The sensation level is measured in decibels. It is frequency specific and starts at 0 dB SL (sensation level) where 0 dB SL is the softest the patient can here a specific frequency. This means that the SL will be different for different people. Everyone's SL will start at 0 but my SL at 1000 Hz may be at 23 db IL and another's may be at 54 db IL.
Put more simply, dB SL is the difference between a person's threshold of hearing and the presentation level of a stimulus. If a person's speech reception threshold (SRT) is 10 dB on an audiometer (dB HL) and you present a speech stimulus to them at 50 dB HL on the audiometer, their sensation level would be derived thus: 50 dB (presentation level) minus 10 dB (threshold), which equals a sensation level of 40 dB SL. This is also the procedure for a frequency-specific stimulus (e.g. pure tones): subtract the audiometric threshold (dB HL) from the supra-threshold presentation level (dB HL) and the difference is the sensation level (dB SL).
