GB2106649A

GB2106649A - Fly-under noise measurement apparatus

Info

Publication number: GB2106649A
Application number: GB08227644A
Authority: GB
Inventors: Thomas Anthony Holbeche
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1981-09-29
Filing date: 1982-09-28
Publication date: 1983-04-13
Also published as: GB2106649B

Abstract

In noise measurement apparatus for measuring noise above an aircraft by a fly-under measurement an acoustically rigid spherical diffraction body (30) acts substantially as a reflector over a frequency range of interest to prevent suspending aircraft noise reaching transducers (33, 34) situated at a small azimuth to the suspension axis and at a position of minimum noise. The invention overcomes the problem of corruption of fly-under noise measurements by suspensing aircraft noise. The invention may be used in a noise assessment method involving simultaneous fly-under and fly-over noise measurement. <IMAGE>

Description

SPECIFICATION Noise measurement apparatus This invention relates to noise measurement apparatus, and in particular to apparatus for the measurement of in-flight exterior aircraft noise.

Aircraft noise is a topic of much debate, and as standards are set to reduce ground exterior inflight noise so more accurate techniques for noise measurement are required, and reliable techniques have been developed for the ground measurement of fly-over noise.

Recent attempts at reducing ground level fly-over noise have centred on the possibility of using airframe components as shields, such that noise is reflected upward. The exact form and placement of such shields is currently the subject of much experiment. Unfortunately, ground measurement of noise is not convenient for the assessment of shielding, since fly-over measurement on an unshielded aircraft must be compared with a subsequent shielded fly-over measurement. This is an expensive operation and beset with measurement uncertainties of repeatability, calibration and background noise.

Simultaneous noise measurement above and below an aircraft inflight offers a reliable solution to the measurement problem, since upward deflection of noise may be readily computed. Unfortunately, a measurement above an aircraft presents considerable problems. A transducer must be positioned above the test aircraft, and practically this requires carriage by a second aircraft, itself generating noise. A simple baffle board could be used to shield noise from the suspending aircraft, but with a complex and detrimental effect on transducer response. Such a baffie board would also present considerable flight problems for the suspending aircraft if suspended, for example, from a cable.

According to the present invention noise measurement apparatus, for noise measurement over a frequency range includes: an acoustically rigid spherical diffraction body; and a transducer substantially flush mounted with the spherical diffraction body surface; the spherical diffraction body diameter being such that it behaves substantially as a reflector over the frequency range of interest.

In use, the diffraction body may be suspended from an aircraft having the transducer situated in the body lower surface such that reflector action shields suspending aircraft noise from the transducer and augments noise to be measured incident from below. The transducer is advantageously positioned on the body lower surface at a small azimuth.

An important feature of the present invention is that it provides noise measurement apparatus which has flight characteristics suitable for aircraft suspension, in particular by a helicopter, so that useful measurements above an aircraft inflight may be made.

In order that features and advantages of the present invention may be appreciated embodiments will now be described with reference to the accompanying diagrammatic drawings, of which: Figure 1 represents a spherical diffraction body; Figure 2 represents a graphical presentation of the acoustic performance of the sphere of Fig. 1, and Figure 3 represents a diffraction body in accordance with the present invention and in operational form.

In an acoustically rigid diffraction body 10 (Fig. 1) a transducer M is flush mounted at 11 to receive acoustic signals from a noise source S at 1 2. The diffraction body 10 is a sphere of radius a and positioned such that at the centre of the sphere the transducer subtends an angle 8 with the plane containing the centre and the source S.

It can be shown by applying reciprocity (see for example "Radiation from a point source of sound on the surface of rigid spheres and discs", National Physical Laboratory Report AP21 (1965) by Delany, Burton and Rennie) that the sound pressure variation (D(B,r)) over the surface of a rigid sphere is given by

where Prn(cos0) is the Legendre polynomial of order m, and hm is the spherical Hankel function of order m.

A useful parameter for assessing the effect of the frequency of sound (wavelength A) emitted by source S is wave number Ka, where 2na Ka = h For distant operation, the distance of the source from the sphere tends to infinity and values of D(fl,) may be computed for different values of wave number, Ka. Results are most easily presented in graphical form (Fig.

2).

In accordance with the present invention, diffraction body 10 is to behave substantially as a reflector over the frequency range of interest, typically between 50 Hz and 10 kHz.

