10.1121@1.4904542

The subjective effect of low frequency content in road
traffic noise
Antonio J. Torijaa)
Department of Electronic Technology, University of Malaga, Higher Technical School of Telecommunications
Engineering, Campus de Teatinos, Malaga 29071, Spain
Ian H. Flindell
Institute of Sound and Vibration Research, University of Southampton, Highfield, Southampton SO17 1BJ,
United Kingdom
(Received 21 June 2014; revised 12 October 2014; accepted 18 November 2014)
Based on subjective listening trials, Torija and Flindell [J. Acoust. Soc. Am. 135, 1–4 (2014)]
observed that low frequency content in typical urban main road traffic noise appeared to make a
smaller contribution to reported annoyance than might be inferred from its objective or physical
dominance. This paper reports a more detailed study which was aimed at (i) identifying the
difference in sound levels at which low frequency content becomes subjectively dominant over mid
and high frequency content and (ii) investigating the relationship between loudness and annoyance
under conditions where low frequency content is relatively more dominant, such as indoors where
mid and high frequency content is reduced. The results suggested that differences of at least
þ30 dB between the low frequency and the mid/high frequency content are needed for changes in
low frequency content to have as much subjective effect as equivalent changes in mid and high
frequency content. This suggests that common criticisms of the A-frequency weighting based on a
hypothesized excessive downweighting of the low frequency content may be relatively unfounded
in this application area. VC 2015 Acoustical Society of America.
[http://dx.doi.org/10.1121/1.4904542]
[SF] Pages: 189–198
I. INTRODUCTION
It is well known that the lower frequencies produced by
internal combustion engines in typical road vehicles, particu-
larly buses and trucks, tend to be physically dominant over
mid and high frequencies under low speed and often con-
gested urban operating conditions.1,2
Torija and Flindell have
shown that this physical dominance does not necessarily lead
to subjective dominance.3
These findings are consistent with
those of other studies4,5
where the higher frequencies were
observed as more significant for road traffic noise annoyance.
Low frequency content is relatively more dominant
indoors and behind noise barriers because walls and screens
are generally more effective in attenuating mid and high fre-
quency content than low frequencies.6
For example, Nilsson
and Berglund7
observed negligible attenuation at frequencies
below 200 Hz from a medium height noise barrier (2.25 m
high) which was nevertheless effective at mid and high
frequencies. Several researchers8,9
have suggested that low
frequency content could be relatively more significant
under these types of conditions potentially leading to feel-
ings of irritation, undue tiredness, concentration difficulties,
sleep disturbances and annoyance induced by rattle or
vibration.6,10–12
The A-frequency weighting which is widely used for the
assessment and regulation of road traffic noise downweights
low frequency and higher high frequency components within
the overall spectrum approximately according to the
curvature of equal loudness contours at lower sound levels.13
As such, it has been criticized on the grounds of over-
compensation for the known reduced sensitivity of the audi-
tory system at low frequencies.14–17
We hypothesized that the subjective importance of
changes in the relative frequency content of road traffic noise
in different parts of the auditory frequency range would be
dependent to at least some extent on the relative dominance
(both objective and subjective) of each part of the spectrum,
divided into low, mid, and high frequencies. In other words,
changes in any one part of the spectrum would be relatively
more significant if that part of the spectrum was already sub-
jectively dominant. From this point of view, a key issue is
measuring at what point in any objective scale does any one
part of the spectrum become subjectively dominant. To
investigate this issue, subjective listening tests were con-
ducted in two parts: (a) test 1 investigated the effect of
subjective dominance on loudness and annoyance for a range
of physical dominance of low frequency (þ10, þ20, and
þ30 dB) and (b) test 2 investigated the effect of overall
sound level [$40 and $50 dB(A)] under typical indoor spec-
trum conditions. Test 2 was carried out because of the well-
known increasing curvature of the standard equal loudness
contours at lower sound levels. This suggests that, if there is
a problem with using the A-frequency weighting for low fre-
quency dominated road traffic noise at higher sound levels,
the problem should diminish at lower sound levels where the
A-frequency curve more closely approximates to the inverse
of the standard equal loudness contours.13
At higher sound
a)
Author to whom correspondence should be addressed. Electronic mail:
ajtorija@ugr.es
J. Acoust. Soc. Am. 137 (1), January 2015 VC 2015 Acoustical Society of America 1890001-4966/2015/137(1)/189/10/$30.00
Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 129.81.226.78 On: Tue, 12 May 2015 17:30:45

levels, the standard equal loudness contours tend to be
increasingly flatter. Thus, the B- and C-frequency weightings
were originally proposed for evaluating medium and high
sound levels respectively, by defining filter characteristics
that more closely approximate to the inverses of the standard
equal loudness contours at medium and high sound levels.6
II. METHODS
A. Approach
The listening tests were carried out under laboratory
conditions using repeated measures to maximize experimen-
tal control and to be able to average out across individual
differences in sensitivity and use of the subjective scales.
