Sound power level against sound pressure level

Sound power level is a property of the machine. Sound pressure level is what you would measure standing somewhere near it. Confuse the two in a plant noise submission and the answer moves by around 8 decibels, which is enough to turn a low impact conclusion into an adverse one under BS 4142.

The error is common because manufacturers publish both, often on the same page, and label them badly. A datasheet reading "sound level 54 dB(A)" tells you nothing until you know whether that is power or pressure, and if pressure, at what distance and in what conditions.

What each quantity describes

Sound power level, written LW, is the total acoustic energy a source radiates, expressed in decibels relative to one picowatt. It does not change with distance, with the room, or with where you stand. A condenser unit has the same sound power level on a roof, in a courtyard or in a laboratory.

Sound pressure level, written LP, is the fluctuation in air pressure at a particular point, in decibels relative to 20 micropascals. It is what a sound level meter reads. It falls with distance, rises near reflecting surfaces and depends on the direction you are standing in relative to the source.

Only one of those is a property of the plant. The other is a property of the plant plus the place plus the geometry.

Why the distinction decides submissions

BS 4142 needs the specific sound level at the assessment position, which is a pressure level. You get there by taking the source's sound power level and propagating it: subtracting distance attenuation, adding directivity, allowing for barriers, ground effects and screening.

If the starting number is actually a pressure level at 1 metre and you treat it as a power level, you propagate a figure that is already about 8 decibels lower than the true source output, and the calculated level at the neighbour comes out correspondingly low. Do it the other way round and you overstate the plant and specify an enclosure nobody needs.

Environmental health officers check this first. It is the fastest way to find out whether the assessment was done by someone who understands the method.

Reading manufacturer data without being caught

Work through the datasheet in this order.

• Find the label. LW or Lw with a subscript means power. LP or Lp means pressure.
• If it is pressure, find the distance. One metre, three metres and ten metres are all used.
• Find the conditions. "Free field" means radiating in all directions with no reflections. "Hemispherical" or "hemi-free field" means the source is on a hard ground plane, which adds about 3 decibels compared with free field at the same distance.
• Find the operating point. Most published figures are at nominal or full duty, in standard ambient conditions.
• Find the octave band table. A single A-weighted figure is not enough for anything except the crudest screening.

Octave band data from 63 hertz to 8 kilohertz is what lets you calculate barrier insertion loss, facade sound reduction and the attenuation of a duct. It is also what lets an assessor test for tonality using the objective method in the standard. Where a manufacturer will not supply it, treat that as a reason to choose a different manufacturer rather than a reason to guess.

Converting a power level to a level at the receptor

For a point source radiating into free space, the relationship is LP equals LW minus 20 times the logarithm of the distance in metres, minus 11 decibels.

Where the source sits on a hard surface such as a flat roof or a yard, it radiates into a hemisphere, and the correction becomes minus 8 rather than minus 11. That is the case for most plant, so it is the one to use as a default.

Plant is almost never in free field. A chiller in a recessed plant enclosure with walls on three sides is in a corner condition, and an assessment that ignores that understates it by up to 9 decibels before any other error.

Multiple sources add logarithmically. Two identical units give 3 decibels more than one, four give 6 more, ten give 10 more. Two units are not twice as loud in decibel terms, and counting only the largest one is wrong.

Part load, night mode and what the condition secures

Published data is usually at full duty. Plant that runs quietly at night does so because it is in a setback mode, at a lower fan speed or with a reduced compressor step.

If the assessment relies on that mode, three things have to follow. The manufacturer has to publish data for it, the mode has to be capable of being locked rather than being a setting anyone can change, and a condition has to secure it. A submission that quotes night mode data and then offers no mechanism to keep the plant in night mode is offering a number rather than a commitment.

The same applies to duty and standby arrangements. Where two units are installed with one on standby, say whether both can run together, because they will during a heatwave.

Illustrative example. A datasheet gives 58 dB(A) at 1 metre, hemispherical, at full duty. The equivalent sound power level is roughly 66 dB(A). Sited in a corner rather than on open ground, the effective radiated level at a neighbour 12 metres away is several decibels higher than a naive free field calculation suggests. Treating the 58 as a power level would have produced a figure around 8 decibels too low. The numbers are illustrative.

The source schedule that makes a submission checkable

Set out one table covering every noise-generating item, and put it in the report rather than in an appendix nobody opens. For each item give the description and model, the quantity, the position and height, the octave band sound power levels, the operating hours, the duty cycle, the mode assumed and the datasheet reference and date.

Then show the propagation for at least one item in full, so a reviewer can follow the arithmetic rather than take the model output on trust. Where software has been used, name it and state the parameters.

That level of transparency is what a Noise Impact Assessment needs to survive scrutiny. Councils expect a suitably qualified acoustician for this work and professional review is strongly advised, particularly where the scheme is marginal against the background. The pattern is the same one that applies to other derived technical measures in planning, such as the vertical sky component, where a single figure carries the decision and the method behind it has to be visible.

Anyone scoping a scheme can check which technical reports apply before commissioning the acoustic work, which usually avoids paying for a survey in the wrong season or against the wrong receptor.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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