Reflector arrays and canopies

akuTEK investigates several features of concert hall reflector arrays and orchestra canopies, to gain knowledge on design that provides adequate reflections.

Orchestra Canopies by M Skålevik was presented in a Stage Acoustics session at the 162nd ASA meeting in 2011.

A paper on canopies and reflector arrays was presented at BNAM 2006, read the paper here.

· Canopy reflectors can be an important element in Stage Acoustics and in some cases in auditorium acoustics

· A low density canopy or canopy array can effectively prevent echoes from above, even without changing stage acoustics.

· Not every hall does needs a canopy, it can even make things worse

The low frequency limit for the useful reflections from a reflector array is discussed in this Paper and this Poster, as presented at IOA Auditorium Acoustics in Copenhagen 2006. A simple geometric estimator for the low cut frequency is deduced from measurements and theory. A paper on this topic was presented at ICA 2007 in Madrid: Low Frequency Limits of Reflector Arrays(pdf). Presentation: PDF-version (261kB). The formula suggested by Skålevik has been verified several times, more recently by Szelag (2016), and applied in design, e.g. ref. ’Acoustical design of the new canopy for the Ginastera Hall of the Teatro Argentino of La Plata’, the paper by Basso 2016.

A status report on Frequency Limits of Reflector Arrays was presented on the conference Auditorium Acoustics, by IOA and NAS, Oslo 2008. Paper. Abstract:

The useful range of reflector arrays is restricted by two independent low frequency limits and one high frequency limit. Flat panel arrays have inherently narrow frequency range, and it is therefore important to predict and to control its frequency limits. A suggested predictor for F_c, based on array geometry, has been verified by measurements at normal incidence and for certain geometrical patterns. Further work should aim for general confirmation of this predictor, or an improvement if needed. It is recommended to develop array design with a more even frequency response within ±3dB deviation from the ideal level, and with a frequency range of at least the 250-4kHz octave bands. This requirement may imply that the flat panel in its simplest form is not adequate.

The existence of a reflection filter related to element size and geometry, and its predictable corresponding cut off frequency is supported by several scale model measurements, and recently by a student project on NTNU, Trondheim, Norway, by M. Dalåmo: Frequency Response of Reflector Arrays .

Angle dependency of the reflection filter is yet to establish and will be further investigated. A recent investigation of incidence angle on square panels using BEM is presented here.

The inherent narrow frequency range of reflector arrays when used as orchestra canopies can be widened by using curved panels. The effect of convex edges and curved panels is presented here.

Edge perimeter and edge diffraction is significant to the low frequency response of reflectors and reflector arrays. External research paper: Fast time-domain edge-diffraction for interactive acoustic simulations (2.6MB pdf). Here, one presentation by Svensson and Calamia.

When designing a reflector above the orchestra pit in an opera house, it is essential to define its purpose, since the balance between singers and orchestra is very sensitive to its shape and position. A case study can be found in this paper.

Pioneer work

An array of triangular panels forming a canopy in the 6000 seat Tanglewood Shed was completed in 1959. The main purpose was to provide early reflections, less than 50ms delayed, reducing the initial time delay gap - ITDG.

Since then panel arrays have become common in concert halls, small or big.

Caveats and pitfalls

In typical wide post-war auditoria, canopies were often used as a remedy to compensate for the lack of early reflections, suppressing the already weak early lateral reflections even more, resulting in very low early lateral fraction (LF). Related: Origin of Spaces; The Queen Elisabeth Theatre - Spatial Sound in a Wide Room; Acoustical problems in large post-war auditoria.

An overly dense and low orchestra canopy acts like a low ceiling—tending to separate the podium acoustics from the auditorium acoustics, resulting in too strong or unbalanced orchestra sound, and lack of reverberance and late energy. Related: Orchestra Canopies—Significant Features (Paper 2006) (Presentation 2006); Stage conditions for orchestral performance (paper) (presentation).

Relevant akuTEK links:

An improved low frequency radiation model for finite sound reflectors (presentation)

Frequency Limits of reflector arrays—status report (paper) (poster)

Frequency Response of Reflector Arrays

Low Frequency Limit of Reflector Arrays (Paper 2006)

Low Frequency Limit of Reflector Arrays (Poster 2006)

Low Frequency Limits of Reflector Arrays (Paper 2007)

Low Frequency Limits of Reflector Arrays (Presentation 2007)

Orchestra Canopies—Significant Features (Paper 2006)

Orchestra Canopies—Significant Features (presentation)
Reflecting Disc - and the Equivalent Dipole Source
Reflector above the opera orchestra pit

Reverberant Space (demo)

Sound Coloration