Acoustic chambers for animal experimentation are used for research in the reduction of the risk of hearing loss due to exposure to intense noise, which is the cause of more than one third of acquired deafness in developed countries and is a relevant concern for occupational health in some jobs.
Anechoic acoustic chambers eliminate any sound reflection and therefore each spatial point is reached by only the sound directly emitted by the sound source. On the contrary, any spatial point in reverberant chambers is reached by sound directly emitted by the source as well as by all sound reflections generated by the chamber walls.
In noise exposure tests with animals, hearing loss depends on the sound level and the exposure time. Thus, designing acoustic chambers with the highest sound level in each point is essential to minimise the experiment duration. This level must also be homogeneous in the area where the animals are located, to ensure the actual sound exposure.
The procedure to design reverberant acoustic chambers consists of:
1. Establishing one the three spatial dimensions, preferably the dimension to be the smaller.
2. Choosing the number and position of the sound sources, number of modes, and factor for signal mitigation due to the chamber walls, and estimating the spatial area where the heads of the animals are usually located.
3. Obtaining the functions of the average acoustic field and of the average quadratic deviation in that area.
4. Using these functions, optimising a merit function to obtain the most homogeneous acoustic field, and estimating with this function the other two chamber dimensions, preferably following the criterion of Bolt (1946, J. Acoustic Soc. Am., 18, pp. 130-133).
5. An alternative, more time-consuming method is to generate maps of the average values of the acoustic field and of the quadratic deviation for the two dimensions being estimated in a range containing the Bolt values. These maps are visually compared to select the optimum.
In noise exposure tests with animals, the hearing loss caused is directly proportional to the product of the sound pressure level by the exposure time. Thus, the higher the sound pressure level the lesser the exposure time required to produce a prescribed hearing loss. Therefore, one of the main features of an acoustical chamber for noise exposure with animals is to generate a controlled sound field as intense as possible.
The main goal of the designed reverberant chamber is to generate an area with maximum sound field and minimum spatial variability. The optimization procedure calculates maps of the mean and minimum square deviation as a function of the proportions between the dimensions of the chamber. Finally, a configuration is chosen which affords the maximum mean sound pressure field with the minimum square deviation.
Innovative Aspects:
" The acoustic field is maximised and homogenised in all the spatial area where the animals are located" The sound spectrum is optimised for the lower frequencies and uniform for the higher frequencies" The size of the resulting chambers is optimised" Experimental duration can be reduced (twelve hours duration experiments have been reduced to a half hour, diminishing the level of animal stress too)
