Why is proper classroom acoustics important?

In the nearly six thousand educational institutions operating in Hungary, the acoustics of the classrooms are mostly not yet adequate. In addition to the inadequacies of sound insulation (noise from the outside), this is mostly due to inadequate room acoustics. Direct effects of insufficient classroom acoustics:

• increased noise in the classroom
• difficult speech intelligibility towards the back of the classroom
• more exhausted students and teachers, damaging health effects
• impaired ability to concentrate.

Without acoustic solutions, for example, in classrooms longer than 7 metres, about two-thirds of the teachers’ speech is not understood. On average, only half of the speech can be understood in the back of the classroom. It is easy to imagine that the appropriate acoustic conditions also have a significant impact on the learning outcomes.

Modern room acoustic design in Fazekas Mihály High School’s history classroom

In 2017, we decided to create an acoustic reference classroom, using the results and experience of several of our previous school acoustic design works and a related research project, and in order to support the room acoustic standardization process we have started in Hungary, for which we were looking for sponsors. With regard to school selection, we followed simple principles: we wanted to fix the worst classroom in the best school. This is how we got to the 315/A classroom in the old building of Fazekas Mihály Primary School and High School in Budapest. This room is also special because not only students are learning here, but it is also used for teacher trainings.

The classroom is not newly built, so we had to assess the current conditions with objective methods before starting the design process. This was achieved by acoustic measurements, which served as a basis for the creation of a computer calibrated room acoustic model. Calibration was performed using a genetic algorithm for frequency-dependent sound absorption properties of existing substances. This is a search procedure whereby the data of the modeling is calibrated to the measurement data, so that we get a condition in the model that is extremely close to reality.

The purpose of the measurement and design was to reach and exceed the specified value of 0.75 of the speech transmission index (STI), in the case of voice without amplification, or so called “raised vocal effort” from the teacher’s desk. The goal was to reach or exceed this value in as many places in the classroom as possible. The speech transfer index describes the possibility and acoustic limitations of speech intelligibility in a complex way, and this is currently the most appropriate objective parameter for the sizing speech spaces. The quantity, quality and type of casings to be used can be determined most efficiently and with the greatest certainty by computer design – with numerical simulations.

In general, the ideal classroom acoustic design can be achieved by using three main solutions together to solve an optimization task:

1. Differentiated sound-absorbing-reflecting suspended ceiling
2. Sound-absorbing wall coverings
3. Providing deep sound absorption

Differentiated sound-absorbing-reflecting enclosure means that the ceiling is not entirely homogeneous in the case of acoustic properties, but is sound-absorbing where necessary while elsewhere reflective, to aid sound propagation in the case of silent speech sources, but to absorb reflections that are detrimental to speech intelligibility. This is especially important for larger classrooms, in the current situation it was sufficient to use a pure sound absorbing suspended ceiling. Voice-absorbing wall coverings in the rear rows significantly increase speech comprehensibility – without this, adequate speech comprehensibility cannot be ensured in many cases. In general, the attainment of bass absorption is critical in terms of limiting background noise, or when using loudspeakers, but there are also dominant frequencies in this range for male teacher sounds.

When the measurements were made, it became clear that the speech transmission index in the room was low, and the reverberation time was extremely high due to the presence of stone floors, plastered walls and other hard, sound-reflective surfaces. The easiest acoustic parameter to measure for such tasks is the reverberation time, which, when graphically expressed, shows how long it takes for the sound pressure level generated by a suddenly deactivated sound source to decrease in the room to one millionth (60 decibels). This value, of course, depends on the location and the pitch of the listener, but from averaging it, a characteristic number can be obtained for the room, which creates an objective image of the acoustic viscosities of the room in a significantly simplified way, and still orients the designer. It turned out that we have to reduce the reverberation time to a quarter in order to achieve the desired speech comprehensibility value. This is not a trivial task, but with the available headroom and the possibility of placing wall coverings, the task proved to be solvable.

As the sound absorbing materials operating in a wide range of sound heights are generally soft, we chose a special coated material for the wall cover, which is impact-resistant, and it even can be pinned, so it could also replace wall board in the back of the room without losing its function.

Due to the materials and solutions used, the base noise in the room was reduced by more than 6 dB without having to change doors and windows or perform mechanical noise reduction. This demonstrates that room acoustic solutions address several problems at once: they improve speech comprehensibility, and reduce noise and reverberation time. And if you also add that these solutions don’t cost a lot of money, it becomes obvious that it’s worth applying. If required by the Hungarian regulations, the appropriate acoustic design of classrooms will not only be worth it, but will also be self-evident.

In addition to the acoustic solutions, we also renewed the lighting. Modern dimmable led lamps were added to the classroom, so now you can not only hear but also see properly in the classroom.

The implementation of modern room acoustic solutions is indispensable in the case of classrooms. Not only in public spaces, but also in smaller rooms and corridors for noise reduction reasons. Even in technically challenging renovation situations, school-built infrastructure can be updated with modern design parameters and methods.