In Short
The silencer a generator needs is decided by a few facts about the engine, the noise limit and where it applies, and the site, not by a grade label such as hospital grade. Stating those facts in the enquiry lets a supplier propose a silencer that is neither more than the site needs nor too little.
A common enquiry asks for a "hospital grade silencer" and nothing else. Sometimes that is far more than the site needs, and sometimes it is still not enough, because the grade is only a label for a range of quietness. The silencing a site needs follows from a handful of facts that are usually already on the table. This guide explains which facts they are and why each one matters, so that an enquiry carries what a silencer proposal needs.
Start From the Engine
It starts with how loud the bare exhaust is. Engine makers publish the exhaust noise of the engine measured at the open outlet, and the figure depends on the engine's size, rating and speed. It also depends on how the figure is stated, and mixing up the different ways of stating it is one of the most frequent mistakes in the data sent with enquiries. Send the engine data sheet and say which figure you mean.
Then the Limit and Where It Applies
The sound limit is set at a place: at the boundary of the site, at the nearest dwelling, or at a stated distance from the equipment. The further away that place is, the more the sound has dissipated on its way, and the less silencing the exhaust has to supply. The position of the outlet matters too: an outlet above a roof or next to a wall, the surroundings of the receiver and any screening between them all change what reaches the measuring point. The limit may also differ between day and night, and the stricter value is the one that decides.
Everything Else on Site Counts
The exhaust is never the only noise source. The enclosure or container radiates engine noise, the radiator fans add their own, and the limit applies to the sum of all of them. An exhaust that alone is just at the limit leaves nothing for the rest, so the exhaust has to sit below the limit with room to spare. Beyond a certain point of silencing, the noise that radiates through the walls of the exhaust pipe and the silencer shell becomes louder than what leaves the outlet, and a larger silencer stops helping until the pipe and the shell are insulated.
The Low Frequencies Decide
A single decibel figure hides where the problem is. The exhaust of a diesel engine is loudest in the low frequencies, and those are exactly the frequencies a silencer attenuates least. Two silencers with the same grade can behave very differently there, which is why a single total figure is not enough. If the low rumble stays too high, the limit is not met however good the rest looks, and the part of the silencer that handles it is the one that has to grow.
When the engine data, the limit and its distance are sent with an enquiry, AXON prepares the proposal for the whole package and names the silencer grade, the number of stages and the insulation that follow from it.
What the selection covers
- Engine exhaust noise at the actual rating and speed
- The sound limit, the place where it applies and the time of day
- Allowance for the enclosure, the radiator and other sources on site
- The low rumble of the engine, not only a single total figure
- Noise radiated by the exhaust pipe and the silencer shell
- Back pressure and space for the silencer
What to send with an enquiry
- Engine model, rating and speed
- The engine data sheet with its exhaust noise figures
- Sound limit, where it applies and when
- Outlet position: roof, wall, height above ground
- Other equipment on site that counts towards the same limit
- Back pressure limit and the available space for the silencer
Questions we are often asked
How much silencing does a generator exhaust need?
It depends on the engine, the sound limit, the distance to where the limit applies and the other noise sources on site. AXON puts these together, and the silencer grade follows from the result.
Why is "hospital grade" not enough as a specification?
Because a grade is only a label for a range of quietness. The site decides what is needed, and one figure does not tell whether a limit is met.
Why should the exhaust be quieter than the sound limit itself?
Because the enclosure, the radiator and the exhaust add up. An exhaust that alone equals the limit leaves no room for the rest.
Why do low frequencies matter most?
Because the engine's exhaust noise is concentrated there and silencers attenuate least there. They are what decides whether a strict limit is met.
What should I send to get a silencer proposal?
The engine data sheet, the sound limit with its distance, the outlet position and the available space. AXON returns the grade, the number of stages and the insulation.