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Characterising cabin noise
Directives to assess and minimise cabin noise have become a top priority for the aerospace industry. From large commercial jets, to helicopters, to propeller-driven general aviation, the need is the same; reduction of cabin noise. High-amplitude acoustic signals, whether in the audible ranges of the human ear (20Hz to 20kHz) or beyond human hearing capability (infrasound and ultrasound) can range from being noisy and causing mild discomfort, to resonating human body components and inducing headaches or nausea. Noise testing is critical to the commercial success and competitiveness of the aerospace manufacturing industry and its desire for passenger comfort.
Deriving the noise source
Noise source location and source strength must be derived before acoustic engineers can improve the condition. Preliminary data can be obtained in wind tunnel testing, but for the most realistic noise and the best characterisation, an in-flight study is preferred. Noise can be derived from many sources. Some of the most common are from fuselage structural vibration, exterior wind noise leaking through windows and door seals, gearbox noise, propeller noise, engine noise, and general squeak and rattle. Knowledge of the acoustic field inside the fuselage can direct the noise abatement procedures for lighter and more efficient damping/insulation solutions. Noise generated from the passenger activities also needs to be accounted for in order to predict and control interior cabin abatement.
Detecting the noise can be difficult when measuring sound power entering the cabin at different locations and from multiple external sources. They tend to become polluted due to hard wall surfaces and reverberant components within the cabin. Early noise source identification methods included accelerometer measurements and sound intensity measurements using two “phase matched” microphones spaced closely apart. Although easy to use, they were time consuming, lacking in resolution, and provided limited information.
Modern methods using a large array of strategically placed microphones strategically, along with complementary software for data reduction, enable test engineers to obtain a greater amount of information, in a fraction of the time. Spherical beamforming, the HELS (Helmholtz Equation Least Square) method, and other acoustic holography methods have been implemented for improved sound pressure mapping, acoustic pressure, surface velocity, acoustic power, and intensity measurements.
The HELS method
HELS is based on nearfield acoustical holography (NAH) and allows the visualisation of acoustic pressure, normal intensity, and normal surface velocity mappings. It differs from the Fourier transform NAH method by analysing the acoustic field through an expansion of spherical wave functions, which greatly simplifies reconstruction and enables complex problem-solving on arbitrarily shaped surfaces with fewer measurement points, saving both set-up time and material costs.
According to Manmohan S. Moondra of SenSound LLC, with HELS, “The acoustic pressure is expressed as an expansion of the particular solutions to the Helmholtz equation. Under spherical coordinates, these particular solutions lead to the spherical wave functions. The coefficients that are associated with the expansion functions are determined by solving an over-determined linear system of equations obtained by matching the assumed form solution to the measured acoustic pressures, and the errors incurred in this process are minimised by least squares and can be expressed as:”
This lends itself well to low- to mid-frequency measurement requirements, as required by propeller or engine noise, which is typically in the 50-300Hz range or normal surface velocity between 1Hz to 1kHz.
Test and set-up
A medium-sized business jet was used in the test, with all interior panels in the passenger cabin removed. In-flight testing was conducted at a flight level of 30,000 feet and Mach 0.73. The closed surface included the forward cabin skin and floor, aft cabin skin and floor, and two closing surfaces (between the cockpit and the forward cabin and at the aft divider location). A conformal circular microphone array of 60 microphones was built (see Figure 1) to cover the circumferential measurements, and a planar microphone array of 50 microphones was built for the closing surfaces measurements. The circumferential measurements were taken every 2cm in the longitudinal direction, located 2cm from the skin.
Figure 1 (left): Circular microphone array, PCB T130D21 microphone
Figure 2 shows the grid with measurement points. Placement areas of the highest interest and likely location of passengers should be assessed, and the reconstructed location of the fuselage skin and the closing surfaces then derived. The interior acoustic pressure field was reconstructed in seven interior planes, including setup of custom microphone arrays and fixtures for circumferential and longitudinal measurements of acoustic pressures along the fuselage body. Identification of “hot spots” in the cabin skin where noise is more likely to be transmitted into the cabin can then be determined.
Figure 2 (left): Fuselage grid with measurement points
Once the noise is identified, engineers can take appropriate steps to minimise them. There are active and passive methods to combat the noise sources, and each method has its advantages. An example of an active method would be to place speakers in strategic areas and broadcast counteractive noise signals to cause destructive interference. When performing passive methods, consideration should be made to weight reduction, which also reduces gas cost and travel time. Some examples of passive methods include special panels, coupling between exterior and interior transmission of sound so it is impaired before it enters the cabin, and other dampening materials.
Figure 3 (left): Distribution of surface velocity at 155Hz
HELS and other NAH-based methods can provide noise source locations in order to improve the sound inside today’s aircraft and helicopters, enabling manufacturers to reduce noise for their passengers. Microphones have been proven to provide the appropriate acoustic pressure response required for HELS and NAH measurements. With the advent of lower cost, prepolarised array microphones, flight test engineers are now able to characterise many of the likely passenger locations during a single test flight, saving time and money for the programme.
29 August 2012
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