Acoustic Considerations Involved in Steady State 

 Loud Speaker Measurements 



By L. G. BOSTWICK 



Synopsis: Certain difficulties encountered in acoustic measurements of 

 the performanceof loud speakers are described. Because of the nature of 

 these difficulties it has not yet been possible to specify a complete and simple 

 set of measurements or conditions which will completely express the per- 

 formance of a loud speaker. Data are given showing the performance of 

 two representative types of loud speakers both when measured in outdoor 

 space free from reflections and when measured under varying conditions in a 

 specially treated acoustic laboratory. The diflterences serve to emphasize 

 the importance of certain precautions in the making of indoor acoustic 

 measurements. 



Scope 



IN view of the general misconception of the meaning of many claims 

 which are made regarding the operation of loud speakers, it appears 

 desirable to discuss in some detail the requirements which should be 

 taken into account in making measurements for setting up ratings of 

 loud speaker performance. For example, claims to "uniform response 

 at all frequencies in the audible range" or "flat characteristic" can not 

 be accurately applied to loud speakers which have thus far been made 

 available. In many cases the claims for a loud speaker are based upon 

 carefully made electrical measurements but these are often obtained in 

 such a manner and under such conditions that they do not represent 

 the performance of the loud speaker as it would be normally observed, 

 and therefore are misleading. The main consideration in making loud 

 speaker measurements is not the electrical circuit arrangement or 

 apparatus of the measuring system but rather the acoustic conditions 

 under which the magnitude of the sound output of the loud speaker is 

 determined. It is the purpose of this paper to discuss steady state 

 loud speaker measurements particularly from the standpoint of the 

 more important acoustic factors which are involved and which must be 

 properly considered in order to be able to interpret the significance of 

 any measurements obtained. 



Loud Speaker Performance Indices and Factors Involved in 

 Their Determination 



Efficiency. — In power engineering and other branches of engi- 

 neering, efficiency (a measure of the degree to which a device performs 

 the functions for which it is designed) is defined as the ratio of the 

 power delivered to a load to the power absorbed from a source of 



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