What is a Hush Kit? 

As a kid, my dad flew out of seatac airport several times a year. At the time 707's were a common sight, and it was okay to go out to the observation deck. Out there we could see directly, and listen to to those beasts taking off. I was completely enthralled, and thought over the years that i must have misremembered just how loud that was. Now I understand that it was as loud as i used to think it was.

I grew up about 2mi off the end of a runway at ORD O'hare Intl in the 70's, and I'm here to testify that, boy howdy, it's hard to describe just how loud a 707 (still flying then) or a 727 ( well into the 90's) was on climb-out. The windows rattled in my bedroom for a few of the heavy hours every day. Now it's just the turbine whine in the same spot. Edit to add: and I was an areospace kid, I knew what they were and liked them, but if you were standing on the sidewalk with yer buds, you had to stop talking for about 15 seconds till the jet moved a mile further away to continue the covo.

I remember attending religious conventions at Yankee Stadium back in the 70s. The speakers had to pause for 60-90 seconds when the 707s departing LaGuardia came overhead at TOGA power. The noise was incredible, and very memorable. The snapping and popping is what stands out. I now know how lucky I am to have been able to experienced that in person. That's a historical time that won't be experienced again. Imagine being in an acoustic fishbowl while turbojets like the 707 fly over every few minutes at TOGA power. It was incredible.

I spent a few years of my youth in the final approach path for the most-used runway at Mitchell International in Milwaukee. Lots of DC-8 and a few 707 approaches with their very particular kind of high-frequency intensive sound (at least the low approach passes were at relatively low thrust compared with the ridiculous sound at takeoff). Even old 747 videos remind me of those days. Aside from the many turboprops still in service the majority of the equipment landing was 727 and DC-9 variants which also made that high-frequency heavy sound. I LOVE aircraft of just about every kind and yet I don't miss that particular noise -- today when I'm traveling in markets that have larger commercial airliners in the air I'm amazed at how much less obtrusive the sound is from those lovely high-bypass turbofans. What a difference from the old days!

J79 is a beast. You don't just hear it, you feel it when close enough. I did 6 months line servicing with 439 Sqn on CF-104 in Germany back in the early '80's. Being a rookie I was 'back end' man when we started the aircraft. On the 1st start of the day & for tactical exercises the bird was started in the Hardened Aircraft Shelter (HAS). Open front & rear doors of the HAS & light 'er up. The hydraulic panel below the power plant at the rear of the aircraft is lowered during start up so the 'back end' man, yours truly, can check for leads after the engine is started. Once all is good back end boy swings the panel up & latches it, then the 5 finger check coordinated between pilot & front boy (girl in some cases even back then ;) ) proceeds before the rocket is sent on its merry way. Anyhow, as back end you get to experience the J79 up close & personal at idle, which I think is somewhere around 60%, perhaps a little higher. Even at idle one can feel the beast....

Republic ThunderShriek turboprop fighter prototype. Was so loud it made people in hearing distance nauseous. Was tested at Edward’s AFB. The tower required the plane be towed out to the desert to be tested due to noise. This is at a fighter test center.

Hush Kits always reminds me of stories I heard of a William "Winky" Crawford that operated cargo flights for salmon up in Alaska in the 80s. He had a company called "Winky's Flying Circus" that operated a lot of tiny Cessna's, Piper's and DC-3s that went around Alaska picking up salmon, gathering it up for a Douglas DC-8 that he owned and shipped the salmon to Tokyo with. Every takeoff of that DC-8 the FAA fined him 1000 dollars for not having hush kits on it, however Winky never had it fitted, as he deemed it to cost more getting it fitted than paying the fine...

There used to be a private motocross track near Beeville Tx in a cow pasture , and a quarter mile off the end of a Navy runway . The jets would do touch and goes with their nozzles eventually pointed down at you for a moment . So I'm wailing along hanging off the loud handle on my 2 stroke and BOOOOOOOOOM ! I had to stop and it took a bit to figure out just what had happened . Just me and my boy , nobody warned us . lol . Coincidentally, this was down the road a little bit from Major Mitchell's house/ mx compound. He wore well starched khakis with a magnificent handlebar moustache. I never chatted him up , but was warned . No cussin ,no drinkin , and definitely no weed . That was his house and definitely the sweetest old school track in Texas at the time .

