The charts show the intensity of sound at different frequencies (pitches). The horizontal scale shows the audio frequency in Hertz (Cycles Per Second) with low frequencies such as
thunder to the left, and high frequencies such as the crack of a whip to the right. The vertical scale shows the intensity of the noise in decibels(db). Decibels (db) are the standard unit for audio intensity. A db is the
minimum difference a person can detect. The legend to the right of the chart shows the RPMs for a specific line, and the total db reading from the meter for that measurement. The meter has three scales. The low scale is used
for the ambient readings, while a higher scale is used for louder noise. A zero measurement means that reading was below the minimum of that scale.
The following links to unrelated web sites are provided for your
convenience. They will help you understand these measurement units: