Wake Up Sound Effect Free Download !!EXCLUSIVE!!
Bobby Morosco
Sound recorded by the author in March 2002 at JFK International Airport shows that wake vortices in ground effect emit infrasound that is 1) more than 40 dB stronger than audible wake vortex sound; 2) substantially stronger than the infrasound component of wind noise and airport noise; and 3) comparable to, and often stronger than, the infrasound component of aircraft noise. Spectra and time plots of the magnitude of wake-vortex-generated sound are presented for aircraft landing on JFK runway 31R.
Wake vortices are compact regions of air characterized by strong rotational motion. They create a strong velocity flow field that, in the case of an aircraft having an encounter with them, can lead to a severe upset that manifests itself in a vertical downwash or rapid uncommanded movements of rolling or pitching. Air traffic control has defined a classification that prescribes minimum distances between successive aircraft to avoid hazardous encounters. Although the separation distances ensure safety, they have an adverse effect on airport capacity, which becomes more and more important with continuously growing air traffic.
Many sensor technologies for detecting wake vortices have been studied and evaluated over the last 25 yr, including microwave and millimeter-wave radar, RASS, sodar, lidar, and anemometer-based ground wind lines. Each of the above falls short with respect to one or more of the following acceptance criteria: vortex detection sensitivity, vortex track capability, all-weather operation, automatic operation, real-time measurement, airport safety constraints, and cost. Of the preceding sensors, lidar has demonstrated particularly robust detection of wake vortices in clear weather and in light snow and rain. RASS (Rubin 2000) has been shown to satisfy all of the above criteria except cost. As a result, there is a continuing interest in new wake vortex sensor technologies.
Wake vortices in ground effect are often heard to emit audible sound at airports. Sound recordings made by the author in March 2002 at JFK International Airport indicate that wake vortices emit infrasound that is more than 40 dB stronger than audible wake vortex sound. In this context, infrasound refers to sound frequencies below 20 Hz. The primary objectives of the effort reported on herein was to determine whether infrasound emitted by wake vortices is a reliable indicator of wake vortex presence and whether wake vortex infrasound could be reliably detected in a background of airport and wind noise.
Previous wake vortex studies have presented acoustic spectra for wake vortices out of ground effect. For example, Michel and Böhning (2002) have published sound spectra using a non-real-time phased microphone array. These spectra were obtained 6 and 12 s following aircraft passage with the array focused at an altitude of 40 m.
The present paper differs from the existing literature in four ways. First, a single microphone at ground level was used to make all recordings. Second, acoustic spectra are presented for wake vortices in ground effect. Third, wake vortex spectra are compared to sound spectra generated by aircraft and ambient wind. Fourth, plots of infrasound emitted by wake vortices following aircraft passage are presented for several types of aircraft.
The recording microphone was enclosed in a two-stage microphone blimp for all measurements. The blimp reduced wind noise and had no effect on microphone response up to about 8 kHz. Wind data were collected at three sites: Mount Washington, Jones Beach, and JFK Airport.
At JFK, sound was recorded during aircraft arrivals on 5 and 6 March 2002 at the U.S. Department of Transportation (DOT) Volpe Center Wake Vortex Test Site, which is adjacent to the middle marker of runway 31R. Aircraft are 60 m above the ground at the middle marker. To prevent vortex flow from reaching the microphone, the microphone-blimp assembly was completely enclosed in a 5-cm-thick porous-foam-covered enclosure whose frame was made of polyvinyl chloride (PVC) pipe. The enclosure sat on a flat sandy surface about 120 m from the glide slope centerline and 1.05 km from the runway threshold.
All sound spectra were computed using the fast Fourier transform (FFT) algorithm. For this purpose, Sony recordings were played through a PICO ADC-100, 12-bit A/D converter, which is flat from 0 to 50 kHz, into a USB port of a Windows XP Gateway 700X computer. The FFT size was 4096 and the sampling frequency was 10 528 Hz. PICO and Excel software were used to make the spectral plots, which were corrected for Shure microphone and amplifier falloff below 20 Hz.
