Choosing a Silent (Canopied) Generator

When choosing a silent (canopied) generator, three things matter: at what distance and under what conditions the noise level in the data sheet was measured, how the canopy is designed and where the set will be placed on site. The dB(A) value in the catalogue is usually given at 7 metres; in a real environment, however, the sound you hear varies considerably with position, reflective surfaces and distance.

Reading the dB(A) value correctly

There are two different quantities related to generator noise:

• Sound pressure level (Lp): The value perceived by the ear, measured at a given distance. Catalogues commonly give it at 7 metres, in dB(A).
• Sound power level (LwA): The total acoustic energy emitted by the source; it is independent of distance. In Europe, regulations require this value to be shown on the label for generators of certain power classes used outdoors.

The two values should not be confused. 95 dB(A) LwA and 70 dB(A) at 7 metres may be two separate pieces of information for the same set. When comparing, check:

• Measurement distance (1 m, 7 m, 10 m?)
• Load condition (no load, 75% load or full load?)
• Measurement environment (free field?)
• Whether the value is an average or the highest point

What you need to know about decibels

The decibel is a logarithmic scale, so our intuition is often misleading:

• Each time the distance doubles, sound pressure falls by approximately 6 dB in open field.
• When two sources at the same level run side by side, the total level rises by approximately 3 dB.
• A difference of approximately 10 dB is perceived by the ear as roughly twice as loud or quiet.

Distance	Set at 70 dB(A) at 7 m (approximate, open field)
7 m	70 dB(A)
14 m	64 dB(A)
28 m	58 dB(A)
56 m	52 dB(A)

This table is an idealised calculation; building façades, the ground and background noise change the result.

What should a good canopy have?

Silence is not achieved just with a thick sheet-metal box. The following elements matter in canopy design:

• Sound-absorbing insulation: Materials such as rock wool or acoustic foam on the inner surfaces, with an oil- and fuel-resistant facing.
• Air inlet and outlet attenuators (baffles): The engine and radiator need a large volume of air. Sound-attenuating labyrinths in the air ducts let the air through while blocking the noise.
• Exhaust silencer: Industrial, residential or critical-grade silencers provide different levels of attenuation. In residential areas, a silencer with higher attenuation is preferred.
• Vibration isolation: Mounting the engine and alternator to the base frame on anti-vibration mounts reduces structure-borne noise.
• Cooling capacity: If the temperature inside the canopy rises, the set may have to derate. It should be checked that the canopied set can deliver full power at the stated ambient temperature.
• Durability: Galvanised or suitably pre-treated sheet metal, outdoor-grade paint, sealed door gaskets and lockable doors.
• Service access: Wide doors giving easy access to oil, filters, battery and control panel determine how easy a canopied set is to maintain.
• Transport and lifting: Lifting points, forklift pockets and, where required, a fuel tank integrated into the base frame.

Placement: half the noise is in the layout

The same set is perceived as much quieter when it is placed correctly:

1. Distance: Place the set as far as possible from sensitive areas such as bedrooms, offices and patient rooms.
2. Direction: Point the exhaust and radiator air outlet away from sensitive areas.
3. Reflective surfaces: Squeezing the set into a corner formed by two walls can raise the noise level through reflection.
4. Ground: A flat, solid concrete plinth reduces vibration transmission.
5. Barriers: An acoustic screen wall can be used if needed, but it must not restrict the cooling airflow.
6. Regulations: Limit values set by municipalities and environmental noise regulations should be checked at the design stage, especially for night-time operation.

Canopy or generator room?

• Canopied set: Installed outdoors, needs no separate room, is transportable and quick to commission. Common for construction sites, residential complexes, homes, commercial buildings and rental.
• Open-type set and generator room: Preferred for large power ratings or where space has been allocated inside the building. Sound insulation is provided by the room’s acoustic design; ventilation and exhaust are designed separately.

Short selection list

• Is the target noise level at the installation site and the measurement point defined?
• Are the measurement distance and load condition of the data sheet value known?
• Can the canopy deliver full power at the highest ambient temperature on site?
• Can the exhaust and air outlet be directed away from sensitive areas?
• Has enough clearance been left for service doors and refuelling?

Next step

See our generator canopy page for canopy options and design details; find the required power with the power calculator and list suitable models with the generator finder. Model-specific noise values are given in the technical tables on the product pages; for projects with a target noise level, contact us.