Different levels of background noise were shown to influence learning and behaviour in rats (CastelhanoCarlos and Baumans, 2009). Guinea-pigs showed the most marked reduction in activity when exposed as compared to rats which appeared least affected by noise exposure. After the noise was turned off there was an increase in the general activity of mice as well as of rats. At the beginning of the exposure all the animals huddled in a group. Another reaction of mice and rats was freezing into a motionless stance. When rats were exposed to 95 dB at 0.5-5 kHz for two 5-minutes‘ periods per day for 28 weeks, their behaviour changed and above all they became aggressive (Algers et al., 1978). Sudden sounds are probably also more startling than those with gradual onsets (Burn, 2008).