144th ASA Meeting, Pittsburgh, PA

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Kids + Cafeteria = NOISE!

School cafeteria noise - The impact of room acoustics and speech intelligibility on children's voice levels.


Joseph F. Bridger - joe@stewartacousticalconsultants.com
Stewart Acoustical Consultants

P.O. Box 30461
Raleigh, NC 27622


Popular version of paper 4pAA6
Presented Thursday afternoon, June 6th , 2002
143rd ASA Meeting Pittsurgh, PA


Cafeterias are often synonymous with noise. If you do an Internet word search on "cafeteria noise" or "noisy or loud cafeteria" you would find over 200 entries. A search on "quiet cafeteria" turns up about 40. Dictionaries often use noisy cafeterias as an example of the use of the adjective noisy. Those who study the ability to understand speech in noise often use what is called "cafeteria noise" as the noise source. It is a recording of many people talking at the same time.

This is especially true of the school cafeteria. Parent Teacher Associations (PTA's) and others call acoustical consultants looking for answers to this problem. Some cafeterias exceed the OSHA noise level of 85 dBA and causing cafeterias to begin accumulating noise dose. When visiting and measuring noise at one school, a frustrated teacher screamed into the microphone on the sound level meter (paraphrased) "It sure is loud in here isn't it?"

So what is the primary underlying phenomenon? Research has shown that signal to noise ratio (S/N) is the number one factor contributing to changes in voice levels used by adults. S/N is simply the difference in sound level between the signal (what someone wants to hear) and the background noise (what is interfering with someone hearing the signal). The S/N in a cafeteria is the difference in sound level of the person talking to the noise of everyone else talking measured at the listener's ear. When the S/N is poor, someone cannot be heard over the "cafeteria noise", the response is often to raise one's voice. This makes sense, after all S/N is also the primary factor affecting speech intelligibility (the ability to understand speech) in this setting. It has been thought that adults will all then raise their voices to some maximum sustainable voice level, until listening conditions improve again.

S/N in a cafeteria is controlled by the number of people talking, the total sound absorption in the room (some surfaces absorb sound, some mainly reflect sound), and the seating density (how close the different noise sources are). We know that small changes in S/N can result in large changes in the understanding of speech. This could possibly mean people may modify their voice levels in response to even small changes in S/N.

Click on the three audio file examples below with different S/N ratios to hear the difference a small change in S/N can make.

Poor Signal to Noise    Marginal Signal to Noise Better Signal to Noise

Why are some restaurants quiet and school cafeterias noisy? For starters, classroom acoustics research has shown that children have a much greater difficulty understanding teachers and each other in noise and reverberation than adults do. Reverberation is the time rate of sound decay in a room. It is the liveliness or 'echoey' (made up word) character that is found especially in large rooms with mostly hard surfaces such as the typical large gym. In addition, the number of children in a cafeteria and seating densities are much greater than in adult cafeterias and restaurants. Quiet restaurants and adult cafeterias also have full acoustical ceilings, carpet, and padded furniture. Most school cafeterias have no more than a basic acoustical ceiling with mediocre sound absorption properties. Less absorption, more people talking, greater seating densities, and greater inherent difficulty understanding speech all mean much louder cafeterias.

What can be done about it? There has been anecdotal evidence that adults respond to improved S/N by lowering their voices, even if conditions are still not ideal. If this is true, and if it is also true for children, it could mean that, even a small improvement in S/N would result in children also lowering their voices, and therefore lower noise levels. To test this idea, noise levels and other relevant information to determine the acoustics of the space were collected and analyzed from 19 schools, of which 13 were elementary schools. 5 minute median broadband A-weighted noise levels varied from the lower 70's (dB) to the upper 80's (dB). The analysis of the data showed that children's voice levels are strongly a function of S/N. Reverberation may also have a lesser role. Reverberant spaces also tend to have poor S/N making evaluation difficult. This work showed that better acoustics (better S/N) result in less noisy cafeterias and children using lower voices.

How can the S/N be improved? S/N can be improved by reducing the number of children talking (quiet periods, reading time etc.), reducing the number of children, adding sound absorption, or having a lower seating density. Many of these and other ideas can be implemented in designing better school cafeterias in initial construction. For instance, the use of more absorptive and full acoustical ceilings, and the addition of absorptive wall treatments. Reduction of seating densities. Reducing the number of kids talking may involve having outdoor play areas, or spreading lunch service over a larger period. With existing schools, often answers are limited to looking at adding sound absorption.



This is what happened at Dillard Drive Elementary School in Raleigh, North Carolina. The A-weighted noise levels were generally above 80 dB and up to 86.5 dB for 5 minutes at a time during recent measurements (even higher during the larger study last year). This school had the highest 5-minute median sound levels in the study. The room had up to around 230 children at a given time with a seating density as little as under 15 sq.ft. per person. The only absorptive surface (other than the kids themselves) was a partial acoustical ceiling that had very mediocre sound absorption (probably chosen for fire-rating, not acoustical quality). The room was very reverberant when empty. It was a day lit room with a high ceiling (over 30 feet in height at it's peak). The fire rating required in the existing ceiling made replacing the panels with a highly absorptive (lower fire-rated) fiberglass panel not feasible. Instead, a fiberglass ceiling was added in a lower ceiling area and similar fiberglass panels added to the walls. Armstrong provided the material. If children did not change their behavior, the treatment (calculated) would have reduced levels only 1.7 dB. Instead, levels were reduced between 3-3.5 dB. This can only be from reduced voice levels. This 3-3.5 dB is typically considered a barely noticeable change, but remember what a small change in S/N can do. So far, many at Dillard feel it was a definite improvement. One teacher felt she can now actually tell who is talking too loud and correct them. One of the PTA members shared high praises about the change. Of course, with that small a change in sound level, some have not noticed as much a change. The data from the other schools suggested a 5 dB change (instead of 3-3.5 dB) would result. The Dillard measurements after room modifications clearly is not typical of the other data collected. It may be that the children will adjust their voices further as time progresses.

So the answer to the question: Are all school cafeterias created equal? NO. Although quiet cafeterias may be too much to ask for, certainly less noisy and more pleasant school cafeterias are achievable.

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