64 Lecture 3 
from the cylindrical spreading law right out tothe extreme 40-mile range. How- 
ever, at the higher frequencies there is additional attenuation. Figure 3.8 illus- 
trates depth dependence in a North Sea area and shows the second stage of pre- 
sentation when transmission loss itself is plotted. This is an improvement, but 
it is recommended that in suchcases the assumed source level always be quoted. 
(Figure 3.8 assumed source levels and depth dependence based on Weston [10]; 
see also Table 3.III.) It may be seen that the transmission loss increases as 
either boundary is approached, because each boundary acts as a free surface. 
This is true for the bottom because the measurements were made in August 
when there is apparently a large quantity of gas held there. The details of the 
curve shapes may be explained as due to the addition of a number of modes. In 
August there is a strong thermocline at 15 fathoms. Thus the lowest modes 
are trapped below the thermocline and tend to produce a signal maximum at 
about 25 fathoms: some intermediate modes produce a maximum above the 
thermocline near 7 fathoms, and the higher modes show little depth dependence. 
It is possible to make the arguments for Figs. 3.7 and 3.8 quantitative, and learn 
about bottom character, etc. 
3.8. RELATIVE ADVANTAGES OF UNDERWATER EXPLOSION SOURCES 
The competitor to the underwater explosion source isthe projector radiating 
continuous waves. Comparative measurements of transmission loss have been 
made at various times using projectors and explosives, and when done carefully 
they agree on average. However, a pure-tone continuous wave transmission will 
often show very large fluctuations due to interference effects, which are smoothed 
out when using the larger bandwidth in the charge experiment. Thus, to study 
average transmission loss one should use shots or a projector radiating band- 
limited noise; whereas to study fluctuations one would probably use a pure tone 
from a projector. 
In summary, most measurements may be made witheither source. There are 
many measurements that can only be made witha continuous wave source. There 
are a few measurements which virtually can only be made with explosions; for 
example, to study low-level arrivals one needs the very high pulse power of an 
explosion. In practice, experimental convenience plays a big part; a shot needs 
no auxiliary equipment and is almost unrestricted in depth. 
In passing, I wantto comment on Dr. Schofield's [7] remarks on the necessary 
sizes of projectors for reasonable efficiency, and to consider production of 
acoustic energy at 1000 cps. Here the electroacoustic device needs to be about 2 
ft across, whereas a spherical charge of only about 2 in. in diameter (say, of lb 
TNT) converts about half its chemical energy into acoustic energy centered at 
1000 cps. This high efficiency at low frequencies is only possible because of the 
finite amplitude effects near the source. I could have stretched my case further 
since the same 2-in. charge would have a bubble-pulse frequency of the order 
of 20 cps, but this would be cheating since the 20 cps is really associated with 
the maximum bubble diameter of a few feet. 
In conclusion, to be well equipped for research one needs to be able to use 
both continuous wave and explosion sources, depending on the experiment. 
Personally, I consider this likely to be the position for a long time to come. 
