What Background Noise Does That Is Actually Useful

Why this matters

A quiet space is not automatically a good space, and in an open plan a low background is the reason people can follow a conversation four desks away. Background noise is the denominator of every speech intelligibility ratio in a building, which makes it a design element rather than a defect. But it is only useful when three properties hold at once: the right shape across the octave bands, a level inside a narrow usable window, and near-uniformity in space and time. Miss any one and you have added noise without buying privacy, which is the worst available outcome and is what a badly commissioned system delivers. The record below is what a commissioned system looks like written down.

What masking actually does, and what it does not

Masking raises the level of the sound that is already there so that unwanted speech falls closer to, or below, the level at which words resolve. It does not cancel anything, it does not remove anything, and it does not care where the speech came from.

It works because intelligibility is a ratio. A voice arriving at a listener at 47 dB, A-weighted sound pressure level re 20 micropascals, overall, sits 9 dB above a 38 dB background and is easy to follow; the same voice sits 2 dB above a 45 dB background and mostly does not resolve into words. The voice did not change.

Two things follow immediately and both get missed:

• It degrades wanted speech identically. There is no filter that knows which talker you care about. That is why the level has a ceiling and why zoning matters.
• It does nothing for a structure-borne complaint. Footfall from above, a pump rumble through a slab, a slammed door felt as much as heard: these are low-frequency and impulsive, and a broadband masking spectrum shaped for speech has very little energy where they live.

The three properties that decide whether it works

Spectrum shape. Masking energy has to sit in and just below the bands that carry speech information, which are the 500, 1000, 2000 and 4000 Hz octaves. A published masking contour is typically fairly flat through the low mid range and then rolls off on the order of 4 to 5 dB per octave above about 500 Hz. Too little roll-off and it reads as hiss, which occupants notice and complain about by name. Too much low-frequency content and it reads as rumble and masks nothing useful, because the consonants that carry intelligibility live high.

Level, with a floor and a ceiling. Systems are commonly commissioned somewhere between about 42 and 48 dB, A-weighted sound pressure level re 20 micropascals, overall, equivalent-continuous, measured at seated ear height. Below roughly 40 dB there is not enough energy to move any intelligibility ratio meaningfully; above roughly 48 dB people begin raising their voices to be heard over it, which puts the speech level back up and undoes the gain while adding fatigue. The exact target is a specification decision, not a universal number, and it belongs in writing before commissioning starts.

Uniformity in space and in time. A system that varies by more than a couple of decibels as you walk the floor becomes audible as a system, and anything audible as a system gets blamed for everything. The same applies to time: masking that steps rather than ramps draws attention to itself. A level change slow enough that nobody can point at the moment it happened is the design intent.

Getting to the loudspeakers

Above-ceiling work means a portable ladder set per 29 CFR 1910.23 and eye protection per 29 CFR 1910.133 before a tile is lifted. In a building whose vintage is not documented, material above the grid is presumed asbestos-containing under 29 CFR 1910.1001(j) until sampled, and that is an inhalation route: do not drill, cut or brush it, and stop if you meet it.

Terminating at the amplifier or its branch circuit is electrical work: de-energize, lock, tag and verify absence of voltage under 29 CFR 1910.333(b)(2) for general industry or 29 CFR 1926.417 for construction, proving the instrument live, dead and live again per NFPA 70E-2021, 120.5, which reaches you through your employer's electrical safety program in the edition it adopted.

The commissioning record, field by field

Blank, this is what the artifact asks for. Every field exists because a system has failed for want of it.

• Target total background, at seated ear height. Level, quantity, weighting, reference, bandwidth, time basis. All six or the field is not filled in.
• Existing background with the masking system off, mechanical running in its normal occupied state, same position, same conventions. This is the number the system has to be added to.
• Required masking-only contribution. Derived, not measured, because levels combine on energy rather than arithmetically.
• Measured masking-only level, system on with the mechanical off where that is possible, same position.
• Per-octave-band measured levels against the system's own published contour, with the allowed deviation stated per band.
• Spatial uniformity, as the spread across a stated number of positions on the same zone.
• Ramp behaviour, the time taken to move between the day and night settings.
• Zones excluded, with the reason.

The record, filled in

A 46-person open plan, one zone, target total background 45 dB, A-weighted sound pressure level re 20 micropascals, overall, equivalent-continuous over 15 minutes, at seated ear height.