The stimuli were road traffic sounds reproduced through
high quality loudspeakers and the tests were carried out in a
sound-proofed room to eliminate any effect of extraneous
sounds. Each stimulus was limited to 12.5 s duration
because, based on pilot tests, this was found to be long
enough for listeners to be able to decide and report appropri-
ate subjective ratings while minimizing listener fatigue.
Moreover, several authors4,18
found, in listening studies of
this same generic type, that the stimulus duration did not
have any significant effect on subjective assessment.
In listening laboratory tests the stimuli are necessarily
of very short duration compared to long time real-life expo-
sures and listeners cannot avoid paying attention to the
sounds in ways that are not representative of real life.
Absolute judgments made in a listening laboratory might
not, therefore, be particularly representative of the corre-
sponding real life situations. On the other hand, there is no
reason to doubt the relative rankings of subjective judgments
of loudness and annoyance of sounds heard in the laboratory
for the corresponding relative rankings that would be likely
for corresponding real-life sounds. Listening laboratories
permit detailed investigation of relative differences in sound
quality that are not feasible in real-life field studies but
which might then need to be calibrated against real-life
subjective judgments to obtain absolute validity.
B. Stimuli
The sounds were all based on two original recordings of
typical low speed busy road traffic noise recorded at 3.5 m
from the edge of the roadway in an open area in a medium
sized city in southern Spain. Mid and high frequency content
was marginally more prominent in the second recording,
used in test 2, than in the first recording, used in test 1, as
can be seen in Figs. 1 and 2. The two recordings were other-
wise subjectively similar with all extraneous non-road traffic
or impulsive sounds avoided as far as possible. The record-
ings were mainly of continuous road traffic noise with indi-
vidual vehicle pass-by events being subjectively audible but
not physically dominant at a rate of every 2.5 to 4 s. The
time average sound levels at the recording positions were
63.8 LAeq (original sound 1) and 68.9 LAeq (original sound 2)
with variation within each sample within a range of plus or
minus 2–3 dB.
Figure 1 shows the filtered frequency spectra for the dif-
ferent sounds used in test 1 which was divided into three
sub-tests, denoted T1.C1, T1.C2, and T1.C3, as shown in the
figure. For the original sound 1 used in test 1, the low fre-
quency range was approximately þ5 dB dominant over the
mid and high frequency ranges. Three different reference
sounds were prepared for the three sub-tests as follows. The
left hand side charts denoted T1.C1, T1.C2, and T1.C3 show
the reference sounds with the low frequency content (LF,
20–250 Hz) boosted by þ2.5 dB, þ7.5 dB, and þ12.5 dB and
the mid and high frequency content (MHF, 315 Hz to
20 kHz) cut by À2.5, À7.5, and À12.5 dB, respectively, to
produce overall physical dominances of LF content relative
to MHF content of þ10 dB (T1.C1), þ20 dB (T1.C2), and
þ30 dB (T1.C3). The A-weighted time average sound levels
of the reference sounds used for each sub-test (T1.C1,
T1.C2, and T1.C3) were 62.2, 62.6, and 67.5 dB(A),
respectively.
