Most single engine aircraft have the engine on the nose or somewhere on the fuselage in the centerline. Some have both at the same time, the Cessna 336 and 337 Skymaster is a good example. And if you are flying twin piston aircraft then sitting almost next to the propellers can be deafening.
Kit aircraft builders have the option to install sound proofing so that the cockpit will be quieter during flight and this helps fighting pilot and passenger fatigue.
Owners of certified aircraft will need to work with a certified company to reduce cockpit noise. But the results can definitely be worthwhile.
Of course, a good headset will work wonders especially if its an active noise canceling type. Test and try a couple in the aircraft you fly most. Good ones are David Clark, Lightspeed, Sennheiser and Bose.
Prepare yourself to get used to some silence in the cockpit!
As most glider pilots will attest to, engines create a lot of noise. During your flight training and practicing engine out landings you are aware that engine noise/sounds are absent or much less. The cockpit is so much quieter except for wind noise.
There are several sources of noise in an aircraft. These include canopy leak hiss or door leakage, propeller noise, engine exhaust, other engine noise including accessories, and wind noise coming from retracting the gear and such.
Propeller noise changes with engine RPM if you have a fixed propeller. Constant speed propellers keep their RPM constant within a certain range. Trying to reduce this type of noise means changing blades with other planforms or even adding a blade by installing a different propeller. For aircraft builders: a thicker windshield helps against propeller noise.
Door and canopy leaks create hissing sounds which increase with airspeed. A good seal should be provided by the kit manufacturer but if its not can be installed easily with common rubber door seal products.
Most sounds produced by the engine comes from the exhaust and can be suppressed a great deal by using a good designed exhaust muffler system. This also reduces the aircraft sound profile as can be heard on the ground.
When a sound wave hits a wall (aircraft engine firewall, bottom fuselage) it transmits or conducts the sound to the other side. Only heavy thick walls block sounds, but in aircraft this is not practical. We need therefore a way to mimic the sound reduction characteristics of a thick wall.
A special foam is used to dampen sounds and stop the vibration of the airframe structure. It acts to absorb and dissipate sound energy and is usually made of polyurethane foam with an aluminum foil on one side and strong adhesive on the other side. This permits quick and easy installation as the material can be cut with a sharp knife or fine hacksaw.
It adds a little bit to the weight of the aircraft, but the comfort of a quiet cabin is priceless. A number of aircraft part supply shops and stores sell this sound damping foam.
To get the most from a sound damping material it must have some special properties: low density and high compressibility are the important ones. It also may not be flammable or conduct heat easily and moisture should also be rejected.
You may have to look around and see other builders what they have used to dampen sound in their aircraft, the solutions can and will vary among aircraft, even in aircraft of the same type!
The design of the cockpit can have a big influence in the amount of noise it dampens. A C-172 is inherently quieter by design than the cockpit of a DynAero MCR-01 which is all carbon fiber and has a large Plexiglas canopy. The noise in the MCR-01 keeps on reflecting in the cockpit until it fades out, which it somehow never does.
A good noise canceling / reduction headset works wonders for the ears of the crew. In the good old days, headsets used to be passive noise canceling but the last couple of years manufacturers are using special designed electronics to actively cancel out noise in the headset. A minor drawback is that these will need electrical power in the form of batteries or be wired from the panel.
I personally use the Lightspeed Zulu headset with a pair of rechargeable Sanyo Eneloop batteries and they last me at least six months with a full charge.
For this to work the headset has two extra microphones near the ear cup picking up noise, which is then fed 180° out of phase with the sound coming in from the normal microphone boom. The result is really, really amazing and yes, you can still hear the engine running.
Most models have extras like left/right volume controls, frequency equalizers or tone control and even Bluetooth to connect a mobile phone as an extra option or selling point for the customer.
Basically, these are well worth the money you will spend on them.