Water hammer (new washer-dryer)

It can be any hose.

AIUI the piston separates the water from the air 'cushion'. A diaphragm does much the same thing. Again AIUI, if the two aren't separated, the air in the 'cushion' slowly dissolves in the water and eventually disappears, allowing the hammer to return.

On that basis, I would think that anyone with an unvented water system and expansion vessel would be protected against hammer.

Just to follow up, the arrestor [1] doesn't completely silence the water hammer but does make a big difference. I'm going to try securing some of the cold water pipes too.

I've found clamping pipes well to the wall with vibration absorbing lumps sometimes works. A series valve turned partly off often works. And an upright dead leg /can/ work. Any of these things can reduce the q of the resonant structure, which is what you're aiming for.

NT

A section of cold water pipe in the crawl space under the dining room that had some play in it, so I strapped it up towards the joists in a few places with some "Sabrefix Builders Band Stainless Steel 20mm" (around the foam insulation that was already on the pipe). That hasn't eliminated the noise but has reduced it significantly further.

If the pipe is only feeding the dryer a valve would be the easiest surest plumbed fix. I always figured squeezing the pipe to reduce flow a bit should work, but never tried it.

NT