The test sounds for comparison against the three refer-
ence sounds were then produced by applying low pass and
high pass shelf filters set at 315 and 250 Hz as shown on the
right hand side charts for each set of sub-tests denoted
T1.C1, T1.C2, and T1.C3. The shelf filters were set with 0.1
octave transitions to apply À6 dB and þ6 dB boost or cut to
produce 12 differently filtered test sounds for test 1. The pur-
pose of testing the four different shelf filter conditions for
each reference sound was to investigate the relative signifi-
cance of small changes in either the low frequency range or
the mid and high frequency ranges depending on the degree
of subjective prominence of those different frequency ranges
considered overall. Applying a range of different shelf filters
to original sounds with relative differences in low frequency
content obviously affects the A-weighted sound levels differ-
ently in each case and these are shown in Table I.
Test 1 was carried out at sound levels in the listening
laboratory which would be representative of sound levels
and frequency spectra as typically heard outdoors. Test 2
was therefore extended by indoor-filtering the original sound
to simulate typical frequency dependent attenuation of
double glazing sealed units made up from 3 mm glass, 3 mm
air gap, and 3 mm glass, according to the values reported
by Quirt.19
The simulated transmission loss spectrum was
approximately flat up to 500 Hz and then increases by
þ15 dB up to around 2000 Hz and is then approximately flat
at increasingly higher frequencies.19
Artificial reverberation
at 0.5 s reverberation time was added to the filtered signal
using digital software to increase the subjective realism of
what was intended to be an indoor simulation. Test 2 was
then divided into two sub-tests denoted T2.C1 and T2.C2
with reference indoor filtered sounds reproduced at 49.5 LAeq
(T2.C1) and 39.6 LAeq (T2.C2) as shown in Fig. 2, to cover a
representative range of typical indoor sound levels for build-
ings facing onto busy main roads. Low pass and high pass
shelf filters set at the same 315 and 250 Hz frequencies with
0.1 octave transitions as in test 1 were then applied sepa-
rately at À9, À3, þ3, and þ9 dB to the simulated indoor
filtered sounds. The 8 different shelf-filters applied to the 2
un-shelf filtered indoor sound levels produced 16 test sounds
for test 2 for comparison against the two reference sounds.
190 J. Acoust. Soc. Am., Vol. 137, No. 1, January 2015 A. J. Torija and I. H. Flindell: Effect of low frequency in road traffic noise

Amplifier gains were set in each case so that the origi-
nally LF boosted (test 1) and indoor-filtered (test 2) sounds
would be reproduced at the sound levels specified above.
The various shelf-filtered test sounds were then reproduced
at higher and lower sound levels without changing the ampli-
fier gain (see Table I).
C. Listeners and equipment
Thirty-three volunteer listeners with normal hearing
took part divided randomly into 11 groups of 3 listeners for
each trial. All listeners either confirmed normal hearing abil-
ity or were directed to the Action on Hearing Loss charity
telephone test at 0844 800 3838 or an online test20
in case
they felt any doubt about this requirement. Two volunteers
initially expressed doubts about whether they had normal
hearing or not, and subsequently produced emails from
Action on Hearing Loss confirming normal hearing after tak-
ing the online test. The age range was 18–65 and there were
18 males and 15 females. All listeners were paid a thank you
gift (£15) as an encouragement to actually turn up once they
had initially agreed to take part.
The listening test used two high quality active monitor
loudspeakers set up in a sound-proofed audiometric test
room at the University of Southampton. All audio signals
(.wav files) were generated via a mainstream laptop with a
good quality sound card, and then sent to the two loud-
speakers (Behringer Truth Model B2031A) via a USB audio
FIG. 1. Frequency spectra of the origi-
nal sound 1, the reference sounds and
the À6 dB and þ6 dB filter gain set-
tings for LF and MHF, in the tests
T1.C1 (upper), T1.C2 (middle), and
T1.C3 (lower).
J. Acoust. Soc. Am., Vol. 137, No. 1, January 2015 A. J. Torija and I. H. Flindell: Effect of low frequency in road traffic noise 191

interface (Edirol USB Audio Interface UA-1X) and a small
high quality audio mixing console (Yamaha Mixing Console
MG10/2). The reproduced sound levels were calibrated im-
mediately before each trial using a type 1 sound level meter
(Norsonic Environmental Analyzer type 121), with a
Norsonic free-field microphone type 1225 placed at the posi-
tion which would be occupied by the center listeners’ head.
The listener’s chairs were positioned so that any differences
in sound levels between the three positions were negligible.
