Reverberation time Archives

Posted by IT MACH

Lecture Theatre Design with Acoustic Modelling

3rd April 2014BB93, Lecture theatre, Resources, Reverberation time

MACH Acoustics were appointed to carry out acoustic design for the refurbished Clattern Lecture Theatre. MACH created an acoustic model of the lecture space, calibrated by acoustic testing of the existing site. The design report concentrated on the two main factors effecting lecture theatre acoustics:

Speech Ineligibility

A standard requirement for a lecture theatre is to achieve good speech intelligibility of the speaker throughout the whole audience within the space. Speech intelligibility is maximised by increasing the signal to noise ratio, that is by maximising the loudness of speech and minimising the ambient noise level. Thus, the acoustical design of rooms for speech reduces to the consideration of four factors:

Providing optimum reverberation time.
Eliminating acoustic defects such as echoes and flutter echoes.
Maximising the loudness in the audience.
Minimising the background noise level in the room.

Speech intelligibility can be measured by the ‘Speech Transmission Index’, a percentage between 0% and 100%. BB93 requires that areas designed for speech should achieve ‘good’ speech intelligibility, corresponding to an STI of at least 60% across all areas of the audience.

Reverberation Time

Reverberation time is not only important for speech, but also important for audio visual presentations, as well as for amplified speech. For example if a film or a music track is being played from the AV system it is desirable to have a low reverberation time to ensure that amplified sound is clean and intelligible by reducing the build up of reverberant noise that can muddy the sound. This is seen clearly in spaces such as cinemas, which are heavily treated with absorptive material.

Onsite Tests

In order to accurately construct the model, MACH Acoustics carried out tests to determine the current levels of speech intelligibility across all areas of seating. Sound pressure levels were measured at various points around the room with a loud speaker playing in the position of a lecturer. The results are shown below. To put this into context, a 10 dB difference is usually perceived by the human hearing as a doubling in loudness.

The results show that levels in the wider areas, even when at a comparable distance to a position in the centre of the room are significantly lower. Levels at the back of the room are very similar to those as wide seating positions in the first few rows. This occurs mainly for two reasons.

The seating arrangement spreads wide from where the lecturer would typically be stood. Hence the lecturer must constantly turn to address different sections of the audience in order to be heard. Consequently when he or she is facing towards one side of the room, the level heard on the other side of the room is lower. MACH would usually advise that a seating arrangement is a) in front of the speaker and b) has an angle from the speaker to the widest seating position that does not exceed 30○ (currently as much as 70-80○).
The front portion of the ceiling is angled and reflective. MACH would usually advise this type of arrangement, as it aids in reinforcing sound levels towards the rear of the theatre by angling early sound reflections towards the back (as illustrated below). Hence as in this case, the sound is more evenly distributed across the majority of the audience behind the first few rows. This can be achieved by a sloped ceiling as in this case, or by suspending angled reflective panels (available in a range of aesthetically pleasing designs) above the area where the lecturer would usually stand.

Acoustic Model

An acoustic model was created, based upon the test data. The model allows us to assess the space by estimating the reverberation time and the spread of sound across the space, as well as the ‘Speech Transmission Index’ (STI) for speech intelligibility.

Ideally MACH Acoustics will usually target an STI of at least 65%, ‘ideal’ speech intelligibility and it can be seen that this was the case in all areas of the theatre. The reverberation time in the existing space is on the high side for amplified AV presentations. It was therefore deemed desirable to reduce this in the design of the refurbished theatre.

New Design Options

Using the model, MACH Acoustics developed a number of design options for Kingston University, providing varying levels of acoustic treatment to reduce the reverberation time and allow for better travel of speech within the space.

All options offered reduced reverberation time from 0.7s to 0.5s and under, with increased STI at back and room edges.Whilst MACH Acoustics had a recommended best options, the various methods allowed the client to select the best design style for the build.

Posted by IT MACH

Lecture Theatre Design – Bath University – Part I

3rd September 2010Lecture theatre, Reverberation time, Room Acoustics

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.