You may recall back in the day when the HAI initiated its 'Fly Neighbourly' (or 'neighborly' as they spell it in the US) campaign.
In an effort to try and reduce noise the Snohomish (really, where do they get these names) County Sheriff's Department 'stuck' on the asymmetrical 500 t/r assembly.
How much noise reduction it actually provides I honestly don't know!
Earl