Page 31 - January 2007

to 88 dBA Leq, 30 min, associated with traffic noise in living
and studying environments in Hong Kong have been report-
42
ed. While being immersed in the I-5 noise field normal
voice communication is clearly impacted such that one must either shout or use a loud voice, and be generally closer, to communicate effectively with another person. This is evident by the three black curves in Fig. 3, which are the third-octave band spectra for human speech at range 1 m from the speak- er’s mouth, for normal, loud, and shouting vocal efforts
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some simple ways to quantify speech intelligibility using A-
weighted measures. For such a quantification the A-weighted
spectral density of the signal and the noise must be roughly
similar (in shape), and this is approximately the case for the
highway noise and loud vocal effort. The corresponding A-
weighted broad band SPL for the loud voice is 74 dBA, which
is not significantly less than the unweighted SPL owing to the
shift to higher frequencies associated with the increase in
vocal effort (Fig. 3). The four of us at the I-5 site carried out
strained conversation at a position ~3 m further offset from
where the measurements were made, which would put the
traffic noise level at this point equal to 85 dBA. A minimum
signal-to-noise ratio equal to negative 6 dB is needed for ade-
quate intelligibility according to the criterion that 60% of the
sentences are correctly identified in the presence of back-
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fic noise level.
For reference, Fig. 3 also shows a spectrum of background
noise measured outside a residential home in northeast Seattle at 5:00 p.m. on the Saturday evening following the I-5 meas- urements (by the first author and under similar meteorological conditions as the I-5 measurements), and of background noise from Hermit Basin in Grand Canyon National Park,45 repre- senting an extremely quiet environmental noise background in air. For the residential case there remains a hint of the din of lower-frequency engine noise from I-5, located 4 km to the west, and perhaps from Interstate-405, located 6 km to the east, but otherwise the higher frequency tire noise has been largely attenuated, and the total SPL is 60 dB re 20 μPa, which corresponds to 46 dBA. This dBA value is typical of that rep- resenting a quiet residential environment2 and the high-pass filtering influence of A-weighting is significant given that the noise contribution is predominately from frequencies < 1000 Hz. For Hermit Basin, the total SPL is 27 dB re 20 μPa, which corresponds to 17 dBA.
Our simple demonstration of a noise impact insofar as the approximate quantification of reduced speech intelligibil- ity and subsequent adaptation (raised voices and closer range), is straightforward enough to do given the ANSI stan- dard plus direct measurements of the noise environment in effect. Our intention is not to imply this quantification is equally straightforward in the case of underwater noise and marine mammals. Nor do we indicate this kind of impact represents a perfect analogy, e.g., in addition to communica- tion impacts, the underwater ambient noise field can impact a number of important marine mammal functions, e.g., feed-
according to ANSI standard 3.5.
In the first issue of Acoustics Today, Long44 discusses
To obtain adequate intelligibility, we needed to close our speaking distance to about 1/2 m, putting the loud voice SPL closer to 80 dBA, and within 6 dB of the traf-
ground noise.
Fig. 4. A-weighting expressed in decibels as a function of frequency.
hearing sensitivity in this frequency band, and a small gain is applied to frequencies between 1 and 5 kHz that corresponds to the frequency range of greatest hearing sensitivity. Thus, the engine noise spectral peak would not usually show up in an A-weighted traffic spectrum owing to the ~20 dB attenu- ation applied in this frequency range.
The ordinate in Fig. 3 is labeled “third-octave band sound pressure level (dB),” meaning the values represent the mean-square pressure output from each of the sound level meter’s third-octave filter bands, expressed in dB re 20 μPa. This is to be contrasted with the pressure spectral density of Fig. 2, a display more common to the underwater acoustics community. Third-octave filtering, however, is a method common to both air and underwater noise analysis, and orig- inates from studies on human hearing for which the ear can be viewed as a series of band pass filters (called critical bands) about a third octave wide. The bandwidth is a con- stant fraction of the band’s center frequency (about 23% for third-octave bands), e.g., at the center frequency of 1000 Hz the bandwidth is 230 Hz. Thus, for the third-octave meas- urement data the measurement bandwidth is increasing with frequency, and were the data in Fig. 3 to be expressed in terms of a pressure spectral density both spectral level and spectral slope would change. To obtain the total (broad band) SPL, i.e., one that includes contributions from all frequencies measured, the sum of the third-octave band levels (SPLi) is taken as follows,
(5)
which gives 92 dB re 20 μPa for this location. The correspon- ding A-weighted value, for which the truck noise is effectively removed, is 88 dBA. (Given the averaging period of 10 min, this number would thus be commonly be reported as 88 dBA, Leq,10 min for A-weighted, equivalent continuous noise level.)
The noise field at the I-5 location would exceed typical upper bounds of tolerable community noise. More represen- tative of traffic noise impacting living areas is the spectrum measured in a residential area close to a busy road (prior to construction of a noise barrier)41 that is very similar in shape to the I-5 spectrum in Fig. 3 but with spectral levels 18-20 dB less. On the other hand, measured broad band noise levels up
Underwater Ambient Noise 29