ESDU Engineer

Aircraft Cabin Noise

Aircraft cabin noise can be a source of irritation for passengers trying to sleep but it can also lead to flight crew scheduling constraints due to health and safety requirements for exposure to high levels of noise, with severe cost implications. While the level of cabin noise has been reduced dramatically since the first commercial aircraft entered service, the quest for the “silent aircraft” continues to drive improvements in noise levels both within the aircraft and externally as environmental noise pollution regulations are tightened, particularly for night take-off and landings.

Aircraft noise is generated both by the power-plants and by the airframe as it passes through the air. A major source of aircraft noise is the aircraft propulsion system. Propulsion systems can take several forms but it is common to divide aircraft into those powered by jet engines and those driven by propellers. The noise levels in the cabins of turboprop aircraft may be as much as 10 to 30 decibels higher than noise levels in commercial jets. With jet engines the introduction of the high-bypass ratio turbo fan has led to significant reductions in noise both outside and inside the aircraft when compared to earlier turbojets. In addition to improvements in power-plant noise levels aircraft cabins now incorporate more advanced materials included in sound-absorbent seats, cabin walls, and partitions.

Cabin noise is due to boundary layer noise, airborne noise and structure-borne noise. Particular attention must be paid to structural design and material selection so that vibrations are transmitted less effectively.

As human hearing shows greater sensitivity to frequencies in the 1 kHz to 4 kHz range it is also important to consider the subjective response of those inside the aircraft. Various models have been developed to assess the subjective response of sound propagation and thus comfort levels inside the aircraft.

Advances have been made both in the types of passive materials used for aircraft acoustic insulation and in highly complex, electronic noise cancellation systems currently under investigation by many aircraft manufacturers. Much of this progress has been driven by the demand for low levels of noise in high-end corporate aircraft.

Commercial airline operators are reluctant to add additional weight to their aircraft and so they employ only the minimum required acoustic insulation. However, the advances made in the corporate market are yielding materials which are acoustically more effective for a given weight.

A new Data Item with associated software on Noise Transmission into Aircraft Cabins has just been issued in the ESDU Aircraft Noise Series. ESDU 07001 provides the user with a means of estimating the reduction in noise levels through a fuselage wall with various levels of cabin acoustic insulation.

The particular strength of this prediction method is that the Item permits the modelling of a cabin wall with sidewall trim elements in any combination, up to a maximum of ten elements including the fuselage skin. This modelling is performed at every frequency by combining the trim transfer matrices of each element in the order corresponding to the trim model of interest, resulting in a single overall transfer matrix. The method can deal with both discrete frequency plane waves and with broadband noise in a diffuse field and is intended for application to noise radiation from various types of aircraft engines into the aircraft cabin.

The program requires as input the cabin geometry, material properties, details about trim properties and acoustic and structural loss factors. The user has the option of using either one of three provided cabin wall trim models or of modelling the cabin wall trim with any combination of three trim elements, with up to a maximum number of ten elements. A routine to estimate acoustic loss factors can be employed if required. The program can also be used to estimate space-averaged interior sound pressure levels if exterior levels are input. Two additional programs to facilitate the generation of input files are included.

Queries regarding this article should be directed to Dr Cyrus Chinoy, Head of the Aircraft Noise and Structural Dynamics Group: cyrus.chinoy@ihs.com