Research

Historical

Outdoor acoustics have created musical languages around the world. One example is yodelling in the European alpine countries. A similar musical form is the Swedish "kulning" used by the women of Dalarna to herd and call their cattle and to communicate with other farms. Kulning and yodelling developed through interplay with the landscape. This shrill and staccato form exploits the echo found in mountainous topography. Neither would be possible in the flat countryside of the Netherlands for example.

In Italy it is likely that Italian gondoliers learned early on to exploit the echo on the narrow canals of Venice passing between the tall, tightly packed buildings. The environment itself inspires song.

Indoors, echo can create a problem and various solutions have to be applied, for example, so that speech can be understood without effort.

Throughout history churches have acted as classrooms for purposes other than theological. Current requirements in terms of room acoustics and teaching are not particularly compatible with the echoes in churches. Organ music, on the other hand, is often adapted to church acoustics. If the music is to be enjoyed, the tempo and the length of notes have to be selected on the basis of the particular acoustics of the church, as Bach and Handel knew. Similarly, priests have been forced to speak slowly and almost over-clearly so that their words reach the congregation.

With the Reformation, knowledge and teaching became more important. The spoken word took on increased importance and more attention was paid to the negative effects of long reverberation times in teaching situations.

Research indicates that good classroom acoustics are vital for learning. However, in New Zealand very little attention has been paid to this in the past. The 1970 Codes for primary and intermediate schools made mention of the acoustic requirements for teaching spaces and the supporting Architectural Guidelines gave an outline of the problem with advice on the control of noise but did not set any standards regarding reverberation times. Acoustic treatment in secondary schools was not considered important enough with only music rooms being considered worthy of nominal treatment. Arguably many schools built since the 1970 Codes were introduced, particularly the open plan designs, still have acoustic problems despite the availability of basic information on the importance of acoustics.

The Ministry's Design Standards Guidelines requires that teaching space acoustics comply with AS/NZS 2107:2000. Acoustics - Recommended design levels and reverberation times for building interiors. This standard recommends design sound levels and reverberation times for classrooms and other areas, such as, assembly halls, laboratories, music studios and "manual arts" areas

Reverberation time

In a room the sound waves are reflected (bounce) off surfaces such as walls and ceilings, causing one source of sound to become several. All the sounds in a room therefore have a particular reverberation time, in other words the sound is heard for a while after the sound source has been turned off. The sound lives on in the form of an echo, a phenomenon known as reverberation.

Reverberation time is taken as the time it takes for the sound pressure level to fall by 60dB after the source of the sound has ceased. In theory, the reverberation time can be calculated on the volume of the room using Sabine's formula. Reverberation times are normally described in six frequency ranges (octave bands) between 125 and 4000 Hz.

FIG 1: Hard surfaces prolong reverberation

Early reflections of sound (e.g. via a blackboard) are good. These produce an impression of space, help to reinforce and clarify speech and convey the feeling that the voice carries. These reflections are particularly important for those at the back of the room where the direct sound is muted if the teacher or lecturer is facing the board. Direct sound is the sound that carries, for example, from the lecturers' mouth directly to the students' ears and it is crucial to good speech intelligibility. Too long a reverberation lime and late reflections, on the other hand, impair speech intelligibility. Fig. 1. for instance shows a gymnasium with hardly any absorbent materials on the walls, floor and ceiling. Consequently, the reverberation time for this hall would be in the order of 1.5 to 2 seconds which is well above the recommended maximum of approximately 0.8 seconds.

Long reverberation times also affect speech intelligibility. A word that does not have time to finish reverberating before the next word reaches the listener causes poor word recognition.

Our ability to comprehend sound is also affected by the relationship between the strength of the sound and the strength of the background noise (signal to noise ratio) such as ventilation noise, footsteps, talking, etc. which is always present.

The geometry of the room, the acoustic characteristics of its boundary surfaces (see the section titled Acoustic Design Solutions for descriptions of different absorptive surfaces), the voice of the speaker and the hearing of the listener are other factors which affect speech intelligibility. ( "Echo - a plus and a minus" in Don't Limit Your Senses, Saint-Gobain Ecophon AB 2002, Hyllinge Sweden, pp. 73-74).

