Part Two - Applied Acoustics:

Reprise

In the first lecture we considered space, the materiality of air in dynamic equilibrium, and sound as information of the air. We considered sensations of space, hearing, listening and the human body and our resonances and responses to sound. We stressed the importance of having the human being, human body, and human activity at the centre of our enquiry.

We finished by looking at reverberation: the idea of sound as energy that bounces around in a space until absorbed by its surfaces. The reverberation time is the time it takes for a sound to 'die down' in an enclosed space. Musicians call reverberant spaces 'wet' and absorbent rooms 'dry'. We saw that this varied with the volume of the space - the larger the volume the longer the reverberation - the 'wetter' the acoustic. We did a simple Sabine equation calculation which takes the volume of a space, and divides this by the total amount of absorption to calculate the reverberation time. This is a good approximation for a normal shaped room.

But it ignores:

  • the shape of the space
  • the relationship of one space to another
  • the positions of the sound-event and listeners
  • the character of the sound event

all of which we know are important to appreciating space and understanding the city.

All actual sound is spatial, specific to a particular location (and time).

So we introduced the idea of sound-mapping as a way of tying sound into a particular location, beginning to describe and understand spatialised sound in space.

In this lecture we will look at the acoustic character and quality of spaces and the connections between one space and another, the idea of buffer spaces, masking, and landscape character, developing the concept of sound mapping so that we can apply them to the design of spaces and cities.

A method: activity, sound, enclosure, behaviour

The standard model for architectural acoustics is this:

  • a sound is made
  • it bounces around the room
  • the modified sound is perceived by the listener.

This perfectly describes the relationship between a loudspeaker and a microphone, but it doesn't help much when considering humans because we modify behaviour in response to our environment. So I suggest a slightly different way of looking at acoustics, the Beale method.

Consider, as a designer of space:

  • the activity
  • the sound of the activity
  • the enclosure and how this modifies the sound
  • how the modified sound affects behaviour.

Note this is a feedback loop: humans are included in this model and they are both speakers and listeners.

Let's look at some examples:

Example: Libraries

Precedent1: The library at Trinity College, Cambridge. It is in two parts: a grand classical building designed by Christopher Wren, with a marble floor and high ceilings, and attached to one end of this a modern library - a series of low-ceilinged spaces adapted from residential accommodation in the adjoining courtyard.

In both, the activity is quiet, contemplative study, a little spoken communication, footfall, the opening of books.

In the grand reverberant space the slightest sound is reflected and taken up into the vast volume of air, but diffused, scattered into a thousand reverberations, so that it loses intelligibility. In the smaller spaces, with low ceilings and bookstacks formed into little bays, the volumes are small and sound is 'deadened' or absorbed quickly.

Paradoxically the modern library with its 'dry' acoustic encourages conversation and promotes more sound, even though it disposes of it quickly. The large reverberant space - a 'wet' acoustic - in this case encourages quiet behaviour, promotes silence, for one is very aware of the sound one is making.

A further question, perhaps unanswerable but very pertinent to libraries, what kind of thought does each space encourage?

The Wren Library at Trinity: Note the high ceiling, the marble floor, and the difference in acoustic between the lower and upper parts of the room. For sale see the figure at the right hand of the door.

Precedent 2: The Courtauld Institute at Somerset House, London

From 1987-1990 the North Block of Somerset House was converted into the Courtauld institute and Galleries. I was one of the team at Green Lloyd Architects creating libraries, teaching and conservation spaces for the Institute.

Here the library is underground, converted from vaults set beneath the famous courtyard. Large. Piranesi-like spaces with curved barrel vaulted brick ceilings within which are two levels of bookstacks, with a crisp, minimal mezzanine.

The preparatory spaces are the archway, the entrance lobby, the staircase down, a narrow passage and then the open vaults, a dramatic progression.

Example: The Library at the School of Pharmacy, University of London.

In 2008 we were asked to design a new library for the School of Pharmacy at London University, working within an existing building. So we put these observations into practice. The brief called for separate zones:

  • for quiet study,
  • for group study,
  • bookstacks, and
  • approach, welcome and printing areas.

Each was treated differently. In the approach areas and bookstacks, we used a rubber floor covering - quite reverberant - and exposed the high plaster ceilings, uplighting them to emphasise the apparent height of the space. Here, although there was plenty of movement and activity, we were maximising reverberation to encourage a self conscious quieting-down of behaviour.