The magnitude of sound recorded during each JFK aircraft landing was plotted versus time in two ways, again using PICO and Excel software. The first set of plots was obtained by playing Sony recordings through a 200-Hz analog low-pass filter into a PICO ADC-100 converter that sampled the data at a 500-Hz rate. The converter was connected to a USB port on a Gateway 300X computer with a Windows 2000 Professional operating system. To generate the second set of plots, the digitized sound already stored in the computer was processed by a filter that approximately matched the infrasound spectrum generated by wake vortices. A matched filter optimizes signal detection in a background of white Gaussian noise (DiFranco and Rubin 1968). The matched filter passband was determined by a digital filter that cut off sound above 20 Hz at 18 dB per octave and the Shure microphone, whose response falls off rapidly below 7 Hz.
Figure 1 shows that wind sound is loudest in the infrasound band and that the spectral shape of wind sound is essentially independent of wind speed and geographic location. If the corner frequency of each spectrum is defined by the intersection of a line approximating spectral falloff above 50 Hz with a horizontal line approximating spectral amplitude between 4 and 10 Hz, the corner frequency decreases with increasing wind speed. This implies that the peak intensity moves toward lower infrasound frequencies with increasing wind speed.
The vortex pair descends because of mutual velocity induction. The initial descent rate is proportional to aircraft weight and inversely proportional to flight speed and the square of the wingspan. Near the ground, the descent rate decreases and the vortices level off at a height approximately equal to one-half of their initial separation, hereinafter referred to as equilibrium height. A vortex enters ground effect when its height is about a wingspan above the ground, which is approximately equal to 2.5 times equilibrium height. This means that the vortices of heavy aircraft are mostly in ground effect right after they are generated at the middle marker.
Figure 8 shows the effect of a vortex-matched filter on the sound plotted in Fig. 2 that was recorded during the arrival of a B747. The peak aircraft sound around 17 s is reduced by a factor of 10 between the two figures. This reduction is substantially understated because the sound in Fig. 2 has already passed through a 200-Hz low-pass filter before being sampled. Figure 8 also shows that the peak B747 aircraft sound recorded 120 m away is smaller than the peak vortex sound, suggesting that noise from other aircraft around the airport, including reverse engine thrust noise, is also significantly attenuated by wake vortex-matched filtering.
As Fig. 8 shows, the recorded infrasound peaked during the time the vortex pressure field was physically sensed, between 30 and 35 s. This is the only example where infrasound totally disappeared after the vortex pressure field ceased to be sensed. Figure 8 also shows two small pronounced sound peaks at 21 and 26 s. Based on their timing relative to the aircraft sound peak, it is conjectured that the first small sound peak was generated during vortex rollup and the second during early descent of the primary vortex pair.
To obtain the spectra in Fig. 14, the spectrum of B747 aircraft sound was computed around 17 s, B747 vortex sound around 33 s, and wind sound around 40 s, with reference to the plot in Fig. 8. The shapes of the B747 vortex spectrum and the wind spectrum are similar except for a pronounced peak in the vortex spectrum between 10 and 20 Hz. Figure 14 shows that B747 sound in the infrasound band is smaller than that for vortex sound, which is consistent with the magnitude relationship shown in Fig. 8. Figure 14 also shows a slower falloff rate of aircraft sound in the audio band compared to vortex sound, which confirms the benefit of eliminating sound above 20 Hz.
Figure 3 shows sound recorded during an MD80 landing at JFK before being matched filtered, and Fig. 9 after being matched filtered. The aircraft sound peak at 6 s in Fig. 3 is again reduced by a factor of 10 in Fig. 9.
In Fig. 15, the MD80 aircraft sound spectrum was computed around 6 s, the wind spectrum around 25 s, and the MD80 vortex spectrum around 53 s, with reference to Fig. 9. The vortex spectrum shape, as before, is similar to the wind spectrum shape except for a small peak between 10 and 20 Hz. The slower falloff rate of the aircraft spectrum in the audio band relative to the vortex spectrum again shows the benefit of vortex-matched filtering.
Figures 4 and 10 show sound recorded during a B767 landing. Vortex-matched filtering once again reduced the aircraft sound peak around 17 s by a factor of 10. In this example, as Fig. 10 shows, the peak vortex infrasound was smaller than the peak aircraft infrasound. This may be related to the fact that a weak vortex pressure field was sensed in this case. Also, vortex infrasound continued after the vortex pressure field was no longer sensed.
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