**7. Predicting measures for checking acquired acoustic quality in a classroom**

The basic measurement of acoustic quality of a room is the measurement of reverberation time as the most important objective parameter of the acoustic quality of a room since it contains characteristics of a room such as dimension and volume and shape and absorption. Measuring reverberation time can be carried out in various ways and different types of measuring signals and sources of signal measurement are used. Basically, all types of measurement of reverberation time differ according to the principle of big difficulties created by the level of sound pressure of 60 dB above the noise level of the area which is needed according to definition for establishing reverberation time. The oldest way of measuring reverberation time is using the burst (pistol or similar) or noise (noise generator) which through frequency analyzer and logarithmic printer give a curve-like lowering of the sound pressure in a space. The Figure 4 graph shows the reverberation time measuring a protractor – a transparent board with scales for reading the reverberation time. In this type of measurement it is advisable that the level of actuation signal is at least 40-50 dB above environmental noise level.

Stochastic Based Simulations and Measurements of Some Objective Parameters of Acoustic Quality: Subjective Evaluation of Room Acoustic Quality with Acoustics Optimization in Multimedia Classroom … 221

**8. Predictions, stochastic based simulations and measurements in** 

The task is to make a prediction, stochastic based simulations and measurements of (some objective parameters of) acoustic (quality) in a sample multimedia classroom (classroom at the Department of Electroacoustics, Faculty of Electrical Engineering and Computing, or FER). Measurements and simulations of some objective parameters of acoustic quality in order to establish opinion whether the sample multimedia classroom is acoustic (according to some rules and conditions) or not, and if not, what measures should be taken in order for that classroom to meet the condition of acoustics. If the same measuring conditions are ensured, objective parameters of must be repeatable and be direct indicators of acoustic quality. The first measurable parameter is the reverberation time (time necessary for level of sound pressure or sound strength is lowered for 60 dB as

Figure 5 shows the layout of the sample multimedia classroom. The numbers refer to positions in the room where reverberation time was measured, and the sound source is also marked with a symbol. Table 3 shows the results of the measurement of reverberation time

Frequency Position 1 Position 2 Position 3 Position 2 Position 4 Position 5 62,5 Hz 1.14 s 1.44 s 1.24 s 1.44 s 1.22 s 1.33 s 125 Hz 1.32 s 1.50 s 1.37 s 1.50 s 1.22 s 1.54 s 250 Hz 1.67 s 1.55 s 1.62 s 1.55 s 1.59 s 1.67 s 500 Hz 1.50 s 1.55 s 1.48 s 1.55 s 1.57 s 1.56 s 1 kHz 1.40 s 1.38 s 1.38 s 1.38 s 1.35 s 1.36 s 2 kHz 0.99 s 1.18 s 1.14 s 1.18 s 1.14 s 1.17 s 4 kHz 0.89 s 0.82 s 0.85 s 0.82 s 0.83 s 0.83 s 8 kHZ 0.67 s 0.63 s 0.99 s 0.63 s 0.64 s 0.65 s

The second measurable parameter is the distribution (division of the sound pressure). Sound pressure at a point in the room area is the result of the interference of direct and reflected sound and therefore in the spatial allocation of the sound pressure there are areas of minimum and maximum of sound pressure which are especially expressed in resonant frequencies. This parameter was measured at various frequencies in 30 points defined with introduced coordinates, shown in Figure 6. The results are shown in

depending on various frequencies of sound source in the real system.

**Table 3.** Results of reverberation time measurement

Table 4.

**the sample multimedia room** 

sound emission is stopped).

**Figure 4.** Graphical layout for direct reading of reverberation time using a protractor

Based on the Schroeder integration, a special way of measuring reverberation time from the impulse response possible to implement into a digital sound measuring device was developed (e.g. B&K 2231 using a module for measuring reverberation time BZ 7108). The results are directly read from the instrument according to the octal and terca (tertiary bands) within the set frequency area. Reverberation time is calculated (extrapolation) based on 10, 20 and 30 dB of lowering of the signal level and thus avoiding the need for a 60 dB signal dynamics. The digital sound measurer or SPL meter (B&K 2231 using a module for measuring reverberation time BZ 7108), depending on the environmental noise level, itself generates the required signal level sufficient for measuring, based on the signal received with appropriate indicators of a low level or pre–actuation, independently carries out level correction. Due to the lowered influences of possible mistakes in measurement, four subsequent measurements within the entire frequency area, whose results are stored in digital sound measuring device memory and through statistical analysis can yield results of proper accuracy.
