Bounce a laser off a mirror attached to a vibrating membrane and watch sound turn into shapes. How do you change the patterns?

This is the most complex of the seven problems both in terms of the experimental set up and the number of variable parameters to investigate! Read the problem in full for details. Only change one variable at a time. And be very careful with lasers.

Problem in full

This is problem 6 of 17 for IYPT 2027

“A thin rubber membrane is stretched over a circular frame and driven into vibration by a loudspeaker. A small mirror is attached to the membrane, and a laser beam is reflected off it onto a distant screen. Under certain conditions, the laser spot traces out complex, steady, or evolving geometric patterns. Investigate the relationship between the driving parameters, the membrane’s properties, and the resulting light patterns.”

Specialist equipment

• A low power laser. NB: importing a hand-held, battery-operated laser pointer with an output power over 1 milliwatt (mW) without Custom’s consent is illegal.
• Membranes and a frame that holds them
• A small mirror and a way to attach it to the membrane
• A projection surface

Safety first!

• NEVER look directly into a laser or shine it at someone’s face
• NEVER shine a laser at aircraft, helicopters or any land based vehicles
• Lasers sold online or at dollar shops may in fact be of a higher power than advertised to get over the import controls. Get them checked professionally if you are at all concerned.
• Be careful when reflecting the laser beam. Keep all observers out of the path of the beam, including yourself. Only look at the pattern on the projection surface.
• Take care there are no other shiny surfaces, such as a mirror or a laptop screen, in your workspace in a position where they could accidentally reflect the laser.