Slide 2 MEASURING SURFACE CHARACTERISTICS R. lvarez de
Sotomayor, S. Yanguas, J. Gonzlez & L. Parra Pavement Surface
Characteristics Department Researchers, CEDEX, Spain
[email protected] [email protected]
[email protected] [email protected] Slide 3 KEY WORDS 1.-
Loudspeaker-microphone probe in surface extended method 2.- Sound
power reflection factor and sound absorption coefficient 3.-
Subtraction technique 4.- Type of signals 5.- In situ measurements
6.- Background noise 7.- Soundproof box 8.- Achieving maximum
accuracy Slide 4 1.- Loudspeaker-microphone probe in surface
extended method Extended surface method given by The International
Standard ISO 13472-1 Measurement of sound absorption properties of
road surfaces in situ. Part 1: A sound source driven by a signal
generator A microphone is located between the source and the
surface. The direct path, and the reflected path. Slide 5 2.- Sound
power reflection factor and sound absorption coefficient With
suitable time domain processing as a signal subtraction technique,
the transfer functions of the direct path, after a Fourier
transform H i (f) and of the reflected path H r (f) are obtained.
The ratio of the squared modulus of these transfer functions gives
the sound power reflection factor Q w (f) from which the sound
absorption coefficient can be calculated, apart from a factor Kr
due to geometrical spreading. Slide 6 3.- Subtraction technique The
impulse response of the direct path is not extracted from the
overall impulse response, instead, it is removed from the overall
impulse response by subtraction of an identical signal made in free
field. This technique allows a longer sampling interval, necessary
for low-frequency measurements. Direct componentReflected component
Time window (Twd)Time window (Twr) (Twd) = (Twr) Slide 7 4.- Type
of signals MLS or Maximum Length Sequence is a binary pseudo-random
sequence with specific properties that make it a suitable
excitation signal for deconvolution based on impulse response
measurements. Other internal sinusoidal test signals have been
analyzed as well: The LIN-sweep and the E- sweep. The frequency of
the LIN-sweep increases proportionally with time, while the
frequency of the E-sweep increases exponentially with time. Slide 8
5.- In situ measurements The tests carried out have consisted in
using three different kinds of signals on three different types of
pavements, under traffic noise influence. The period lengths used
with the three signals were 0.34 seconds. All signals on pavement
were repeated ten times for each point as in the case of the free
field measurement. Ten tests were carried out per point and all the
results were averaged too, therefore the result was made with a
hundred measurements per test point. Slide 9 5.- In situ
measurements BBTM 11B Aggregates size0/12 mm Porosity17,1 %
Thickness4 cm Lifetime4 years Actual conditionsgood Mixture
temperature 11,68 C PA 16 Aggregates size0/18 mm Porosity20,5 %
Thickness4 cm Lifetime4 years Actual conditionsgood Mixture
temperature 10,93 C AC 16 Aggregates size0/18 mm Porosity3,5 %
Thickness5 cm Lifetime6 months Actual conditionsgood Mixture
temperature 11,12 C Slide 10 5.- In situ measurements Slide 11 6.-
Background noise The traffic noise is not continuous neither in
frequency nor in intensity. It has been found out the importance of
doing the same number of measurements on air than on pavement, to
reduce the background noise effects. Slide 12 7.- Soundproof box
How the signals were influenced by the background traffic sound
pressure levels?. A soundproof box was built with polystyrene
sheets, in order to compare how the background traffic noise
affected absorption coefficient results. Slide 13 7.- Soundproof
box Slide 14 8.- Achieving maximum accuracy As more measurements
are made the deviation from the average could be minor. When
measuring under traffic conditions it is advisable for each shot to
do a free field measurement. All measures must have low period
lengths, 0.34 seconds in our case, to reduce the non continuous
behaviour of background traffic noise. The E-sweep signal was the
unique signal able to detect different porosities offering coherent
coefficient absorption values on three surfaces. Slide 15 8.-
Achieving maximum accuracy It has been painted as a traffic light
colors the results explained along this presentation. PAVEMENT
TYPES PA 16 BBTM 11BAC 16 KIND OF SIGNAL - Very high sound
absorption coefficient values even in low frequencies - Porosity
undetected -Most sensitive to the background noise MLS -Lightly
high sound absorption coefficient values even in low frequencies -
Porosity detected -Medium sensitive to the background noise - High
absorption coefficient values even in low frequencies - Porosity
undetected -Medium sensitive to the background noise LIN-SWEEP
-Good sound absorption coefficient values even in low frequencies -
Porosity detected -Less sensitive to the background noise E-SWEEP
Slide 16 Many thanks for your attention! It will be a pleasure to
answer any question MEASUREMENT OF SOUND ABSORPTION COEFFICIENT OF
ROAD SURFACES IN SITU R. lvarez de Sotomayor, S. Yanguas, J.
Gonzlez & L. Parra Pavement Surface Characteristics Department
Researchers, CEDEX, Spain [email protected]
[email protected] [email protected]
[email protected]