The pre-test calibrations were carried out using an 80 dB(A)
pink noise.wav file signal to ensure sufficient headroom
above the low frequency boosted reference and comparison
sounds to avoid distortion.
D. Procedure
The aims and procedures were carefully explained to
each listener on arrival and voluntary consent obtained and
documented. It should be mentioned that all the participants
received exactly the same information. For test 1, listeners
FIG. 2. Frequency spectra of the original sound 2, the reference sounds and the À9 dB, À3 dB, þ3 dB, and þ9 dB filter gain settings for LF (left) and MHF
(right), in the tests T2.C1 (upper) and T2.C2 (lower).
TABLE I. A-weighted sound level (LAeq) of each of the reproduced experimental sound.
Test 1 Test 2
T1.C1
(LF À MHF ¼ þ 10 dB)
T1.C2
(LF À MHF ¼ þ 20 dB)
T1.C3
(LF À MHF ¼ þ30 dB)
T2.C1
[Reference
sound ¼ 49.5 dB(A)]
T2.C2
[Reference
Gain
setting LF MHF LF MHF LF MHF LF MHF LF MHF
À9 dB — — — — — — 46.8 48.3 36.9 38.3
À6 dB 61.8 58.6 61.0 61.9 65.8 67.0 — — — —
À3 dB — — — — — — 48.0 48.7 38.1 38.8
þ3 dB — — — — — — 51.7 51.0 41.7 41.0
þ6 dB 64.5 67.7 67.6 66.6 72.7 71.4 — — — —
þ9 dB — — — — — — 57.1 55.6 47.1 45.6

were instructed that the sounds were recorded outdoors
alongside a busy main road in an open area in a city and that
they should judge the sounds in that context. For test 2, lis-
teners were instructed that they should judge the sounds as if
they were indoors and the road traffic sounds were coming
indoors from a main road nearby and outside. Moreover, par-
ticipants were asked to keep the same position, looking at
front and without moving their head while they were listen-
ing to the test sounds in order to maximize comparability
between them. The relative magnitude estimation method21
was selected for reporting the subjective magnitudes of each
test sound. According to this method, listeners rate subjec-
tive loudness and annoyance of each test sound numerically
against a defined reference sound which is given an arbitrary
rating of 100. It should be mentioned that no restriction on
number values was indicated to the listeners, but they were
just required to rate numerically a given stimulus, so that the
numerical difference between such stimulus and the refer-
ence sound (modulus ¼ 100) reflects any perceived or sub-
jective differences in sensation. In each case, the reference
sounds were as described in Sec. II B above.
For each test condition, the reference sound was pre-
sented first (12.5 s) then the test comparison sound (12.5 s)
with a short 1 s gap in between. A ten second gap was
allowed for recording subjective judgments and then the
next reference and test comparison pair was presented and
so on. It should be mentioned that each test sound was pre-
sented, and judged by the listeners, only once. The order of
presentation was fully randomized between the different lis-
teners, and the overall duration of both tests was less than
20 min, including short time gaps between different test
sequences, thereby minimizing listener (and experimenter)
fatigue as far as possible.
