A unit of relative intensity or amplitude. Usually abbreviated to dB. When dealing with intensity ratios the decibel is defined by the formula 10 log(I1/I2), where I1 and I2 are the intensity values of the two sounds concerned. For amplitude ratios, the formula is 20 log(A1/A2). A1 and A2 are the amplitude values for the two sounds. For the measurement of speech sounds, the unit dB IL (Intensity Level) or dB SPL (Sound Pressure Level) is often used. In this case the I1 or A2 is the intensity or amplitude of the speech sound concerned and I2 or A2 is a reference representing the intensity level 10-16watts/cm2) or amplitude value (0.0002 dynes/cm2) of the threshold of human hearing. On this scale, normal conversation has a level of approximately 60dB, shouting or singing a level of about 75dB and a sound loud enough to cause physical pain a level of about 130dB.