Quality Classroom Acoustics help Education
The speaking person transmits three types of information to the listener. First, we learn what a person is talking about. This is a logical, or semantic, information. It is transmitted through the selection of a semantic content of words, their order, syntactic structures. Secondly, whoever speaks, thanks to the individual characteristic of the voice of each person. This is an identifying information. By voice, you can determine the physical and psycho-emotional state of a person, his/her intentions and some character traits.
That is why, during student lectures in classrooms, serious attention should be paid to the creation of a comfortable acoustic environment. In the current established standards, there are a number of sound insulation requirements that can be applied to lecture halls considered, for example, as high school classes.
Every responsible designer envisages the decoration of walls and ceilings of classrooms with specialized sound-absorbing materials that correct the acoustic environment of these classrooms. At the same time, it is important to separate auditoriums with live acoustics (without sound amplification) and auditoriums with sound amplification. All of the above about the use of sound-absorbing materials refers to the last case.
Good audibility is one of the most important requirements that must be met by educational classrooms. It allows all students to comfortably perceive the speech of a lecturer or other audio information in the educational process. The requirement of good audibility can be fulfilled if the sound arising at one point in the classroom is perceived without distortion at any point of the room (without echo and with a favorable reverberation duration).
Benjamin Boone, a Pro-Papers resource specialist who assists in writing students’ technical essays and had defended a graduation thesis on acoustics asserts that audibility depends on: 1) the shape of the room; 2) its size; 3) a constructive solution; 4) placement of the sound source; 5) reverberation time.
#1. The shape of the room
A rectangular or trapezoidal shape of the classroom plan is advantageous. In the latter case, the direction of sound should coincide with the height of the trapezoid. The square, round and oval-shaped rooms, as well as coatings in the form of large curved surfaces (domes, cylindrical vaults, etc., concentrating reflected sound waves), shielding surfaces (long-reach tiers, deep niches) are unsatisfactory in terms of acoustics. Rows of seats rising from the scene, separation of the walls and ceiling surfaces are favorable for audibility.
#2. The size of classrooms
The limit of audibility of normal speech in its direction is between 20 and 30 m, to the sides is 13 m, in the opposite direction – 10 m. The height of classrooms is desirable to be no more than 8 m.
#3. Constructive solution
Massive ceilings and walls, as a rule, are less acoustically favorable than resonating facings (wooden or artificial slabs). When arranging heating and ventilation systems, thermal up-currents in the path of sound from the source to listeners should be avoided.
Placement of additional seats in the aisles and the rise of rows from the stage has a positive effect on acoustics of the hall. Exceeding the rows with seats for students by 8 cm (according to French norms) provides every student with good audibility.
#4. The sound source placement
The sound source should, if possible, be placed near an acoustically rigid wall. With a high altitude of the classroom an acoustic visor should be arranged. Multiple sources of sound should be concentrated in one place; Speakers for speech reproduction should be placed in rooms no further than 34 m, for music – no further than 24 m from the sound source.
#5. Reverberation time
The phenomenon of reverberation occurs when the sound reflects the enclosing surfaces of the rooms. The listener perceives this phenomenon as attenuation of sounds.
The phenomenon in which the reflected sound is perceived in time separately from the direct sound (when the path of the reflected sound is ≥ 34 – 24 m) is called echo. Consequently, the reverberation time increases with increasing room sizes. In each classroom, there is an optimal reverberation time, which depends on the volume and purpose of the room (speech or music).
#6. Acoustical absorption
Sound waves, meeting walls or other objects on their way, are partially reflected by them (and the angle of incidence is equal to the angle of reflection), are partially absorbed (with the conversion of sound energy into heat) and partially penetrate through the barrier.
To calculate acoustical absorption, coefficients that have a certain value for various materials are used. As acoustical absorbing materials can serve:
- Porous materials whose acoustical absorption increases with sound frequency increasing. To absorb low-frequency sounds, one should use loose fibrous materials, laying them in layers of sufficiently large thickness (100 mm). The inclusion of air gaps by, for example, mounting porous materials increases their efficiency. When painting them do not use dyes that create a dense layer;
- Perforated plates. The most effective plates with holes with a diameter of 4 mm and a distance between them of 10 mm.
- Special acoustic plates. In addition to perforated plates, plates with grooves, and recently, continuous asbestos, mineral wool and glass wool plates are used;
- Thin plates-membranes that are freely attached to the wall on the ground; under the action of sound waves, they come into oscillatory motion and absorb sound.
In order to avoid distortion of the tone quality, it is necessary to carefully select materials for interior decoration. Porous materials absorb higher-frequency sounds, while dense materials absorb low-frequency sounds.
Acoustical source power
Each acoustical source corresponds to the maximum allowable volume of the room. So, for a human voice, the maximum volume of a room is 3000 m3.
Sound strength and reverberation duration are influenced by sound-absorbing materials, the choice of which depends on what type of sound you have to deal with: air, accented sound, noise or vibration.