The relative subjective magnitude estimations for loud-
ness and annoyance were normalized onto common scales
for test 1 and test 2 separately taking into account the three
different reference sounds in test 1 and the two different ref-
erence sounds in test 2 using Lawless’s method based on
logarithmic transformation.22
III. RESULTS
A. Test 1: LF subjective dominance
Figure 3 shows mean normalized subjective ratings for
loudness and annoyance for each of the 12 test conditions
included in test 1. The homogeneity between individual par-
ticipant’s subjective ratings was encouragingly high with
between-participant variability (measured as the ratio
between the inter-quartile range and the median value) less
than 0.31 in all cases (see Table II). As can be seen in Fig. 3,
the effects of applying the shelf filters (LF below 315 Hz and
MHF above 250 Hz) at þ6 and –6 dB on subjective loudness
and annoyance vary depending on the extent of LF domi-
nance in the overall frequency spectrum, i.e., þ10 dB
(T1.C1), þ20 dB (T1.C2), and þ30 dB (T1.C3). For each
case, increasing the LF shelf filter from –6 to þ6 dB has a
linearly increasing effect on both subjective loudness and
subjective annoyance with increasing LF dominance in the
overall frequency spectrum. However, increasing the MHF
shelf filter from –6 to þ6 dB does not have a similarly line-
arly increasing effect on subjective loudness and subjective
annoyance with increasing LF dominance in the overall fre-
quency spectrum. For the lesser amounts of LF dominance
in the reference sound (þ10 dB), subjective loudness and
subjective annoyance are both marginally more sensitive to
the MHF shelf filter than to the LF shelf filter. However, for
the greater amounts of LF dominance in the overall fre-
quency spectrum (þ30 dB), subjective loudness and subjec-
tive annoyance are marginally more sensitive to the LF shelf
filter setting than to the MHF shelf filter setting. This is con-
sistent with the hypothesis that under typical outdoor condi-
tions, subjective loudness and subjective annoyance are
generally less sensitive to changes in the relative amount of
LF content than to physically equivalent changes in the rela-
tive amount of MHF content, and only become similarly sen-
sitive under conditions where the relative dominance of LF
content within the overall frequency spectrum is much
greater than would normally occur under typical outdoor
conditions.
FIG. 3. Relative magnitude estimation (reference value ¼ 100) for each stimulus (À6 dB and þ6 dB in filter gain at LF and MHF) in the three conditions
(T1.C1, T1.C2, and T1.C3) tested in test 1, as to perceived loudness (left) and perceived annoyance (right). It should be noted that the range of values, as
reported by the listeners, was 43.9–219.4 for perceived loudness and 33.6–209.6 for perceived annoyance.

B. Test 2: Indoor conditions
Figure 4 shows the equivalent relationships for the two
indoor-filtered test conditions, with reference sounds at 49.5
(T2.C1) and 39.6 (T2.C2) dB(A), respectively. Figure 4
shows marginally greater increases in subjective annoyance
than subjective loudness for increasing amounts of both LF
and MHF content within the overall frequency spectrum.
Under the conditions tested, subjective loudness appears to
be equivalently sensitive to changes in LF and MHF content,
whereas subjective annoyance appears to be relatively more
sensitive to changes in LF content than to equivalent changes
in MHF content, particularly at the higher sound level. The
statistical significance of these observations is supported by
the multiple linear regression analyses and analysis of var-
iance (ANOVA) reported in Tables III and IV. Both statisti-
cal tests were conducted with the average responses of each
group of 3 listeners in the 11 sessions of the listening
experiment.
C. A-weighted scale for assessing sound exposure in
indoor environments
Figure 5 shows the linear regressions and correlation
coefficients for the relationships between mean normalized
subjective loudness and mean normalized subjective annoy-
ance for A-frequency weighted and C-frequency weighted
sound levels (12.5 s LAeq and LCeq) for each of the 16 test
conditions (8 test conditions for the 39.6 dB(A) indoor refer-
ence sound, and 8 test conditions for the 49.5 dB(A) indoor
reference sound). The C-weighting scale has already been
adopted as a standard descriptor for assessing low frequency
noise in some regulations in the Nordic Countries,23
mainly
because it is felt that the more conventionally used A-fre-
quency weighting downweights LF content excessively.
However, Fig. 5 shows that for these particular test condi-
tions, the A-frequency weighting achieves higher correla-
tions with mean normalized subjective loudness and mean
normalized subjective annoyance than the C-frequency
weighting even where the amount of LF dominance in the
overall frequency spectrum has been significantly increased
over and above normal outdoor conditions. For complete-
ness, Fig. 6 shows the differences in overall frequency spec-
tra between the eight test conditions of the test T2.C1 plotted
under A and C frequency weightings. It should be noted that
the differences in overall frequency spectra between the
eight test conditions plotted under A and C frequency
weighting are exactly the same for tests T2.C1 and T2.C2.
IV. DISCUSSION
The relative magnitude of the MHF content of the simu-
lated road traffic sounds tested in this laboratory study was
TABLE II. The inter-participant variability (ratio of the inter-quartile range to the median value) in responses as to perceived loudness and annoyance for each
experimental sound in test 1.
