Applying Common Sense to Intelligibility

If you’re reading this blog, you’ve no doubt heard about and know what the definition of intelligibility is, but may not be sure of how to achieve it. Most of the time, the quest for intelligibility is met with “It depends…”. That is true, but more often than not, that applies to unique situations. Since speakers are generally found on the ceiling, the general rule of thumb to achieve voice intelligibility is to space the speakers at twice the height of the ceiling with the same, lower wattage setting. That’s ‘sound’ advice for audio systems that are constantly in use and theater type systems. However, for emergency voice systems, it’s over kill and can lead to over designed and over budget systems.

I’ve recently helped with the design of a hospital voice evacuation system and found that by laying out and measuring at specific distances, the sound levels of 10′ high, ceiling mounted speakers, a constant decibel level of 77 db can be attained at 5′ above floor from any point in a corridor of normal width by spacing the speakers at 30′ and tapping them at 1/2 watt. Since ambient sounds levels in the corridor are about 55 db, that puts you 20 db over ambient. Not too loud, not too soft.

Sometimes designers forget that these speakers will not operate in a vacuum and that sound loss due to distance is mitigated by reflection and the presence of other speakers. By the book, speakers lose 6 db for every doubling of distance from the speaker, starting with the rated distance. Rated distance is what the speaker’s output is as measured at a certain distance, almost always 10′. If a speaker’s output when tapped at 1/2 a watt is 83 db at 10′, in a test lab it would measure 77 db at 20′, 71 db at 40′, and so on. However, the actual loss in a corridor with other speakers will be about 3-4 db. By plotting out the decibel level at key distances, factoring in angular sound losses and accounting for actual conditions, we can see that 30′ spacing will give an even sound level of 77 db at 5′ above floor. Don’t get trapped by a loose application of general intelligibility practices. Spacing these types of speakers with just a rule of thumb as a guide can do more harm than good.

Affiliated Fire Systems has the equipment, expertise and support you need to install, test & inspect a problem free fire alarm and/or emergency voice system. If you have any questions regarding your life safety needs, contact us via our ‘Contact Us’ page on our website, or email me directly at generowe@affiliatedinc.com.