Experiences with soundproofing

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So, while my neighbours are quite odd (they have certain ritual ‘chants’ that they shout throughout the day.. I’ll leave it there) they have actually been very agreeable since they moved in a year ago - minimal noise (besides the periodic chanting), not up super early or late, don’t use their garden, etc. Tbh I could barely hear them.

Then a few weeks ago a deep bassy and LOUD impact noise started coming from their kitchen - it begins in the morning and goes off every 20 minutes until 10pm-2am. I have absolutely no idea what it is, how it can be so loud and how they’ve lived there for a year and not heard anything of the sort but now it is constant. Unlike talking noise, we can’t just move rooms as it vibrates through the whole house. You can even hear it from outside, so it is loud and definitely a result of a deliberate action. It’s keeping me up at night and waking me up in the morning.

I’ve gone round 3 times to discuss and they are refusing to answer the door to me..? Also popped a note through the door to no avail. The caginess is making me think it is a new ‘ritual’ of theirs and is unlikely to stop.

Therefore, does anyone have any experiences with soundproofing a party wall to share? Specifically for impact noise (I understand this is much more difficult to manage) and not talking/TV as we have no issue with this. I hear such varying reports that I’m not sure it would be worth the money.
 
When my parents and I moved into a semi - detached, our neighbours were musicians, think the were in an orchestra and also the Salvation army band . We got the idea from somewhere that fitted wardrobes full of textiles absorbs noise. We could hear their music more in the room that didn't have these wardrobes.