Brüel & Kjær Sound & Vibration Measurement A/S

Sonic booms – environmentally friendly?

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Courtesy of Courtesy of Brüel & Kjær Sound & Vibration Measurement A/S

JAXA and Brüel & Kjær Join Forces to Measure Sonic Booms

Brüel & Kjær and JAXA join forces in a research project to measure sonic booms. Side-by-side with JAXA’s ABBA sonic boom system, Brüel & Kjær’s PULSE LAN-XI based data acquisition and analysis system is used to study the effects of sonic booms on buildings.

When Concorde first broke the sound barrier back in the 70s, it was banned from supersonic flight overland; the shock waves from a sonic boom could shatter windows and cause considerable noise nuisance for the public. Now, once again, aircraft manufacturers are looking at overland supersonic flight. But just how can supersonic aircraft be made environmentally friendly?

If airplanes are to fly supersonically over land, then there must be no significant impact on our daily lives. So to quantify this we must ask the questions:

  • What level of sonic boom is acceptable?
  • What is the relationship between psychological impressions and the physical metrics of sound?
  • What effect do sonic booms have under indoor conditions?

To this end, JAXA and Brüel & Kjær have agreed to conduct joint research on sonic boom measurement systems. JAXA is actively investigating these phenomena to understand sonic booms and their effects on the environment. JAXA’s knowledge and experience with sonic boom research and investigation, together with Brüel & Kjær’s knowhow about the equipment and principles for sound and vibration measurement, make for a perfect partnership in the quest to understand the sonic boom. The recent testing in Sweden was aimed at looking into the effects that sonic booms have under indoor conditions. Whilst the direct audible noise from a sonic boom may be reduced by filtering by the building structure, building walls and windows may vibrate, generating added noise in the form of rattling or the like. “Do sonic booms cause a higher impact than other types of noise?”

ABBA in Sweden

JAXA (Japan Aerospace Exploration Agency) is looking for technologies that will drastically reduce the sonic boom nuisance. As a step on the way, JAXA has been trying out their full-scale, sonic boom test system at a remote test-site in northern Sweden. JAXA invited Brüel & Kjær along to measure side-by-side with a PULSE LAN-XI based system, to compare the Brüel & Kjær system with their own sonic boom system – the Airborne Blimp Boom Acquisition (ABBA) system.

The ABBA system will be used in the flight tests of JAXA Silent Supersonic Technology Demonstrator (S3TD).

The Measurement Scenario

The test setup was created to measure sonic booms inside and outside of a building, plus the vibration of the building's walls and windows.

Equipment inventory:

  • LAN-XI system Type 3050
  • 4 × ½” pressure-field Microphone  Type 4193 – both inside & outside the house 
  • 2 × Surface Microphone Type 4948
  • 4 × ICP Accelerometer Type 4508
  • IRIG-B Generator

JAXA also measured sonic booms above the ground, up to an altitude of 1000m, using an inflatable blimp from which hung measurement microphones and a data recorder.

Measurement Setup

Aircraft are expensive to run and need refuelling, so with in-situ field tests like these, timing is of the essence if the operational window is to be grasped. There is a lot of preparation for a short measurement event, therefore, speed and ease of setup are important. The weather can play an all important role in timing of the tests, and typically one only has a few hours to prepare the systems.

The ease of use and quick setup of the LAN-XI system lends itself admirably to this challenge:

  • TEDS – Transducer Electronic Data Sheet
    Input modules support TEDS transducers. This allows automatic front-end and analyzer setup based on information stored in the transducer, such as sensitivity, serial number, manufacturer and calibration date.
  • Dyn-X
    Dyn-X provides input modules with a single analysis range exceeding 160 dB. This eliminates the need for an input attenuator for ranging the analysis-system input to the transducer output.
  • POE - Power Over Ethernet
    PoE allows the power needed for each module to be carried by LAN cables rather than by separate power cables. This minimizes the number of cables required and results in lower cost, less downtime, easier maintenance and greater installation flexibility.
  • PTP – Precision Time Protocol
    PTP makes it possible to synchronize the clocks in the system components with sub-microsecond accuracy. Provides the ability to use any LAN-XI module as stand alone, in a frame or in a distributed system. High accuracy measurements are possible over long distances with only a LAN connection, and modules can be placed close to the measurement object.
  • CIC – Charge Injection Calibration
    Provides easy microphone calibration and system verification
  • Listening to signals – enables the verification of measurements whilst they are happening

Challenges of Measuring a Sonic Boom

An aircraft travelling faster than the speed of sound will produce a thunder-like sound – the sonic boom. As the aircraft passes through the air, it creates a series of pressure waves in front of it and behind it that travel at the speed of sound. As the aircraft itself passes the speed of sound the waves are forced together, or compressed, because they cannot 'get out of the way' of each other. These eventually merge into a single shock wave that produces the boom.

The sharp rise in pressure at the nose of the aircraft decreases steadily to a negative pressure at the tail, followed by a sudden return to normal pressure after the object passes. This 'overpressure profile' is known as an N-wave because of its shape. The 'boom' is experienced when there is a sudden change in pressure, so the N-wave causes two booms, one when the initial pressure rise from the nose hits, and another when the tail passes and the pressure suddenly returns to normal. This leads to a distinctive 'double boom' from supersonic aircraft.

Measuring and analyzing a sonic boom is not an easy matter and places high requirements on the measurement equipment. The sonic boom is characterised as simultaneously a loud, low-frequency and impulsive noise. The pressure increase through the shock waves takes place over a very short time – in the order of a few milliseconds – and the overall peak overpressure is of the order of 50 to 100 Pa. It is low frequency because the main part of its frequency spectrum is in the infrasonic or low audible frequency range (1-30 Hz).

The curve shows a comparison of JAXA made measurements with Brüel & Kjær made measurements. The comparison is uncannily good! 

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