It is the brain child of Three Spires Accustics, an independent firm of consultants specialising in environmental acoustics and other areas of pollution and regulatory control.
I have not used the app myself but apparently you can record noise, use it to keep a diary, and then when you have sufficient evidence, send it off to your Local Authority Environmental Health dept.
The app has a complete list of all UK Environmental Health Dept contact details for you to choose from, together with useful information about noise nuisance generally.
So if you live next door to some noisy tenants, and their landlord is refusing to do anything about it (not that they can do very much in most cases) – it might be worth a look. Its only £2.49 after all.
There is also some helpful information on their website here.