Wasn't going to bother doing this, but, thought I might as well further respond in order to complete this story (which I thought up in around 5 minutes flat) .... I.E extending this noise suppression, and later Hush-Kit implementation, topic to cover the CV880's and DC-8's (plus other period aircraft types) since it's all relative.
As I commented above (per my last response): "NOISE" had been an issue since jet aircraft types began entering commercial service during 1950's. This remained so throughout the 1960's and 70's too, but, "NONE" of the major aircraft, and engine, producers were particularly interested in addressing these matters despite acknowledging existence of such issues .... probably for reasons of technological limitations during this earlier period, the cost of developing solutions (versus their preference/s to apply resources to further develop money-making projects) then passing on costs of such developments to customers (resulting in more expensive products and prospectively limiting sales opportunities), and "probably" also because the prevailing attitudes of the day were founded largely along the lines of ....
if people "choose to" develop land and live next door to airports then "NOISE" is something they need to contend with" 
As also mentioned above: During the 1960's and 70's "NOISE" began to be addressed (of sorts), by airport authorities (primarily in response to action/s of public lobbyists) and which resulted in implementation of evening curfews and/or noise abatement procedures over/near residential areas located near airports .... and which persist even today also
(in an age where aircraft noise has been "dramatically reduced" from what it ever used to be).In comparison with P&W JT3C/JT4A/GE CJ805/and RR AVON "turbojet" engines (of early B707's/B720's/CV880's/DC-8's/and DH COMET's) and GE CJ805/P&W JT3D/JT8D/RR CONWAY and SPEY "fanjet" engines (of later B707's/B720's/B727's/B737's/BAC ONE-ELEVEN's/CV990's/DC-9's/HS TRIDENT's/and VC10's) .... each of which entered service during 1950's through 60's. The big GE CF6/P&W JT9D/and RR RB 211 "turbofan" engines (of wide-body A300B's, B747's, DC-10's, and L-1011 TRISTAR's) .... that entered service from the late 1970's period .... were not only much more powerful, but also, considerably quieter (for the time) too.
Aircraft "NOISE" remained an issue though and it wasn't until the early 1980's that previously non-existent Hush-Kits began to be "independently developed" for B707's/B720's/BAC ONE-ELEVEN's/and DC-8's primarily (and later for B727's/B737's/and DC-9's too) given popularity of these aircraft which extended through the 1980's and into the 90's .... although the attitude of major aircraft manufacturers continued to wane between luke-warmness to almost complete disinterest in regard to development of noise suppression options for these essentially aging 1960's era aircraft types. It seems their preference was simply to address "NOISE" through marketing of their latest/more advanced/quieter/cleaner/more fuel efficient A300B/A310/B747/B757/B767/MD-11/and MD-80 developments of superior logistical capabilities .... each of which were aimed at replacing older, much noisier, and less economic civil aircraft types.
COMMING BACK TO NOISE REDUCTION IN REGARD TO BOTH CV880's AND DC-8's (in particular) ....As was additionally mentioned above (in regard to both the B707's/B720's/and COMET's): The GE CJ805 "turbojet" engines on CV880's also featured a congregation of daisy/flower petal-like tubes located at the hot/exhaust end of each engine nacelle and which were similar, but not identical, to those featured on engines powering early B707's/B720's/COMET's .... as evidenced per the following images ....
CV880/880-M
The engines of early DC-8-10's/20's/30's/40's also each featured similar, though different again, engine exhaust tubes .... as evidenced per the following images ....
DC-8 SHIP ONE 1958 (P&W JT3P "turbojet" engines/not T-Rings or reverser equipped)
DC-8-10/20/30/40 (P&W JT3C/JT4A "turbojet" and RR CONWAY "fanjet" engines/T-Rings and reverser equipped)
In addition to these tubes the engines on series 10/20/30/and 40 DC-8's featured T-Rings/Ejectors too (all of which combined were an early attempt at "NOISE" suppression although the T-Rings/Ejectors also assisted functionality of reverse thrust as well) .... as evidenced per the following images and descriptions ....
DC-8-10/20/30/40 (P&W JT3C/JT4A "turbojet" and RR CONWAY "fanjet" engines/T-Rings retracted)
DC-8-10/20/30/40 (P&W JT3C/JT4A "turbojet" and RR CONWAY "fanjet" engines/T-Rings extended)
The HS 121 TRIDENT featured similar flower petal-like noise suppressing engine exhaust treatments as well (and which seemed to be common to some civil jet aircraft types of the 1960's era) .... as evidenced per the following images ....
TRIDENT 1/2
TRIDENT 3B
Despite this RR SPEY engines of TRIDENT's (and BAC ONE-ELEVEN's) still earned these aircraft a reputation for being one the noisiest civil jet types operating in, and out, of both British and European airports throughout the 1960's/70's .... and to such an extent noise abatement procedures were routinely practiced following TO. Although the TRIDENT remained in British service until 1985 these aircraft (and VC10's which remained in civil service until 1981) were never Hush-Kitted. US enforced noise regulations were implemented earlier than became the case within other global regions.
