706 MFXHANICAL DESIGN AND PACKAGING 



the reference sound pressure. The reference pressure usually used in 

 referring to pressure waves transmitted through air is 0.0002 microbar. 

 This sound pressure level provides a good reference since it is audible to less 

 than 1 per cent of the human population. The general equation for the 

 sound pressure level is: 



Sound pressure level (db) = 20 log]o/)2//>j (13-15) 



where p2 = rms induced or measured pressure, microbars 



p\ = reference pressure = 0.0002 microbar. 

 Some useful conversion factors are: 



1 microbar = 1 dyne/cm^ = 1.45 X 10-^ lb /inch^ 



Acoustic noise levels reaching the area housing the electronic equipment 

 are reduced by the atmosphere primarily by two mechanisms, absorption 

 and deflection. Absorption losses can result from many effects, but the 

 most important of these is the reduction by water vapor and oxygen gas in 

 the air of the sound energy generated by the source. The deflection losses 

 of major importance are caused by the scattering of the sound rays by air 

 gusts and the general turbulent condition of the air. 



The acoustic noise environment is responsible for erratic operation of 

 airborne equipment, especially tubes and certain types of relays and 

 accelerometers. High-frequency resonance of tube grid supports and relay 

 contact arms is due to acoustic noise. Sound pressure levels between 110 

 and 140 db appear to be critical. Frequencies below 500 cps are serious for 

 tubes. Some frequency bands existing above 1000 cps cause resonance on 

 parts of relays, resulting in unsatisfactory performance of the relay. Besides 

 the malfunctioning of electronic equipments caused by this environment, 

 structural failure of the support structure can occur, especially at riveted 

 and bolted locations. 



Two methods of approach can be taken to provide satisfactory operation 

 of airborne electronic equipment subjected to the acoustic noise environ- 

 ment. Control of the sound pressure level at the equipment can be accom- 

 plished by sound absorption or sound isolation techniques. The use of 

 mechanical and electrical components not sensitive to this environment 

 provides a second approach. 



Transistors, magnetic amplifiers, and ceramic tubes are examples of 

 components that can be subjected to high sound-energy levels before 

 malfunction occurs. Improvements in fabrication techniques of the support 

 chassis for electrical components will result in increasing the resistance to 

 acoustic noise. For example, the use of material bonding techniques in 

 place of riveted and bolted assemblies appears to be a satisfactory method 

 of fabrication. 



