One day soon, res­i­dents who live close to air­ports will say “Thank you, Ben­jamin”. For three years now, Ben­jamin Poiri­er has been analysing the sen­si­tive issue of air­craft noise abatement. 

“In 2005, after I had grad­u­at­ed from UTC major­ing in Mechan­i­cal Engi­neer­ing, and after start­ing a bolt-on cur­sus at a French Air Force pilot train­ing school, I opt­ed to do a Master’s degree in Acoustics Research with the UTC-Rober­val Lab­o­ra­to­ry”. The top­ic I chose was how to use Her­schel-Quincke tubes to con­trol jet engine fan noise emis­sions. In short, this is the large fan that draws air into the reac­tor com­pres­sors stages. This fan alone, with its large spin­ning blades, can gen­er­ate a noise lev­el of up to 100 dB. 

 “We machined the tubes and test­ed them in ane­choic cham­bers to get a bet­ter under­stand­ing as to how they can reduce noise with­out tak­ing into con­sid­er­a­tions of weight or size, for the moment”, explains Ben­jamin Poiri­er. “The acoustic wave trav­els along the tubes and gen­er­ates a sub­trac­tive inter­fer­ence with the main fan noise source”.  The gain, acousti­cal­ly speak­ing, is sig­nif­i­cant – some­where between 3–8 dB. From the per­ceived noise lev­el (the psy­cho-acoustic lev­el), the tubes allow for a gain of 50%. 

“This does not mean that the noise lev­el is divid­ed by two”, warns Ben­jamin Poiri­er. Noise, per se, is a sum­ming of numer­ous, com­plex fac­tors that depend on our dis­tance from the air­craft body, on wind con­di­tions and ambi­ent tem­per­a­ture. In sci­en­tif­ic terms, the Her­schel-Quincke tubes have an effect on cer­tain fre­quen­cies, but not all. But anoth­er note­wor­thy advan­tage in using them is that it also allow the oper­a­tors to reduce the quan­ti­ties of gas­es emitted. 

The dis­cov­ery made by Her­schel and Quincke, in fact, goes back to the 18^th Cen­tu­ry. The under­ly­ing prin­ci­ple is to divert noise and make it trav­el in anoth­er chan­nel so as to be able to can­cel the main source by inter­fer­ence.  “This prin­ci­ple has been applied to car exhaust muf­flers and for ven­ti­la­tion ducts. Our inno­va­tion  will con­sist of applying the same prin­ciel to air­cr­fat and air­cr­fa engines.