How to Place Outdoor Equipment Inside Noise Limits

Why this matters

The cheapest time to fix a noise problem is before the pad is poured. After install, the options are a barrier the manufacturer may not allow, a relocation the customer pays for twice, or a neighbor who calls code enforcement every hot night. A heat pump, pool pump or standby generator that is legal in the back corner can fail the ordinance in the side yard ten feet from a bedroom window, and the difference is arithmetic you can do on the estimate visit with the spec sheet and a tape measure.

What you need before you start

• The adopted noise ordinance for the jurisdiction the neighbor's property sits in. What a limit is written against (receiving district, measurement position, day and night periods, descriptor, total level versus the source's contribution, tone and impulse adjustments) is owned by the card on community noise limits; this procedure assumes you have read that clause.
• The unit's published sound rating. For outdoor air conditioners and heat pumps that is the A-weighted sound power level rated under AHRI Standard 270, often printed in bels (7.4 bels is 74 dB). Pool and generator equipment publish their own figures; note whether each is sound power or sound pressure at a stated distance.
• The plat or a sketch with lot lines, the house footprint and the neighbor's windows. See related: How to Read a Plat and a Survey.

The procedure

1. Find the limit that actually applies, and write it down with its period. Read the receiving district's number for the period the equipment will run in its loudest mode. For cooling-dominant equipment that is often the night limit on a hot night; for a heat pump in a cold climate it can be a winter night in defrost. Reason: the night limit is lower, and the day figure is the one people quote. Skip this and you design to a number five or ten decibels too generous.

2. Locate every receiving position the clause allows. If the clause measures "at the property line", the nearest point of each neighbor's line is the position. If it allows "any point on the receiving property", the officer can stand at the closest point, which is usually the same line. If it measures at the exterior of the nearest dwelling, measure to the neighbor's wall or window. Reason: distance is the dominant term in the estimate, and a placement checked against the far neighbor and not the near one has not been checked.

3. Get the right sound rating for the operating condition. The AHRI 270 figure is rated at one standard condition. Variable-speed and two-stage units often publish levels at several speeds, and the highest one can sit several decibels above the rated figure. Use the highest level the equipment will reach during the limiting period. Reason: an estimate built on the rated point can pass while the unit fails at full speed on the one night that generates the complaint.

4. Convert sound power to sound pressure at each position. Sound power (Lw) is what the machine emits; sound pressure (Lp) is what a meter at a position reads. For a unit on hard ground with nothing between it and the position, the hemispherical spreading estimate is:

Lp = Lw - 20 x log10(r) - 8, with r in meters, which in feet is Lp = Lw - 20 x log10(r) + 2.3

That geometry assumes the unit sits on a reflecting ground plane with open air around it. Add about 3 dB when the unit sits within a couple of feet of one hard wall, and about 6 dB in an inside corner of two walls, because the reflected sound adds to the direct. Each doubling of distance takes off about 6 dB under the same geometry. Reason: you need the number at the neighbor's position, not the number on the brochure. Skip the reflection term and the placement tight against the house reads 3 to 6 dB better than it will measure.

5. Add the other sources if the clause limits total level. If the ordinance limits everything the meter hears from the property, a new unit is not judged alone: the customer's existing pool pump or a second condenser counts. Decibels add on an energy basis, not arithmetically; the method is owned by How to Add and Subtract Noise Levels Correctly. Reason: two sources that each pass can fail together.

6. Apply a margin and any character penalty. These estimates carry a few decibels of uncertainty either way: grass absorbs, a fence reflects, the published figure has its own test tolerance. Hold a design margin of 3 dB below the limit as a starting default, and tune it once your shop has measured a few of its own installs. If the adopted text adds a penalty for a prominent tone or impulse, add it before comparing. Compressor hum is the usual tonal trigger; the trigger test belongs to the tonal-noise card. Reason: a placement that passes by half a decibel on paper is a coin toss in the field.

7. Fix a failing placement in order of how reliably each fix works.

• Distance. The most reliable fix; it needs no maintenance and cannot be defeated.
• Geometry. Move the unit off the inside corner, or turn the discharge and the compressor side away from the receiving position, where the manufacturer's clearances allow.
• A quieter unit or a speed limit. A lower-rated unit is a real fix. A "quiet mode" that caps fan or compressor speed at night helps only if the unit still meets the load at that cap, and the shop should confirm it does rather than assume it.
• A barrier. A solid barrier that breaks the line of sight between the unit and the position commonly buys about 5 dB, more with height, and seldom more than 10 to 15 dB outdoors. It must have no gaps (a slatted privacy fence leaks), and it must sit outside the manufacturer's minimum airflow clearances, because a barrier that makes the unit recirculate its own discharge air trades a noise complaint for a capacity and efficiency loss.

