Active noise control system

• US 4,689,821 A
• Filed: 09/23/1985
• Issued: 08/25/1987
• Est. Priority Date: 09/23/1985
• Status: Expired due to Term

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Salikuddin, Mohammed, Tanna, Himatlal K.
• Primary Examiner(s): Rubinson, Gene Z.
• Assistant Examiner(s): Schroeder, L. C.
• Application Number:
  US06/778,822
• Time in Patent Office: 701 Days
• Field of Search: 381/71, 381/73, 381/94
• US Class Current: 381/71.9
• CPC Class Codes: B64C 1/40 Sound or heat insulation , e.g. using insulation blankets
• CPC Class Codes: G10K 11/17823 Reference signals, e.g. ambient acoustic environment
• CPC Class Codes: G10K 11/17853 of the filter
• CPC Class Codes: G10K 11/17857 Geometric disposition, e.g. placement of microphones
• CPC Class Codes: G10K 11/17881 the reference signal being an acoustic signal, e.g. recorded with a microphone
• CPC Class Codes: G10K 11/17883 the reference signal being derived from a machine operating condition, e.g. engine RPM or vehicle speed
• CPC Class Codes: G10K 2210/1082 Microphones, e.g. systems using "virtual" microphones
• CPC Class Codes: G10K 2210/121 Rotating machines, e.g. engines, turbines, motors; Periodic or quasi-periodic signals in general
• CPC Class Codes: G10K 2210/1281 Aircraft, e.g. spacecraft, airplane or helicopter
• CPC Class Codes: G10K 2210/3028 Filtering, e.g. Kalman filters or special analogue or digital filters
• CPC Class Codes: G10K 2210/3032 Harmonics or sub-harmonics
• CPC Class Codes: G10K 2210/3045 Multiple acoustic inputs, single acoustic output
• CPC Class Codes: G10K 2210/3216 Cancellation means disposed in the vicinity of the source

Abstract

An active noise control system reduces cabin noise levels for a propeller or propfan aircraft due to the propeller noise signature by making the output from a secondary noise source identical in amplitude but opposite in phase to the propeller noise signature.

Claims

1. An active noise control system for reducing the propeller noise signature impinging on an aircraft fuselage due to a revolving propeller comprising:

(a) a microphone for measuring the propeller noise signature generated by the revolving propeller and for providing a microphone output signal equivalent to the propeller noise signature;

(b) a time delay unit for receiving said microphone output signal and for delaying said microphone output signal by a predetermined time delay interval to provide a time delay output signal;

(c) a programmable gain amplifier for receiving said time delay output signal and for amplifying said time delay output signal by a predetermined gain to provide a secondary noise source input signal;

(d) a secondary noise source for receiving said secondary noise source input signal and for providing an acoustic output in accordance therewith; and

(e) an array of microphones for measuring the noise at a plurality of locations on the aircraft fuselage due to the propeller noise signature and the secondary noise source output, and for providing an output equivalent to the measured noise at each location;

(f) control means for receiving said output from said array of microphones and for controlling the time delay interval of said time delay unit and the gain of said programmable gain amplifier whereby said microphone output signal is modified in phase and amplitude so that the secondary noise source output reaching the aircraft fuselage surface reduces the noise level of the fundamental blade passage tone as well as its associated harmonics.

2. An active noise control system according to claim 1, wherein the control means comprises:

(a) a noise level determining means for determining the average noise level, F(A,φ

), at the fuselage surface, as measured by said array of microphones, as a function of the secondary noise source input signal amplitude A and the phase difference between the propeller noise signature signal as measured by said microphone and the secondary noise source input signal;

(b) gradient determining means for determining the sign, plus or negative, or the gradient (∂

F/∂

A).sub.φ and

(∂

F/∂

φ

)_A respectively; and

(c) regulating means for varying the predetermined time delay interval of the time delay unit and the gain of said programmable gain amplifier whereby (∂

F/∂

A).sub.φ and

(∂

F/∂

φ

)_A are zero.

3. An active noise control system according to claim 2 further comprising a plurality of analog-to-digital converters for receiving the output signals of said array of microphones and inputting those digitalized signals to said control means.

