AcousticalMechanism

Content

1. Introduction

2. Problem in acoustic design

3. General principle of acoustical design

4. Sound Propagation in an Auditorium

5. Criteria for good acoustic

6. Case study (Opera house, Sydney)

7. Conclusion

8. References

Introduction

The architect designs a great looking and comfortable structure . The people attend the grand opening and are impressed with what they see, but they have gathered for more than a dazzling display of architecture, lighting, electronics, carpets, glass, surface textures and paint. The outer beauty of an structure is recognized by how it looks, but the inner more lasting beauty of the structure is truly known by how it sounds.

Architectural acoustics is the science of controlling sound within buildings.

Noise transmission within building:1. By external means (through building skin envelop)- main noise paths are roofs, eaves, walls, windows, doors and penetration.2. By internal means- Typical sound paths are room partition, acoustic ceiling panels, door window etc.

These different reflecting sounds create standing waves that produces an annoying sound.

There are three ways to improve workplace acoustics and solve workplace sound problems – the ABC’s.

A = Absorb (usually via ceiling tile)

B = Block (via workstation panels, wall placement and workspace layout)

C = Cover-up (via electronic sound masking)

Introduction

Problems in Acoustical Design1. Lots of sound, but little is heard A sound wave start at the loud speaker and

only 0.17% of the sound is directly heard by the audience. Direct sound will decrease by 6 dB for each doubling of distance propagated

2. Noise blocks our ability to hear The auditorium is meant for understanding

speech. Noise destroys sonic clarity

3. Background noise Background noise is all the sounds one

hears when the lecturer is not saying something

4. Acoustic noise, echoes and reverberation

General principle of acoustical design

1. Site selection and planning Site should be away from external noise pollution. 2. Volume volume should be according to number of

audience, intensity of sound,.

3. Shape Shapes are decided after tracing the path of

reflected sound wave and concentration of sound wave.

4. Sound absorption sound absorbing material should be given to

control reverberation.

5. Seating arrangement, seats and audience staggered sitting arrangement distance from row to row should be 450mm-

1000mm

Some Absorption Coefficients

Frequency (Hz)Material 125 250 500 1000 2000 4000

Concrete/brick 0.01 0.01 0.02 0.02 0.02 0.03Glass 0.19 0.08 0.06 0.04 0.03 0.02Plasterboard 0.20 0.15 0.10 0.08 0.04 0.02Plywood 0.45 0.25 0.13 0.11 0.10 0.09Carpet 0.10 0.20 0.30 0.35 0.50 0.60Curtains 0.05 0.12 0.25 0.35 0.40 0.45Acoustical board 0.25 0.45 0.80 0.90 0.90 0.90

Sound Propagation in an Auditorium

1.As sound waves travel at about 345 meters/second, the sound coming directly from a source within an auditorium will generally reach a listener after a time of anywhere from 0.01 to 0.2 seconds.

2.Shortly after the arrival of the direct sound, a series of semi-distinct reflections from various reflecting surfaces (walls and ceiling) will reach the listener. These early reflections typically will occur within about 50 milliseconds.

3. The reflections which reach the listener after the early reflections are typically of lower amplitude and very closely spaced in time. These reflections merge into what is called the reverberant sound or late reflections.

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

5.For impulsive sounds, the reverberant sound begins to decay immediately.

reverberation

Sound waves

Criteria for Good Acoustics

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

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

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

Reflection from Flat surface

Reflection from convex surface

Criteria for Good Acoustics

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

5. 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.

Echoes

Sound shadow

Sydney Opera House(Icon of 20th century)

Case study

Sydney Opera House ArchitectJørn UtzonHall, Todd & Littlemore

EngineerOve Arup & Partners

BuilderHornlbrook Pty Ltd

LocationBennelong Point, Sydney

Land coverage1.8 hectares (4.5 acres) of land

Dimensions183 metres (605 feet) long and about 120 metres (388 feet) wide

StructureIt is supported on 588 concrete piers sunk up to 25 metres below sea level.

Power supplyIts power supply is equivalent for a town of 25,000 people. The power is distributed by 645 kilometres of electrical cable

Interior and Acoustics

The rich and extensive timber interiors

An exemplary example of the use of plywood and laminated hardwood.

Timber is used as the primary material for the interiors, with the warmth, colour and tactility of timber providing a contrast with the heavy, monochrome, load bearing concrete of the podium and sails.

Each beam was to be made up of two plywood box beams bolted together, with acoustic insulation in the cavity inside each beam.

The ceiling was to consist of a series of plywood box beams

Section of opera house

Spanning horizontally between the box beams was to be panels of plywood reinforced with hot bonded aluminium. These horizontal elements were attached to the top of one beam and the bottom of the next creating a stepped form to the ceiling.

On the top of these panels was to be bonded 2mm of lead for low frequency sound insulation.

the ceiling of the performance halls give a feeling of a floating cloud.

Built scheme

Throughout the interiors, prefabricated panels of laminated Brush Box were used for flooring, stair treads and risers and wall panels. An extremely hard and dense timber, Brush Box was chosen for its warm, rich colour and grain, acoustic performance and high durability.

Sydney Opera House shell ribsSydney Opera House shell ribsSydney Opera House shell ribs

A Strategy for Design with Timber

Timber and AcousticsTimber has historically been used for acoustic applications for a number of reasons. A timber surface does not just reflect sound, but resonates slightly, giving it a particular quality and colour. A hard surface such as concrete reflects sound with a hard and sharp quality. In the Sydney Opera House, the musicians were happy to have timber used as it was a material they understood, many of their instruments being made from timber. They were accustomed to the quality of sound timber spaces created. This surface treatment with panelling breaks the sound, reducing echoes.

References

Sydney opera house -Tom Fletcher

Design for good acoustics and noise control - moore, john edwin.

www.sydneyarchitecture.com/roc/qua01.htmwww.wikipedia.com

E-References