Acoustics

Question 1

Explain the term Reverberation Time and describe the two main factors which determine the reverberation time within a space.

Answer 1

Reverberation time is the duration it takes for sound to decay by 60 dB in a given enclosed space, indicating how long sound lingers and reflects within that environment.

• Directly proportional to room volume
• Inversely proportional to area of absorption

Question 2

Explain the difference between sound insulation and sound absorption.

Answer 2

Sound Insulation: Ability of sound transmission to penetrate through a material/between spaces.

Sound Absorption: Reducing the reflection of sound (echo, reverberation).

Question 3

Give an example of a material that provides a high level of sound insulation.

Answer 3

Concrete, brick, glass, plasterboard + air gap. hard. i.e. non permeable materials

Question 4

Give an example of a material that provides a high level of sound absorption

Answer 4

Polyester, wool, fiberglass batts, carpet. i.e. Porous materials.

Question 5

Rank the following enclosures from best to worst (1 = best, 4 = worst) for reducing noise received outside the enclosure.

Answer 5

1. Batts then plywood
2. Plywood then Batts
3. Plywood
4. Batts

Question 6

Identify and describe two beneficial acoustic design elements of a Speech Room (e.g. lecture theatre or meeting room).

Answer 6

Goals: Reverberation control (low RT) and avoiding focusing and echo.

Absorptive Treatment (perforated wall panels) - Reduce the reflection and hence reverberation of sound which improves clarity.

Early Refection’s (overhead materials and walls) - At least 3 early reflections to help with clarity of speaker.

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Question 7

explain if these features would be beneficial or detrimental for an open plan office and why.

Answer 7

Sound Panels: Roof panels would be beneficial to an open plan office space provided that the seating arrangement is not too close together.

Absorptive Wall Panels/Materials (perforated): Would be beneficial as they would reduce reflection of sound within the office, improving clarity.

If the room is too effective at absorbing sound then the office space will be too quiet for private conversations

Question 8

Give an example of an ‘acoustic defect’ and explain how to avoid this defect when designing a new lecture theatre

Answer 8

Echoes: Delay from sound reflecting from rear wall facing speaker. Creates a noticeable delay in speech and can decrease clarity.

Flutter Echo: Reflection of sound between to surfaces close together.

Focusing: Sound is louder of quieter in specific areas of a room

Room Modes:

Background Noise:

Question 9

Give an example of what building material may provide low frequency absorption in a lecture theatre

Answer 9

Wooden paneling over cavity

Plasterboard Walls

Question 10

Give an example of what materials may provide high frequency absorption in a lecture theatre

Answer 10

Curtains, carpet, felt

Question 11

Describe two key acoustic features of an Innovative Learning Environment (ILE).

Answer 11

Absorption – Low RT 0.4s
Low background noise
Flexibility

ANSWER NEEDS ELABORATION

Question 12

Describe the Mass Law of sound insulation for single leaf constructions. Answers should include:

(i) What is effect of increasing mass (be specific if possible)
(ii) Limitations of the mass law

Answer 12

(i) The Mass Law: Doubling the mass increases the STC by 6 points.

Example: Gib board has a STC rating of 26. Therefore two layers of Gib provides a rating of 32.

(ii) Increasing mass is an inefficient way of increasing STC. Large air cavities combined with layers of mass are significantly more effective.

Question 13

Describe alternative construction that can be used to beat the mass law.

Answer 13

Use of large air cavities, reduction of solid connections through independent studs and rubber isolators.

Question 14

Draw a diagram of a typical construction that would comply with Clause G6 of the NZ Building Code.

Answer 14

G6 addresses airborne and impact sound

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Question 15

Describe the significant difference in the sound insulation performance (sound transmission loss) of light weight walls when compared to concrete or block walls.

Answer 15

Thin slabs are very problematic for airborne and impact sound transmission. In practice slabs must be thicker than 120mm for compliance with G6 horizontally.

Heavy weight walls (brick/concrete) are less absorptive than plasterboard/timber walls.

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Question 16

Describe the Lombard effect.

Answer 16

People talk louder in environments where background noise is elevated.

Question 17

Describe the café effect. Your answer should include two factors that drive the café effect.

Answer 17

Sound levels build up in reverberant spaces. Driven by the Lombard effect and use of reflective (non absorptive materials) i.e. polished concrete, glass, brick, open kitchens

Question 18

Describe how the effect can be minimized through acoustic design.

Answer 18

Sound absorbing materials? (ceiling tiles, acoustic screens, separation distance

Question 19

Would increasing the distance between tables in a restaurant help to avoid the café effect or make it worse? Explain why.

Answer 19

Separation distance ideally greater than 10m^2 per person.

When tables are spaced farther apart, it reduces the direct sound path between individuals and decreases the level of overlapping conversations. This can result in a decrease in overall noise levels and better clarity of individual conversations, making it easier for people to understand each other without excessive background noise.

Question 20

Roughly sketch the impulse response below in your answer booklet and circle and describe/label three areas of the room impulse response below (for a music source).

Answer 20

Direct Sound: From the source to the listener

Early Reflections: off rear and side walls

Reverberation: Persistence of sound due to multiple reflections.

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Question 21

Discuss the main change in room acoustics for music performance spaces from the Renaissance period to the Baroque period. Discussion could include the type of building and/or the change in reverberation time.

Answer 21

Renaissance: Large reverberant spaces (churches and cathedrals) with long reverberation times.

Baroque: Small audiences (palace ballrooms, reception rooms. Acoustically intimate spaces with moderately low reverberation time, high definition and clarity. Composers wrote music to suit the acoustics of these smaller spaces.

Question 22

What shape of concert hall was typical during the 19th Century, often described as the golden era of concert halls?

Answer 22

19th Century Shoebox Halls/Rectangular Halls
- Proportions of 2 cubes (1:1:2 H:W:L)
- 1500-2000 seats
- Shallow Balconies
- Decorated surfaces (diffusion)
- Full reverberant sound with clarity
- Reverberation time less than 1.8 seconds

Question 23

Describe how intimacy and reverberation were achieved in the design of Philharmonie de Paris

Answer 23

Intimacy relates to the initial time delay gap between the direct sound and the 1st reflection.

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Question 24

The Building Code Clause G6 contains minimum performance standards for two areas of acoustic design. What do the terms STC and IIC stand for and what type of sound do they relate to?

Answer 24

STC: Sound Transmission Class - Sound transmission of walls, floors and ceilings shall be no less than 55

IIC: Impact Insulation Class: A measurement of noise caused by a standard impact source.
- Impact sound of floors shall be no less than 55.

G6 applies to habitable spaces: Bedrooms, living rooms/lounges, kitchens and studies.

Question 25

Name two household noise sources that are controlled by STC?

Answer 25

Talking, stereo, tv etc.

Not applied to: plumbing, lifts, air conditioning.

Question 26

Name two household noise sources are controlled by IIC?

Answer 26

Walking, dropping objects?

Question 27

List three aspects of acoustic design of an apartment building that are not covered in the Building Code.

Answer 27

Horizontal transmission of sound.

Internal noise sources such as plumbing, lifts, air conditioning

External noise sources such as traffic, aircraft or
industrial noise

CHECK

Question 28

The National Standard AS/NZS 2107:2016 provides guidance on which two elements of acoustic design?

Answer 28

Reverberation time and background noise.