Jet engines are noisy - even a modern high-bypass turbofan is still deafeningly loud compared to pretty much anything except the even-louder low-bypass turbofans or a turbojet (much louder still than even a low-bypass turbofan). Unsurprisingly, making jets quieter has been a major concern for the entire jet age; as the majority of a jet engine's noise comes from turbulence at the boundary between the hot core exhaust, on the one hand, and the ambient air (turbojets) / bypass airflow (turbofans), on the other (along with, for turbofans, a small contribution from turbulence at the boundary between the bypass airflow and the ambient air), the primary means of doing this without needing a turbine-incineratingly-high bypass ratio1 is by making it so that the two or three exhaust streams mix smoothly, and preferably do so before going out the tailpipe (so that any noise that is generated during the mixing process can be trapped within the nacelle).
To this end, early jetliners used intricate forced-mixer tailpipes, which were extremely effective at mixing the exhaust and the ambient air, and, thereby, reducing noise; they could either be built into the engine at manufacture time, or retrofitted later on as a "hushkit". These could be seen both on turbojets...2
...and on turbofans.3
With the switch to high-bypass turbofans in the 1970s and 1980s, the dramatically-increased bypass ratio of the new engines was enough all by itself to produce a large drop in noisiness, such that even a plain, unmixed tailpipe was acceptable, and the forced-mixer tailpipes of the earlier engines slowly disappeared from view as the numbers of aircraft using the older engines dwindled.
However, since then, the NIMBYs have gotten more exacting and noise standards have continued to grow stricter and stricter, forcing jets to again use exhaust mixing to make them quieter. Two methods for doing this are in common use:
These sawtoothed tailpipe rims enhance the mixing of the core, bypass, and ambient air, reducing noise:4
These engines simply send both the core and bypass airflow through a single, long tailpipe, where shearing forces at the boundary between the two airstreams create turbulence that causes them to mix to some extent before exiting the engine:5
Both of these methods, however, are fairly inefficient at mixing the different airstreams; a scaled-up forced mixer would be far more mixy (and, therefore, more noise-reducingy). So why do modern high-bypass turbofans still use inefficient methods of exhaust mixing, instead of the more effective forced mixing?
1: A higher bypass ratio requires (for a given thrust rating) more power per unit turbine, which requires the turbine to run hotter.
4: Exhaust mixer of a General Electric GEnx-2B high-bypass turbofan from a Boeing 747-8I (image by Olivier Cleynen at Wikimedia Commons); note the chevrons on both the fan cowling (here opened to appease the peanut gallery), for mixing the bypass airflow with the ambient air, and on the core tailpipe, for mixing the core exhaust with the bypass airflow.
5: Exhaust mixers of two General Electric/SNECMA CFM56-5C high-bypass turbofans from an Airbus A340-300 (image by Hansueli Krapf at Wikimedia Commons); two more A340s, along with their engines and said engines' exhaust mixers, are also visible in the background (one each at upper left and upper far right; please ignore the 747 at upper mid right).