Then a laser, directed through a rotating prismatic mirror, scans its narrow beam across the part. Microphones terminate festooning cables. The beam scans the part 6,250 times a second, while a stepper motor moves the part vertically. There's no way for a leak to escape us unless it finds a way to be on an interior surface that is not illuminated by the laser. The process is one of leak detection followed by leak location.
Sensitive microphones measure the sound wave.
The sound appears at frequencies of 6.25, 12.5, 18.75 kHz, and higher multiples. We might measure harmonics up past 10 or 12, depending on how loud the leak is. Any test harmonic that has an amplitude two or more times the baseline harmonic flags a leak, Dowling explained. Measuring background noise is an important step in the process, Dowling said. It can include noise from the equipment.