834 
34 IN Ve 
thin, tight winding of high grade rubber tape over a thin 
layer of rubber cement such as Bostik 292 (BB Chemical 
Company, Cambridge, Massachusetts). 
Where the cable or other mounting is rigid (as in the 
case of copper tube cables), successful coatings have 
been obtained by dipping the unit several times into a 
molten mineral wax (such as Zophar Mills C276) and 
allowing a thin wax coating to be built up. 
Successful, but fairly bulky coatings have been ob- 
tained by cold-molding of Thiokol (Minnesota Mining 
and Manufacturing Company). Molding in plastics has 
Fic. 2. Stages of construction of single-ended tourmaline blast 
gauges. 
1—Raw tourmaline crystal. Cuts are made perpen- 
dicular to the principal (Z) axis. 
2—Slab of tourmaline before dicing. 
3—Dicing tool (periphery charged with diamond dust). 
4—Tourmaline slabs cemented to glass plate for dicing 
operation. 
5—Stacking of disks and central tab. (Silver electrodes 
have been applied.) 
6—Element after being sweated together in oven. 
7—High lead connecting intermediate pairs of faces. . 
8—Center conductor of cable. 
9—Brass tube. 
10—Special lead-sheathed microphone cable. 
11—Various sizes of gauges which have been constructed. 
Diameters, left to right : 13, 14, }, 4, and } in. (4- and 
3-in. gauges are used for underwater measurements). 
12—Central tab soldered to brass tube. 
13—Latex edge insulation. 
14—Painted silver shield providing contact between 
outer faces and ground. : 
been unsuccessful because of the brittleness and poor 
adhesion of thin layers of these substances. Molding in 
rubber has proved impractical because the accompany- 
ing high temperatures damage the gauge elements. 
In air or other gaseous media the coating serves an 
additional important purpose in that it provides a meas- 
ure of thermal insulation to the crystal element, thus 
ARO NS> AND RH. COLE 
preventing serious pyroelectric interference from changes 
in ambient temperature and changes due to temperature 
rise of the gas on adiabatic compression in the shock 
wave. 
V. TRANSIENT RESPONSE CHARACTERISTICS 
OF GAUGES 
A. High Frequency Response 
We shall first analyze the effect of the geometrical 
size of the gauge upon the fidelity of its peak pressure 
reading. 
(1) Frequency response. Assume a simplified ideal 
case in which the gauge is represented by a circular disk 
of radius a, and negligible thickness, and assume the 
gauge to be oriented edgewise to the direction of propa- 
gation of a sinusoidal standing wave Po cos(27x/d), 
where «x is the distance from an axis through the center 
of the gauge and J is the wave-length. 
If c is the velocity of propagation, it is readily shown, 
by integration over the surface of the gauge, that 
R (f)=2J,(2nfa/c)/(2xfa/c), (1) 
where f is the frequency of the standing wave, J:(2fa/c) 
is Bessel’s function of the first kind, index one, and 
R({) is response of the gauge, defined as the ratio of the 
actual maximum force it experiences to the ideal maxi- 
mum force given by the product of Po and the gauge 
area. R(f) is plotted as a function of af/c in Fig. 3. It is 
seen that the response falls to zero at the frequency 
fo = 0.61¢/, a. 
At higher frequencies the response undergoes suc- 
cessive reversals of phase and gradual decay. It should 
be noted that ordinary steady-state response measure- 
ments are insensitive to phase, and the negative portions 
of the curve would be measured as positive. This 
rectified form is indicated by the dashed curve of Fig. 3. 
As a numerical example, the response in water of a 
gauge 12.6 mm in diameter will be half its static value at 
a frequency of about 60 kc/sec. and zero at a frequency 
of about 100 kc/sec. 
A number of gauges were calibrated for the UERL 
group by the Columbia University Underwater Sound 
Reference Laboratories, the frequency-response curves 
being obtained at acoustic levels by using the gauge to 
pick up the signal of a standard projector previously 
_calibrated by means of the reciprocity principle. A 
comparison of the theoretical calculation with experi- 
mental results is given in Fig. 4, and it is seen that the 
two are in generally good agreement. The hump in the 
experimental record for Gauge 5A53 has the charac- 
teristic appearance of a mechanical resonance and is 
probably due to a cantilever vibration of this particular 
gauge on its 5-in. length of copper tube cable. 
(2) Step response. If a gauge is to be used for transient 
pressure measurements, it is desirable to analyze its 
performance in terms of its response to simple forms of 
traveling wave. The simplest useful case is that of a step 
