Introduction to Acoustics

Table 23.8 Crossover speeds for various cases. According
to Sandberg [23.99] “The table assumes that ‘normal’ tires
are used, and that the road surface is a dense asphalt concrete
or an SMA with max 10-14 mm chippings. ‘Cruising’ is
constant speed, and ‘Accelerating’ means an ‘average’ way
of accelerating after a stop, but not as much as in the ISO
362 driving mode.” (After [23.99] Table 5.1)
Vehicle type Cruising Accelerating
Cars made 1985-95 30–35 km/h 45–50km/h
Cars made 1996- 15–25 km/h 30–45km/h
Heavy trucks made 40–50 km/h 50–55km/h
1985-1995
Heavy trucks made 1996- 30–35 km/h 45–50km/h
of the tire treads, axial vibrations of the sidewalls, air
pumping due to the deformation of the tread by irregularities
in the road surface, and possibly vibration of the
rim of the tire. At low frequencies, there is evidence that
the tire treadband can be considered to be a cylindrical
beam elastically supported by the sidewall [23.100].
A quantative description of tire–road noise is best
obtained by development of a model of the sound radiation.
A history of past modeling attempts has been
given by Kuijpers and van Blokland [23.101], and more
generalized models are also available [23.102, 103].
The road surface itself has a significant effect on the
generation of tire noise. The roughness of the surface
affects the excitation of the tire and subsequent generation
of noise; the porosity of the surface effectively
changes the compressibility of trapped air, and if the
acoustic impedance is in the range to absorb sound, energy
can be absorbed by the surface. Information on the
characteristics of porous surfaces is given in [23.99].
Tire–road interaction noise is currently the subject
of intensive research. Bernhard et al. have given a summary
of research [23.104], Sandberg et al. [23.105]
have discussed poroelastic road surfaces, and Thornton
et al. [23.106] have studied the variability of existing
pavements. Although porous surfaces have been shown
to provide noticeable reductions in traffic noise levels,
questions remain about the durability of these surfaces,
especially in cold climates. The durability of such surfaces
in a warm climate has been studied by Donavan
et al. [23.107].
Traffic Noise Prediction Models. The US Federal Highway
Administration has released version 2.5ofthetraffic
noise model, whichmaybeusedforawidevarietyof
calculations related to traffic noise. The characteristics
Noise 23.2 Noise Sources 989
of the model are described in Sect. 23.3.1 on outdoor
noise barriers.
Aircraft Noise
Exterior noise emissions of aircraft are of major importance
to persons on the ground, and interior noise emissions
are a concern of both airline passengers and crews.
Aircraft Noise Emissions. Noise emissions from civil
aircraft have been regulated for more than 35 years, first
by the Federal Aviation Administration in the United
States, and later, internationally, by the International
Civil Aviation Organization (ICAO). Nevertheless, although
the noise from individual airplanes has been
significantly reduced, the total exposure to aircraft noise
for residents in communities near many airports has
increased because of the growth in the number of
aircraft operations at airports throughout the world. Consequently,
operators of the airplanes and the airports
continue to face limitations because of aircraft noise.
This section discusses the noise emissions of aircraft;
noise at the receiver is discussed in Sect. 23.4.4. Requirements
for the noise levels that may be produced by
aircraft are given in title 14 of the US Code of Federal
Regulations, 14CFR part 36 [23.108] and in annex 16 to
the Convention on International Civil Aviation (ICAO
annex 16) [23.109].
The methodology of determining the noise emission
of aircraft for certification purposes is well developed.
Certification noise levels are not intended to, and do
not, represent noise levels that may be measured in
communities around airports.
Certification noise limits, in terms of effective perceived
noise level (EPNL), are specified at three points
– known as lateral, flyover, and landing approach.
The limits are a function of the maximum design
takeoff gross mass, and apply to both jet-powered
and propeller-driven transport-category airplanes, business/executive
airplanes, and helicopters. The limits
apply to new-design airplanes and to retrofit modifications
of old-design aircraft.
Perceived noise levels are determined from 500 msaverage
one-third-octave-band sound pressure levels, at
500 ms intervals, and are adjusted for the additional
noisiness caused by prominent tones, if present, or
other spectral irregularities. For each of the three noisemeasurement
points, effective perceived noise levels are
determined from the time integral of tone-corrected perceived
noisiness and adjusted to reference atmospheric
conditions, reference flight paths, reference airspeeds,
and reference engine power settings.
Part G 23.2