MODELLING SOUNDSCAPES

Presentation by

Rob Bullen

Is it a concert

hall?

YDetailed modelling of reverberation characteristics, clarity, spatial separation, etc. Audible simulation (?)

Is it a lecture theatre, place of

worship or similar?

Calculate STI, check reverberation, maybe some other features depending on $

Y

N

Is it an apartment

?

Discuss AAAC star rating, select partitions accordingly (& meet BCA)

Meet the BCA & AS 2107

N

Y

N

Architectural Acoustics

cafe, hospital ward, call centre, dance studio, library, gym, foyer, courtyard, …

Can we do more in the “also-ran” spaces?

Rather than just meet criteria, can we think about and evaluate the soundscape in the space?

The level and quality of sound from all sources – including differentiated & undifferentiated, conscious & unconscious

To do this we need:

• indicators of goodness – preferably numerical

• ways to evaluate a soundscape that doesn’t exist yet

• design tools to achieve the desired soundscape – these are just normal acoustic tools – partitions, finishes, sound masking, etc.

The Physical Soundscape

Roof/Ceil

Hroof

Window

Hwind

paircraft

ppeople

ptraffic

hroom

hroom

hroom

hroom

hroom

hroom

The Mental Soundscape

Conscious

(Conscious Awareness)

Unconscious

Mood

Automatic behaviour

Performance effects

Stress?

Speech

Baby

AircraftInformation

Aesthetics

Annoyance

The Task of Acoustic Design:

Engineer the physical soundscape in such a way that it will produce a desired mental soundscape.

Physical Mental

Sound pressure at ears

The Interface Between Physical and Mental Soundscapes

• detection – frequency analysis of the incoming signals in slices of about 30 ms to produce a form of spectrogram

• “scene analysis” – breaking the spectrogram into streams in such a way that they probably represent objects in the real world

• informational and attentional masking – picking out the few streams that will be reach conscious awareness. The rest will form part of the unconscious.

Formation of a mental soundscape seems to involve at least 3 separate processes:

Some Preliminary Assumptions and Rules of Thumb for Designing a

SoundscapeUnconscious Sound

1. In any soundscape the streams with low SPL (as well as unstreamed noise) will generally be relatively stable with time. Call this the “background” and identify its level with either the total LAeq of these low-SPL streams or the LA90 of the total noise. This sound is always unconscious (after some time in the space).

2. For most spaces, the only control you need on background sound is on the level. The range of levels in AS 20107 is a reasonable guide to acceptable levels of background (unconscious) sound.

Some Preliminary Assumptions and Rules of Thumb for Designing a

SoundscapeConscious Sound

3. For a time-varying sound source, if LAmax is at least 10dB below the background, it will be energetically masked (i.e. inaudible). [For unusual spectra, e.g. very low-frequency sound, you may require the SPL to be 10 dB below background in each octave band.] Otherwise the source will form a stream and has the potential to become conscious.

4. If LAmax of a time-varying source is at least 5dB above the background, assume that the stream will become conscious. [Again, to be more conservative you can apply this to each octave-band separately.]

5. A source that is both conscious and unwanted will cause annoyance.

Some Preliminary Assumptions and Rules of Thumb for Designing a

SoundscapeSound Quality

6. For non-speech sound, to a first approximation sound quality is described by reverberation time. For example:

< 0.5 secs = “soft”, “sophisticated”, “dead”. People will tend to speak more quietly. Required for amplified music.

> 1 sec = “buzzy”, “live”. People will tend to speak more loudly. Required for classical music.

7. Similarly, to a first approximation speech quality is described by STI.

In short, I propose the following general specifications for a

soundscape:

• Background SPL

• What should be inaudible and what should be not intrusive?

• RT

• Where do you want speech to be intelligible / not intelligible?

Evaluating a Soundscape

• You can calculate the total background level, the LAmax from time-varying sounds, the RT and the STI, then see whether the soundscape will fit your design intent

Or…

• You can simulate the soundscape.

[Demo of SoundScape program here]

SoundScape: A few technical details

Sound file

reader

Splitter

Mixer

TL Filter

Splitter

Reverb-eration

Direct

Mixer

Amp: distanc

e

Mixer

Mixer

To reverberation

To direct

Amp: source level

To sound paths

From sound sources

From sound paths

SOUND SOURCE

SOUND PATH

ROOM

SoundScape: A few technical details

TL Filter: Fits a digital filter to an arbitrary specified set of TL values, approximately. (Filter is 2 conjugate poles + 2 conjugate zeros.)

Reverberator: Feedback delay network

delay 1

delay 2

delay 3

…

filter 1

filter 2

filter 3

…In Out

Average delay

= mean free path / c

= 4 V / S c

Filter reduction, dB

= delay * 60/RT

In fact, the ability to add sources at different levels is useful for other

things …

• Simulations of environmental noise for potential residents? Experience indicates this is dangerous and difficult to get right, but in some limited scenarios could be useful (e.g. difference between single and double glazing).

• When does a noise sound tonal?

• How well can you REALLY measure a sound in the presence of background?

Conclusions …

• For acoustic design of “also-ran” spaces, it’s possible to do better than just meet AS 2107 and BCA. The acoustic properties of these spaces are important to most people in their normal lives.

• Because these spaces are so diverse, we need a general-purpose methodology to set acoustic goals and evaluate a design. The methodology should allow non-experts (e.g. architects) to communicate with experts in a common language.

• The concept of a soundscape can do this, with a few descriptive parameters:

• background SPL

• audibility and intrusiveness of foreground sources

• RT and STI

Conclusions …

• From this, an expert can do calculations and produce a design that is likely to satisfy the requirements.

• Alternatively or additionally, simulation of the soundscape can help greatly both in defining the requirements and in achieving them. Where necessary, in simple cases a non-expert can produce a design that is, at least, a lot more satisfactory than if they had ignored acoustics altogether.

• What’s more simulations are fun. Which makes them good for students.