As additionally stated above too: How effective these early noise suppression attempts were is a debatable point. Neither of these endeavour's should be referred to as Husk-Kits though (and which is precisely what they weren't) .... by virtue of the fact Hush Kit type noise suppression treatments never came into existence until the early 1980s.
By this period (the 1980's) "almost all of" the noisy "turbojet" powered B707's/B720's/CV880's/DC-8's/and COMET's had already been retired from fleets of major operators around the world (it was escalating fuel costs and other economics factors during the 1970's that encouraged this more than noise), and even "fanjet" powered B707's/CV990's/and DC-8's were also, by this time, well on their way toward being retired from regular commercial service too .... although larger DC-8-61's/62's/and 63's remained popular among some major operators, during this period, given these aircraft had the right capacity and range for routes to which they were assigned, however, upon the introduction of FAA mandated Stage 2/3 noise regulations these aircraft then needed to be made noise compliant in order to (legally) remain in service. This is how, and why, true "HUSH-KITTING" began to take effect from the early 1980's.
Hush-Kits were (again) "independently developed" by a number of aerospace focused engineering companies and applied to B707's/B720's/DC-8's/and BAC ONE-ELEVEN's. In the case of B707's and BAC ONE-ELEVEN's the most commonly adopted Hush-Kit treatments were very conspicuous .... being located at either the intake, or exhaust ends, of the engine nacelles on these particular aircraft .... as evidenced per the following images ....
B707 (Hush Kit/Stage 2)
B707 (Hush Kit/Stage 3)
BAC ONE ELEVEN 200/300/400/475 (Hush Kit)
BAC ONE ELEVEN 500/510ED (Hush Kit)
BUT .... in the case of DC-8-50's/61's/62's/and 63's such Hush-Kitting (or other acoustic noise suppression treatment) was less externally conspicuous. In the case of both DC-8-50's and 61's an obvious external tell-tale indication, of these particular aircraft having been Stage 2 Hush-kitted, was a gold/brass coloured rim around the leading edge of each engine nacelle (suggesting interior acoustic treatment/s having been fitted) .... as evidenced per the following DC-8-50 images ....
DC-8-50 (Stage 2 acoustic treatment)
Another, and much more conspicuous, Stage 3 noise suppression treatment was also later developed, but, only ever applied to FINE AIR DC-8-50F's/61F's .... and as evidenced per the following images ....
DC-8-50F (Stage 3 acoustic treatment)
DC-8-61F (Stage 3 acoustic treatment)
Hush-Kit modifications applied to DC-8-62's/63's were different again. These were similarly not so externally conspicuous either, but, could be assumed per natural metal (or sometimes coloured) sleeves around the entire forward section of each engine nacelle .... as evidenced per the following DC-8-62/63 images ....
DC-8-62 (acoustic treatment)
DC-8-63 (acoustic treatment)
However .... the only truly effective, but also much more costly (though a far cheaper option than replacing older fleets with more advanced aircraft types) noise compliance solution for DC-8's was upgrading these aircraft with (then) modern CFM-56 "turbofan" engines. Throughout the 1980's a total of some 110 DC-8-61's/62's/and 63's (only) were re-enginned accordingly .... thus being rebranded DC-8-71's/72's/and 73's respectively .... as per the following images ....
DC-8-71
DC-8-72
DC-8-73
These CFM-56 turbofan engines were not only quieter, but also, hugely more fuel efficient, and cleaner burning, whilst additionally promoting greater power, with corresponding and significantly improved logistical aircraft performance/endurance capabilities too. Although the CFM-56 turbofan was extensively used among military C-135 type aircraft it was never applied to any civil operated B707's .... beyond a single airframe (temporarily re-branded "B707-700"). This particular aircraft was used, by BOEING, in order to flight test this (then) new turbofan engine technology. Upon the conclusion of flight testing this same aircraft was converted back to standard B707-type P&W JT3D type power and sold to a civil operator ....
B707-700 (CFM-56 powered)
C-135 (CFM-56 powered)
A particular disadvantage for the B707 (in respect of its suitability for CFM turbofan re-enginning) was that these aircraft, having shorter legs, lacked adequate ground clearance "for civil type operations" .... given the much larger diameter of the CFM-56 turbofans. The cost of implementing taller landing gear to increase ground clearance, along with necessary wing strengthening too, in order to accommodate these superior engines was prohibitive .... and justification for such conversions were also largely invalidated anyway given the majority of B707 operators were, by the early 1980's, already committing to more advanced and superior performance twin-enginned B757's/B767's.
One will find all of the details discussed within my above 2 responses are "relatively well represented/animated" among each of the corresponding HJG offered 3D models Mark C
AKL/NZ
