Engines, exhaust systems, propellers, cabin vents and aging door and window seals are just a handful of sources that can make the typical aircraft cabin a miserable place to spend hours on end. It’s also a path to irreversible hearing damage. How do you know if your cabin is too noisy? Fly in a comparably designed newer one, or one with cabin upgrades like that Piper Saratoga cabin in the main image below.
Part of the problem is that noise is one of those things that gradually sneaks up on an aging aircraft, and you might not realize it. It’s worth fixing because excessive noise is fatiguing for you and your passengers—even when wearing flagship ANR headsets. The good news is that soundproofing doesn’t have to cost an arm and a leg, and there are some DIY options. Ahead of our interior upgrade market survey, here’s a primer to get you thinking more about including soundproofing in your next interior upgrade.
CABIN NOISE 101
Put on your sound engineering cap. The force of sound waves is measured in decibels—and the scale used is logarithmic. For reference, while individual reaction to sound intensity varies, 70 dBA is often the basis for sound comparison—it is about the level at which most people keep the audio on a TV. Sound in the high 70s dBA is annoyingly loud to most people; 80 dBA is twice as loud as 70 and is the sound level of garbage disposals; 90 dBA is four times as loud as 70—it’s the sound of a motorcycle passing 25 feet from you. Some are louder than others—you know, loud pipes saves lives kind of thing.
In airplane cabins—especially exceptionally loud ones—hearing damage is not to be taken lightly. We know plenty of hearing aid-wearing pilots who admit to trashing their hearing because of years of not wearing good (or any) headsets. We too remember those bad old days. It doesn’t take long for damage to occur. Sound at 100 dBA is eight times as loud as 70 and will cause serious hearing damage in as little as eight hours of exposure; 110 dBA is 16 times as loud as 70 and is what you hear during a runup with the windows open. Think about that the next time you run up on a hot day, elbow hanging out the window frame.
The cabin air distribution system can be a major contributor to the noise level, especially on airplanes where the intake is within the prop arc and there are sharp turns in the path the high-speed air has to follow.
No good deed goes unpunished. Because weight is the enemy of general aviation aircraft design, a lot of cabin noise is the result of efforts to reduce weight. The light aluminum skin vibrates and resonates with the energy generated by each passage of a prop blade. Each power pulse from the engine, blasting out of the exhaust, further activates the aluminum. The movement of the airplane, shoving the air out of the way, generates significant noise. Ever fly a glider? Sometimes the claims of “silent flight” just aren’t true.
DON’T OVERTHINK IT
Frankly, there are some things you can do to reduce cabin noise without spending a dime. Start simply by taking a close look at how you operate the aircraft. Propeller noise can be reduced by turning the prop slower; using the POH, select the lowest RPM for a given power setting on an airplane with a constant-speed prop. Don’t worry about an “oversquare” power setting; that OWT is a myth. Many airplanes have a particular RPM at which overall vibration is lower, allowing for a lower perceived sound level.
Switching to a three-blade prop reduces cabin noise because the blades are shorter so the tips are moving more slowly and generating less noise. Three-blade props usually mean smoother operation, a benefit for the pilot and airframe. But for aging airframes, you might have to dig deeper.
What can be done to cut the racket? There is a combination of three approaches: Reduce the intensity of the sound at its source, reflect the sound away from the cabin and absorb the sound before it gets to the cabin.
One source is wind noise. If you’ve flown high-wing Cessnas, you may have noticed the noise coming from the can-style air vents. They can be replaced with the STC’d (for pre-1981 models) Precise-Flow Air Vents. They’re available from Sporty’s for $450 and also used by the interior refinisher AirMod. The adjustable flow-rate nozzles rotate 360 degrees for more precise airflow, plus new O-rings eliminate leaking in cold weather ops. You might have to relocate the outside air temp gauge on some models. And while you’re dealing with wind noise (and cold cabins), take a close look at door and window seals. Time spent on sealing the cabin doors may prove worth the investment.
