707 without hushkit:
http://www.airliners.net/open.file/543237/L/
707 with hushkit:
http://www.airliners.net/open.file/321118/L/
Notice the difference in the nacelles......
Now one should take the word "Hushkit" on a 707 with a grain of salt. Especially when it comes to the Burbank Nacelle conversion. There is nothing quiet about a blow in door nacelle.
Very few hushkits really do nothing but spread the noise around a bit. Hushkits that also effect performance (ie. thrust required) are the only ones that really show realistic improvement.
Jet Engineering recently certified a hushkit for the MD-80. If you were to fly an MD80 by without this kit and then by with the kit nobody on the face of the planet would be able to tell the difference. Even a monitor will be hard pressed to notice any real difference.