For use in aircraft fly-under measurements, accurate measurement is required over an adequate flight distance. It will be appreciated (Fig. 2) that for KaA4 the pressure level D(S,) is substantially uniform up to an angle of approximately 60 , which gives an angular coverage of 1 20' (ie 60 either side of vertical).It will further be appreciated that suspending aircraft noise (now considering this noise source to be at 8 = 0 ) is severely attenuated in the measurement range of 120 to 180'. In order to avoid the "bright spot" of noise at exactly 180 the transducer M is mounted at a small azimuth of 10 (170 on Fig. 2) in a region.of high attenuation.

In order that the present invention may be fully appreciated a further example will now be described.

A diffracting body 30 (Fig. 3) consists of two hemispherical shells 31, 32 each made up from a laminate of several layers of resin bonded glass fibre fabric and reinforced with internal struts (not shown). The shells 31 and 32 are bolted together at their circumference to form an acoustically rigid sphere. Both shells are filled with polyurethane foam to enhance rigidity.

The body 30 carries two transducer condenser microphones 33 and 34 mounted at a small azimuth (on). Microphones 33 and 34 are themselves flush mounted with the sphere surface, but conventional wind shields 35, 36 are fitted externally. Supporting wires, such as wire 37 are cleat jointed to the sphere 30 and shackled to a ball bearing swivel 40, allowing rotation about main suspension cable 41, to relieve any twist. A telemetry link is provided for the transducer signal via stub aerial 42, telemetry electronics was installed behind access panel 43.

An operational sphere has been constructed as described above, having a diameter of 1.83 m. The sphere was for helicopter suspension by a cable of 1 50 m, thus the microphones 33 and 34 were at an azimuth of 82 with respect to the distant helicopter so that helicopter noise was attenuated as described above. Taking Ka = 4 for this sphere a response uniform to within 2 dB was obtained down to a frequency of 100 Hz. At an upper measurement frequency of 10 kHz, Ka = 1 70.

This size of sphere thus provides an accurate measurement signal over a useful range of frequencies.

The operational sphere was spray coated with a conductive coat of zinc to provide electrical shielding and facilitate radar tracking. The sphere was finished in fluorescent orange with a black sector, so that any spinning motion could be detected. It will be appreciated that the present invention provides noise measurement apparatus which is suited to helicopter suspension.

Signals produced by one or both microphones carried on the sphere may be received by telemetry for ground processing in accordance with known principles. A particular feature of the diffraction body as described is that due to behaviour as a near perfect reflector for incident noise, gain varies little with frequency over the frequency range of interest. Measurements made with the sphere therefore require little correction.

A particular aspect of the present invention is use in a noise measurement method involving the steps of measuring noise above an aircraft in flight as hereinbefore described, simultaneously measuring noise below the aircraft; and comparing the noise measurements.

Preferably the aircraft and the suspending aircraft are flown clear of noise ground effects. Advantageously the relative positions of the sphere and aircraft with respect to the ground equipment are similarly recorded, for example, by radar tracking or Kinetheodolite.

It will be realised that the present invention is not limited to the examples described above. A spherical diffracting body in accordance with the present invention may be chosen for use in other measurements to shield a noise source which would otherwise corrupt the measurement.

Claims

CLAIMS The matter for which the applicant seeks protection is:

1. Noise measurement apparatus for noise measurement over a frequency range including an acoustically rigid spherical diffraction body and a transducer substantially flush mounted with the spherical diffraction body surface; the spherical diffraction body diameter being such that it behaves substantially as a reflector over the frequency range of interest.

2. Noise measurement apparatus as claimed in claim 1 or aircraft suspension and having the transducer situated in the body lower surface such that in operation reflector action shields suspending aircraft noise from the transducer.

3. Noise measurement apparatus as claimed in claim 2 and wherein the transducer is situated at a small azimuth relative to the suspension axis.

4. Noise measurement apparatus as claimed in any preceding claim and wherein the diffraction body behaves substantially as a reflector over a frequency range of 50 H2 to 10 KH2.

5. Noise measurement apparatus substantially as herein described with reference to the accompanying diagrammatic drawings.

6. An aircraft noise measurement method involving the steps of measuring noise above an aircraft in flight with noise measurement apparatus as claimed in any preceding claim, simultaneously measuring noise below the aircraft, and comparing the measurements.