A downhole tool comprises a pulse generator that can generate longitudinal pulses in a drill string. It is determined whether or not there is a first spatial element Q1 that satisfies a contact condition that there is a possibility of contact with a reference spatial element Q0 on a discriminant plane. In addition, the devices may be used to sample tissue during biopsy or other medical procedures where perforating a blood vessel could result in injury to a patient. The focussing frame is releasably mountable to the light frame. The present invention also relates to various methods of countering the harmful waves with the counter waves by such source matching or wave matching and various methods of providing the counter units as well as counter waves.