System and Method for Noise Cancellation of Low Frequency Noise in Ventilation/Fluid Systems

The present invention provides a system and method for cancellation of noise from noisy elements, e.g. ventilation fans or pumps, placed to transport fluid, e.g. air, between two separate rooms. Focus of the method is to provide an effective attenuation of high level acoustic noise at low frequencies, i.e. below 200 Hz, utilizing standard components and only requiring limited space.

In fluid machines and ventilation systems, the ventilators, heat exchangers, ducts, etc. tend to induce fluctuations in the air/fluid flow, thereby producing low frequency noise, which often can be problematic to deal with within the maximum available space. 

  • Passive silencers to effectively attenuate such low frequency components have very large dimensions, very often making it impractical in practical use.
  • Active noise cancellation is an alternative. However, still, high output at low frequencies, requires a loudspeaker system with a large total diaphragm area. And in order to provide an acceptable electric to acoustic transfer efficiency, such loudspeaker system will require a cabinet with a large volume, such as a closed box or a bass reflex box, if it is to be achieved with standard, lower price components.
  • However, still the size of such loudspeaker cabinet is a problem in many applications, since most often e.g. ventilation ducts are fitted into a rather small space. Thus extra space for a large loudspeaker cabinet is difficult or even impossible to find.
  • Finally, while it is possible to obtain a large low frequency acoustic output from a loudspeaker in a small cabinet, this requires a powerful, expensive power amplifier together with a loudspeaker driver with a high power handling capacity. Very often making the solution too costly.   

This invention offers a very simple, low cost solution

It can be used in both passive and active noise cancellation solutions. By removing the need for a loudspeaker cabinet, and placing the loudspeaker including diaphragm, so that one side is in contact with the room being ventilated, and the other side is vented out to free air, and moving the diaphragm in antiphase, the low frequency noise is cancelled out for the rooms which would otherwise experience this low frequency noise.

Many practical designs are conceivable, all depending on the fluid machine/ventilator system where noise is to be attenuated. An example might be a small ventilation system intended for ventilation of e.g. classrooms or larger offices, where there will typically be fans for inlet and outlet, combined with a cross-flow heat exchanger. Here can the speaker may be placed parallel to the inlet side fan and heat exchanger i.e. as a shunt in relation to the main air flow, and correspondingly on the outlet side.


  • Suppression of low frequency noise in systems with noise generating elements like ventilators, heat exchangers, pumps and duct systems, including e.g.
    • Air ventilation systems, or
    • Systems made for transporting gases or fluids

Granted US patent:
9,165,548 “System and method for attenuating noise from a fluid machine or a turbulent noise source”

AAU Tech ID: 2008-521/10-0073

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