Acoustic sounding

Details Acoustic sounding Acoustic sounding

2002-09-03

2004-06-29

Lefkowitz, Edward (Department: 2855)

Measuring and testing

Fluid flow direction

With velocity determination

C073S170110, C073S587000, C073S598000

Reexamination Certificate

active

06755080

ABSTRACT:

TECHNICAL FIELD
This invention relates to the use of acoustic pulses for atmospheric sounding or probing. It is particularly useful in measuring the height and other characteristics of thermal inversion layers [TILs] and other discontinuities in the lower atmosphere. TILs can affect radio transmission as well the transport and/or dissipation of pollutants, while other discontinuities such as windshear and clear-air turbulence near airports can affect aircraft safety.
Another technical field to which this invention may be applied is the investigation and characterization of the acoustics of buildings or the built environment.
In this specification “sound” will be used as a verb meaning to acoustically probe or explore, and “sounding” will be used as a noun to indicate the result of acoustic probing or exploration. If it is necessary to employ “sound” to mean an auditory sensation or the vibrations capable of causing such sensations or, indeed, to mean the generation of such vibrations (as in ‘to make a sound’), the word “sound” will be suitably qualified to make that meaning clear.
BACKGROUND TO THE INVENTION
The conventional method of sounding the lower atmosphere is to use a radio sonde carried by a balloon to telemeter temperature and moisture measurements to a ground station that is able to track the balloon or its load by radar. This can provide accurate identification of TILs and the windshear occurring in so-called ‘ducts’ between adjacent TILs. The measurements are useful in weather prediction, plume dispersal prediction and in the characterisation of ducts for the siting of terrestrial microwave communications and other purposes. However, disposable radio sondes, along with their associated radar tracking stations, are expensive. They are also unsuited to frequent use near airports where such soundings are most needed.
It is known to measure the height of a TIL and the wind velocity above it by directing high energy single-tone acoustic pulses upwards at a high angle of elevation and analysing the time delays and Doppler shifts in a received signal after reflection and/or refraction in the atmosphere. A sounding system of this type was published in the Australian Engineer of October 1997 and was applied for the prediction of smokestack plume dispersal. In that system, a high-energy monotone acoustic pulse was directed at an angle to the vertical and the transit time and Doppler shift were detected by a sensitive receiver placed some hundreds of meters away. In order to obtain the necessary signal-to-noise [S/N] ratio at the receiver, a transmitted acoustic pulse of some hundreds of watts was directed through a large horn (antenna). A similar large horn had to be used at the receiver because of the large attenuation of the transmitted pulses in the atmosphere. The bulky transmitter and receiver elements had to be moved about to obtain measurements at various azimuth angles to discern the direction and velocity of the wind in the layer of interest. Such a system is obviously unsuitable in built-up areas because of the level of noise generated.
It is also known to sound or investigate the acoustic properties of a concert hall by feeding acoustic test signals through loudspeakers located on stage (or at other selected locations in the hall), detecting the signals received at various specific locations in the hall (usually in the audience seating) and analysing the received signals to determine the principal reflected signals and their contributions to multipath distortions and reverberation times. Expensive and sophisticated computer analysis of the composite received signal by experts is necessary because of the highly complex nature of the received signal.
OUTLINE OF THE INVENTION
The present invention involves an acoustic sounding system wherein the component tones in a transmitted chirp are mixed, differenced, correlated or otherwise compared with the component tones in an echo chirp resulting from the reflection, refraction and/or scattering of the transmitted chirp. In this way, chirp transit times (and therefore the location of reflecting or refracting discontinuities in range) can be indicated as a frequency difference between the transmitted and the received chirps at any given instant. Furthermore, phase jitter or variation in an echo tone can be detected and displayed to indicate variation in velocity of the reflecting or refracting discontinuity with respect to the transmitter and/or receiver. The transmitted acoustic chirp can be generated by feeding a loudspeaker with an electrical input signal from the sound card of a computer (for example), while the echo chirp can be detected using a microphone that generates an electrical echo signal. Though the effectiveness of both loudspeaker and microphone can be enhanced by using suitable reflector dishes, the acoustic power required in the pulse is tiny in comparison to that required for the single-tone pulse of the art.
It will be appreciated, however, that many echo chirps will be generated by a single transmitted chirp because there will normally be many atmospheric discontinuities—or TILs—within range. While the comparison can be done with analog systems using known mixer circuits, they may not be able to provide the discrimination required in demanding situations. It is therefore preferable to compare the input and echo signals in the Fourier domain using DSP (digital signal processing) techniques, the Fourier-transformed digital signals being subjected to complex multiplication to yield complex sums and differences from which the difference signal is normally selected. The result can be subjected to inverse Fourier transformation to generate an amplitude vs. time series in which the amplitude coordinate is the difference component (indicative of the discreteness of the TIL discontinuity) and the time coordinate is indicative of the distance of the respective TIL discontinuity from the transmitter and receiver.
In general, the chirp should have a tonal range (ie, acoustical bandwidth) suited to the object being sounded. Low level atmospheric TILs are best sounded at the lower end of the audible range; for example, 500-5000 Hz, more preferably between 800 Hz and 3 kHz and most preferably between 1 and 2.5 kHz. On the other hand, chirps used for the sounding of concert halls will generally have a wider tonal range, or successive soundings will be made using chirps having a succession of narrow tonal ranges.
The tones in a chirp can be distributed in many ways. Most commonly, the frequency of the tones will increase or decrease linearly from the start to the end of the chirp. In this case, it is desirable to attempt to achieve a uniform rate of phase-shift from the start to the end of the chirp. Such linear chirps are more easily processed, especially using analog techniques. However, many other tonal sequences can be employed. For example, the frequencies can vary in a cosine manner, in steps or even in a random or pseudo-random manner. It is practically essential to process more complex chirps of this type using DSP and Fourier techniques.
Generally speaking, the longer the duration of a chirp the greater the potential processing gain of the system when using DSP and Fourier techniques. However, the processing power required to handle Fourier transformations and Fourier domain manipulations is also positively related to chirp duration. We have found that current readily available FFT algorithms, chips and DSP techniques known in the art cannot handle chirps much longer than about 30 s duration in a practical manner. New generation FFT chips and techniques are likely to allow chirps of more than a minute to be processed.
Another consideration affecting the duration of the chirp is whether the echo signals are to be processed in real-time or off-line. The simplest approach is to process the echo signals in real-time and to make the transmitted signal (and chirp) of sufficient duration to ensure that echo signals start arriving before the input signal has finished. In this way, the frequency di

Affiliated with

Also associated with

Rating

Acoustic sounding does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Acoustic sounding, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Acoustic sounding will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3331491