For some customers, it's the ultimate white noise. For most, however, the incessant droning experienced in an aircraft cabin is not just annoying, it’s a potential health hazard.
According to a bbc.com article, cabin noise at cruising altitudes is around 85 dB, a noise level that can cause temporary hearing problems such as tinnitus, or ringing in the ears, following a long flight. In fact, the FAA warns that noise exposure higher than 90 decibels for eight or more hours per day may lead to permanent hearing loss. Noise levels can often get close to this threshold for passengers seated toward the back of a plane.
And the problem of aircraft cabin noise is of growing concern. Our quiet aircraft initiative has resulted in noise-reduction innovations and created an opportunity to define baseline specs for hydraulic system noise.
Cabins in today’s modern aircraft are louder than yesterday’s plane cabins, particularly from systems generating tonal acoustic signatures such as the hydraulic system.
- Higher-pressure hydraulic systems such as the 5,000 psi system first used on the Airbus A380 – or the three 5,000 psi systems currently used on the Boeing Dreamliner – are creating more noise and vibrational energy.
- Newer, thinner structural materials like composites are more readily transmitting mechanical and other noise-creating vibrations to interior cabins.
- More-electric or point-of-use hydraulics, now located in the fuselage area, are also creating additional sound challenges.
Combine the above with new environmental laws requiring quieter aircraft, increasing customer demands for a more comfortable cabin experience, and the high cost of addressing noise issues following entry into service, and you have the reasons why cabin noise abatement is top of mind for today’s aircraft and rotorcraft original equipment manufacturers (OEMs).
It’s an issue our Hydraulic Systems Division (HSD) has been extensively researching – and resolving – for almost a decade.
Our quiet aircraft initiative
Begun in 2009, Parker’s quiet aircraft initiative has focused on developing quieter hydraulic systems. The goal of the initiative is to understand how hydraulic noise is generated, transmitted, and received in an aircraft, and to develop standard solution/design protocols to minimize the resulting cabin noise.
Parker Aerospace noise research has focused on three basic elements of noise problems: source, path, and receiver. The optimal design solution is often a unique combination of minimizing emitted energy from mechanical components, controlling acoustic energy flow through transmission paths, and limiting exposure to undesirable noise levels.
For example, hydraulic-pump-generated flow pulsations, also known as fluid-borne noise (FBN), can lead to vibration of system components - such as hoses, tubes, fittings, valves, filters - and ultimately cabin noise. Similarly, varying internal pressures transmitted to the pump casing can cause vibrations that generate cabin noise by energy transfer through mounts. This is known as structure-borne noise (SBN). These vibrations can also cause airborne noise (ABN) when emitted as acoustic energy off the pump.
Quiet aircraft innovations
Through our quiet aircraft engineering efforts, Parker HSD has developed industry-leading, aircraft-level expertise in hydraulic system noise, creating quiet design technology that has allowed us to reduce cabin noise and system vibration by as much as 12 decibels.
This technology includes:
- Advanced, computer-based optimization protocols.
- A semi-anechoic chamber used for sound pressure and power analysis.
- An aircraft test bed for transmission path analysis.
- A specialized test rig for acoustical modeling.
- A proprietary methodology determining the strongest vibration transmission path on an aircraft.
Using this technology and our extensive understanding of hydraulic noise, we’ve been able to produce quieter pumps, optimizing them for minimum source noise and vibration. We’ve also been able to identify key design drivers of system-level hydraulic resonances and predict changes in resonance frequencies; these capabilities facilitate optimal system design and pump integration.
The result has not only been reduced cabin noise and vibration. Our almost decade-long pursuit of cabin noise abatement has led to our inclusion in and support of a Canadian-sponsored project named Development of engineering methods for SBN specifications and quantification of aircraft and rotorcraft cabins.
Our CRIAQ and CARIC partnership
Currently underway, the three-year research project is the combined undertaking of two nonprofit Canadian organizations – the Consortium for Aerospace Research and Innovation in Canada (CARIC) and the Consortium for Research and Innovation in Aerospace in Quebec (CRIAQ). The project’s goal is to develop and validate engineering methods for predicting structure-borne noise generated in aircraft/rotorcraft cabins by integrated vibrating systems, specifically the helicopter main rotor transmission and the aircraft hydraulic system.
Bombardier Aerospace, Bell Helicopter Textron Canada, and Parker Canada are partners in the undertaking. Efforts will focus on investigating, extending, and validating noise metrics and methodologies, with the resulting development of engineering tools and a knowledge toolbox that will facilitate a better engineering integration of structure-borne noise design challenges all along the product design cycle – from early design through validation and verification.
Using engineering methods to master structure-borne noise (SBN) during the whole development life cycle will allow OEMs to specify system, structure, and interface behaviors; verify compliance; and ensure the vibro-acoustic integration quality of the hydraulic and main rotor transmission systems in the aircraft/rotorcraft. One of the important project deliverables is a new tool to be used for specifying dynamic force requirements from pumps into the structure. This will go a long way to mitigating annoying SBN. At this time, no such tool and/or structure-borne noise evaluation methodology is used or available to the aerospace industry.
With improved SBN-related technical specs from OEMs to suppliers, the expected benefits of the CRIAQ and CARIC project include:
- The reduction of design risk and cost.
- Design with minimum added weight.
- Increased core knowledge and relevant tools used from design through validation to lessen structure-borne noise for quieter cabins.
For additional information on Parker Aerospace systems and capabilities, please visit our website.
This post was contributed by Engineering Manager of Research and Development Rick Klop from Parker Aerospace.