4. An active noise control system for reducing the propeller noise signature impinging on an aircraft fuselage surface due to a revolving propeller comprising:

(a) a microphone for measuring the propeller noise signature generated by the revolving propeller and for providing a microphone output signal equivalent to the propeller noise signature;

(b) band pass filters for receiving said microphone output signal and for separating each tone of the measured propeller noise signature to provide individual output signals equivalent to each separated tone of the propeller noise signature;

(c) a multichannel time delay unit for receiving each individual tone signal and for delaying each tone signal by an individually selected predetermined time interval to provide a plurality of time delay unit output signals;

(d) a multichannel programmable gain amplifier for receiving each individual time delay unit output signal and for amplifying each signal by a selected gain to provide a plurality of gain amplifier output signals;

(e) a summing amplifier for receiving each individual gain amplifier output signal and for summing each individual gain amplifier output into a single secondary sound source input signal;

(f) a secondary sound source driven by said secondary sound source input signal for providing an acoustical signal directed towards the aircraft fuselage;

(g) an array of microphones positioned adjacent to selected positions on the aircraft fuselage for measuring the resultant signal due to said secondary sound source output and the propeller noise signature and for providing an output indicative thereof; and

(h) control means for receiving said array of microphones output signals and for providing appropriate signals to said time delay unit and said programmable gain amplifier whereby the noise level at the fuselage surface is minimized.

5. An active noise control system according to claim 4, wherein the control means comprises:

(a) a noise level determining means for determining the average noise level, F(A,φ

) at the fuselage surface, as measured by said array of microphones, as a function of the secondary noise source input signal amplitude A and the phase difference between the propeller noise signature signal as measured by said microphone and the secondary noise source input signal;

(b) gradient determining means for determining the sign, plus or negative, of the gradient (∂

F/∂

A).sub.φ and

(∂

F/∂

φ

)_A respectively; and

(c) regulating means for varying the predetermined time delay interval of the time delay unit and the gain of said programmable gain amplifier whereby (∂

F/∂

A).sub.φ and

(∂

F/∂

φ

)_A are zero.

6. An active noise control system according to claim 5, further comprising a plurality of analog-to-digital converters for receiving the output signals of said array of microphones and inputting these digitalized signals to said control means.

7. An active noise control system for reducing the propeller noise signature impinging on an aircraft fuselage comprising:

(a) a tachometer for providing a reference output signal equivalent to the position and speed of rotation of a powerplant for driving a propeller;

(b) a multichannel oscillator for providing a plurality of individual output signals including the fundamental blade frequency and the harmonics thereof, said oscillator outputs being triggered once during each period of said tachometer output signal;

(c) a time delay unit for receiving each individual oscillator output signal and for delaying each individual oscillator output signal by a predetermined time interval;

(d) a programmable gain amplifier for receiving each individual time delay unit output signal and for amplifying each individual time delay unit output signal by a predetermined gain;

(e) a summing amplifier for receiving each individual gain amplifier output signal and for summing each individual gain amplifier output into a single secondary sound source input signal;

(f) a secondary sound source driven by said secondary sound source input signal for providing an acoustical signal directed towards the aircraft fuselage;

(g) an array of microphones positioned adjacent to selected positions on the aircraft fuselage for measuring the resultant signal due to said secondary sound source output and the propeller noise signature and for providing an output indicative thereof; and

(h) control means for receiving said array of microphones output signals and for providing appropriate signals to said time delay unit and said programmable gain amplifier whereby the noise level at the fuselage surface is minimized.

8. An active noise control system according to claim 7 wherein the control means comprises:

(a) a noise level determining means for determining the average noise level, F(A,φ

), at the fuselage surface, as measured by said array of microphones, as a function of the secondary noise source input signal amplitude A and the phase difference between the propeller noise signature signal as measured by said microphone and the secondary noise source input signal;

(b) gradient determining means for determining the sign, plus or negative, of the gradient (∂

F/∂

A).sub.φ and

(∂

F/∂

φ

)_A respectively; and

(c) regulating means for varying the predetermined time delay interval of the time delay unit and the gain of said programmable gain amplifier whereby (∂

F/∂

A).sub.φ and

(∂

F/∂

φ

)_A are zero.

9. An active noise control system according to claim 8, further comprising a plurality of analog-to-digital converters for receiving the output signals of said array of microphones and inputting those digitalized signals to said control means.