Ask for a generator exhaust silencer and the first question back is usually "which grade?" - industrial, residential, critical or hospital. The names are useful shorthand, but they are often taken for more than they are. Here is what they say, and what they do not.
Four names for rising attenuation
The grades describe how much exhaust noise a silencer removes, in rising order. An industrial-grade silencer is meant for sites where the exhaust noise only has to be taken off its peak - a plant with other machinery, far from anyone who would complain. Residential grade is meant for sets near houses and offices. Critical grade is for sites where the exhaust has to be quieter still, and hospital grade is the top of the range, for installations where the set must be close to inaudible outside its enclosure.
Why the names are not a specification
There is no single standard that fixes the attenuation of each grade. Every maker publishes its own figures, per octave band, and two silencers sold under the same grade name can differ by several decibels at the frequencies that matter. The grade tells the buyer roughly which family of silencer to look at; the selection is made on the octave-band data against the noise limit of the site.
That limit is a sound level at a distance - at the property boundary, at the nearest window, at 1 m or 7 m from the set, depending on the permit or the standard. It is the figure to put in the enquiry. "Residential grade" alone leaves the supplier guessing what the site needs.
Low frequencies decide
Exhaust noise has a low-frequency part, the pulsation of the cylinders, and a high-frequency part from the gas flow. The low rumble is what carries over distance and through walls, and it is the part a thin silencer handles worst. A unit whose single-number rating looks adequate may still leave a hum at the neighbour's window if its low-frequency attenuation is weak. Reading the octave-band data, especially the lowest bands, is the only way to know.
The grade costs back pressure
More attenuation means more resistance to the gas. Each step up in grade adds pressure drop, and the engine maker sets a limit on the total back pressure the engine may see. A hospital-grade silencer on a long pipe with several bends can push the system over that limit, and an engine over its back pressure limit loses output and runs hot. The silencer grade, the pipe diameter and the route are selected together; the article on exhaust back pressure explains that side.
The silencer is one of three
A generator set radiates noise from the engine itself, from the cooling air openings and from the exhaust. Upgrading the exhaust silencer by two grades does nothing for the fan noise leaving the air outlet. A quiet installation is designed as a whole: the acoustic enclosure, the air inlet and outlet silencers and the exhaust silencer, each sized for its share of the limit.
AXON Technic supplies exhaust silencers in all four grades, together with the piping, insulation and stacks, and the enclosures and air silencers that complete a quiet generator package. Send the engine data sheet and the site noise limit and the selection is made on the octave-band data, not on a grade name.