David Lubman and Louis C. Sutherland
a Acoustical Consultant, 4301 Middletown Lane, Westminster, CA 92683-4514 USA
b Acoustical Consultant, 27803 Longhill Drive, Rancho Palos Verdes, CA 90725-3908 USA
The incremental costs for achieving good classroom acoustics is a small fraction of total costs of new school construction. But since school construction funding is limited, good acoustics must compete with other programs intended to improve the education and lives of citizens. Good acoustics can survive this competition if decision makers are convinced that its costs are justified by their economic benefits. A proper economic study of this question has never been attempted. However, informal engineering estimates made by acousticians, audiologists, and material vendors suggest that the case for good classroom acoustics is strong. This paper identifies and estimates some of the costs for good acoustics in new construction. It also identifies and estimates some previously unrecognized economic benefits of good acoustics, as well as some of the hidden costs of marginal or poor acoustics. Costs and benefits are compared using recent economic data from the USA. Results suggest that the economic benefits of good acoustics far outweigh its costs. Therefore, a case can be made that good classroom acoustics is a good economic investment. The authors intend this paper to inspire others to more fully study the economic, social, and educational benefits of good acoustics.
The education of its citizens is essential to all modern societies. Most formal education takes place in classrooms. Classroom learning typically involves intensive speech communication between teachers and students, and among students. The effectiveness of this communication, and hence, the effectiveness of the learning environment is mediated by acoustical conditions in the classroom. Good classroom acoustics greatly facilitates learning. With good classroom acoustics, learning is easier, deeper, more sustained and less stressful. Excessive noise and reverberation in classrooms are barriers to learning to the extent that they degrade or inhibit speech communication.
Poor classroom acoustics degrades the educational process for all students and teachers. It is also true that noise and reverberation are selective barriers to learning. Young children, adult learners, and persons with hearing, language, speech, attention deficit or other learning disabilities are especially vulnerable to marginal or poor acoustics.
The educational benefits of good classroom acoustic environments have not been quantified. This creates a problem. Absent clear statements of their benefits to learning, the features necessary to ensure good classroom acoustics are often omitted from classroom design specifications. Even when acoustical features are initially included in design, they are often removed in value engineering design exercises because they are perceived as costs without quantifiable benefits. It is important therefore to quantify the cost benefits of good classroom acoustics. This paper previews some exercises intended to quantify the benefits.
Figure 1 shows recent costs for K-12 (Kindergarten through 12th grade) US classrooms for the year 2000.
Some classrooms suffer from excessive reverberation due to insuffi-ient sound absorption. If the ceilings are not too high, a cost-effective way of introducing sound absorption is with an acoustical ceiling. Table 1 shows that the cost difference between a mediocre acoustical ceiling of NRC (Noise Reduction Coefficient) 0.55 and a good ceiling with NRC 0.75 is less than $1 USD per student, when amortized over a 20 year lifetime. It also shows that unoccupied classroom T60 for the mediocre ceiling is 0.75 sec., somewhat too high for a learning space. The better ceiling reduces T60 by almost 0.2 sec, which can make a noticeable improvement.
The maximum T60 recommended for unoccupied classrooms of about 10,000 ft 3 is o.6 sec. Table 1 shows a T60. of 0.75 sec with the NRC 0.55 ceiling, which exceeds this recommendation.. If the classroom has a fairly high ceiling of about 11 or 12 ft (not recommended), T60 may even exceed 0.8 sec. Even if the SNR (speech-to-noise ratio) is an excellent 15 dB, the RASTI (Rapid Speech transmission Index) score will be only about 0.6, which is marginal for learning. Upgrading the acoustical ceiling reduces T60 by nearly 0.2 sec., which increases RASTI scores by about 0.05. The lower T60 improves recognition of unfamiliar sentences by about 3% to 4% for normal listeners, and even more for listeners with communication disabilities. The benefit is even greater at low SNR.
The cost for educating a child in a mainstream classroom is about $7000 USD per year per student. With the better ceiling, about 3% more unfamiliar sentences are understood correctly by normal students. A measure of the economic value of this improvement is (0.03)* $7000 or $210. The additional $1 cost for the better acoustical ceiling is relatively insignificant. By this measure, the economic value of the better ceiling exceeds its cost by a factor of 200. That is surely a good investment! But unless value-engineering reviewers are told the economic benefits, they are likely to reject the better ceiling on costs alone.
A metric is proposed by which the annual economic value of quiet can be estimated. Assume that AAE (Acoustical Access to Education) is nil when classroom ambient noise levels are 65 dBA and over. Also assume that AAE is 100% when classroom ambient noise is 35 dBA or less. Assume a linear relationship between AAE and noise level over the 30 dB range. Then every 3 dB of noise reduction between in this range improves AAE by 10%. If the annual cost of education is $7000 per student, the value of a 3 dB classroom noise reduction is $700 per student. The value for a class of 20 students is thus $140,000 USD per year. This argument can be used to justify paying more for quiet HVAC or other noise-making products. It can also guide purchasing decisions when quieter but more expensive products are available alternatives.
There were 2.9 million public school teachers in the US in the Year 2000 (National Center for Education Statistics) Teachers lose an average of 2 days/year for vocal fatigue caused by raising their voices to talk over noise. Recently, the cost for substitute teachers was about $220 per day. The national cost for teacher vocal fatigue is estimated at $638M USD. A large fraction of that cost could be saved each year if schools were quieter. Teacher satisfaction and the amount of verbal interaction between teachers and students would also improve. Dividing $638M among the 85000 US Public Schools would provide an annual budget of $7500 per school for educational purposes.
Current methodologies for estimating the costs and benefits of good acoustics are inadequate. Studies are needed to determine the full impact of acoustics on learning, the full social costs of poor acoustics, and the cost benefits of good acoustics. In the absence of methodical economic studies, crude estimates were made here. (Other examples will be shown in the oral presentation). By standardizing and proliferating such measures, good classroom acoustics can be given the economic foundation it now lacks.
1An earlier version of this paper was presented at the 138th Meeting of the Acoustical Society of America, J. Acoust. Soc. Am, Vol 106, No 4, Pt. 2, October 1999 1