In the group study areas, we created semicircular free standing partitions, highly reflective on the convex, outer side (veneered, varnished timber), and highly absorbent on the concave, inner side (fabric over mineral wool), within which the groups were to sit. Rather than taking the partitions to full height - the sense of light, openness and connected-ness was important - we treated the ceiling with absorbent materials (perforated plaster with an air gap behind) and floated 'acoustic clouds' (curved panels of compressed mineral wool over a steel plate, suspended on wires from the ceiling) above the space. The combination of these created an almost perfectly absorbent ceiling, an 'artificial sky'. With no reflections from above, the sound in these booths is markedly different from the main space, and students can converse at a normal level without disturbing other groups. Again, we uplit the acoustic clouds, taking advantage of their space to diffuse the light and drawing attention of users to the strange shapes which create such an aura of calm.

This example can be used to illustrate some important techniques in sound design.

  • First, try and imagine the activity in the space, in your minds eye be there and imagine what it is like.
  • Then decide what kind of enclosure you wish to create. What kind of sonic space suits the activity - what acoustic atmosphere you wish to create.
  • Choose the dimensions of the room, the overall shape and curvature and
  • then choose the materials of the space to suit.

So how do we choose the materials?

You will remember from lecture one that sound rushes out at 333 m/s until it meets a change in the medium, for example a wall, floor or ceiling. Some is absorbed and some reflected back. The range of materials we have at our disposal allows us great flexibility in design.

For example:

A plastered masonry wall will reflect nearly all the sound - 99% of a 2.5m wavelength sound falling to 95% for a 10cm wavelength sound.

Plasterboard over timber studs will reflect only 70% of the 2.5m sound, rising to 95% for a 10cm sound. Although both materials look identical, this is because the stud wall and its cavities act as resonators, taking some of the energy out of the sound wave.

Plasterboard with little square holes in it, [e.g. Gyptone from British Gypsum] with an air space or mineral wool behind will reflect less than 50% of the 2.5m sound, rising to 70% for a 10cm sound.

If you can accept slots in the finish, there are a wide variety of absorbent materials using the air space behind to absorb the sound.

If you wish to use fabric, you can put this on a solid backing, reflecting, again, nearly all the sound, or stretch it over a frame with mineral wool behind, reflecting only

stretch this over

For example:
Walls can range from being almost completely revereberant to completely absorptive. In a sense if they are absorptive they are irrelevant for acoustic effect, it is like a quiet open sky, giving nothing back

The big spatial determinants are:

is there a ceiling. If not, if we are open to the air, there is 0% reflection

Activity

31. City sounds
sound-scape
sounds that are not sounding at present
Actual sound and potential sound Aristotle.

Sound
32. Sound sources
elements
living things
machinery
alarm sounds
catalogues of sound

Introducing sound into the building
- mechanical systems
- bells
- water
- hinges
- floor finishes and footfall:
gravel, boards, carpet, rubber, stone.

Enclosure

33. Localised environments
Theatre
dining hall
kitchen
sleep duvet

34. Room acoustics
performer speaker here
wall here or here
ceiling here or here
symmetric and asymmetric environments

breaking up the sound good and bad reflections: specular reflections
absorbing the sound letting it escape
Convex and concave surfaces
ceilings apses focus
first reflections late reflections

rhomboids

vaults

domes

fans

Behaviour

The cocktail party effect - people can't hear each other and talk louder and louder to hear each other which makes background sound even louder. So they talk louder - the ability to separate out our friends voice from the background.

Affecting acoustic behaviour
A notice saying ‘silence’
Colours and acoustics
Jean Nouvel theatre concert hall dark blue bad acoustics grey walls good acoustics
understanding the sound you hear

Vauxhall station effect: high pitched sounds, mole scarers

tribal effect of classical music

music to buy with

muzak

rowdy lads and self awareness

Philophony

- promoting sound
- appropriate to context
making a building appropriate to context
a parallel in townscape:
- shouting iconic
- or storytelling,
stitching together

39. Noise control
Physiological effects of noise.
Aircraft and traffic noise. -
Impact sound -
Airborne sound
Stuff that stops sound: mass, isolation.
Fields of baffles.
Active acoustics (cars and Bose headphones)
Soundscape -
The one sound - private acoustics (walkman) - civic acoustics - parliament.
City a collective organism.
Air and water. feng shui.
____ Further resources can be found at www.philophony.com
____ About the speaker. Marcus Beale is practitioner of architecture and music. He is director of Marcus Beale Architects . Before becoming a professional architect he was a composer of ballets and professional violinist, performing with the Penguin Cafe Orchestra, with whom he toured in Japan USA an Europe from 1981-1985. He is one of few architects to have played the BBC Whistle Test and to have had his music performed on BBC Radio 3. Further information at www.marcus-beale.com