Acoustics and EVAC II
Installations and measurements of sound evacuation systems in case of emergencies (the second part)
Acoustic design of EVAC Systems
There are various types of environments where it is necessary to install an EVAC system: large and small, industrial, civil, commercial etc, as specified in the Box at the side.
In the new law UNI CEN/TS 54-32:2015 Fire detection and fire alarm systems -Part 32: Planning, design, installation, set-up, running and maintenance of the voice alarm system all reproduce the information reported in previous legislation and refers to two methods of design: prescriptive and detailed. The prescriptive method is based on simple rules and can be used in simple acoustic cases, where the following conditions are fulfilled:
- reverberation time less than 1,3 s at frequencies of 500, 1.000 and 2.000 Hz;
- background noise level less than 65 dB (A);
- voice message sound level greater than 75 dB (A);
- height of the speakers not greater than 5 m;
- maximum distance between uni-directional speakers (bi- directional) 6 m (12m);
- distance between speakers and listeners (in the absence of obstacles) not greater than 4,5 m (uni-directional) and 6 m (bi-directional).
In all other cases, detailed design is necessary, according to two reported methods of the legislation: analytical calculation based on parameters predicted statistically and acoustic CAD stimulations.
The acoustic expert, who is in charge of the design of a new EVAC security system, has to put into practice the conditions, from the manufacturer, to access the data on materials used in the construction project and planning phase, including the volumetric dimensions of the various areas. Thus, the acoustic CAD can programme. It requires this information in addition to the characteristics of the noise sources, in order to provide the background noise levels and reverberation times, as well as all other acoustic parameters.
On the other hand, in the case of a project or the testing of an EVAC security system already in existence, the technician must be able to carry out preventative measures, for reverberation time and background noise levels.
This allows you to choose how to proceed, depending on the specific acoustic properties of the environment.
An environment which presents with even just one of the problems explained in the graph (Graph 1), such as long reverberation or a raised level of background noise, is a problematic environment for the installation of an EVAC system. For this reason, a design based on an analytical provisional model is necessary with acoustic CAD, given that there can be multiple variables. The simulation helps the designer with two distinct functions which are connected: choosing the most suitable system (brand and modern, as well as sizing and pointing), and correcting the acoustics of the room with the appropriate materials in order to increase the STI.
The acoustic stimulation should be carried out by an acoustic expert, or better yet, by an acoustic designer possibly also with expertise in electro acoustics, so that he is capable of choosing and optimising the audio system or at least the speakers. If, however, the stimulations are carried out by designers without detailed knowledge of acoustics, they need to be assisted by acoustic designers who can at least point out the absorption ratio of useful materials to the internal acoustic treatment, to be inserted into the acoustic CAD (and which are always important to assess). In practice, the EVAC systems have often relied on studies which deal with special systems, but which display no expertise in acoustics and, in most cases, are not executed in the right way...or sometimes are not executed at all. More often than not the competent authorities do not possess the specific expertise themselves.
The design of an EVAC system, unlike what it often thought, must take into account all the rooms and spaces
of a building, and, in particular, those through which evacuation will take place. For example, a school building seems to fall under the category of “simplified system” (given that generally speaking, school rooms are not large). However, the building as a whole may present acoustic problems, especially in common spaces (canteen, gym, lecture halls) and in the sound distribution areas (high and bulky, important in the case of evacuation because most of the escape routes pass through here). For this kind of environment a “simple design” is therefore impossible and the acoustics of each room must be studied in detail as well as assessing the levels of background noise with an acoustic CAD.
In the table below, you will find reverberation times and the STI measured in the canteen of a school in Lucca, before and after acoustic treatment. After the treatment, it can be noted a significant improvement in the intelligibility in the canteen, from an average value of 0,42 to 0.76, and indicator of great acoustic comfort. The percentage of phonemes lost during conversion, for example, goes from an average value of 17 to around 3. This feature allows you to not have to increase the volume during a conversion, thus avoiding an increase in background noise which is disturbing, especially during meal times.
To obtain the results shown in Graphs 1 and 2, a correction to the ceiling and walls was carried out.
The problem is less prevalent for clothing shops, where reverberation times are usually shorter thanks to the clothes on display which have sound-absorbing properties.
The design of an EVAC system can be simplified by placing the speakers at the minimum distances. Large department stores and supermarkets are more problematic: usually they have high ceilings, hard, reflecting floors. Furthermore, the exposed materials don’t have sound- absorbing properties. In these cases, a preliminary stimulation phase is useful in order to evaluate the specific treatments.
Sports halls, swimming pools, auditoriums etc are problematic in terms of sound and should be analysed with an acoustic CAD stimulation. Even in industrial fields, where the environments are large and machinery tends to raise the level of background noise, the treatment is complex.
An extreme case
The analyses of the intelligibile of speech in road tunnels was Laura Mecca’s graduation dissertation topic, an intern at the Studio Sound Service and co-author of this article. In fact, the tunnel represents one of the most difficult cases for EVAC systems: due to the nature of the environment, built with reflective materials (asphalt and cement), which generate high reverberation, and due to the high levels of background noise generated by fans and the cars. The preliminary study revealed that the noise of the fans always dominates that of the car engines while the analyses carried out on the ratio of absorption of materials (such as asphalt), results in a notable difference capable of determining, depending on the porosity, real changes in the STI of more than 10 percentage points. In terms of sound diffusion, (a trumpet from B&C Speakers was used) a change of speaker, with different pointing or positioning it further away (50-75 m) may influence up to ±2% of the maximum. in short: transducers and speakers should have specific features of directivity and power, but the difference in results is linked to the acoustic properties of the environment.
Mandatory EVAC; where and how?
The Italian legislation defines the environments where it is necessary to provide an EVAC system: here is the list with the relative ministerial decrees:
- shopping centres larger than 1.500 m2 or height greater than 30 m (DM 10/03/98);
- school buildings (classes 3-4-5) with a capacity greater than 500 people (DM 26/08/92);
- commercial activity in buildings greater than 400 m2 (DM 27/07/2010);
- office with more than 100 people (DM 22/02/2006);
- public show spaces such as theatres, cinemas, conference rooms etc. (DM 19/8/96); - hotel structures with more than 25 beds (DM 9/4/94);
- health structures and hospitals (DM 18/9/02);
- sports facilities with space for more than 100 spectators (DM 18/3/96);
- historical and artistic buildings: museums, galleries, libraries etc. (DM 20/5/92 n.569 for museums and DPR 30/6/95 n.418 for libraries);
- underground stations (DM 11/1/88).
- Laura Meucci: Sistemi di sicurezza EVAC di Segnalazione vocale per l’evacuazione di grandi ambienti: progettazione, messa in funzione e collaudo (con il metodo STI), in ambito acustico, relativi ad alcuni studi e progetti in corso d’opera; Degree thesis in Engineering;
- European legislation EN 60849, from April 1998, Sound systems for emergency purposes, recognised in Italy in 1999 as the regulation CEI EN 60849 Sistemi elettroacustici applicati ai servizi di emergenza;
- Legislation UNI ISO 7240-19 Fire detection and alarm systems - Part 19:
- Design, installation, commissioning and service of sound systems for emergency purposes, Sistemi fissi di rivelazione e di segnalazione allarme d'incendio - Parte 19: Progettazione, installazione, messa in servizio, manutenzione ed esercizio dei sistemi di allarme vocale per scopi d'emergenza;
- European legislation EN 54 Fire detection and fire alarm systems, in particular parts 2, 4, 16, 24, which the 7240-19 makes specific reference to equipment making up the sound system.