The Gold Line DSP2B is a dedicated meter to measure Speech Intelligibility using OPT STICis intelligibility software option.
Speech intelligibility is not a physical quantity like Amperes, Volts or Btu's. It is the degree to which we understand spoken language. Speech is not necessarily intelligible simply because it is audible. Everyone has experienced speech signals that are loud enough but overly reverberant, suffer from echoes or distortion and therefore are not understandable. Announcements in airports, train stations and places of worship often suffer from this problem.
Intelligibility is measured by sending a test signal through the sound system and into the listening environment. The test signal is an artificial reproduction of a range of modulated sound frequencies important to human speech. The meter measures the received signal, compares it to the signal specification and computes the Modulation Transfer Function (MTF). The MTF is a measure of the change in or distortion of the signal as it passes through the sound system and as it is affected by the listening environment. The MTF is used to compute and display the Speech Transmission Index (STI) on the meter. The meter can convert the STI to a generic Common Intelligibility Scale (CIS), which permits direct comparison to other test methods.
It should be noted that any "measurements' are actually predictions of voice intelligibility. Subject based test methods can gauge how much of the spoken information is correctly understood by a person or group of persons for that particular test. When properly done, that resulting value is a prediction of how much of the spoken word will be correctly understood by others at some other time. Intelligibility meters have been developed and use a test method that has been correlated with accepted subject based test methods. Therefore, the numbers presented by a meter are also predictions. Still, it is common to refer the results as measurements. This is exactly the same as audibility measurements. The definition of a decibel for sound pressure levels is the ratio of the measured sound pressure level to the threshold of human hearing. That is, zero decibels is the threshold of audibility. However, that threshold is a subjective number determined many years ago by testing a large group of people with "normal" hearing.