There are several sounds, with defined spectral compositions, that have been assigned colors for identification.  These colors invoke the rainbow of the visible light spectrum to facilitate comparative references to the spectral nature of various sounds.  The analogy is not completely accurate, however.
     Each named color in the visible light spectrum consists of a single frequency of radiation, the equivalent of a pure tone sound.  A named color of sound, in contrast, refers to a specific combination of frequencies.  It should also be noted that the characteristics and effects associated with certain colors by traditional color theory are not applicable to sound.  There are some interesting parallels, however, between color theory and colored noise.

     The discussions of occupational soundscapes throughout this series have implied a certain perspective, either that of one exposed to noise or that of one responsible for its management and control.  In some cases, both perspectives are taken simultaneously.  Other perspectives are also possible, however.
     For anyone not engaged in the activities that generate noise of concern, the nature of a soundscape changes.  As an example, consider a professional auto race or outdoor concert.  Direct participants (e.g. racers, performers) experience occupational noise exposure, as do support personnel (e.g. course marshals, sound technicians, security).  Spectators are exposed to recreational noise; that is, the exposure is optional.
     Sound that escapes the event venue to the surrounding area becomes community noise.  The same is true of commercial and industrial facilities, where continuous operation can cause greater impacts on nearby communities than discrete events.

     The third noise control objective (see Part 11) is to “protect receivers from sound immission.”  This is somewhat misleading, as receivers, or “listeners,” are actually the first priority in the workplace.  However, modification of sound exposure at the receiver is the last resort to ensure protection from hearing loss and other effects (see Part 8).  If source and path control efforts are insufficient, receiver noise control becomes the critical “last stand” against damaging effects of sound exposure.
     Receiver noise control is not comprised exclusively of the use of hearing protection devices (HPDs), but HPDs often dominate the discussion.  Here, the discussion includes the types of HPD available, rating systems, selection criteria, and challenges in implementation.  Though options may be limited, engineering and administrative controls are also discussed.