Sound has a profound ability to alter your brainwave state particular when it has a rhythm to it. Our ancient ancestors knew this.
African tribes would use special ritual drumming techniques to take the members of their ritual into a deep hypnotic state. The medicine men would use this to invoke deep visions, aid the recovery from sickness and injury, even communicate with the spirits of the native lands.
Like the pulses of vibration that produces sound on a guitar string, your brain also creates pulses of vibrations. These vibrations are electrical impulses. Their activity can be measured by using an EEG (electroencephalogram) that measures the frequency of these pulses.
You measure the frequency of these pulses in Hertz (Hz). Your brain actually has a predominant frequency that it operates in any given moment, and this can be associated with your state of mind.
So the emotional state of your mind in any moment such as feeling happy, sad, frightened, sleepy or excited can be measured as a frequency.