Stimulus
Perceived loudness Perceived annoyance
T1.C1 T1.C2 T1.C3 T1.C1 T1.C2 T1.C3
(LF À MHF
¼ þ 10 dB)
(LF À MHF
¼ þ 20 dB)
(LF À MHF
¼ þ 30 dB)
(LF À MHF
¼ þ 10 dB)
(LF À MHF
¼ þ 20 dB)
(LF À MHF
¼ þ 30 dB)
LF À 6 dB 0.09 0.13 0.22 0.14 0.18 0.26
LF þ 6 dB 0.20 0.23 0.23 0.20 0.17 0.20
MHF – 6 dB 0.21 0.31 0.18 0.26 0.25 0.17
MHF þ 6 dB 0.13 0.20 0.16 0.17 0.25 0.14
FIG. 4. Relative magnitude estimation (reference value ¼ 100) of perceived loudness (left) and perceived annoyance (right) with variations of À9 dB, À3 dB,
þ3 dB, and þ9 dB in filter gain at LF and MHF, for the two conditions (T2.C1 and T2.C2) tested in test 2. It should be noted that the range of values, as
reported by the listeners, was 25.2–220.7 for perceived loudness and 17.5–242.7 for perceived annoyance.

TABLE IV. One-way ANOVA results (mean square and F-value) for testing the statistical significance of the effect of the filter gain setting in each frequency
range on perceived loudness and perceived annoyance for each condition tested in test 2.
T2.C1 T2.C2 T2.C1 T2.C2
[Reference
[Reference
[Reference
[Reference
LF Between group 3301.71 2215.89 8951.54 5310.43
Within group 67.65 95.13 159.28 298.43
F 48.81a
23.29a
56.20a
17.79a
MHF Between group 3174.36 3514.28 2730.64 2332.92
Within group 75.85 103.08 152.42 117.25
F 41.85a
34.09a
17.92a
19.90a
a
p 0.01.
FIG. 5. Linear relationship between the A-weighted sound levels (᭺) and the C-weighted sound levels (ٗ) and both perceived loudness (left) and perceived
annoyance (right), in test T2.C1 (upper) and test T2.C2 (lower). Filled symbols correspond to MHF filter gain and unfilled symbols to LF filter gain.
TABLE III. Results (r2
and b values) of the linear regression analysis (N ¼ 44) for estimating perceived annoyance and perceived loudness from the filter gain
setting in each frequency range for each condition tested in test 2.
T2.C1 T2.C2 T2.C1 T2.C2
[Reference
[Reference
[Reference
[Reference
LF r2
0.762 0.633 0.795 0.569
Standardized coefficient (b) 0.873a
0.796a
0.892a
0.754a
MHF r2
0.758 0.705 0.548 0.579
Standardized coefficient (b) 0.871a
0.839a
0.740a
0.761a
a
p 0.01.

found in all cases investigated to have at least an equal or
greater effect on subjective loudness than the relative magni-
tude of the LF content, except only when the LF content was
around 25 to 30 dB higher than the MHF content (test T1.C3
and test T2.C1). Under these LF dominant test conditions,
the relative magnitude of the LF content was slightly more
influential on reported loudness than the relative magnitude
of the MHF content. This finding is generally consistent with
the standard equal loudness contours for pure tones even
though typical urban road traffic noise is qualitatively very
different from the pure tones used to derive equal loudness
contours in the first place.3,13
A different pattern of results was observed for subjec-
tive annoyance. In test 1, the relative magnitude of the LF
content had a greater effect on subjective annoyance than the
relative magnitude of the MHF content only when the LF
content was around 30 dB higher than the MHF content (test
T1.C3). In test 2, the relative magnitude of the LF content
had a greater relative effect on subjective annoyance for the
higher indoor sound level (T2.C1) at 49.5 dB(A) reference
condition than at the lower indoor sound level (T2.C2) at
39.6 dB(A) reference condition. This finding appears to be
generally consistent with the research of Nakamura and
Tokita on low frequency noise thresholds.24
Note that
Nakamura and Tokita24
proposed different threshold curves
for different subjective attributes (detection, annoyance,
displeasure, oppressiveness, and vibration) associated with
the sound pressure level in the LF range.25
Torija and Flindell3
found that under typical road traffic
noise (as measured outdoors), loudness reported by the par-
ticipants was similarly affected by variations in the relative
amounts of LF and MHF content, while reported annoyance
was dominated by the MHF region. Our research hypothesis,
that when MHF content is subjectively important, variations
in LF content tend to be irrelevant in explaining perceived
annoyance, appears to be supported by both sets of data.
