For an experiment on the impact of traffic noise on bird behaviour, I tried to playback some low frequency traffic noise with a fixed amplitude. While (trying to) adjust the noise amplitude, I realized that the loudspeaker was quite unreliable: the amplitude of my playback varied by 3-10 dB from one day to the next although I had not changed any parameter or touched any button (and I always made sure that the battery was fully charged before use).
Has someone already experienced this? Any idea where this could be coming from and what to do to fix it?
Has someone any suggestion for a reliable device that would accurately render low frequency sounds?