What makes a particular noise louder than another is how much energy is put into producing it, and we measure and express this loudness in decibels. We can measure decibels with a decibel meter. Sound is a vibration passed from one molecule to another and when it reaches our ear the vibration is converted into a sound by our brain. The larger the vibration the louder we interpret the sound. A decibel meter contains a very sensitive microphone that acts a little bit like our eardrums and measures the size of the vibration caused by the sound wave. This is then interpreted into an electrical signal and gives out a reading in decibels. Essentially, the more the microphone vibrates the larger the decibel reading.
Laura Brettle, Science Museum