אדריכלות ישראלית

Acoustic Comfort

Architecture of Israel #

108

February

2017

page

english

acoustic bridges, second article in the series

snarky noise

bends round corners

Sound travels through materials, liquids,

solids, and gases in all directions. One

interesting characteristic attributed to the

movement of sound is its ability to by-pass,

bend and bounce. This is why it’s possible

to hear a car hooting on the other side of

a building, or the regional alarm system,

although the space between the source and

the target is invisible. And this is also the

reason for the flawed effectivity of acoustic

barriers, such as those installed along a

road, or when one can still hear street noise

when opening a window that doesn’t face

directly onto the street.

In order to understand how sound bends,

one needs to understand that sound

is not established on the matter it first

encounters, but rather generates a chain

movement to adjacent matter, which in turn

causes movement in all adjacent particles.

Therefore, when sound encounters an

acoustic barrier, the particles that pass

through the barrier vibrate and move the

particles surrounding them, so the sound

actually by-passes the barrier and reaches

the area behind it.

It is essential to consider this principle

when striving to achieve acoustic comfort

in buildings. Since, similarly to the barrier

example, noise is capable of passing through

cracks and holes in walls – especially those

left around the frames of windows, doors, or

any other crack left during construction.

Thus, when comparing “acoustic comfort”

to “climate comfort”, it is possible to relate

to “sound bridges” in analogy with heat

bridges, which enables an unwanted

passage of heat or cold, mainly between the

building and its acoustic surroundings (see

previous article in AI #107).

A common failure regarding acoustic comfort relates to the passage of

sound and noise between a building and its surroundings, and between

its internal spaces. A famous image that describes sound travel is a drop

falling into water, creating a ripple. The problem with this two-dimensional

image is that traveling sound is rather multi-dimensional, more like a ball.

However, one must bear in mind, that

sound behaves differently from heat. A

common problematic sound bridge is that

of in-wall windows, since, as mentioned,

sound travels in all directions, and when

it meets the window it can easily evade it

through the gap left in the wall for its free

movement, even when the window itself is

double glazed.

Sound doesn’t require a large permeable

gap in order to be heard close to the source.

Therefore, sound bridges exist even when

the crack is invisible, since in many cases

it is hidden by a thin plaster layer, that even

if it is sufficient to prevent the penetration of

cold or heat, it is not adequate to prevent

sound from passing through.

This is frequently evident in clinics or medical

practices, where total privacy is required –

and yet sounds carry from the therapy room

into the waiting area and vice versa.

Intuitively, one would say that in most cases,

this is due to hollow doors that enable

unwanted passage of sound, although

climatically they may be reasonably

insulated.

In other cases, gaps between the frame and

the walls are left during installation, then

aesthetically sealed by a plank of wood or

metal, but do not prevent the passage of

sound and noise.

Ignore for a moment the low acoustic

capability of conventional concrete blocks,

sound bridges can constitute any unsealed

gap between them. A discrepancy between

wall size and block module entails on-site

hand cutting or gap stretching, left to be

sealed later on with a thin plaster layer.

Eng.Omri

Abas

Although this may look aesthetically whole,

it does not prevent an unwanted passage of

sound.

Sound bridges are also formed in bare

concrete walls when wires or screws that

hold molding boards during casting are

transferred through pipes, leaving holes

after the concrete dries.

Here again, a thin layer of plaster is

insufficient to prevent invisible acoustic

bridges that transfer voices and noise.

Sound bridges are also found in various

parts of a structure after quarrying in order

to lay water, electricity or communications

pipes that are then covered by a thin layer

of plaster.

Israeli Standard for green building, requires

that the building envelope be planned and

built in such a way that the noise level inside

a building does not exceed 40 decibels

when windows are closed – a theoretically

attainable result, provided that the number

of sound bridges is significantly low.

However, standards essentially deal only

with minimal required conditions, not

optimal situations, which in fact constitute

the essence of good architecture.

One can live with noise – that’s a fact. But

the question is whether we have to, while

we can easily achieve much better acoustic

comfort with sound aware architecture.