Back in the early 80’s I flew SIC in a Gulfstream II, this aircraft was equipped with Rolls Royce Speys. Least to say it was quite loud from what I was told. We flew out a midwestern universities airport and because some developer built a bunch of homes about a half mile from the end of the runway and the residents were complaining, the university placed signs along side the runway that read, "NOISE ABATEMENT TAKEOFFS PLEASE". Well every pilot who flew the early jets said BS, not going to risk a multi million dollar jet for some idiots who bought homes at the end of a runway, we had a habit of flipping the bird at the sign as we went full power for take off. Fast forward to 2023, was sitting at a combo military and civilian airport waiting for a shuttle bus and the Airforce was conducting nite practice and let me tell you, those F16 taking off full power were very painful to my ears and the terminal, parallel taxiway, runway were between the military runway which was situated parallel with the civilian runway did zero at blocking the noise. Also back in the 60’s we lived by another university airfield where some pilot brought in a 707 at night and thought it was the main airport which back then has a similar runway configuration. Woke us all up but the best part came when they had to take off from that airfield, passengers were bussed to the correct airport with their luggage but mechanics had to be brought in to strip out all of the seating to cut the load on the aircraft. The airline even dispatched a tanker truck to offload fuel since it was a ten mile ferry flight. Well everyone knew when they left the university airfield, you could hear those engines winding up for a good three minutes to attain full thrust before they left and this airfield was only five miles from our neighborhood. Word was the airline promptly fired the pilot but he was hired by another airline within a few days. They found his skill at landing on a runway that was about a thousand feet too short was good enough for them.

thanks so much for going thru all you have here to tell us an show us how hush kits work, its really interesting just how we went about trying to numb that sound of the old jets, my father told me when he was in vietnam the jets back then were so loud it would blur your vision an make you unstable in your balance, some times he said it made folks feel sick..

I do some maintenance for our military and i had an opportunity to stand fairly close to the runway when they started a mig-21. The noise is overwhelming,i would describe it as a acetylene torch sound but millions times louder,and you can feel it in every part of your body. It's an amazing experience

I remember book of records stating that Saturn V is the loudest sound produced by man, measured at 210dB. Which gives me an idea - you could invite some folks from the rocketry youtubers to your test cell so they can practice how to record loud noise with proper dynamic range so they're ready for next Starship launch or whatever. And you could get some proper recordings of your afterburners ;)

I can attest to the sound level of the 707s. I grew up in the 50s 2 miles off the threshhold of one of O'Hare's runways in Park Ridge Illinois. Park Ridge actually built a plexiglass domed van with an observer with binoculars and a sound level meter to gather lawsuit evidence against airlines. My dad glued down the knick-knacks my mother had put on glass shelves to keep them from vibrating off and breaking.

True story. My early A&P days at IAD. Mid ‘80’s. We had service contracts for the RAF VC10’s and Air France Concordes. Both extremely loud on takeoff. The 4 Conways on the VC10 was louder at takeoff then the 4 Olympus’s were on the Concorde.

When i was a kid in the late 80's, i can remember the BAC 1-11 with Speys taking off from Manchester Airport UK and they would shake the whole airport terminal. They sounded like the most violent thunder even from inside the terminal behind the thick glass viewing windows. Im sure this was with the earlier type hush kits fitted so i can only imagine what the sound must have been like without them. Of afterburner feels like twice as loud then it must be like the end of the world loud

United decided to install hush kits on 727 JT8Ds. They were a pain to install. Also very time consuming. Not friendly to fuel consumption rate. At the time I was working in the jet shop at OHare Chicago.

And 2 J79s in full afterburner are even louder... Worked F4 for a year, and another year on F105. Really loud is when the sound shakes you internally and affects your breathing. Those aircraft do that. Great video!

If you haven't, you really need to see a couple top fuel dragsters running side by side at some point in your life. It's an experience to say the least!!

Ejectors/augmenters could produce extra thrust under some conditions (probably mostly low speeds) with the right Venturi shape. They would draw in extra air mass, accelerate it through the converging section, accelerate it further in the throat by interacting with the higher speed jet exhaust, and sometimes slow it back down a little bit through a diverging section. They work kind of like a very inefficient LP turbine and fan. Regardless, the result is a higher volume of air at lower speed, which is naturally quieter.

JT3Cs and JT4A's on water injection are INSANELY loud...and smokey LOL. I remember seeing the odd one when I was a little guy. The noise was ear-splitting!!

Your choice of questions to answer are usually very interesting. Thank You for the time spent trying to inform us. Best Wishes to You and Your Family.

The last 722-Adv had some pretty cool hush kits, but alas, even with those they were loud. Appreciate the video.

Regarding turbojet/afterburner loudness - the best description I've heard is that it's so loud that it takes all your willpower to override your instinct to run away as fast as you can.

I used to work at a place that was near a big airport. Got used to the passenger and cargo planes flying over. Figured that was pretty normal noise and wasn’t bad. Then a fighter jet flew over and seemed to rattle the whole shop. We all went out to see what was up because it was so loud. And that was it basically just cruising around overhead. Would certainly be an experience near one at full power and full afterburner.

