Besides getting louder, there are other ways to help make the cellphone vibration sound like a growl. You can see in the pictures for the story that Will has toys on the floor, and some are peeking out from under the bed. The toys can vibrate too and add rattles to the sound as it bounces around under the bed. If the floorboards are loose, even more vibrations will be added.
If you put the cellphone on the top of a guitar, it will sound even louder. A guitar is a box with a hole in the top and when the top vibrates, usually by plucking the strings, the vibrations go into the box, bounce all around, and then come out the hole.
Here's how a cellphone can actually sound like a growl.
If you hold a cellphone while it is vibrating, you can hardly hear it at all. If you put the cellphone on a table, it will sound a little louder. The vibrations from the cellphone make the table vibrate. Then the table is vibrating and so is the cellphone. So, there are more vibrations moving through the air to your ear. The vibrations add together and sound louder.
Based on a True Story
Altogether, these different vibrations created a growling sound. And that’s the way it actually happened.
Drums have a vibrating top over an open box where the sound moves out from the bottom. As the sound bounces around in these boxes, the sound vibrations add together even more and sound even louder.
This picture shows what it looks like underneath a typical floor. The floor is resting on beams.
A floor doesn’t look like a guitar or drum, but it can treat sound the same way a guitar or drum does. The cellphone vibrations shake the floor a little. Underneath the floor are the wooden boards that hold up the floor. They make a kind of box like the sound box of a guitar or a drum. The vibrations bounce around under the floor, vibrate the floor more and get louder.