To further reduce cabin door noise, we like inflatable door seals, such as Bob Fields’ “Ultimate Inflatable Door Seals” (www.aeroaccessories.aero/). They’re a bladder that goes around the door and are inflated either by hand or electrically once the door is closed. Prices start at $499 for manual systems and there are a wide range of applications—from Mooneys to the Beech 35/36 Bonanza or 55/58 Baron. The Ultimate Electric kit for a Cessna P210, as one example, is $2600.
While we have not seen hard data on door seal noise reduction, in the Piper, Beech and Cessna twins we’ve flown that had them, we could clearly tell the noise level difference when we waited until cruise to inflate the seal. We have been told that a pilot generally cannot detect less than a 2 dBA sound change in flight. All of the company’s door seals have FAA-PMA approval and are made of a vulcanized silicone rubber compound. The Texas-based company has been offering the seals for over 45 years.
Interior shops and techs tell us that the most cost-effective method is sound damping by installing a sound-absorbing material against the inside of the skin of the airplane, completely around the cabin. For a little more money, using a material that is specifically designed for sound absorption combined with a foil that reflects sound away from the cabin just about doubles the effectiveness.
From an economic perspective, the time to install soundproofing is when you’re upgrading the interior or avionics so at least part of the cabin is going to be opened to the stringers and skin. It’s a lot of work with sizable teardown, and there might be some surprises along the way. Be prepared for expenses beyond the soundproofing work when you pull everything out. It’s almost an imperative to have someone check the bare interior for needed repairs when it’s all exposed. This could be the first time it’s seen daylight since being sealed up on the factory line 40-plus years ago. Corrosion issues are common, as are frayed wires that could pose a fire risk.
The first step in installing any sound damping material is to remove the interior (something you don’t want to do if the interior is pretty new) and peel off the factory-installed material (usually a thin layer of Fiberglas) and whatever adhesive was used. Every shop we interviewed said the odds are high that there will be corrosion caused by the moisture-wicking effect of the factory soundproofing and the adhesives that were used to install it. We learned that corrosion treatment and soundproofing go hand in hand.
Moreover, you tend to get what you pay for with soundproofing materials. You also add more weight to the airplane, which is something to consider.
At a basic level, plain old Fiberglas does a reasonable job for a reasonable price. Shops tell us that Fiberglas tends to be the best bang for the buck. An inch to 1.5 inches of lofted Fiberglas costs a few hundred dollars for materials and requires an experienced technician about eight hours to install in a four-place single. It adds five to six pounds over the weight of factory soundproofing in a Cessna 182 or Beech Bonanza and, in tests by interior provider Air Mod, gives a 4-dBA noise reduction.
There are other solutions, and a quick browse of the Aircraft Spruce and Specialty and Wicks Aircraft websites, to name two suppliers, reveals there are plenty of materials for DIYers.
But choose soundproofing materials carefully. You’re looking for flame-retardant materials that are manufactured to meet requirements for FAR 23.856 and FAR 25.856 Part II.
One popular insulating product is SoundEx, which has excellent vibration damping and noise absorption properties. It may be preferred to other non-aviation products on the market because of its thermal qualities, it adds less weight to aircraft (6.11 ounces per square foot for 1-inch thickness, and 5.8 ounces per square foot for 3/4-inch thickness), plus it’s reasonably affordable. Aircraft Spruce sells a basic package that will do a Cessna 172 for $965, a Beech F33 for around the same price and an older-vintage Cessna 310 for around $1400. There are plenty of other available applications.
The SoundEx installation is “press fit,” no adhesive is used and the material does not absorb water so it does not increase the corrosion risk to the airplane. Further, it can be removed and reinstalled easily during future maintenance.
GO ALL IN
As you’ll see in our upcoming report on interior upgrades, soundproofing can add to the project’s bottom line, but we think for the majority of aging aircraft, the payback is big.