I worked for burbank aeronautical Co years ago we built hush kits for 707 dc8 aircraft we did flight test for faa and installations ' this was for the P&W jt3d3b we remanufactured the inlet biducts and inlet bullet and later did kits for dc9 and 737 with that experience and my manufacturing experience at lockheed I qualified for school to get my A&P TICKET thanks !

Loved watching CPAir 707s trundle along the ramp from Pearson airport old parking garage

I grew up (and still live next to) next to Warner Robins AFB in Georgia. Back in the early-mid 70's, RAFB housed a number of older B-52's (I'd reckon probably G series). Even if they were flying overhead at 5,000' and at highly subdued speeds, those things were inSANEly loud! Absolutely piercing as hell! Even today, they are home to a lot of JSTARS, and compared to everything else, they're really loud. Sometimes my wife will complain about them, and I've told her "Enjoy it! We're one of the few places in the world where we can get to hear engines like that anymore!"

Im getting the idea that jet engines are really loud.

When I was in the USAF, I was assigned to a SAC base that had KC-135A and B-52G models that had J-57 engines which were extremely loud during a dry takeoff. When the takeoff weight exceed a certain weight, we used water injection it increase thrust which amped up the sound to an almost unbearable sound. An 8 engined BUFF doing a water takeoff is something you can't imagine until you hear it in person.

I remember commercials about the “whisperjets” as a kid.

Many thanks for answering my question🙂

I used to hear KC135's take off when I worked at General Dynamics, in FtWorth. The 135's were louder than a F16 at Full Burner. They weren't as loud as a Top Fuel Dragster, if you were standing on the Starting Line but, they were UNBELIEVABLY LOUD. I had access to the Flightline so I was within about 4 or 500' of them. They made A TON of Black Smoke to go with it. I'm assuming THAT was from Water Injecting the Engines on takeoff but, I don't know for sure. I'm guessing the AirForce was still using Turbo Jets, at that time. Late 80s

Well done once again J

Imagine what the first reengined B52 will sound like ... BR710 is a remarkably quiet engine. It'll be quite the difference.

blowgun experiment is a simple venturi and in some shop equipment can be used to generate a fairly strong vacuum as well as the increased flow reduced speed you mentioned

One example of a blow gun with a Laval nozzle is the Silvent 501. It claims 78 dB at 6 bar air pressure.

And again, another very interesting video.

What a great topic! I grew up in a military low fly area so used to loud jets, and still visits to airports when I was a kid were quite incredible, I think only people living near military bases these days would have a clue & even those jets are generally low bypass fans. Still remember Concorde & the daily Aeroflot deafening London in the early 90s... There's a 727 used for pollution incidents here with kitted engines, I think that's the only thing I've seen in years. Those old Tridents & 1-11s & F28s, good grief. If they start using geared LP fans ( more? I don't know if there are any in service yet ) we won't even have the supersonic fan tip brrrr on takeoff. Thanks for the book recommendations! I am a dirty engineering dropout but I still love reading about the engineering. I'm a little curious if the ejector-style is at least drag-neutral given the addition of super hot exhaust gas to the part.

I'm with you on the offensiveness of the high frequencies. It was always the crackling that was most offensive.

I was fortunate enough to work at KIAD . my 30 years on the field I witnessed and heard the Concord take off. Watched and heard the SR71 DO a tower flyby with full Afterburner when its was donated to the Smithsonian. I was present when the 777 for United airlines took off for its inguinal flight and even at 300 yards from the runway all you heard was hissing air. I remeber the nite a British airways Airbus took off with engines that were stage 4 compliant and all you heard was the locust chirping away and watched the last cargo specific 727-200 with the most powerful jt8 Whistler's take off to Memphis for retirement . Jet noise is not a sound to be hushed it was music you could identify the plane by. Sadly today everything sounds like a hairdryer.

Totally agree about high frequencies feeling way worse

The silencer baffles on the old 707slook almost like a milkweed blossom

Awesome video

The early low bypass engines were still noisy. I’m thinking of the Conways on the VC10. Saw a RAF VC10 take off from EDI and it shook the entire airport

A top fuel dragster up close and personal is around 150 db's. But only for about 5 seconds.

My favorite sounds, F-4D/E, F-111A, water burning KC-135's, B-1B, C-141 and FRED with the original engines.

18:50 The tube demonstrated a shift in the sound frequencies like the flower nozzle, though!

Can’t wait to watch this one! Always good watches, keep up the good work! I wanted to ask, is there any particular other reasons you don’t work on larger or newer engines? I would guess it has to do with manufacturers doing repairs themselves or large differences between different engines making it easier to specialize on specific engines. Not to mention technology and costs associated with newer engines. Did I hit the nail on the head? I’m assuming I’m missing something more obvious

AgentJayZ, This made me so happy! I liked and subscribed!

