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Aero Acoustics 101

The builders of experimental aircraft and the restorers and fabricators of commercial aircraft face a number of issues when it comes to the control of aviation noise, vibration and heat (thermal dynamics). While that experimental aircraft is far more graceful than your earthbound luxury sport sedan, the creature comforts are far less sophisticated than the standards to which we have grown accustomed in our automobiles, making the flight less pleasurable.  We have taken the thermal-acoustic research and the technology that has become commonplace in the automotive field and adapted it to the small aircraft industry.
Historical Perspective
Historically, aircraft manufacturers have paid little attention to interior acoustics and environmental conditions in the passenger cabin. The "insulation" and sound dampening material originally installed in commercially built small aircraft was marginally effective in its day. Even in today's new small aircraft, interior acoustics are low priority until you get into the luxury level models.  The simple solution to noise abatement has been a set of tight fitting headphones with some sort of “active noise cancellation” electronics.
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Here are typical complaints restorers and experimental aircraft builders encounter when it comes to sound and heat management:

•Engine, wind and exhaust noise invade the passenger cabin.

•Heat from the sun radiates into the passenger cabin.

•HVAC system do not effectively moderate cabin temperatures.

•Noise from the thin aluminum body shell invades the passenger cabin.

•Heat and noise comes through the firewall.
Noise invades the aircraft passenger cabin through one of two paths:
•Air-Borne Path--Noise is generated by aero mechanics—like the engine— which becomes “air-borne” and is carried into the passenger cabin through openings in the firewall, floor pan joints and thin body skin of the aircraft.

•Structure-Borne Path--Noise is generated by aero mechanics or wind friction which is transmitted into the passenger cabin in the form of vibration. This causes the aircraft’s body panels to flex and move against each other and/or resonate squeaks, buzzing, rattles, humming and other irritating noises. This “structure-borne” noise causes various rates of vibration in the firewall, floor and baggage pans, body panels, doors, and roof, etc.
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Current Acoustic Technology in vehicle design incorporates sound control systems and acoustic material throughout the vehicle—from the engine bay to the rear compartment. The purpose of these materials is to dampen, insulate and absorb the heat and cold, as well as noise before it affects the passenger cabin. In a typical luxury vehicle, sound and heat control material is incorporated into the interior bodywork of the vehicle. Its primary mission is to protect the passenger cabin from external noise. The ultimate goal is to create a “sound proof box.”

Borrowing concepts from state-of-the-art automotive acoustical research and technology, there are things you can do and materials you can install during the restoration or building process of small aircraft to soften the ride, dampen the noise, isolate the mechanical sources of heat and control the external environment with a minimum amount of additional weight.
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Here is a brief description of the materials used in acoustic management:

Damping is the reduction of vibration and noise generated by resonant vibration in areas like body panels, door panels, floor pans and roof panels. Damping material should be at least twice the thickness of the material to which it is being applied and cover approximately a quarter to half of the surface area. The most effective material used in damping is a self-adhesive rubberized asphalt material that is acoustically “dead” because of its dense mass and weight—about two pounds per square foot. The best automotive dampers are water repellant.

Absorbers are materials that do not block sound, but rather, "soak up" sound and prevent sound waves from richeoshay off opposing body panels. Absorbers are made up of dense fibrous materials with open pores. Open cell foams, fiberglass and fibertech are examples of sound absorbing materials. Thick materials absorb low to high frequencies: thin materials mainly absorb medium and high frequencies.

Reflective Barriers have come of age as the result of the aerospace industry. Reflecting heat away from critical equipment as well as passengers is the primary objective of the reflective barriers, which usually consists of highly reflective aluminum.

Damper-Absorber-Reflective Barrier Combinations have been developed in the past decades in the form of a shielding material that serves multiple purposes. Insulation products that dampen, absorb and reflect heat consist of aluminum sheeting bonded to a fibrous padding with fiberglass mesh reinforcement to achieve high strength. These materials are highly reflectivity of radiant heat and achieve maximum sound dampening. High quality combination shields are thin in design but produce high “R Factor” ratings and high acoustical NCR ratings.
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What Acoustic Controls Will Not Do
Before any acoustic treatment can have a major impact on the passenger environment, the aircraft’s body panels must be aligned and sealed and structural mechanics must be in tip-top-shape. A noise leak is a hole, (or gap between body panels) which offers little or no resistance to the flow of air-borne noise from entering the passenger cabin. Air gaps can be found in such places as:

•Excessive gaps in the body structure.

•Poor or inadequate alignment where body and floor panels come together.

•Inadequate sealing of metal joints such as mechanical access panels.

•Misalignment of doors, vent and engine cover panels.

•Poor fitting grommets and boots around cables, pedals, steering column and shift levers.

•Inadequate sealing of weather strips around doors, windows and engine cover panels.
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Pre-Cut Acoustishield Kits vs. Universal Acoustishield Kits

To manage these problems, we have created a series of Acoustishield Kits which deal with specific noise, vibration and heat problems commonly found throughout the aircraft. Each of these kits have been developed as part of a menu of options that allow you to pick and choose the solutions you need for the problems you face in your restoration or experimental aircraft project. Each of our kits are modeled on actual aircraft and are easily modified to fit custom applications.

Pre-Cut Acoustishield kits are available for growing number of popular experimental and civil aircraft and are packaged to insulate the following areas of the passenger cabin:

•Roof
•Front and Rear Floor

•Under Belly (sub-floor) Body Panels
•Firewall
•Body Panels, Cowl and Rear Bulkhead
•Door (as required)
 
Universal Acoustishield Insulation Kits are available for aircraft for which we do not currently have pre-cut kit patterns.  Universal kits include all of the materials that are found in our pre-cut kits along with highly detailed and illustrated instruction sheets with step by step instructions and diagrams to make installation easy--from preparation and layout, to cutting and installation in a typical aircraft environment.

In a bare metal aircraft body shell, installation time is about two to four hours for each kit using simple hand tools. If you need to remove upholstery or interior panels, more time will be required.

All of the materials included in our products meet Federal Aviation safety standards.
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What you can achieve with a properly insulated vehicle?
Quiet Flying comfort in the form of reduced interior decibel levels and virtually no exterior heat or cold issues.

Quiet Flight Solutions performs pre and post tests on a number of aircraft each year in our continuing evaluation of our products and techniques. The results we get from these tests are surprisingly consistent from airplane to airplane as shown below.
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Secondary Benefits to Aircraft Insulation
Along with the obvious benefits of aircraft insulation, our customers report that their passenger cabin comfort is greatly improved because the cool and warm air produced by HVAC systems is captured and maintained inside the cockpit. Customers tell us that they operate the HVAC controls at lower levels after installing Acoustishield.

General passenger conversations are much easier according to other customers. When the aircraft body is completely insulated, sound wve transfer is virtually eliminated and human voice and clarity is greatly improved. Our customers report that they can actually carry on a conversation with the person in the adjoining seat without having to raise their voice or shouting.  Amplified speaker volume in radio communications equipment can be reduced by as much as 50 percent with the installation of Aircraft Acoustishield.
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© Quiet Flight Solutions 2007