Although most of the sounds we hear are transmitted through the air, air is not the only carrier of sound waves—nor is it the best. A ticking clock can be heard through the air if you’re close enough, but put your ear to the table with the clock on it and the ticking will sound much louder.
When something vibrates, the strength of the vibration and the length of time the vibrations continue can vary quite a bit, depending on the materials involved. Hit a piece of wood with a stick and the sound lasts for just an instant. Hit a metal gong with the same stick, and the sound may continue for many seconds. Water is another good transmitter of sound.
Why the difference? In some materials, the molecules are tightly packed together; in other materials, the molecules are more loosely arranged. How close the molecules are to one another can affect how easily they can bump into each other to start a vibration moving along.
When you hit the coat hanger against another object, it starts vibrating. The vibrations in the metal travel through the string and into your fingers. The vibration is transferred to your head through solid objects, not air. Compare the sound of the coat hanger swinging into a chair or desk without holding the string against your ears. The sound is much duller. This demonstrates how the same vibration sounds differently when it travels through different materials.