Why A vibrating body produces sound but we cannot hear the sound of vibrating pendulum?

Human ears can only hear (i.e., detect vibrations) in a range of about 20-20,000 vibrations per second. A pendulum will usually vibrate at much less than 20 vibrations per second. But please note that a vibrating object (such as a plucked string) may also produce secondary vibrations, other than the main frequency.On the other hand, a vibration must also be strong enough, in order to be heard.