Why Are Lawn Mowers So Loud?

Yea, I hear ya, the sound of a mower at full tilt, especially early in the morning can feel like an attack on the senses. Surely if they can make a truck quiet, they can make a mower quiet, right?

So why are they lawn mowers so loud? Mowers are loud because mufflers fitted to most engines are a cheap basic type known as – Absorptive type mufflers, they create very little gas flow restriction which is great for power but bad for noise.

Manufacturers could make a mower less noisy, but they don’t  because they don’t want to sacrifice cost and engine power.
 
The Outdoor Power Equipment Institute (OPEI) which is the association of outdoor equipment makers, decided voluntarily back in 1974 to set a noise level target of 95dcb for mowers. 

Noise Source and Solutions

Although the engine makes most of the noise you associate with a mower, a surprising amount comes from a spinning blade, much like the blades of a helicopter cutting through air. Next time you see an electric mower stop and listen, most of the noise you hear, is the blade cutting through air and grass. Electric mowers are not as quiet as you might think.

Anyhow mower noise comes from four main sources, listed below are possible solutions for excessive noise. If looking to go below 20 decibels, I suggest a donkey.

Mower muffler

Exhaust/Muffler – Exhaust heatshields become loose and baffles inside become loose also. To test, give the muffler a tap with the handle of a screwdriver and listen for the tell tale rattle.

Damaged muffler

Check your Muffler for damage, these guys get very hot and are prone to cracking and corrosion.

You already know mowers cause lots of vibration, and stuff just comes loose, that’s why it’s a good idea to check over your mower regularly looking for loose stuff. The baffles live inside the muffler and sometimes break free causing a thin metallic sounding rattle. Mufflers get really hot and vibration can cause them to crack. They can be repaired by your local muffler shop.

Exhaust gasket

Gaskets – Gaskets are used to mate the muffler to the engine. They create a seal, and as you can imagine when it breaks down you get lots of noise and fumes. Gaskets are easy to replace.

Blade

Blade Noise – Blades make a surprising amount of noise. Blade tips cut through the air at over 200 mph and most lawn tractors will have 2 and maybe 3 blades.

Bare deck

Bare Metal – The underside of mower decks are just painted, debris thrown against the deck resonates, like a bell.

Mower deck painted

You can DIY this one, at the auto parts store you can buy spray on bed liner which adds sound deadening and metal protection to your deck.

Just make sure the deck is clean and dry before painting outdoors. You can check out a video on that subject right here.

Also, try sticking sheets of self adhesive bitumen car sound deadening material in a few places on the deck topside. You can pick these up in an Auto parts store. Sure it might look a little odd, but it does help.

Engine – Obviously the engine is a major contributor to noise levels. Valves, rockers, camshaft, crankshaft and especially the fan (located on top of the engine) can be considerable. There are things you can do to help minimize the noise. Valve lash should be checked and adjusted every year, it doesn’t take long. Not only will it cut down on noise, it’ll give you more power and better gas mileage.
 

Mower engine fan

Engine Fan – Most small engines don’t have coolant they are air cooled and so they need a fan to pull cool air across the engine, and fans are noisy.

Check out “Valve lash adjustment”, it’s for a walk behind mower, but the process is the same for any OHV engine. Engine oil is another opportunity to help reduce noise. When oil gets old it gets thin which causes engine ratel. Your mower needs a tune-up at the beginning of every season. Check out “Tractor mower tune-up”. 
 

Valves

Valve Lash – Valve train will be noisy if there’s excessive lash.

Of course your engine may rattle because it’s worn, if you think that may be possible, try using a thicker oil or try Lucas oil treatment, it’s great stuff, you will notice a quieter engine, I promise. The engine fan is needed to cool the engine, so its got to stay. But try putting self adhesive bitumen on the underside of the hood, really does help reduce noise.

Mower body

Body – Body panels, deck linkages levers etc. will rattle and squeak as the engine and blades cause them to vibrate. Greasing all metal deck arm contact points will reduce noise, spraying with WD40 will help also.

Check your hood and seat rubber stops, replace with a DIY fix if needed. Run a blade down some old rubber hosing, great for pushing onto the edge of a rattling hood, MacGyver style.

