Two researchers carried out an investigation into the noise exposure of one of the aquariums due to the visitors in the Schönbrunn Zoo in Vienna, and found some surprising results.
They hid an underwater microphone in a one cubic metre aquarium to register the effects of visitors tapping on the glass. Helmut Kractochvil and Harald Schwammer investigated this activity for four weeks, counting the taps per 100 visitors between midday on Monday to midday on Friday, and then from midday to Friday to midday on Monday. They then introduced three different signs with the inscriptions: “Tapping on the glass kills the fish”, “Only idiots tap the glass”, and “Please do not tap the glass”.