630 BELL SYSTEM TECHNICAL JOURNAL 



in which the distribution of levels in this group of 25 tubes is compared 

 with that of the total group as tested on the apparatus rack. The 

 noisiest tubes in the selected group are from 6 to 8 db quieter than the 

 noisiest tubes of the unselected group, and in the selected group, 

 there are none of the tubes which make up the worst 18 per cent of the 

 whole group. 



In several commercial situations where microphonic disturbance 

 was at one time troublesome, this type of selection has proved to be of 

 practical value. In these situations, the number of quiet tubes re- 

 quired is only a small percentage of the manufactured output. Fur- 

 thermore, only a small percentage of the normal output of tubes are 

 found to be prohibitively noisy. Under such circumstances, it is 

 found that when selected tubes from the quietest 25 per cent of the 

 manufacturers stock are used in the positions most sensitive to mechan- 

 ical shock, the disturbance in these types of equipment either disap- 

 pears entirely or recedes to such a level that it is no longer troublesome. 



Protection from Shock 



Where selection of quieter tubes is not feasible or is not sufficiently 

 effective, further reduction of microphonic noise may be achieved by 

 protecting the tube from mechanical and acoustic shock. A very 

 efficient agency for protection from mechanical shock is a well-designed 

 cushion socket. The effectiveness of such a socket depends on its 

 vibrational transmission characteristics considered in relation to the 

 response characteristics of the tubes used. Considerable improvement 

 is usually obtained, however, whatever the combination of tube type 

 and socket type. Figure 9 shows two typical cases of microphonic 

 improvement obtained by using one of several good types of cushion 

 socket which have been tested. The curves drawn in solid lines repre- 

 sent the distributions of microphonic noise levels of a group of No. 

 102F Tubes tested in one instance in a rigid socket, and in the other 

 in a cushion socket. The mean improvement here due to the cushion 

 socket, is about 30 db. The dotted curves represent similar tests 

 made on a group of No. 262A Tubes and show a mean improvement of 

 about 18 db. 



In cases where the noise must be reduced to very low levels, it may 

 not be sufficient to protect the tube from disturbances transmitted 

 mechanically through its base and socket. Except in a perfectly quiet 

 location, there is always some disturbance produced by sound waves 

 impinging directly on the bulb of the tube. Ordinarily this disturbance 

 is negligible, but where the base is sufficiently well cushioned, it may 

 be of controlling importance. It can be reduced only by reducing 