Existing background, masking off, mechanical in occupied state: 38 dB, same conventions.

Required masking-only contribution, derived. Levels add on energy, so the masking contribution is the energy difference, not 45 minus 38. Ten raised to 4.5 is 31,623; ten raised to 3.8 is 6,310; the difference is 25,313; ten times log10 of that is 44.0 dB. So the system alone has to produce 44.0 dB, A-weighted, overall, equivalent-continuous, not 45 and certainly not 7.

That correction is the one this record exists to force. Setting the system to 45 dB alone would have landed the room at 10 times log10 of (31,623 plus 6,310), which is 45.8 dB total, most of a decibel over target and audibly closer to the hiss complaint threshold. It is a small error here because the two levels are far apart; where the existing background is within 3 dB of the target, the same mistake doubles the energy and the system ends up 3 dB hot.

Measured masking-only, mechanical off, mid-zone position, octave bands, unweighted sound pressure level re 20 micropascals, equivalent-continuous over 5 minutes. The target column is this system's own published contour, reproduced as the acceptance criterion for this job. It is not a universal curve and it does not transfer to another system.

Allowed deviation, plus or minus 2 dB per band. All eight bands pass.

Overall from the target contour, A-weighted. Applying the A-weighting adjustment to each band and summing on energy gives 43.8 dB, A-weighted, overall. Against the derived requirement of 44.0 dB the system is 0.2 dB light, which is inside the plus or minus 1 dB acceptance on the overall figure and is on the conservative side of it.

Total, both sources. Ten times log10 of (ten raised to 4.38 plus ten raised to 3.8) is 44.8 dB, A-weighted, overall, equivalent-continuous over 15 minutes. Target 45. Accepted.

Spatial uniformity. Nine positions across the zone, spread of 1.6 dB, A-weighted, overall. Acceptance was 2 dB. Passed.

Ramp behaviour. Day setting to night setting over 20 minutes, no step exceeding 0.5 dB.

Zones excluded. The training room, because intelligibility is the goal there and masking works against it. The two enclosed rooms on the north side, because they are already isolated and adding masking inside them adds noise with nothing to mask.

Sibling-rule check. Every level in the record carries quantity, weighting, reference, bandwidth and time basis, including in the table header and in the excluded-zones line where plain speech would have dropped them. The band table is unweighted per octave and the overall is A-weighted, and the two are never compared directly. The masking contribution is derived as an energy subtraction, printed, rather than an arithmetic one, which is the same rule the level-addition article states. The direction matches the speech-intelligibility article and the open-plan article: raising the background lowers intelligibility for every talker in the zone. Rounding on the overall figure went to the light side rather than the flattering side.

Where a raised background is the wrong answer

Where a tonal component already exists. Masking a tone with broadband noise raises the total level while the tone still stands out, because the ear finds tonal content in noise very well. Fix the tone first.

Where the complaint is structure-borne. Footfall, a rumble through a slab, a door strike. The masking spectrum has almost nothing at those frequencies, and the fix is on the impact side.

Where the space is judged against an ordinance boundary. A community noise limit is a local ordinance rather than a federal number, and the total at a property line is what gets measured. Adding a broadband source to satisfy an interior complaint has to be checked against whatever the local limit measures, which is often per octave band and often different at night.

Where the occupational exposure question is live. 29 CFR 1910.95 sets the federal floor with an 85 dB A-weighted eight-hour time-weighted average action level and a 90 dB permissible exposure limit at the same averaging. An office masking system operates 40 dB below that and is not an exposure question, but adding broadband noise in a space that already sits near the action level is, and it goes on the exposure assessment rather than into an acoustics conversation.

References

• 29 CFR 1910.95 for occupational noise exposure including the action level and the permissible exposure limit; 29 CFR 1910.23, 1910.133 and 1910.1001(j) for ladder, eye and presumed asbestos-containing material controls above a ceiling.
• 29 CFR 1910.333(b)(2), general industry, and 29 CFR 1926.417, construction, for de-energizing amplifier circuits; NFPA 70E-2021, 120.5, for live-dead-live, binding through the employer's electrical safety program in the edition adopted.
• The masking system manufacturer's published contour and commissioning procedure, which owns the target spectrum and the allowed per-band deviation for that system.
• See related: What Speech Intelligibility Depends On; Why an Open Office Is an Acoustics Problem, Not a Layout Problem; How to Add and Subtract Noise Levels Correctly.