Even when LF content is physically dominant, such as under
typical indoor conditions, it is not necessarily any more
significant for subjective annoyance.
A common criticism of the A-frequency weighting scale
is that it underestimates the effect of LF content on subjec-
tive annoyance26
or, alternatively, that it overestimates the
effect of mid and high frequency content.17
For example,
Persson et al.,27
in a listening test using mainly low fre-
quency sounds, found a higher correlation between sound
levels and subjective annoyance when using dB(lin) rather
than dB(A). However, in the listening tests presented in this
paper which use broad-band road traffic sounds with the rel-
ative prominence of LF content increased in the same way as
occurs in real-life indoor situations, and at A-frequency
weighted sound levels representative of real-life indoor
situations, the correlations between A-frequency weighted
FIG. 6. Frequency spectra of the reference sound and the À9 dB, À3 dB, þ3 dB, and þ9 dB filter gain settings for LF (left) and MHF (right), using the A-
weighting scale (upper) and the C-weighing scale (lower).

sound levels and subjective annoyance were higher than
between C-frequency weighted sound levels and subjective
annoyance.
It is interesting to note that, because the subjective effect
of low frequency content in road traffic noise was evaluated
using listening tests carried out under laboratory conditions,
phenomena such as rattle or vibration were not considered.
Under field conditions, LF noise might induce secondary
emissions of light architectural elements of buildings (e.g.,
rattling windows) and of household paraphernalia (e.g., mir-
rors or pictures).28
Such indoor secondary emissions, related
to rattle and vibration phenomena, have been pointed out as
significant contributors of annoyance caused by transporta-
tion noise in residences.28–31
For this reason, in field studies
and where rattle and vibration caused by low frequency
sounds are significant factors, the C-frequency weighting
might have higher correlations with subjective annoyance
than in this paper. On the other hand, in many situations
where rattle and vibration are, or have been, identified as a
cause for complaint, it might reasonably be assumed the
problem might be more effectively addressed by attention to
the structure of the building than to the properties of the road
traffic passing by outside.
V. CONCLUSIONS
Based on previous subjective listening trials,3
we
hypothesized that the subjective importance of changes in
the relative frequency content of road traffic noise in differ-
ent parts of the auditory frequency range, such as occurs
when moving from outdoors to indoors, would be dependent
to at least some extent on the relative dominance (both
objective and subjective) of each part of the spectrum, di-
vided into low, mid, and high frequencies. In other words,
changes in any one part of the spectrum would be relatively
more significant for subjective annoyance if that part of the
spectrum was already subjectively dominant. From this point
of view, a key issue is measuring at what point in any overall
scale does any one part of the spectrum become subjectively
dominant.
In the first part of the study, which was aimed at identi-
fying the difference in sound levels at which low frequency
content becomes subjectively dominant over mid and high
frequency content, we found that differences of at least
þ30 dB between the low frequency and the mid/high fre-
quency content are needed for changes in low frequency
content to have as much effect on subjective annoyance as
equivalent changes in mid and high frequency content.
In the second part of the study, which was aimed at
investigating the relationship between loudness and annoy-
ance under conditions where low frequency content is rela-
tively more dominant such as indoors where mid and high
frequency content is reduced to a much greater extent than
low frequency content because of selective attenuation by
the building envelope, we found that changes in low fre-
quency content appeared to make smaller contributions to
subjective loudness and annoyance than might be inferred
from the implied objective or physical dominance of those
changes. The results suggested that commonly expressed
criticisms of the A-frequency weighting based on an
hypothesized excessive downweighting of the low frequency
content may be relatively unfounded in this application area.
ACKNOWLEDGMENTS
This work was funded by the University of Malaga and
the European Commission under the Agreement Grant No.
246550 of the seventh Framework Programme for R & D of
the EU, granted within the People Programme, Co-funding
of Regional, National and International Programmes
(COFUND), and the Ministerio de Economıa y
Competitividad (COFUND2013-40259).
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