-per-seat values for various auditoria

Type of Auditorium   

  Minimum    Optimum    Maximum 

Rooms for Speech

2.3 3.1 4.3

Concert Halls  6.2 7.8 10.8

Opera Houses 4.5 5.7 7.4

Catholic Churches 

5.7 8.5 12.0

Other Churches

5.1 7.2 9.1

Multipurpose Halls

5.1 7.1 8.5

Cinemas 2.8 3.5 5.6

Page 22: Acoustic Design Ppt

Elimination of Defects1. ECHOES

These 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.

Solution: Either alter the geometry of the offending  surface or apply absorber or diffusion.

2. SOUND CONCENTRATION

Sometime referred to as 'hot-spots', these are caused by focused reflections off concave surfaces. The intensity of the sound at the focus point is unnaturally high and always occurs at the expense of other listening areas.

Solution: Treat with absorber or diffusers, better still, redesign it to focus the sound outside or above the enclosure.

Page 23: Acoustic Design Ppt

3. SOUND SHADOWING

Most 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.

Solution: Redesign the protruding surface to provide reflected sound to the affected seats or get rid of the protrusion.

4. DISTORTIONS

These occur as a result of wildly varying absorption coefficients at different frequencies. This applies an undesirable change in the quality and tone coloration (of frequency distortions) to sound within the enclosure.

Solution: Balance the absorption coefficients of acoustical finishes over the whole audible range.

Page 24: Acoustic Design Ppt

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. As long 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.

Solution: Add some form of acoustic separation (a screen or a door) 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 a significant concern when designing control rooms and recording studios.

Solution: Apply subtle changes in overall shape of the room or find out which surfaces are contributing and use large sound diffusers.

Page 25: Acoustic Design Ppt

Electro-acoustics The reasons for using sound amplification equipment within an architectural contextTo increase the sound level when a sound source is too

weak 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 an

auditoria. To reduce the relative effects of background noise. To provide paging, information or warning facilities. To reproduce electronic or recorded material.

Page 26: Acoustic Design Ppt

Speaker PlacementThere are essentially three types of

loudspeaker system;1.A centrally located system

.

Page 27: Acoustic Design Ppt

A centrally located system.

Also known as a high level system, this is essentially a single cluster of loudspeakers located near the source. Such a system gives maximum realism as the amplified sound, whilst increasing loudness and clarity, is still associated with the original source

Page 28: Acoustic Design Ppt

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 amplification level to service only a small part of the whole audience

Page 29: Acoustic Design Ppt

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 central system.

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 loudspeaker

system, it does provide high intelligibility where the room is not too reverberant.

Page 30: Acoustic Design Ppt

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

Page 31: Acoustic Design Ppt

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, some members 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.

Page 32: Acoustic Design Ppt

As a result of these problems, line or column speakers are 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 further reflection problems.

Page 33: Acoustic Design Ppt

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.

Page 34: Acoustic Design Ppt

THANK YOU FOR YOUR KIND ATTENTION