-3 series was louder than the JT
-8. However, "louder" is both a quantitative and qualitative term. You can get db values for each engine in their original installations to make the quantitative comparrisons. It should be noted, the original JT
-3C and even JT
-4 turbojets had "organ pipe" sound suppressors on Boeing jets and a similar array plus the ejector ring in the DC-8-10/20/30 installations. Military J-57 and J-75 versions of these engines didn't have any sound suppression and were even louder.
The introduction of the JT
-3D series Turbofan for the Boeing 707/720B and DC-8-50/60 series provided the first reduction in maximum noise. As a listener, the difference would only be apparent by side by side comparrison as they were still pretty loud.
engine was developed for the B727 and DC9 as a neighbor friendly turbofan powerplant that was noticably quieter than all of the JT
-805, and Rolls Royce Conway powerplants in use on the 4 engine airliners. However, as the JT
progressed with higher and higher thrust outputs, again the maximum noise increased. This is why many DC9s seemed so loud when powered with the JT
The requirements for reduced noise introduced Stage II
, and later Stage III standards. Inlet treatments and exhaust modifications reduced the noise values of all of the engines. Unmodified planes were banned from many locations. Because many younger observers have only experienced DC-8s and B707s in recent years that were modified to Stage II
or III standards, it is understandable why they might not perceive the noise differences with the JT
-8 powered planes in compliance with the same standards.
All jets operating today must meet Stage III nearly everywhere. But a new Stage IV
standard is emerging and the cycle of noise reduction and obsolescence starts again.
But, returning to the historic context, remember that Eastern Air Lines was the launch operator of the Boeing 727 with the then new and revolutionary JT
-1 powerplant. They dubbed these planes "WhisperJet" to market their new good neighbor quality.