Don't believe everything the Professor tells you! [Editorial]

Revisiting the bell–jar demonstration

“…In particular, the difference in impedance between air and glass (which makes up the bell-jar) tends to play a major role in this demonstration and interacts with the effect of reduced air pressure [4,7,9,11]. When air density becomes very low, which occurs when air pressure is reduced to about 0.1 mm Hg (the minimum attainable by typical mechanical air pumps), the sound produced by the vibrating source may not be audible to a listener, not because of the absence of air but because of the large impedance mismatch produced by the large differences in densities between the vibrating source and air, and then between air and glass [4,7,9,11]. At these interfaces, reflection of sound occurs, and thus the sound is inaudible outside the jar [4,7,9,11].…”

“…First, even sophisticated vacuum pumps leave some air inside the jar, which is enough to allow for transmission of sound. In the paper by Fleming [7], he highlighted that at room temperature this residual air exerts a pressure of about 10 −2 cm Hg (about 13 Pa); under these 0031-9120/13/020247+05$33.00 c 2013 IOP Publishing Ltd conditions, the mean free path of the air particles is about 7 × 10 −4 m. He noted that if the frequency of the sound emitted by the bell is about 300 Hz, the wavelength comes to about 1.1 m. He emphasized that since the wavelength of the sound is greater than this mean free path of the air particles, the sound can still be transmitted by the air inside the jar. Sprott [8], however, clarified that when the mean free path of the air particles inside the jar exceeds a few centimetres in value, transmission of sound would effectively stop.…”

“…When air density becomes very low, which occurs when air pressure is reduced to about 0.1 mm Hg (the minimum attainable by typical mechanical air pumps), the sound produced by the vibrating source may not be audible to a listener, not because of the absence of air but because of the large impedance mismatch produced by the large differences in densities between the vibrating source and air, and then between air and glass [4,7,9,11]. At these interfaces, reflection of sound occurs, and thus the sound is inaudible outside the jar [4,7,9,11]. To remedy the problem, Dejun [5] suggested adding a microphone inside the bell-jar to avoid the effect of the air-glass impedance mismatch.…”