Ambient Noise Levels and Reverberation Times in
Megan Noelle Lucus
Brett E. Kemker
Dept. of Speech and Hearing Sciences, Psychoacoustics Laboratory
118 College Dr. #5092
Hattiesburg, MS 39406-0001
Popular version of paper 2PAAa11
Presented Tuesday afternoon, Apr 20, 2010
Poor acoustics can make listening difficult. This is true for all settings but is especially important in elementary school classrooms. If a child cant hear or understand the teacher, learning is diminished. Further, students who enter a classroom with a disadvantage for listening, such as hearing loss, will have even more difficulty understanding in a classroom with poor acoustics.
A major contributing factor to this problem is that classroom acoustics are not taken into account during the planning stages of construction. As a result, schools are constructed near busy highways and airports and may have noisy heating and air conditioning (HVAC) units. These conditions may produce an intrusive background noise level, which not only distracts the student but requires the teacher to raise his or her voice in order to be heard, a practice that can lead to vocal problems.
Therefore, every classroom should be evaluated to see if acoustic conditions are conducive to learning. The American National Standards Institute (ANSI S12.60-2002) has published acoustical criteria guidelines for appropriate background noise levels and reverberation times for schools. Background noise and reverberation time can interfere with ones ability to understand speech in two different ways. First, background noise, if sufficiently high, can directly override speech sounds and make them inaudible or unrecognizable. Second, even in a quiet room, reverberation time can interfere with ones ability to understand speech. The interference is due to the presence of reflected speech sounds (echoes) that arrive at the ear later in time than the same speech sounds that take a direct path to the ear. This reverberation effect is sometimes described as smearing of speech sounds. Reverberation time is defined as the time it takes for a sound to decrease in level by 60 decibels. The more highly-reflective surfaces a room has, the longer a sound will continue to bounce around even after the source of the sound has stopped. Thus reverberation can also contribute to the overall background noise level. Noise and reverberation can act in isolation or jointly to reduce the ability to understand speech.
with the ANSI standard is voluntary, so it is the responsibility of each school
district to determine if its classrooms meet the standard. Thus, the purpose of this study was to
determine if a sample of elementary public school classrooms in the
Two acoustical measures (background noise level and reverberation time) were taken in each room. Both measures were obtained with an Audix TR-40 measuring microphone whose output led to SpectraPLUS Version 5 software. This computerized system was used to record and analyze the ambient noise and to measure reverberation time. Three samples of each measure were taken in each room to assure accuracy of the measures.
The acoustical measurement findings from this study are summarized in the figure below. Average background decibel noise levels can be read from the scale on the left. Average reverberation times can be read from the scale on the right. Each point is the average of three measures. The variability of these measures was small as reflected by standard deviations of 1.1 decibels for noise and .13 seconds for reverberation times.
Figure 1. Summary of acoustical measures taken at three Mississippi Schools
The ANSI standard for school classroom background noise is 35 decibels (A-weighted) and .60 seconds for reverberation time in rooms smaller than 10,000 cubic feet. We found that no classrooms met the ANSI standard for background noise level. Average noise levels for each room varied from 38 to 50 decibels (A-weighted). A spectrum analysis showed that most of the noise was in the low frequency range, a finding consistent with previous studies of school classroom acoustics. Eight of nine classrooms met the ANSI standard for reverberation time, ranging from .21 to .62 seconds. All nine classrooms were less than 10,000 cubic feet.
While all but one classroom met the ANSI recommended reverberation time, none met the standard for background noise level and thus these classrooms do not have acoustical characteristics that are optimum for learning. The negative impact of background noise on the learning process is well-documented. There is ample evidence from previous research that classroom background noise levels that exceed 35 decibels are detrimental to hearing and understanding speech and thus diminish the capacity for learning. Therefore, acoustic modifications or classroom accommodations for teachers and/or students may be necessary for all the classrooms measured in this study.
