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Isolated Transient Signals

Reference 1 proposes uniform sound level conventions for both continuous sources and transient sources. Continuous sources, in electrical engineering terminology, emit what are called power signals, i.e., they continually radiate energy and are best described in terms of the time rate of energy released, power [4].

Sources emitting isolated transient sounds, however, are best described as producing energy signals. Such signals are most easily characterized directly in terms of energy, since the integral tex2html_wrap_inline133 is finite. The integral tex2html_wrap_inline133 is infinite for a continuous source. Ref. 4 carefully states that an ``energy signal has zero power, and a power signal has infinite energy." Because transient sources produce zero average sound power, a useful level descriptor for such sources would depend on energy only, not power or intensity.

At first glance one might adopt a convention for transient sources underwater similarly to that used in air. This is the sound exposure level


where tex2html_wrap_inline137 is a suitable reference pressure, tex2html_wrap_inline139 is a reference time (often 1 s), and E is the sound exposure


The difficulty here is that the sound exposure level depends on a specific reference pressure, which would be different for the outdoor and underwater environments.

A better convention for isolated transient sources, similar to those proposed for continuous sources in Ref. 1, would be a sound source energy level (SSEL) defined as


where W is the total energy radiated by the sound source and tex2html_wrap_inline145 is taken as 1 J. This convention would be useful for legitimately comparing source strengths in the air or water. Perhaps initially unfamiliar, this notation already exists in a current ANSI standard [5].

It is true that to completely describe a transient sound source that more information is required than just the radiated energy, such as spectral content, rise time, etc. However, as a lowest common denominator using the SSEL will allow for quick and fair comparisons between similar airborne and waterborne sound sources.

Typical man made underwater transient sounds include those of explosions and penetrating sonic booms [6], which are also familiar airborne sounds. For example an underwater explosion source might be characterized as Sound Source Energy Level, 60 dB re 1 J, (Energy = 1 M J) .

Since a sound source energy level is easily expressed in dB, this standard convention seems superior to that suggested in Ref. 1, i.e., the use of a ``intensity of a transient tex2html_wrap_inline147 " which is not a standard notation. If one uses the new SSEL convention, it is similarly true to Ref. 1 that ``a separate standard for air and water hardly is necessary."

next up previous
Next: Conclusions Up: Comments on ``Standard Definitions Previous: Suggested Convention for Sound

Fri Aug 30 16:31:47 EDT 1996