am i hearing things?

My dad just bought headphones as a gift for my grandmother and I didn't really think about them. However, I heard this irritating high pitched squeal one morning, but when I asked my parents if they heard the noise they said they couldn't hear anything. When I found out the noise was coming from the headphones I told my dad and he thought I was messing with him because he couldn't hear anything. Apparently the frequency of the noise is too high for either of my parents to hear anymore. Like we learned in class, humans can have a hearing range from 20 Hz to 2000 Hz, but it varies from person to person. Once a frequency is outside of a person's range, they no longer hear it. However, since I can hear it, my grandmother's dog probably can, too.