Sound waves

This model plays no audio. Compare the visible trace. This investigation compares fixed frequency stops at 200 and 400 Hz and relative amplitude settings 2 and 3. The approximate human audible range is about 20 Hz to 20 000 Hz, not a hearing test.

The microphone signal is plotted against time, not distance. The 10 ms timebase (1 ms per division) and gain stay fixed. Trace height is proportional to amplitude; a dashed trace preserves the previous reading.

The cone, air and hammer are slowed down. The magnified particles and spacing are not to scale. In real air, wavelength = speed / frequency: at 200 Hz, 340 / 200 = 1.7 m.

Each pump stroke halves pressure from 100 kPa to a 1 kPa floor. The remaining signal factor is 0.05 + 0.95 × pressure / 100, a teaching approximation for remaining air and vibrations through the mounting. It is not a calibrated relationship. In an ideal vacuum there is no sound transmitted through the empty space; supports can still transmit vibrations. A real bell jar is a teacher demonstration.

Reduced motion freezes the cone, hammer and air at representative positions and shows the end of each change straight away.