Topics for today:

  • Hearing:  temporary threshold shift (TTS) and permanent threshold shift (PTS).  The U.S. Occupational Safety and Health Administration (OSHA) has guidelines for industrial and recreational noise exposure. The approach is a "dosage/exposure" model, meaning that the guidelines allow higher levels of noise exposure for short durations, and lower levels of noise exposure for longer durations.
  • Room acoustics:  sound in rooms consists of the direct sound from the source, reflected sound from the various room surfaces, reverberation (long term) sound due to a plethora of overlapping multiple-order reflections, and "noise" due to unwanted sound from mechanical systems, air handling, plumbing, noise infiltration, etc.
  • Reverberation time is one way to categorize room acoustics:  RT60 or T60 is the time in seconds required for a sound to die away by 60 dB. T60 is generally measured at several different frequencies, because the reverb time depends upon the absorption of the room surfaces, which generally vary with frequency. 
  • T60 is generally greater for rooms of large volume, and for rooms with highly reflective surfaces.

Topics for the next lecture:

  • More on the sound in rooms:  absorption, reflection, transmission.

  • Sabine equation for architectural acoustics and reverberation.