Technical Report

Acoustics of an Atrium with Lobby

1    Abstract
2   Atrium
2.1 Reverberation
2.2 Sound Transmission
2.3 Focusing Effects
3   Lobby

1 Abstract

The simulation of the sound propagation within the Atrium shows the following:

The biggest reduction in sound level and reverberation will occur with an absorbent shell wall
The sound level and the reverberation can further be noticeably reduced with perforating the wood panels of the interior curtain wall

2 Atrium

2.1 Reverberation

The dominating measure to describe the general "feel" of a space is reverberation. There are additional measures for special spaces like speech transmission in auditoria or music perception in concert halls. Reverberation is controlled by applying absorbent materials to the surfaces of a room. If the proportion between the volume and the surface area is big, then even making all surfaces absorbent will not reduce the reverberation below a certain value. We have calculated the reverberation in the atrium by simulation testing these options:

Option A: Non-absorbent shell wall
Option B: Absorbent shell wall
Option C: As option B, with 2 microperforated polycarbonate sheets beneath the glass roof (distances 30 mm and 100 mm)
Option D: As option B, with perforated wood panels on the interior curtain wall

With the alterations of the built the sound absorption will be excellent even at low frequencies.

The resulting reverberation times are:

2.2 Sound Transmission

The sound transmitted within the atrium is also dependent on the properties of the surfaces. Surfaces near a speaker will enhance the sound level of a speaker and surfaces further away will guide sound by reflections to a listener.

We have simulated a speaker standing in the south of the atrium speaking with a raised voice. Here again the already above mentioned options were tested. Since the results for some options differ only by a small margin from other options only the most striking options are depicted.

Comparing the pictures it is interesting to note that there is no big difference in sound level between the front part and the back part of the atrium. This is due to the reflecting pixel and shell walls as well as the glass roof which guide the sound to the back part. More quantitatively than the pictures above is a direct comparison of the histogram of sound level in the back part of the atrium.

Picture 5 shows the following:

With an absorbent shell wall the sound level is reduced by about 6 to 7 dBA, which is a noticeable difference.
Additional microperforated sheets beneath the glass roof will reduce the sound level by only a small amount (another 1 dBA)
Additional perforated wood panels will cause a bigger drop in sound level (another 6 dBA)

2.3 Focusing Effects

There will be no focusing in the atrium. The curvatures of the shell and pixel walls are bigger than the size of the atrium and no focusing can occur according to the reflection laws of curved mirrors.

There will be however some mirroring effects. As already mentioned above sound will be reflected mainly by the glass roof from the front part into the back part of the atrium. Also some whispering effects may occur along the pixel wall in the front part of the atrium. Depending whether a "pixel" is in the way due to its curvature sound is guided along the reflecting wall. But this effect can be neglected. It will only occur if the listener and the speaker will stand near the wall and only within the front part of the atrium where the pixel wall is freely accessible.

3 Lobby

The ceiling of the lobby together with the reveal of the skylight and the wood panels of the facade need an absorbent treatment.

Possible measures are:

Ceiling and reveal skylight: BaswaPhon, a thin acoustically permeable, white plaster on a stiff mineral wool. The appearance will be like a plain gypsum. Pigments can be added to the plaster to add colour.

Perforated wood panels on the interior curtain wall: The panels could be perforated with an acoustic mat behind it.

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