All the units are a bit different so this is the best way we can explain it.
The units all make a noise but not all can be heard by humans. What each person can hear is also subject to how old they are. Younger people have a much easier time heraring the outputs from the units and people over the age of 65 often cannot hear any of the models we sell.
The Classic has a range of about 5ft and is designed for small spaces, this can be heard a little buy people under the age of 35.
The PRO Models are very powerful and have 3 levels with a range of about 15′. The highest level can be heard by most people under the age of 60 as a high pitched squeak.
Please note with both of these models if they are placed under the hood of a vehicle for the most part the sounds will be contained.
The MouseBlocker 1000 has a range of about 50′ and this unit is whisper quiet. Most people cannot hear a thing while some when close up hear a quiet chirp.