Mower deck linkages

Linkages – Keep all the metal to metal links well greased, it helps dampen rattling and squeaks. Check that the rubber hood and seat stops are in place.

Muffler Types

The two main types of exhaust mufflers are – Absorptive mufflers and Reflective mufflers. Most mowers are fitted with the less expensive absorptive type muffler. So what the difference between the cheap one and the more expensive one? Design, materials used and execution.

Absorptive Mufflers

An absorptive muffler is a very basic muffler, probably the one fitted to your mower. It doesn’t use any clever engineering, it does a poor job of noise reducing. It will usually incorporate a spark arrester, which is a mesh screen that catches any sparks that might exit the engine.
 
This muffler causes very little restriction to gas flow which is great for power, that’s why racing cars are so noisy. This type muffler is fitted to most lawn mower engines. 

Reflective Mufflers

Reflective mufflers or resonators – Engineered to kill noise using clever acoustic engineering. Sound waves are pushed through perforated baffles in resonating chambers where some noise is cancelled out, known as Destructive interference. Special acoustic suppression temperature resistant material (not unlike rock-wool) is sandwiched between the chambers and the exhaust outer casing, this further suppresses noise. 
 
The larger the muffler the quieter the motor, that’s why high end luxury cars have very large mufflers. The down side to this type of muffler is flow restriction – the baffles and chambers cause restriction to the flow of gases which in turn causes back pressure, and back pressure reduces the power of the engine.

Super Quiet Lawn Mower Mufflers

Here’s a possible solution it’s the Super Trapp Quiet Muffler, I haven’t used it so I can’t comment first hand, but doing some research, it seems to do the business. Check out the YouTube video below. The Super Trapp is a Reflective muffler type, it uses witchcraft and wizardry to make an engine as quiet as a cricket. 

Muffler

Mufflers – Some makers do a better job than others, John Deere mufflers do a first class job.

Lawn Mower Louder Than Usual

Mowers create a lot of noise and vibration, the engine and spinning blades set up vibrations that over time will start to pull your mower apart. A lawn mower can make many different type noises, they can be squeals, squeak, constant howl, cyclical noise, or just a general harsh roughness. Some noises are just impossible to describe and I know describing noises may not be useful to some.
 
What is useful, is to see when the noise is present, is it present as soon as you start the mower, or only when your driving or maybe only when the blades are engaged. This kind of detective work will help you find and fix the problem quickly.

Check

If you feel your mower is louder than normal, you can check a few of the more common noise sources. Some of these won’t apply to walk behind mowers but most will.

  • Oil level ok?
  • Blade(s) loose (cyclical noise)
  • Muffler or brackets loose (loud roar/rattling)
  • Muffler gasket leaks (loud roar)
  • Muffler cracked or broken (loud roar)
  • Hood loose or contacting the body (rattling)
  • Seat brackets loose or rubber bushing worn/missing (rattling)
  • Debris caught in the drive line (cyclical noise)
  • Belt pulley bearings worn (harshness/howl)
  • Blade spindle bearings worn (harshness/howl)
  • Belt worn/damaged (cyclical noise)
  • PTO clutch worn (harshness/howl) (Tractor/Ride-on)
  • Deck carrying arms loose/dry (rattling)
  • Wheel bearings dry (squeal/Squeak)
  • Steering dry (squeal/Squeak)
  • Transmission worn (harshness/howl)

This isn’t a complete list, and as you can imagine there are many possibilities, but these are the more usual causes of noise.  

Blade Spindle

Blade spindle

Blade spindles transfer the the power to the blades. They are bolted to the deck and have bearings top and bottom to provide smooth spinning.

The bearings wear out and can cause a howl roar when the blades are on. The bearings can be replaced, but often replacing the whole spindle makes more sense.

Pulleys

Pulley

Pulleys are used to drive and route belts around the chassis. Most will have bearings and they’re the ones that cause trouble. They’re a common source of noise.

Generally, if you have a worn out belt, then chances are one or more pulleys are also worn, and vice versa. Pulleys are fitted to the driving belt and also to the cutting deck belt system.

Bearings

Most pulleys employ integrated bearings but some are replaceable.

Belt Wear

Mower drive belt

Belt wear or damage will cause a cyclical noise as the damaged area contacts the pulleys. Damaged cutting deck belts will also cause lots of vibration.