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A varying current from “output” transformer passing through the voice coil, varying forces are acting which cause it to vibrate back and forth. As the cone is driven back and forth, its large area sets air into vibration.
It is a well-known fact that large sized loud speakers are especially good at reproducing and dispersing low-pitched sounds. Due to diffraction and interference, however, high-pitched sounds are emitted in a narrow beam and in a forward direction only. Loud speakers with small cones are especially good on the high-pitched notes but are poor at reproducing the low ones.