How you can ensure that sound is produced even in the absence of a material medium, but it is not heard?

When hammer of the bell hits the gong sound is produced due to the vibrations of gong which travel through air to the wall of the jar to vibrate due to which the outside the bell jar is also set in vibrations .thus sound is heard by us. But when the air is removed from the jarwith the help of a vacuum pump sound produced due vibrations of gong cannot travel to the wall of the jar,so no sound is heard.we can see that the hammer is stricking to the gong,but we cannot hear it.