The research reported by the New Zealand Classroom Acoustics Research Group (Wilson et al in "Classroom Acoustics-A New Zealand Perspective", 2002.). and by Dodd and Whitlock (Dodd G and Whitlock J, Auditory and Behavioural Mechanisms Influencing Speech Intelligibility for Primary School Children, V-3581-3582, 18th International Congress on Acoustics, Kyoto, April 2004.) has focused on Primary School classrooms because children of primary school age are at the most risk from the effects of non-optimum acoustical conditions in the classroom.

Acoustical problems of excessive room reverberation and background noise may occur in classrooms for pupils of all ages but older children have largely developed aural and linguistic skills and knowledge which allow them - and most normal hearing adults - to instinctively cope with and correct for these difficulties.

Younger children (<12-13 years) by contrast have yet to fully acquire these skills and therefore cannot make the compensations which are instinctive for adults. In addition it is emerging that the hearing systems of young children take a long time to mature - perhaps this process takes typically into their teenage years before they possess what we consider to be an adult physiology. This immaturity renders them less able to discriminate against noise compared with adults.

Thus young pupils have a particular need for reverberation-free and noise-free conditions in the classroom. The difficulties that non-optimised rooms create may not be perceived by their teachers since, as adults, they possess the skills enabling them to hear well under such acoustical conditions.

Current Research

"Classroom Acoustics - Milestone 6 Report - An Investigation of the Classroom Acoustics Needs of Primary School Children" prepared by Auckland UniServices for the Property Management Group, Ministry of Education. January 2004.

This research is an investigation into two psycho-acoustical phenomena which are hypothesised to play a large part in speech intelligibility and the understanding and production of speech in noise: Integration time within the hearing system, and the so-called Lombard effect. The work has been carried out with a particular focus on children, and with a view to understanding how to optimise the primary school classroom environment for intelligibility of speech.

"Classroom Acoustics - A New Zealand Perspective" Published by The Oticon Foundation in New Zealand, 142 Lambton Quay, PO Box 9128, Wellington.

The project was carried out by a multidisciplinary team of professionals who have a strong belief that good acoustics should be one of the prime design considerations for new classrooms. The Oticon Foundation in New Zealand has funded the project. The Oticon Foundation is a charitable trust that provides grants for projects that increase awareness and knowledge about hearing loss.

"Building Bulletin 93" Published by The Stationary Office UK.

The Building Bulletin 93 forms part of the Building Regulations 2000 (UK) and is an extensive resource for those wishing to emulate the standard required in the United Kingdom.

"Eco Miljo" A monthly web-based newsletter published by Ecophon Sverige, Hyllinge, Sweden. They manufacture a range of acoustic ceiling tiles and wall absorbing materials.

The newsletter contains interesting and informative articles on acoustics in work and teaching spaces. It also documents methods used to improve the acoustics in these areas.

"The Acoustical Environment", Melissa Smith, Doctoral Student, School Design and Planning Laboratory, University of Georgia. Paper Presented to the School Design and Planning Laboratory Seminar on Acoustics in the Classroom May 25, 2002

Smith reviews the literature on acoustics, particularly noise, reverberation, signal-to-noise ratio, task performance. According to the American Speech, Language, and hearing Association (1995) sources of noise inside classrooms including students talking, desks and chair sliding on floors, and books and papers shuffling is the most detrimental to learning because of the similar frequency of the teachers voice particularly if combined with external noise. It explains that the acoustical environment of a classroom is measured by the signal to noise ratio and reverberation time.

"New probe on air noise", Bruce Tober, Noise Management, May 2001.

Aeroplane noise was identified by Evans and Lercher as contributor to impaired learning. They looked at 217 third and fourth grade children in a rural area 22 miles from Munich Germany before and after the opening of a new airport. Children in the chronic noise group experienced modest but significant increases in blood pressure and significant increase in stress hormones, which could have life long effects. Evans and Lercher also compared 115 nine and ten year olds from Alpine villages in Austria looking at the kinds of noises typically found throughout neighbourhoods in Europe. They reported that exposure to lengthy periods of noise, for which children have no control over, appears to lead to "learned helplessness syndrome' a condition usually linked to depression and poverty.

"Chronic Noise Exposure and Reading Deficits: The Mediating Effects of Language Acquisition". Lorraine Maxwell and Gary Evans. Environment and Behavior, 29(5), 1997

A study of first and second grade students at two New York City schools found that students attending the quieter schools scored as much as 20 percent higher on a word recognition test than the students who learned with noisy aeroplanes flying overhead