Reason for the order: the first two fixes cannot be undone by a homeowner or a season; a barrier and a speed cap can.

8. Record the estimate on the job. Write the limit with its period, each position and its distance, the rating used, the estimate at each position and the margin. Reason: if a neighbor complains, the shop can show what it designed to, and a later tech can see whether something changed.

A worked example: one heat pump, three placements

The house sits in a residential district. The ordinance limits sound measured at the receiving property line, on total level from the source property, with a night limit we will call 50 dBA for illustration (the adopted text owns the real figure). The proposed heat pump's highest published sound power is 74 dBA. The customer already runs a pool pump on a night timer that reads 44 dBA at the rear neighbor's line.

Placement A: side yard, the customer's first choice. The unit would sit 1 foot off the house wall and 6 feet from the side neighbor's line.

• Spreading: 20 x log10(6) is 15.6, so 74 - 15.6 + 2.3 = 60.7 dBA.
• One wall within a couple of feet: 60.7 + 3 = 63.7 dBA.
• Against a 50 dBA limit: 63.7 minus 50 is 13.7 dB over.

No barrier fixes 13.7 dB outdoors with the clearances a heat pump needs. Placement A is out.

Placement B: rear wall. Against the rear wall, 25 feet from the rear line and 30 feet from the side line. The rear line is the nearer position, so it governs.

• Spreading: 20 x log10(25) is 28.0, so 74 - 28.0 + 2.3 = 48.3 dBA.
• Wall reflection: 48.3 + 3 = 51.3 dBA, 1.3 dB over by itself.
• With a solid barrier that breaks the line of sight to the rear line, outside the airflow clearances: 51.3 - 5 = 46.3 dBA.
• Add the pool pump, because the limit is total: 46.3 and 44 combine to about 48.3 dBA (10 x log10 of 10^{4.63 plus 10}4.4).
• Margin: 50 minus 48.3 leaves 1.7 dB, short of the 3 dB default.

Note what happened at the last step. Judged alone, the barrier placement passes with 3.7 dB to spare (50 minus 46.3), and a tech reading only the new unit would have called it done. The total-level clause is what fails it.

Placement B with a quieter unit. The customer agrees to a comparable-capacity unit whose highest published level is 68 dBA.

• Spreading and wall: 68 - 28.0 + 2.3 + 3 = 45.3 dBA.
• Barrier: 45.3 - 5 = 40.3 dBA.
• With the pool pump: 40.3 and 44 combine to about 45.5 dBA.
• Margin: 50 minus 45.5 leaves 4.5 dB, which clears the 3 dB default.

The quote carries the quieter unit and the barrier as separate lines, with the estimate written on the job so the customer sees why both are there.

What would change this answer. If the adopted clause had limited only the new source's contribution rather than total level, Placement B with the original unit and the barrier would have met the margin at 3.7 dB. If the adopted text carries a tone penalty and the compressor's hum triggers it, add the penalty before step 6: even the quieter unit's 4.5 dB margin may not survive a penalty larger than that, and the fix becomes distance or a third placement. And if the side neighbor's property were zoned commercial while the rear neighbor's was residential, each line would carry its own limit, because the receiving district sets the number. Distance alone would not tell you which line governs; you would run the estimate against each line with its own limit.

Checking the placement after install

An estimate is a design number. When the unit is running at its highest speed during the limiting period, a measurement at the governing position confirms it. Use the measurement method and background correction in How to Work Out Whether a Noise Complaint Is Actionable, and stand only where you have permission: the neighbor's line is the neighbor's property. If the measured level sits more than about 3 dB above your estimate, look for the reflection or the operating mode you did not account for before blaming the meter.

References

• AHRI Standard 270, sound performance rating of outdoor unitary equipment, and AHRI Guideline 275 on applying those ratings to estimate sound levels at a position
• The noise ordinance as adopted by the jurisdiction of the receiving property
• See related: What a Community Noise Limit Is Usually Written Against; How to Add and Subtract Noise Levels Correctly
• See related: What Tonal Noise Is and Why It Is Judged Differently; How to Work Out Whether a Noise Complaint Is Actionable
• See related: Property Lines, Easements and Where Equipment Can Go