Interesting they ended up with the augmenter design. There was a theory for a long time about Augmenters on Pulse jet engines that it increased performance but ive seen mixed reviews. .The Inlet attenuaters you were rtalking about sound similar to the Generator houses i used to work on . They had Detroit 2 strokes and Cat 399 engines in them but it was crazy how the sound deadening worked .You could walk into one of the rooms with the engines off and even your voice sounded muffled .

Back in my military days, I was helping trim the engine on an A-4M Skyhawk. Pretty fun, but we had a F-4 Phantom next to us, running with full afterburners! As you say, I don't know how to describe how loud that was! It was dusk, and those 'burners looked so beautiful in the low light. I was poking around for some explanation of the noise, and Scott Manley did a nice video on some of the physics of it... ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-BdCizNwLaHA.html

@agentjayz My understanding is that the modern evolution of hush kits are the scallops or chevron's at the rear of a 787 engine nacelle. Apparently they are designed to help with the mixing of the high speed hot engine gases with the outside atmosphere to help attenuate sound.

The 'newer' type reminds me a bit of a glass pack muffler.

The old Air New Zealand 737-200s had to have hush kits put on them. Theres a great video below with a great shot of the engine and hush kit @1:25 in ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-goP9EhL4v94.htmlsi=1u3B0VswYgeDv7Fu

I live only a few minutes from the town of Hucknall, UK where there used to be a Rolls Royce factory and airfield. The "Flying Bedstead" was developed there. There is a commemorative statue/monument in the town market place that incorporates one of those noise suppressor cones. Was it invented by RR I wonder?

Any sense for if/how much performance was affected by adding a hush kit? I’m sure weight and drag play in, but does altering the exhaust flow make a noticeable change in thrust?

Supersonic Noise of a TurboJet is really loud.

Noise limits are 85db for 8hrs or 90db for an hour. Some of the modern narrow body airliners have similar idea as the "hush kits" on the jet exhaust (not the bypass fan) by a corrugated cut of the nozzle. The noise issues where in the cabin (where the body eliminated lots of the high frequency but did very little against number), or in housing under the flightpath (so quite far form the actual aircraft), rather than for ground crews or airport terminal. Air augmenters ect are also a big safety thing because they can reverse, so when idiots poke the nozzle against their skin it just blows the wrong way out its intake rather than into their blood and risking killing them. As much as I think people so untrained should not be in the risk zone to begin with, the penalty should be to their bosses in the form of a fine, not to the untrained person in the form of serious injury.

AgentJayZ, You're amazing! I hit the like button as soon as I saw it!

Do we have any actual (d)B comparisons between a turbojet and e.g. the Atlas-V 1st stage engine?

Here in Palm Springs California our international airport is basically in the middle of the city. Compared to the early 2000’s we barely hear the air traffic unless it’s military. Large or small they are loud. But you really have to be at the right place right time to notice. Playing baseball at a younger age we used to hear what I think I now know as reverse thrusting a form of breaking? I’m not savvy on the lingo.

The workings of the lining material is stated in the book ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Jynh81_zl8g.html . I quote: "These noise absorbing materials convert acoustic energy (air pressure) into heat energy." That means the rest of the design is to get rid of that heat as quickly as possible. Maybe that resulted in that flower design to increase surface area.

Can you please explain why they say that the Saber had a centripetal engine? All of the Saber engines that you show us are annular. Grim Reapers wonders too. What planes do you fly on DCS?

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Molten Sodium Lakes, 🤔I can see that. 🌴

Mentally I am also more scared of high frequencies because they contain more energy which could damage my ears.

So is it loud then?

I know that I'm going to have a tough time muting turbine noises for my future plans, since it is aimed to work entirely on the ground surface.

10dB reduction in noise is exponentially more than half. 3dB reduction is half. 3bB increase is double. 10dB would be half of half of half plus 1/3rd.

Do you use db?

Hmm I cant help wondering if the F86 could have hypothetically taken an afterburner if the airframe was strong enough. Ir would it have messed with the balance too much? IE been too long or put too much weight behind the COG etc. Also there is a channel called Hush Kit that is aeroplane based and personally I find it extremely funny. Well their "worst" series is.

Re loud race cars: Does anyone remember the Indy turbine car? PT 6 powered. Compared to the other cars, someone joked that they should have installed a horn to let the other drivers know it was about to pass them up.

The f 86 has 6 in he's wider and used 20mm cannons not .50s They built about 360 of them I believe it had 9000 lbs of thrust!!!

I love jet noise...

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couldn't the saw-teeth on the exhaust of some modern engines be considered a form of hush kit?

👀👂

Do you have a Minnesota state flag yet? If not, let me know the size you want and address to send it to.

Because decibels are measures on a logarithmic scale, every 3 db doubles the sound, not every 10.