Background Noise Defined

Background noise comes from a number of sources all around, inside and out.

What is “Background Noise?”  It’s the average sound level created by any combination of nearby noise sources as measured in the classroom of interest.

DSW to submit alternate for consideration

All of these possible noise sources, defined below,  can contribute to background noise and should be considered in the placement of classrooms, and in the wall type for separation and exterior walls:

• Outside environmental noise from traffic, aircraft, and industrial sources should be considered during site evaluation and selection.

• Schoolyard and maintenance noise need to be considered, and they can be addressed in the design stage as well as mitigated through appropriate scheduling policy.

• Inside noise from adjacent spaces (classrooms next door, corridors, restrooms, etc.) should be considered in the design stage and they can be reduced or abated by careful placement in the first phases of planning.

All of these sources of noise can be addressed by proper noise and vibration control, including placement of mechanical equipment, proper design of walls, floors, and ceilings, vent layout/HVAC design and plumbing design.  Selection of quiet devices or building systems  (HVAC, plumbing, lighting) can also play a critical roll in noise reduction.

Note: the unwanted sounds from other speakers in the room such as a disruptive student is not addressed in this booklet

The background noise  is the average sound level  measured in deciBels, or dB.  The human ear filters sound somewhat and is less sensitive to low frequencies; this is approximated by weighting the measurement mathematically.  The resulting weighted and averaged sound level is called A-weighted deciBels or dBA and is an approximation of human perception of sound level at normal listening levels.  As sound levels increase, the human ear filter changes, and at higher levels the filter is called C-weighted or dBC.  Let’s take a look at a table from the standard that lists allowable levels to get an idea of the numbers:

Table X: Limits on one-hour-average A- and C- weighted background sound levels from internal sources associated with building services^(a) and utilities or from outdoor sources.

Learning Space	Greatest one-hour-average A- and C-weighted  background noise level (dB)
Core learning space with enclosed volume ≤ 283 m3 (≤ 10 000 ft3) Core learning space with enclosed volume > 283 m3 and ≤ 566 m3 (> 10 000 ft3 and ≤ 20 000 ft3) Core learning spaces with enclosed volumes > 566 m3 (> 20 000 ft3) Ancillary learning spaces (any volume)	35/ 55(b)  35/ 55(b) 40 / 60(b) 40 / 60 (b,c)

Note (a) for table X. The background sound level from HVAC shall be added (on an energy basis) to the level from other building systems such as lights, plumbing etc if applicable.

Note (b) for table X. For background sound from a multiple mode HVAC operating at design or maximum capacity, the background sound level limit is 2 dB higher than specified above. If the multiple mode HVAC system is operating at reduced or lower capacity, the background sound limit is 1 dB lower than specified above.

Note (c) for table X: A corridor used as an ancillary teaching space should adhere to the same level as for any ancillary space. If the corridor is only used for traffic, the background sound limit can be increased by 5 dB above that specified in this table.

So what is 35dBA? 40dBA? Before we get much further, let’s take a look at sound levels that you may be familiar with:

Use fig 1.6 from cavanaugh page 5 thermometer.

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