Franzis 65276 bat detector kit franzis 9783645652766 p the franzis bat detector kit lets you listen to nature with modern electronics human ears cannot hear the cries of bats because they use high frequency sound well beyond our hearing range this is where the bat detector comes in it electronically translates the bat s high frequencies up to 100khz to frequencies we can hear 20hz to 20khz for ease of construction the kit contains a pre assembled circuit board you only have to solde