Once a classroom has been identified as having poor acoustics one must consider the remediation necessary to either comply with the standard or to improve capability for learning. It is important to remember that the overall goal is to improve the speech-to-noise ratio so that children in the classroom can hear and understand the teacher. Previous studies show that a teachers speech needs to be at least 15 decibels above the noise (15 decibel speech-to-noise ratio). This condition is achieved when a teacher speaks at an average conversational level when the background noise level is below 35 decibels. However, in the classrooms measured in this study the minimum 15 decibel speech-to-noise ratio is not met due to the background noise levels exceeding 35 decibels.
There are at least two approaches one may take in dealing with high background noise levels that interfere with speech. The speech signal can be increased or the noise level can be decreased. Thus, the first option to consider for improving the speech-to-noise ratio is to increase the speech level. It is not sufficient to merely request the teacher to speak louder as this can result in physiological damage to the larynx. A common remedy is to have the teacher use an amplification system. Such a system is similar to a public address system in which the teacher wears a microphone and his/her speech is routed to an amplifier. The amplified output is adjusted to present the teachers speech at comfortably loud levels. Loudspeakers are distributed around the classroom in such a way as to enable all students in the classroom to hear at an acceptable speech-to-noise ratio. Although this approach may resolve the background noise problem for one classroom, a teachers amplified speech may contribute to the noise level for classrooms nearby. Another possibility is to identify students who are known to be at risk to noise and to have these students wear personal electronic systems that receive speech sounds directly from the teacher. Again, the teacher wears a microphone but the output from the microphone in this case is transmitted via frequency modulated (FM) wireless technology to a students personal FM receiver with private earphones for each child. In this way, the child can adjust his/her FM receiver to a comfortable listening level. With FM transmitters, different frequency-transmitter channels may be used in each classroom to avoid interference with FM systems in other classrooms. Also, FM systems would not contribute to noise levels in adjacent classrooms. Some of the disadvantages to FM systems are that they are expensive and require professional expertise to assure that all students who use an FM system have it set properly and that it is in good operating condition every day.
The second option to consider in dealing with high background noise is to decrease the background noise level. The literature regarding classroom acoustics consistently identifies HVAC units as a chief source of background noise. The temporal and acoustic characteristics of the noise measured in this study indicate that the primary noise source is the HVAC system. Unfortunately, it is so expensive to replace noisy HVAC systems with quiet ones that this solution is rarely chosen. Therefore, administrators should consider the noise impact of HVAC systems at the planning stage. Other known noise sources such as highway traffic and airport noise may be addressed through legislative avenues. Future school construction must take acoustical characteristics into full consideration. It should become common practice to include acoustical standards in contractors bids for school construction or in modification to existing structures. The overall school location and classroom location within the school are other important noise-contributing factors to consider prior to construction. School boards should consult acoustical experts for advice.
Our findings indicate that the classrooms measured in this study are not optimal for learning because none met the ANSI guideline for background noise. The primary source of the background noise in each classroom was the HVAC system. While improvements in listening are possible by means of amplification systems for teachers and personal FM transmitters/receivers for select students, these remedies are not always practical and often create other problems. It is obvious that if schools are going to meet the ANSI recommended guidelines, then acoustical characteristics of classrooms must be considered paramount by administrators when planning future construction.
Although compliance with acoustical standards is not mandatory, we strongly recommend the following:
1. We urge school districts to conduct acoustic assessments to determine if their classrooms meet the ANSI standard. This recommendation is even more appropriate for school systems that have high failure or drop-out rates.
2. If ANSI S12.60-2002 guidelines are not met, then efforts should be made to either alter the acoustical characteristics to meet the standard or provide accommodations for teachers and students to provide an optimal acoustical learning environment until schools that meet the standard can be built.
3. Future school construction must include acoustical standards that meet ANSI guidelines and are known to be conducive to learning.
4. School boards and administrators should seek advice from acoustical consultants prior to beginning new construction or making modifications to existing structures.