Noise cancellation using a mechanical oscillator

Abstract

A system and method for the reduction of noise radiated by a vibrating structure at a given frequency are disclosed. In systems of the invention, a radiation-cancelling device that acts like a resiliently supported mass with a significant sound radiating area is attached to the exterior surface of the structure to be quieted. Radiation from such a device, when constructed so that the motion of the vibrating structure provides the driving frequency for oscillation of the spring-mass system in the radiation-cancelling device and so that the natural frequency of the spring-mass system in the radiation-cancelling device is lower than the vibration frequency of the vibrating structure cancels the sound radiating from the (preferably) considerably larger vibrating surface to which it is attached. If appropriate, more than one radiation-cancelling device may be attached to a given vibrating structure in a system according to the invention.

What is claimed is:  
     
         1 . A system for cancelling noise radiation from a vibrating structure, said system comprising: 
 a vibrating structure, said vibrating structure having a sound radiating area; and    a radiation-cancelling device, said radiation-cancelling device having a sound radiating area, wherein said radiation-cancelling device comprises a spring-mass system cooperatively attached to said vibrating structure and wherein, further, 
 the motion of said vibrating structure provides the driving frequency for oscillation of said spring-mass system in said radiation-cancelling device; and  
 said radiation-cancelling device is designed so that the natural frequency of said spring-mass system in said radiation-cancelling device is lower than the vibration frequency of said vibrating structure.  
   
     
     
         2 . The system of  claim 1 , wherein the sound radiating area of said radiation-cancelling device is smaller than the sound radiating area of said vibrating structure.  
     
     
         3 . The system of  claim 2 , wherein the sound radiating area of said radiation-cancelling device is less than one twentieth the sound radiating area of said vibrating structure.  
     
     
         4 . The system of  claim 1 , wherein the sound radiating area of said radiation-cancelling device is larger than the sound radiating area of said vibrating structure.  
     
     
         5 . The system of  claim 1 , wherein said vibrating structure is a transformer.  
     
     
         6 . The system of  claim 1 , wherein said vibrating structure is a train wheel.  
     
     
         7 . The system of  claim 1 , wherein the spring-mass system of said radiation-cancelling device is in the form of an inverted metal cup.  
     
     
         8 . A method for cancelling noise radiation from a vibrating structure, said method comprising the steps of: 
 providing a vibrating structure in need of quieting, said vibrating structure having a sound radiating area; and    cooperatively attaching to said vibrating structure a radiation-cancelling device, said radiation-cancelling device having a sound radiating area, wherein said radiation-cancelling device comprises a spring-mass system and wherein, further, 
 the motion of said vibrating structure provides the driving frequency for oscillation of said spring-mass system in said radiation-cancelling device; and  
 said radiation-cancelling device is designed so that the natural frequency of said spring-mass system in said radiation-cancelling device is lower than the vibration frequency of said vibrating structure,  
   wherein, when said vibrating structure vibrates at said vibration frequency, noise radiating from said sound radiating area of said vibrating structure is cancelled by noise radiating from said sound radiating area of said radiation-cancelling device.    
     
     
         9 . The method of  claim 8 , wherein the sound radiating area of said radiation-cancelling device is smaller than the sound radiating area of said vibrating structure.  
     
     
         10 . The method of  claim 9 , wherein the sound radiating area of said radiation-cancelling device is less than one twentieth the sound radiating area of said vibrating structure.  
     
     
         11 . The method of  claim 9 , wherein the sound radiating area of said radiation-cancelling device is larger than the sound radiating area of said vibrating structure.  
     
     
         12 . The method of  claim 8 , wherein said vibrating structure is a transformer.  
     
     
         13 . The method of  claim 8 , wherein said vibrating structure is a train wheel.  
     
     
         14 . The method of  claim 8 , wherein the spring-mass system of said radiation-cancelling device is in the form of an inverted metal cup.