Acoustical considerations for designing a multipurpose venue

The Lazaridis Hall was designed to hold lectures, convocation ceremonies, and musical performances, so it was critical to manage competing acoustic needs and expectations.
The Lazaridis Hall was designed to hold lectures, convocation ceremonies, and musical performances, so it was critical to manage competing acoustic needs and expectations.

When a space is to be optimized for musical performances, additional variables need to be considered because different types of music require varied environments. One primary concern is the reverberation time (RT60) of the space, which is the time it will take for energy to decay in a room by 60 dB. The RT60 can be modified by changing the types or the location of finishes in the room, which introduces or removes acoustic absorption from the space. Physically changing the volume of the space is another possible intervention.

Adjustable acoustic elements can change the sound in a room to better serve different uses. For example, retractable absorptive surfaces, such as curtains or banners, can reduce or increase the RT60 of a space to help with intelligibility, musical clarity and, in some cases, envelopment.

At Lazaridis Hall, curtains on the second floor can be pulled into the room to reduce the RT60. Conversely, removing the curtains allows an increased RT in the room, which may create a more complimentary range if a certain music type is being played. With the curtains present, the lowest RT60 measured in the audience is 1.1 seconds. With the curtains removed, the highest RT60 measured is 1.3 seconds.

Aside from RT, another metric used is distinctness (or: D50), which is the amount of energy arriving at a given position within the first 50 milliseconds of the direct sound compared to the overall sound energy. The design target for Lazaridis Hall was 50 per cent, which represents good intelligibility and effective communication. Speech intelligibility can be optimized in a room by ensuring a low RT60, a quiet background noise level, and increasing early sound reflections from nearby surfaces. In this design, speech intelligibility can be improved in the space by bringing in the curtains to reduce the reverberation time. For types of music thriving on longer reverberation time (when speech intelligibility is not the priority), the curtains can be withdrawn to improve the musical experience, at the expense of natural speech intelligibility.

A number of fixed reflectors were designed and placed in the hall to optimize the D50 and maximize the amount of energy sent to the audience. These reflectors include some of the walls, suspended ceiling reflectors and the front of the balcony facia, which was specifically curved to improve the coverage of useful reflections. The ceiling reflectors are painted black, and hide above the slat ceiling below. The slat ceiling was designed to be acoustically transparent, allowing sound to pass through and be reflected off these hidden reflectors.

The combination of the reflectors and the drapery provides the right balance for the Lazaridis Hall audience. The reflectors provide early reflections of sound to the audience while the curtains absorb and minimize late energy reflections. This, combined with a low background sound level, optimizes the overall speech intelligibility in the space.

An analysis tool was used to design reflection patterns from surfaces such as the ceiling and balcony facia. The tool integrates directly into 3D modelling software and enables quick evaluation and adjustment of reflection patterns based on complex shapes and curves to send as much useful acoustic energy across the audience plane as possible.

Natural acoustics

For music, an acoustically ‘pure’ experience is one where unamplified instruments and voice create an excellent experience with clarity and envelopment. Think opera or drama—at their best, these experiences offer the highest level of engagement and intimacy when the audience knows they are hearing the performer directly. Hearing the actors or performers take deep breaths, or hearing their footfalls on the floor, creates an immersive experience. Large opera houses provide this sense of intimacy even in the back row, whether there are three people in attendance or 4000.

Spaces can almost always be designed to function with a sound system, but depending on the programming and environment, natural acoustics may be an option. For example, lecturers at Lazaridis Hall may not always use a microphone, so the room’s natural acoustics were factored into the design. The main auditorium can use natural acoustics for all of its intended uses: lectures, convocations, and musical performances.

However, natural acoustics might not always work if there are other competing needs for the venue. As desirable as natural acoustics can be, there are venues where the required use cases are so varied they need a completely electronic solution. Similarly, there are venues where natural acoustics may not be desired by users—think a rock music venue or a lecture space for faculty that only wants to communicate through microphones and speakers.

Once the acoustic priorities are determined—including whether natural acoustics are even needed, review the most stringent acoustical requirements first and then work from there. Designing for natural acoustics means limiting the background noise, carefully placing reflectors, and strictly controlling reverberation time, all of which place pressure on budget and co-ordination.

Audience location

Acoustics are greatly impacted when the performer-to-audience relationship changes. Consider a thrust stage, which extends out into the audience. This provides an immersive experience for the audience, but it poses significant acoustical challenges as performers are facing away from parts of the audience. In an audience in the round configuration, the audience surrounds the performer, and we cannot rely on the performer to remain in one location or to project in one direction. In the worst case, certain areas in the audience may receive none of the performer’s direct energy. Acoustic reflector locations and shapes must be designed to encourage useful reflections, depending on a range of performer positions and facings.

Introducing complex curves to the reflectors can increase the exposure the reflectors get to different performer locations, and can also optimize the audience coverage from a single reflector. Another option is to have different reflector configurations—locations, heights, and angles—depending on the audience-to-stage configuration. Also, good acoustic design utilizes reflectors to reflect sound back onto the stage for the benefit of the performers.

Experienced performers will be able to adjust their performance style to cover as much of the audience as possible, for the greatest amount of time. Acoustic designers must determine if the acoustic geometry is optimized even as the location of the performer or audience changes.

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