Great Music in Great Spaces

This innovative series considers architecture from an acoustic perspective, presenting an architecturally significant South Australian edifice as the site of a specially-adapted musical performance. It is an initiative developed by Soundstream Collective and the South Australian Chapter of the Australian Institute of Architects.

Typically, the event opens with an introductory commentary on the historical and architectural significance of the building by an architect. A short recital of specially selected works by the Soundstream Collective follows. An expert in acoustics then describes and explains the acoustic properties of the space, how this is related to the construction and purpose of the building, as demonstrated by the preceding musical performance.

The inaugural event, on Friday 4th November, was a presentation two buildings, the Freemasons Hall (below, left) on North Terrace, built in 1927 and the 2010 Innova21 Building (below, right) at the University of Adelaide North Terrace Campus.

Steve Grieve, of Grieve Gillett Architects, opened the event with a brief history of the building and its distinctive architectural features such as the walls and columns, which appear to be made of carved sandstone, but are in fact made of reinforced concrete covered with a sand-filled render. Adelaide soprano and harpist, Emma Horwood, then performed a selection of songs: O Come Emmanuel (Trad. Arr. C. Sonnemann), Ave Verum Corpus (Mozart), I Dreamt I Dwelt in Marble Halls (Michael Balfe), A Closed World of Fine Feelings and Grand Design (Graeme Koehne), The Flower Duet (Delibes) and Ave Maria (Schubert).

Peter Swift of AECOM then discussed the structural features of the lobby which created a reverberation time of 4 seconds, and how this was perfectly suited to those particular works and instrumentation. The group then left the Hall and walked, in the late afternoon heat, to Innova21, to settle in the covered walkway space between the new building and the adjacent Engineering North Building, and enjoy refreshments of wine and cheese. Ross Chalmers, Senior Architect at Design Inc, presented the significant features of the building, emphasising the innovative technology that had been developed to minimise its environmental impact.

The next performance was a recital by International guitarist Aleksandr Tsiboulski: Cello Suite No.1: Prelude (Bach, arr. Stephen Snook), Guitar Dances (No. 1 – Ross Edwards, arr. Adrian Walter), Prelude No. 1 (Heitor Villa Lobos) and Un dia de noviembre (Leo Brouwer). Following this, Peter Swift explained the acoustic features of the space, why it was so well-suited to the guitar, and how its acoustic properties differed from the Freemasons Hall. The floor was then opened for discussion, and an opportunity for the audience to ask questions.

The Acoustic Features of the Performance Spaces

Peter Swift, Associate Director – Acoustics at AECOM, gave a talk on the general nature of the acoustic features of performance spaces, and described the particular characteristics of both the Freemasons Hall and Innova21. Peter has kindly provided these notes for Soundstream to post on our website.

General desirable features

A few of the attributes required in a performance space are: A single space, minimal overhangs (shallow balconies etc). Heavy walls/ceiling to keep low frequency energy in. A desirable mid frequency reverberation time is volume dependent but generally at least 1.4 sec for smaller spaces and up to 2.2 sec for larger spaces. Seats/audience are the main provider of absorption hence the space should have a volume of approximately 8-10 cubic metres per person to achieve an appropriate reverberation time.

An appropriate mix of early reflected energy (for clarity) and later reflected energy (for envelopment) arriving from all directions including from above. To do this, the space needs appropriate reflectors around and above at different distances. These can be the walls and ceilings and other introduced reflectors, all with diffusive properties (not just large flat areas).

Diffusive elements are: sharp edges and large curves for low frequencies moderate curves and discontinuities for mid frequencies 25-75 mm deep shapes for high frequencies It is also desirable to have early reflections arriving back at the performer. This early energy makes the performer feel good about the sound being produced (like singing in a bathroom). If adequate reflected sound is missing, it feels more like performing outdoors and the performer can tend to try too hard and this affects the quality of the sound being produced.

Freemasons Foyer

The space is approximately 1800 cubic metres therefore with 50 people, we expect the mid-frequency reverberation time to be around 2.6 sec which is too long. For 9 cubic metres per person there would be need to be around 200 people for a single large space to achieve the desirable reverberation. Luckily, because of the large mezzanine overhangs, the space can act as a double volume, a lower volume of around 1000 cubic metres, and a coupled space above connected to the upper space by the ‘opening’ at mezzanine level, and the stairs. This gives an early decay time of around 1.6 seconds in the primary space with a longer late reverberation as sound comes back from the mezzanine area.

People provide the bulk of mid and upper frequency absorption. There are some openings (also letting external noise in) and the relatively lightweight ceiling elements give some low frequency absorption. Overall the reverberation time would be fairly even over the wide frequency range giving good ‘warmth’ (low frequency reverberation similar to or just greater than mid frequency reverberation). The space offers extremely good diffusion, edges of mezzanine balconies for low frequency, columns and coffered ceilings for mid frequencies, and the stonework details on the columns and around the balcony faces for the high frequencies.

For a pleasant mix of clarity and reverberation, good listening positions need to have a direct view to the performer and be preferably at least 4 m away to get mix of early sound and envelopment. The performer also receives a lot of early reflected energy and that allows the performer to feel good about the sound that is being produced.

Innova21 exhibition space

The space is approximately 5800 cubic metres therefore with 50 people, we expect a long reverberation time to be around 3.8 sec. For 9 cubic metres per person there would be need to be around 650 people in a single large space to achieve the desirable reverberation.

People provide the bulk of the mid and upper frequency absorption with some fixed openings at each end and along the edge of the roof. There are some edges of upper open floors between the old and new buildings and a soffit of the third floor giving some low frequency diffusion, and a minor amount of mid frequency diffusive elements on the facade of the old engineering south building, but generally there is not much mid and high frequency diffusion. There are predominantly large flat areas hence most sound heading up will keep going up and come down again quite late. Early reflections off side walls and a little overhead from the soffit of the third level, provide the only early non frontal reflections. So sound is not as enveloping as for the Freemasons foyer. Much of the new wall and roof surfaces are lightweight and low frequency can be ‘absorbed’ (be lost from the space) hence the low frequency reverberation time will be relatively short compared with the mid and upper frequencies. The early sound decay will be short compared with long reverberation time. Hence the space is not as ‘warm’ or the sound as enveloping as is the case for the Freemasons foyer.

The performer will not receive much early reflected energy, only a little from the side walls and third level soffit.The clarity will be high (be closer to listening to a performance outdoors, like listening in an alley with the side walls providing the reflections) but the sound propagating upwards keeps going up. In this space, the sound does come down again but it is too late to add to the clarity of the performance. Theoretically, to listen to a good mix of early and late (reverberant) energy the listener should be at least 15 m away from the performer but at this distance, given the relatively low sound power of the acoustic guitar, the listening level will be low and lost in the relatively high background noise.