RR Viper jet pipe

[ZeBedie]

A bit of a niche subject this, but looking at HS125-600B, some aircraft had a straight, simple jet pipe, others had a diffuser. Why was there a difference?

https://www.prints-online.com/new-images-july-2023/hawker-siddeley-hs125-600b-g-halk-32230610.html

https://abpic.co.uk/pictures/view/1736019

[Self loading bear]

Hush kits?

[spekesoftly]

I believe the diffuser was a customer option (or retrofit) to reduce the viper's noise output. At one time such 'Hush-kits' were quite common on various jet aircraft as airports became increasingly sensitive to noise.

[Quemerford]

I recall that husk kits on One-Elevens made no discernible difference. I imagine that it must have been measurable but my human ear couldn't detect anything.

[Allan Lupton]

I recall that husk kits on One-Elevens made no discernible difference. I imagine that it must have been measurable but my human ear couldn't detect anything.
We were always a bit puzzled that the One-Eleven was so much noisier than our Tridents. I think I recall a member of the Weybridge opposition (later colleagues) telling us that most of the One-Eleven's noise was from the air-driven CSDs in which case a hush kit would indeed make no difference.

[dixi188]

I was at Gatwick when the first 1-11 hush kits appeared, Tarom I think. The difference was only noticeable when the aircraft got some distance away. The noise seemed to be the same on take-off but the hush kit fitted ones faded away quite soon whereas the unfitted aircraft could still be heard when they were near Seaford.

[Viscount Way]

We were always a bit puzzled that the One-Eleven was so much noisier than our Tridents. I think I recall a member of the Weybridge opposition (later colleagues) telling us that most of the One-Eleven's noise was from the air-driven CSDs in which case a hush kit would indeed make no difference.

Quite soon after One-Elevens entered service it was found that acoustic vibration was cracking the horizontal stabiliser structure. The nozzles were replaced to limit the problem. Noise was actually much increased and some power lost I recall. The One-Eleven CSD was actually a CSDS which incorporated an air driven starter. It was the running down of the air driven part which gave the characteristic “whoop, whoop” noise at the end of the engine start cycle. Whatever possessed them to install such a heavy, unreliable contraption will never be known. Nothing like it has been seen since as far as I know.
Trident-wise, I seem to recall early mornings hearing Tridents running engines post-maintenance at Hatton Cross from my house in Hersham. Maybe seven or eight miles away. The aircraft on the Cullum mufflers too!

[Quemerford]

I'm glad to hear that my mind wasn't entirely playing tricks: I do seem to recall seeing a noise footprint at some point, which showed how the dB should have been reduced (possibly in an aft-wards direction as dixi188 alluded to above).

However, slightly off-topic, I really miss the sight and sound of a low-level SOAF One-Eleven!

[tdracer]

A couple things to keep in mind. First, the human ear is not a particularly good sensor when it comes to determining the relative loudness of noise. It takes roughly a 3db change in noise level before the human ear can even detect it (which, given the logarithmic nature of the decibel scale means a doubling (or halving) of the sound energy level - yet obtaining a 3db reduction is a huge engineering challenge. Further, the human ear doesn't detect all frequency levels the same - hence there is a db scale that is "weighted" for certain frequencies.
Second, the official jet aircraft Takeoff and Landing noise levels are determined at fairly specific points along the aircraft flight path - so getting a meaningful reduction in noise level at the official measurement sites doesn't necessarily mean the same thing at other locations along the flight path.