When testing the performance of the NAT Vent Attenuator at Winford Primary School, we performed a noise break in assessment whilst opening a window. The video above illustrates the sound level difference has a steep decay from a small opening.
Speech intelligibility is highly dependent upon the audience being able to differentiate speech from background noise. To enhance intelligibility, speech levels should be maximised and distributed evenly across the seating areas. This is achieved by a combination of methods. First, restricting space increases the amount of sound energy per seat. Secondly, consideration of the length of the hall, to respect distance losses and increase awareness of any visual gestures is advised. Finally increasing the number of early reflections to the audience will increase intelligibility.
Room Shape – Volume
The ideal room volume per seat for lecture theatres is:
The lecture theatre at Bath University holds 350 people and has a volume of 1400 m3, this equates to 4 m3/seat, thus meeting the above criteria.
Room Shape – Length
Visual gestures made by the speaker can greatly improve the recognition of what is said. Although the benefit is not easily quantifiable, it is generally thought that the maximum distance from source to receiver should be no more than 20m. Firstly because this is the maximum distance a typical person can identify these visual prompts, and secondly, it ensures a good direct sound pressure level; essential for localisation.
The Bath lecture theatre had been designed with a maximum distance of 15m, from source to receiver and, depending on the design of a building, this can limit the audience capacity. However, Bath adopted a fan shaped room to maximise the audience capacity yet maintain the source to receiver distance.
Enhancing early reflections
As has been described, reflected sound plays a critical role in maximising loudness levels to the audience. Ideally, it is important to focus these reflections to the rear of the auditorium, since the direct sound at this point will be reduced due to the decay of sound over distance and the grazing effect of the audience. The method by which to focus these reflections is to correctly angle the surfaces at the front of the auditorium so that sound is directed towards the rear of the audience.
Angling the side walls of a given auditorium is often difficult due to the inherent restraints imposed by the building. Similarly in this case, the layout of the Bath University building is fixed, and therefore the option of
adding angled side walls was not explored further. The angle and layout of the ceiling then became the critical factor affecting the acoustic performance of the theatre.
The initial design of the ceiling was not favourable, the sudden step in the bulkhead and the down stand created by a structural steel, created two acoustic shadows on the audience.
To overcome this, the ceiling height was dropped to make the ceiling flat across the structural steel (green line). An angled ceiling was then introduced (blue). The position was calculated using a geometric method such that the audience to the rear of the theatre would receive two separate early reflections, instead of one; greatly improving the speech intelligibility within the lecture theatre.
On a number of projects MACH Acoustics has been requested to provide acoustic consultancy to achieve flexible acoustics within an auditorium. Flexible acoustics is the ability to vary the levels of reverberation within a space to achieve a longer reverberation time for a music and short reverberation time for speech. The table below provides the performance requirements for a range of spaces.
A high reverberation time is required for music, since reverberation will add depth and warmth to the music. On the other hand, a high level of reverberation will have a detrimental effect on speech intelligibility, meaning that a hall specifically designed for music is not suitable for conferences and drama production. Designing an auditorium purely for drama and speech often results is a space where music will tend to sound dead and hollow.
The equation below provides the basic Sabines formula for reverberation. From this equation, it can be seen that to achieve variable levels of reverberation, either the volume of the auditorium and/or the levels of soft treatment within the auditorium, needs to be adjusted.
The designs above work by exposing or concealing the acoustics absorption in a space, and therefore varying the acoustics performance of a building. The key in providing a hall with flexible acoustics is to determine a method of varying the levels of soft treatment. The auditorium within Slough Libary achieves this very cleverly. A future blog entry will be added showing how variable acoustics was added to this space.
Different forms of room acoustic treatments and creative designs are vital when creating spaces with a difference. MACH Acoustics were requested by Allies and Morrison to review the acoustics of their offices and reception spaces. For those who have seen these spaces it is clear that conventional forms of soft treatment would clash with the hard, minimalist feel of this building.
Fundamental to MACH Acoustics was to understand variety of materials used within this development which was basically smooth concrete, thick cast iron details, some furniture and only a few other hard elements. Time was also spent understanding the client, Allies and Morrison’s, expectations, desires and needs. The result of our initial acoustics consultancy is presented in the design report which can be found here;
Creative Room Acoustic Treatments
This report illustrates a respect for architecture and the exceptional beauty of this building. The proposed forms of treatment are seen to compliment the concrete and details of this building. To provide design flexibility, two design options where proposed; the first was to apply acoustic treatment in small quantities to a range of objects, including cupboards, display cabinets and other small elements. The second option was to add soft treatment to one or two areas, where these areas had a sufficient surface area. The second half of this report provides a range of visuals to help the design team understand MACH Acoustics concepts.
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.
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.