Aircraft Noise

CHAPTER 3 CHAPTER 3 Airport Noise Control

3.2 Aircraft Noise

Aircraft noise can be described by measuring the level of noise in terms of sound intensity.

Where a noise-level measure is required, the simple dB(A) method is not entirely satisfactory. Following the introduction of jet aircraft, research carried out at JFK Airport, New York, indicated that the ear summed noise in a much more complicated way than the A-scale weighting of dB(A). As a result, another noise level measure was devised, the perceived noise level (PNL), a D-weighted summation that is sufficiently complex to warrant computer calculation.

Single-Event Measures

Noise intensity by itself is not a complete measure of noise. Intensity requires the factor of duration, which has been found to have a strong influence on the subjective response to noise. The principal measures of single-event noise used are effective perceived noise level (EPNL or L_EPN) and sound exposure level (SEL, sometimes abbreviated to LSE or LAE).

Annex 16 to the Convention on Civil Aviation of ICAO recommends use of EPNL, which modifies the PNL figure by factors that account for duration and the maximum pure tone at each time increment. This measure of the single event therefore incorporates measures of sound level, frequency distribution, and duration. Federal Aviation Administration (FAA) practice, on the other hand, uses a measure based on the sound exposure levels, weighted on the A-scale, over the time during which the sound is detectable. The accumulation procedure takes note of the logarithmic nature of sound addition. Both the EPNL and the SEL are used as bases for developing environmental measures of noise exposure.

Cumulative-Event Measures

In the case of noise nuisance generated in the process of airport operation, it is not simply the magnitude of the worst single noise event that gives a measure of environmental impact. Over the operational day of the airport, many noise “events” occur. Therefore, single-event indices are not useful methods of measuring aircraft noise disturbance, which is related to annoyance and interference with relaxation, speech, work, and sleep.

Quantifying such interference requires noise measurements in terms of instantaneous levels, frequency, duration, time of day, and number of repetitions. Many surveys have been carried out to correlate community response to all these factors.

Day/Night Average Sound Levels (United States)

average sound level (DNL or L_DN), which is computed from

(3.1) where N_D =

number of operations 0700–2200 hours

N_N = number of operations 2200–0700 hours

SEL = average sound exposure level

i = aircraft class

j = operation mode

Partial L_DN values are computed for each significant type of noise intrusion using Ashford et al. (2011). They are then summed on an energy basis to obtain the total L_DN owing to all aircraft operations:

(3.2)

Noise and Number Index

Another cumulative event measure that is widely quoted in airport noise literature is the noise and number index (NNI). This is a rather simple measure that was used widely in the United Kingdom and had limited use elsewhere. Equation (3.3) is the relevant formula for computation:

NNI = _PN + 15 log N − 80 (3.3)

Surprisingly perhaps, the definition of terms within the formula is not completely standardized among users. It is common practice, however, to define N as the number of occurrences of aircraft noise exceeding 80 PNdB, the peak level caused by a Boeing 707 at full power at approximately 13,000 feet (4,000 m) height. L_PN is the logarithmic average of peak levels. NNI has been replaced in the United Kingdom by _den (see below).

Equivalent Continuous Sound Level L

_EQ

Usually specified for a relatively long measurement period, the equivalent continuous sound level L_EQ is defined as the level of equivalent steady sound that, over the measurement period, contains the same weighted sound energy as the observed varying sound. It is stated in mathematical form as

(3.4)

where L(t) is the instantaneous sound level at time t, and T is the measurement period. In practice, this is the same as

(3.5)

which is the summation of the individual aircraft sounds over the measurement period T.

There are several versions of the A-weighted equivalent continuous sound level metrics in use in Europe (L_EQ or LA_EQ). It is accepted by the World Health Organization (WHO) as the preferred metric for noise disturbance.

Because of shortcomings of the NNI, which was not entirely applicable to all airports, L_EQ has replaced the NNI in the United Kingdom and is shown here only for reference purposes. It has been found at Heathrow that the NNI and L_EQ equivalences were

35 NNI 57 L_EQ

45 NNI 63 L_EQ

55 NNI 69 L_EQ

Noise-Exposure Forecast (United States)

Prior to the development of the LDN index, the measure of cumulative noise exposure in the United States was the noise-exposure forecast (NEF), which still occurs in much FAA literature. It is computed from

(3.6) where

EPN =

average effective perceived noise level that is computed from individual LEPN values. This is the EPNL defined previously.

K = 88 for daytime periods (0700–2200)

K = 76 for nighttime periods (2200–0700)

and

(3.7) where (t) = sound level in dB(A) or PNdb

T = 20 or 30 seconds to avoid including quiet periods between aircraft

The combined 24-hour NEF is computed using Eq.(3.8):

(3.8)

A-Weighted Day-Night Average Sound Level (L

D N

or DNL) and