On Sun, 9 Aug 2015 16:42:38 -0500, Muggles wrote:

On 8/9/2015 4:41 PM, Oren wrote:
On Sun, 9 Aug 2015 16:17:04 -0500, Muggles wrote:


I used my knuckles to tap/knock on the wall the same as knocking on a
door, and I knocked on the wall firmly. I just couldn't hear the difference.

As shown. The difference is distinguished by a hollow or solid sound.


My hearing isn't all that great. Sometimes, I thought I heard a higher
pitch, but I just wasn't sure.

Try a mechanic's or doctor's stethoscope. Or have a hearing test?