Lecture Theatre Design - Bath University - Part I

Hello, I’m Andrew, one of the eight people working at MACH Acoustics. I've recently been involved in a project for Bath University. MACH Acoustics were asked by ADP Architecture and Cowlin Construction to work on the acoustic design for a 350 seat lecture theatre. Obviously speech is the important element in a lecture theatre and I thought for this post I'd explain some basic guidelines to follow when conceiving plans for auditoriums used for speech.

Design Factors
 
The key factors to address when designing a space for speech are:

1. Providing an optimum reverberation time.
2. Eliminating acoustic defects such as echoes and flutter echoes.
3. Maximizing loudness in the audience.
4. Minimizing noise levels in the design space.
5. Providing a speech reinforcement system where needed.

Our design intent for Bath University was to provide a lecture theatre without the need of providing a speech reinforcement system so we didn't need No. 5.

No.4 is dependent on the sound insulation separating the lecture theatre with all adjoining spaces, plus any Mechanical and Electrical (M&E) services noise within the space, these are relatively simple to control. No.2 is primarily concerned with late reflections and flutter echoes. These effects are usually dealt with by the placement of acoustic treatment which is required to satisfy No's.1 & 3.

Providing an optimum reverberation time

Different types of auditoria require different acoustic performance criteria. One fundamental criterion is the selection of the optimum reverberation time (RT) - or the length of time it takes for sound to decay by 60dB. Long RT’s are desirable for musical performances but tend to interfere with the spoken voice making intelligibility levels lower. Hence, speech specific auditoria require a relatively short RT. A useful guide to selecting the required RT is provided graphically below:

 Placement of acoustic treatment 

The images below provide generic design advice with respect to the positioning of soft treatments within an auditorium. The surfaces to the rear of the auditorium are typically made soft and absorptive to control the extent of reverberation. This rear location also stops late reflections bouncing off the rear wall and being heard as a notable echo by the front row of audience. The amount of soft treatment required will depend on the absorptive properties of the materials used, the volume of the hall and the chosen reverberation time discussed above.

 (A note to consider where long reverberation times are required, such as musical spaces, is that little to no additional soft treatments are then required. This can expose the rear wall and cause undesirable echoes which the audience will hear. To combat this rear walls can be made diffuse. This is done by making the surface irregular. Off the shelf products may be used or bespoke by using varying sizes of projected wooden battens. The purpose being to scatter the sound in many diverse directions compared to a single strong reflection.) 

Surfaces to the front of an auditorium should be hard to enhance early reflections and increase loudness for the audience.

Methods of maximising loudness/volume and how these were adopted for the Lecture Theatre at Bath University East Building will be discussed in my next post.