Fence to stop air conditioner noise from neighbour?

My mum has a large Mitsubishi outdoor unit and when it's running it sounds like a noisy dump truck idling just outside the room - in fact the first time I heard it I could not understand how a truck could be so close to her house until I went outside to see what the noise was! Although Mitsubishi markets into domestic applications, their stuff is better suited to commercial and industrial applications where noise pollution isn't such an issue.

Sound doesn't just travel straight through the fence; there are many flanking paths over the top of the wall from reflections from the neighbour's wall and eaves above the fence, etc, plus at the top of the fence sound diffracts (bends) over the top. A solid heavy barrier like a brick wall will work best and make sure there are no gaps, e.g. at the bottom. The higher and longer the wall or barrier, the better. Have a look at the walls in front of houses on main roads which have been widened and owners offered noise protection.

I've recently returned from Japan where all of the outdoor units are virtually silent, even though they are the same brands eg Mitsubishi, Panasonic, Fujitsu, etc, so it must be something to do with a lack of performance standards in Australia. Noisey units are cheaper to make.

The good way to reduce noise
https://www.youtube.com/watch?v=naJPJst-RUE

The she'll be right way
https://airc.com.au/reduce-noise-out...r-conditioner/

Not always possible, but we used to quieten down portable generators with timber panels at 45 against the generator. The noise bounces off the panel and into the ground.
By the way, the difference with those super quiet units, is simply a noise suppressing canopy.
Noise suppression is seen as black magic and almost unheard of in Australia.

The best way to deal with noise is not to generate it in the first place. That needs to be done at the design stage by not building sound generators into the design. For example a siren is not that different to a fan and is made by fan blades passing close to an external frame.

Fan blades in outdoor AC units are noisy when not enough attention is given to the blade shape, blade speed, blade clearances and air paths in and around the unit. For whatever reason, the Japanese AC companies sell noisy outdoor units in Australia that they would not sell in their own market.

The Daikin sound cover reduces the sound power emitted from the outdoor unit by 1/2, which is equivalent to ≈3.0 decibels. That sounds impressive until you realise that ±3.0dB is considered to be the smallest change in sound level that most people will notice.

The second link above gives a reasonable explanation of the issues and possible mitigations the OP can take if the neighbour won't help out. None of these methods, or even all of them combined, is likely to solve a problem of the magnitude that comes from the Mitsubishi outdoor unit at my mum's place, and I suspect at the OP's either.

Talk to your neighbour about the problem and see if the unit has a night mode, if it does this usually makes a lot of difference.

I replaced an old Mitzy unit that carked it after 20 years for a new one 7.1 kw for the living room and open kitchen adjacent. It's good for up to 60 m2.
The unit outside is whisper quiet. It points to the Colorbond fence, about 1.5 meters away from it. If I stand in front of it when running I can not hear the compressor, only the fan that has a very low tone,something like ummm and the neighbour has never made a comment about noise.

If talking to the bloke next door doesn't work one thing that may help is tall clumping bamboo in pots, it seems to be quite good in acting as a noise absorber/diffuser

Originally Posted by Cole

Along with other suggestions.. for your deck area, consider attracting birds with plants/water source.. put in a water feature with a lot of splash noise.. a wind chime perhaps.

Yes ... another good one is to stick your fingers in your ears and sing la la la la

Sorry couldn't resist ...