Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
through Icy Strait to Glacier Bay. An average of 2 - 3 cruise ships per day
plus about 8 - 10 ferries per week pass through most of the inside deep water
routes. In addition to this traffic, barges and cargo vessels also use these
routes between the major Southeast Alaska cities and Seattle. Specific source
information is given in Table 5.1.
5.2 Sound Exposure Modeling
Computer-implemented models have been designed to help assess the potential
environmental impact of diverse tvoes of noise sources on the many
species of marine mammals found in ~laskan waters. The Standardized ~bise
Contribution Model (SNC) has been developed to provide a means of comparing
the acoustic energy contributions from all types of sources. The output of
this SNC Model is a logarithmically-scaled number proportional to the acoustic
energy density produced by a specific type of source operating in a defined
reference area. This SNC value is used together with information on hearing
characteristics and population density as an input to a Standardized Exposure
Rating Model (SER) to rate potential response of a specific species to noise
exposure. This SER Model is designed to evaluate the degree of potential
impact of a specific source on a specific species by producing a
logarithmically-scaled number proportional to the degree of matching between a
noise source output bandwidth and a species hearing sensitivity characteristic.
The SNC values for the important sources in specific OCS reference areas
were used in deriving SER ratings for the species within'the areas. The
resulting SER values serve as a means of ranking the potential for an acoustic
interaction between specific sources and species. The procedure used in
developing the SNC and SER models is summarized in the following discussion.
5.2.1 The standardized'noise contribution model
The model uses a spreadsheet format to facilitate data entry, application
of transmission loss information, and estimation of standardized noise spectra
. for a wide range of sources. The procedure involves selection of siterepresentative
source types, transmission paths, source temporal patterns, and
source spatial distributions - including those of moving sources. The basic
concept for the procedure has been developed from industrial noise modeling
procedures used for human population centers. It is based on the concept of
the equivalent sound level for a time-varying or moving acoustic source. This
concept was discassed in Section 3.1.2. The equivalent sound level, Le , is
the constant sound level which produces the same acoustic-energy exposu$e dose
as the actual time-varying sound field.
For prediction of human response to noise, a total exposure period of
eight hours is used to determine the average effective sound level of a
fluctuating or intermittent noise source. This corresponds to the general
period of working or sleeping activity. For marine mammals a shorter period
of time is appropriate since they are not as constrained to a specific location
as humans. The appropriate time period is difficult to determine. Few
data are available on responses of marine mammals to repeated or ongoing
exposure to sounds that, at least initially, cause behavioral responses.
Moreover, the exposure period probably varies for different species and may