Reverberation – the persistence of sound, and suggested treatment with absorption

In section X “on absorption”, the maximum reverberation time for Core Learning Spaces (unoccupied, furnished) in the octave bands 500, 1000, and 2000 Hz were quantified.   Why these octave bands?  These octave bands are the most important to the intelligibility of speech, and if they are controlled, it is likely that the neighboring octave bands will be similarly controlled.  Let’s take a look at where the majority of speech frequencies lie relative to these critical bands:

Insert octave band graph with regions labeled for speech, ANSI reverb octave bands, and NRC octave bands.  The caption will indicate “the 125Hz to 4kHz bands encompass the majority of speech frequencies” and “Below see a definition of the NRC, or Noise Reduction Coefficient.”

Above yet another acronym has been introduced, NRC, or Noise Reduction Coefficient.  This is an arithmetic average of the absorption coefficients of a material or surface in the 250Hz, 500Hz, 1kHz, and 2kHz octave bands.  It is a good indicator of how the material reacts with speech sound energy; a number closer to 1 indicates that it absorbs most of the energy, and close to zero indicates it reflects most of the energy (the range is 0-1).  Basic building finish materials such as ceiling tiles or fiberglass panels will have a NRC rating to help determine their in usefulness in absorbing sound and  controlling reverberation in the speech frequency range.

The following list is meant as a suggested guideline for school absorptive treatments (also see Annex C in ANSI 12.60-2010, Part 1):

1. Corridors: recommended 75% of ceiling is treated sound absorbent, and no less than 50%
2. Large rooms (ie: cafeteria) with ceilings up to 12ft: the full drop ceiling exclusive of ventilation and lighting should have an NRC of 0.7.
3. Ceilings from 12 ft. to 15 ft. should have an NRC greater than 0.7, and/or walls should be considered.
4. Ceilings higher than 15 ft.: it is recommended that a more detailed analysis be performed by a professional experienced in reverberation control.
5. Wherever permitted wall treatment should be combined with the ceiling treatment, and may allow for a reduced NRC ceiling.
6. Where permitted by sanitation restrictions (See local Health Department Codes & Regulations which may require hard surfaces that can be scrubbed clean,) the same requirements should be applied to food serving and preparation areas.
7. Gymnasiums (very high ceilings) require absorbent wall treatment in order for the ceilings to be effective.
8. Lecture halls of this size and auditoriums have special requirements that include HVAC noise reduction, specific seat absorption characteristics, and sound reinforcement: it is recommended that a professional experienced in these matters is consulted. (see table c.1 in ANSI 12.60-2010, Part 1 for more details)

Location and mounting of absorbing material:

• General Classrooms (no fixed lecture position, ceiling @10′) place most if not all absorbing material on the ceiling.
• Lecture Classrooms (fixed lecture position) ring the ceiling and upper walls with absorbent material – save the area directly above and in front of the lecturer.
• Drop ceiling airspace is not to be any smaller than 16”.

DSW to re-draw

Notes:

• Control of reverberation below 500 Hz should be performed by an experienced consultant.

• Manufacturers’ recommendations must be followed when installing sound-absorbing wall treatments.