160 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



Table V 



Measured longitudinal velocity and attenuation at 25°C and 2.5 mc. 



Longitudinal attenuation in DB/cm at 25°C and 2.5 mc except as noted. 



Material 



Dural, 17 ST 



Brass, half hard 



Polystyrene . . . 



Plexiglas 



Tenite II, (cellulose acetate butyrate), 2% 



plasticizer 



Tenite II, 13% plasticizer 



Poly vinal formal 



Polyvinlidene chloride 



Poly N-butyl methacrylate 



Poly I-butyl methacrylate 



Neoprene 



Polyethylene 



Nylon 6-6 (3-30 megacycles) 



Nylon 6-10 (3-30 megacycles) 



Density 



2.7 



1.05 



1.18 



1.23 

 1.21 

 1.24 

 1.71 

 1.05 

 1.05 

 .99 

 .90 

 1.11 

 1.11 



surface. By determining the spacing of these lines the velocity of the 

 shear waves can be determined. 



Another method has also been developed which is more applicable for 

 high loss materials. This is a pulsing method and is a modification of the 

 method proposed by one of the authors for measuring the properties 

 of small crystal specimens." Here longitudinal or shear crystals are sol- 

 dered to the fused quartz rod as shown by Fig. 30 and a sample to be 

 measured is placed between these by means of a liquid such as poly- 

 isobutylene which has a high shear elasticity. If the specimen has a 

 small attenuation, this can be measured by taking the difference in the 

 amplitude of successive reflections. If the specimen has a high loss, this 

 does not work and another method has been used which consists in 

 sending a pulse from both crystals.^^ One crystal is then used to receive 

 and it receives the wave sent through the sample and the wave reflected 

 from the fused quartz-sample interface. By adjusting the amplitude 

 until these two are equal and the frequency or phase of one channel 

 until the waves cancel, a ratio of amplitudes and a frequency of half 

 wavelength are accurately determined. From these the velocity and 

 attenuation can be calculated. 



This method has been applied to measuring the longitudinal and shear 

 velocities of polyethylene and 6-6 nylon. The polyethylene was of "equi- 

 librium" crystallinity and average molecular weight corresponding to 



" H. J. McSkimin, "Ultrasonic Measurement Techniques Applicable to Small 

 Solid Specimens," J. Acoust. Soc. Am., 22, No. 4, July 1950, pp. 413-418. 

 16 H. J. McSkimin, J. Acoust. Soc. Am., 23, No. 4, pp. 429-435. 



