Furthermore, increased flight operations can be conducted around more restrictive airports. The determination of an aircraft type’s noise is a complex process guided by documents such as:
- International Civil Aviation Organization(ICAO) Annex 16 – Environmental Protection, Volume I – Aircraft Noise
- Federal Aviation Administration (FAA)CFR14 PART 36 – Noise Standards: Aircraft Type And Airworthiness Certification
- ISO 5129 – Acoustics – Measurement of Sound Pressure Levels in the Interior of Aircraft During Flight
In order to minimise measurement error and uncertainty, these standards place stringent requirements on acoustic measurement systems and methods. The effect of each element of the measurement chain on frequency response must be known, and compensated. Brüel & Kjær has developed an integrated solution within the PULSE™ data acquisition system called FRC – Frequency Response Correction. Typical uses are aircraft noise certification, interior noise testing and engine testing. However, many other applications can also benefit from frequency response correction.
Aircraft Community Noise Minutiae
Performing aircraft noise certification is a daunting task. Adherence to all applicable regulations must be demonstrated to the appropriate authorities. Hardware system requirements are well defined, but integrating a system also involves many other elements. End-to-end response correction is such a process, and can require a sizeable custom database effort. This is because flight-testing is often performed with more microphones and locations than required for “simple” certification purposes.
The advantages of providing this functionality in a commercial off-the-shelf (COTS) product are obvious. Brüel & Kjær was recently approached by The Boeing Company to make response corrections available as a component of the PULSE data acquisition system, for use in airplane noise testing.
“Various regulations require that system frequency response corrections be applied to spectral data,” says L. Chris Hunting, Boeing Aero/Noise/Propulsion Laboratory Airplane Noise Testing Engineer. “The bulk of the measurement must have both sensitivity and frequency response corrections applied. Further more, these regulations demonstrate that the frequency response of the entire measurement chain needs to be accounted for”.
Mr. Hunting concludes, “Moreover, some of the corrections will be ‘type’ corrections –always the same for a given model number –and others will be ‘specific’ to a given setup or serial number. For specific corrections, the correction must have been determined within a given time period. This implies that multiple corrections may be available with the ability to select one”.
The five response correction elements addressed in PULSE FRC are:
- Transducer: A correction for the transducer’s response as a function of frequency. The correction is general to any transducer assumed to have a linear response. For example, a precision air condenser microphone’s response can be defined by its own electrostatic actuator (EA) response, coupled with the microphone type’s EA to free-field or diffuse-field correction.
Windscreen: A correction for the use of a windscreen or other transducer cover. This is known for standard windscreen types. The user can define new types.
- Sound field: A correction for the usage or mounting of the transducer, defined by its model (type number), configuration (bare or with grid cap), and incidence angle of sound field.
- Line Insert: A correction for the cables or transmission lines in each signal path. This may be negligible for short cable lengths. However, flyover tests can involve very long cables. Typically measured electrically.
- Tape Recorder Pair: A correction for data analyzed with a recorder and reproducer pair. In practice, this is established by recording a known signal on one device and playing it back on the other.
Historically, analyzer frequency response correction was also required. However, most modern digital analyzers have negligible effects and therefore need not be “compensated”.
Realising the User Interface
Once the specifications were well defined, development progressed rapidly. One of the goals of the Brüel & Kjær/Boeing project was to maximise the use of existing PULSE functionality and components. The Hardware Table and PULSE Data Manager (PDM) were great starting points. PULSE FRC uses a structure similar to the Hardware Database to assign corrections used in each signal path. Corrections are stored in a PDM database. Ina PULSE project, data is stored as measured, but can also be displayed and exported with corrections after a measurement. Therefore, the original data is always available should any modifications or errors arise. FRC requirements stem from community noise certification, but a similar process is also used for in-flight acoustic measurements, where PULSE FRC was put to the test in a gruelling flight test schedule (as part of the Quiet Technology Demonstrator Two program in August 2005), near a small town on the American prairie.
Under the Big Sky
The Boeing flight test facility is located 17miles north of Glasgow, Montana (Big Sky Country), at the former home of the 4141stStrategic Wing and the 91st Bomb Wing of the United States Strategic Air Command. Temperature, relative humidity and wind must all be within a prescribed envelope for data to be valid, with appropriate meteorological conditions for flyovers generally being in the very early morning. “One of the suitable days began for me at 03:00 a.m.”, relates Guillaume Bock, Northwest US Sales Engineer, “but a number of the test team were on-site even earlier and had already attended pre-flight meetings!”
During the testing, several PULSE software analyzers with PULSE FRC produced hundreds of measurements recorded aboard an experimental Boeing 777 plane. Brüel & Kjær also supplied a 36-channel PULSE data acquisition system for on-board monitoring of vital acoustic measurements. A prototype PULSE system was also deployed to measure flyover noise. An enthusiastic Guillaume remarks, “This was an impressive technical tour de force by a veteran team of pilots, technicians, and engineers. The stakes were very high to complete testing in the allotted window”.
Several aircraft noise test measurements can be simplified with the PULSE FRC –Frequency Response Correction component. It can provide end-to-end response compensation for noise certification tests, eliminating the need for external databases and corrections. In-flight or test cell signals recorded digitally on AIT tapes can be analyzed using Type7774 PULSE Interface to SONY® SIR-1000,applying any needed corrections as well. Many other uses can be found for this new ability to correct a measurement according to the whole signal path response.