Unit 5 - Air Transport Flashcards
Why does aircraft noise generate higher disturbances
Aircraft Noise affects a relatively small number of people but, typically, the degree of disturbance to these people is much greater due to.
- high levels of the noise produced
- frequency content of the noise generated is in the range to which the ear is most sensitive.
why do Smaller aerodromes give rise to complaints
Smaller aerodromes give rise to complaints from surrounding communities
- deemed to be a nuisance and non- essential.
Noise generated by aircraft
Noise generated by aircraft is mainly related to the air flow velocity within the engine system and over the aircraft frame
- both broadband and tonal In nature.
The parameter used to assess aircraft noise within the noise certification process is the effective perceived noise level (EPNL).
Aircraft - D-weighted sound level (dB(D))
D-weighting reduces the effect of low frequency noise and recognises the potential annoyance at the higher frequencies associated with aircraft noise.
Specifically, compensates for the extra sensitivity of the ear at around 4 kHz, which is a frequency band associated with jet engine compressor noise.
Developed for jet engines rather than for the turbo-fan engines on most modern aircraft, so it is no longer used.
Perceived Noise level (PNL)
Expressed in dB, is a rating of noisiness which has been specifically applied to the assessment of aircraft noise
derived from octave or one-third octave frequency bands sound pressure levels.
It is suitable for broadband noise spectra, but its usefulness is limited when the noise source contains discrete tonal or frequency components
How to calc PNL
- Obtain SPL values for the octave frequencies (63, 125, 250,…..8000Hz)
- Obtain “noy” values from the contours of perceived noisiness
- Calculate the total perceived noisiness Nt using
Nt* = *Nm + 0.3 S(N – N**m) (5.1)
where Nm* is the highest noy value and *N* is the sum of the noy values. The 0.3 factor reflects the relative importance of *Nm in the estimation of N**t
- The total perceived noisiness is then converted to the perceived noise level (in PNdB) using theformulae:
PNdB = 40 + (100/3) log N**t
Noise and Number Index (NNI)
Until the early 1990’s, the NNI was the main index used to rate aircraft noise and the potential for complaints from local residents living near to airports.
NNI has been replaced by L**Aeq
Based upon the two factors from social surveys…
- The average peak noise level PNdB of the aircraft
- The number of aircraft (N) heard during a daytime period of 12 hours
Noise and Number Index (NNI) formula
A single number rating scale (NNI) was established
- NNI* = PNL + 15 log N - 80
- PNL* represents the logarithmic average of the perceived noise levels, such that
𝑃𝑁𝐿 = 10 log(1⁄𝑁) ∑𝑁 10𝐿⁄10
Noise and Number Index (NNI) levels
Levels are often summarised as follows:
35 NNI – low annoyance
45 NNI – moderate annoyance 55 NNI – highannoyance
The use of NNI
The use of NNI has declined since the mid 1980’s and there are five reasons for this.
- The origin of the NNI was based upon noise measurements and a social survey at Heathrow airport and its surrounds in the early 1960’s. Hence, other airports at the time may have yielded a different NNI predictive equation and a different “scale” of annoyance.
- The nature of aircraft noise had changed dramatically since the development of the NNI.
- The number of residential properties around UK airports has increased significantly over the years.
- The use of an average maximum level in the predictive equation has often thought to be a limitation as it masks the highest levels. The use of the maximum level may be more appropriate as this would provide a worst-case scenario.
- The number of flight movements at night has increased significantly, and the NNI may therefore be inappropriate to rate annoyance at night
Equivalent Continuous Sound Level LAeq for aircraft noise
CAA study on aircraft noise and sleep disturbance carried in 1980 concluded that the LAeq,T index was as good as NNI for assessing aircraft noise exposure and correlated well with the level of sleep disturbance
LAeq,T limitations for assessing aircraft noise
LAeq,T may also have limitations for assessing aircraft noise. Two possible limitations are:
- LAeq,T is based on an “averaging” of noise in some respects as opposed to a single event (or maximum) level that may specifically characterise aircraft noise
- LAeq,T may underestimate the level at which people are annoyed or impacted by the noise
noise certification requirement
International Civil Aviation Organisation (ICAO) Annex 16 (incorporated into the national legislation of mostwestern nations)
British Civil Airworthiness requirements BCAR section N (equivalent to ICAO Annex 16)
what is Noise Certification
part of a general strategy to reduce the impact of aircraft noise in the vicinity of airports.
- The legal requirement for noise certification stems from the Air Navigation Order 1990.
- It is a process that applies to all propeller driven aircraft, supersonic civil aeroplanes, microlights,and all subsonic planes which have a take-off distance in excess of 610 m. It also applies to helicopters
The scheme, for jet and propeller driven aircraft above 5700 kg, sets limits for noise levels which must not beexceeded at three monitoring points
- On the approach path to represent the noise generated at a point directly beneath a 3- degree glide-slope and positioned 2000 m from the runway threshold
- On the take-off path to represent the noise received at a point directly beneath the aircraft take-off flight path and positioned 6.5 km from the point of brake release
- At sideline positions to represent the peak noise received at a point along a line parallel to and 450m from the extended runway centre.
Maximum noise levels are measured in terms of EPNL (the tone and duration corrected PNL)
