In order to set noise Standards, an understanding of current research and technology development is imperative. Technological progress continues to push the aviation community to delivering on the ICAO goal of limiting or reducing the number of people affected by significant aircraft noise. ICAO continually monitors research and development in noise reduction technology, and this complements the Standard-setting process.

Aeroplane Noise Technologies

In its eleventh cycle (2016-2019), CAEP conducted an independent expert (IE) review to evaluate aeroplane noise goals by 2027 and 2037. More information on the IE review can be found in ICAO Doc 10127 (2019). The main IE conclusions regarding noise reduction technologies are as follows:

• For modern large aircraft, Single Aisle and Twin Aisle, jet noise is a secondary noise source even at departure, with fan noise dominating. For smaller aircraft, business jets and small regional jets, the noise from the jet may still dominate at departure, as it does for many older aircraft. Jet noise has been reduced by reducing jet velocity to improve fuel burn, but because jet noise is now a secondary source, further improvements in fuel burn will not bring automatic substantial reductions in noise.
   
• Fan noise is the dominant departure noise for modern large aircraft whilst it is important at take-off for small aircraft; fan noise dominates engine noise at approach for all aircraft. Reduction in fan pressure ratio is likely to lead to a reduction in fan noise, both forwards and rearwards. Beyond reducing fan tip speed, further fan noise reductions are challenging. Extensive dedicated research (involving full engine ground and flight test demonstrators) combined with 3D unsteady, viscous analysis of the whole gas-path (air intake + fan + OGV + struts + bypass duct) can be expected to bring some additional gains. Better acoustic liner technology will help, but against this, the intake and bypass duct will get shorter in relation to diameter and this will reduce the area amenable to treatment.
   
• A key technology for reducing fan noise is acoustic wall treatment, and liners in the inlet and bypass duct provide essential attenuation. Work continues to improve liner performance, but the task of maintaining current levels of liner attenuation will be challenging, given the incentives to make the intake and bypass duct shorter in relation to diameter, and to reduce nacelle length for fuel burn reasons.
   
• Airframe noise is the largest noise source at approach for modern large aircraft, mostly from the landing gear. Potential airframe noise reductions are very dependent on aircraft category, design and operational characteristics, and the exploitation of this potential will be driven by multiple constraints.
   
• As engines get larger in relation to aircraft size, corresponding to lower fan pressure ratio, it becomes more important for the engine and the aircraft to be designed together as an integral unit. The optimization of the aircraft needs to include acoustic design as well as design for minimization of fuel burn and emissions.
   
• The scope for noise technology reductions of the conventional tube and wing configuration, particularly in large aircraft, now appears to be limited, and the potential additional benefits of acoustic design optimization will need to be properly assessed. Novel configurations, or even some very advanced tube and wing configurations, may bring new noise reduction opportunities, but at the same time these will introduce significant challenges of different nature, which will also need to be addressed.

Helicopter noise technologies

As part of the technical monitoring effort, CAEP conducted a status review of the noise technology advancements of helicopters between 2000 and 2015 to highlight the developments since the last helicopter noise assessment report conducted in 2001 (during CAEP/5). The review included examining both noise reduction technologies and the costs associated with those technologies. The results of the helicopter status review can be found here. The report includes an overview of international noise technology programmes and research initiatives, key noise reduction technologies of modern helicopters, and the status of advanced noise reduction technologies currently being tested in research programmes. Constraints and challenges to incorporate both current noise reduction technologies and promising new technologies are also considered.