CHAPTER VII 

 DIRECT RADIATOR LOUD SPEAKERS 



7.1. Introduction. — A loud speaker is an electroacoustic transducer 

 designed to radiate acoustic energy into a room or open air. There 

 are two general types of loud speakers in use to-day, namely: the direct 

 radiator and the horn type loud speaker. The diaphragm of the direct 

 radiator loud speaker is coupled directly to the air. The diaphragm 

 of the horn loud speaker is coupled to the air by means of a horn. 

 The direct radiator loud speaker will be considered in this chapter and 

 the horn loud speaker will be considered in the following chapter. 



The almost universal use of the direct radiator loud speaker is due to 

 the simplicity of construction, small space requirements, and the relatively 

 uniform response characteristic. Uniform response over a moderate fre- 

 quency band may be obtained with any simple direct radiator dynamic 

 loud speaker. However, reproduction over a wide frequency range is 

 restricted by practical limitations. The two extreme ends of the audio 

 frequency band are the most difficult to reproduce with efficiency com- 

 parable to that of the mid-audio frequency range. Inefficiency at the low 

 frequencies is primarily due to the small radiation resistance. There are 

 a number of means available for increasing the radiation resistance at the 

 low frequencies. A large radiation resistance may be obtained by using 

 a large cone. A phase inverter consisting of a completelv enclosed cabinet 

 with ports provides a means tor extending the low frequency range. A 

 horn may be used for presenting a large radiation resistance to a dia- 

 phragm, at the low frequencies. The efficiencv of a direct radiator loud 

 speaker at the high frequencies is limited by the mass reactance of the 

 vibrating system. There are a number of arrangements suitable for re- 

 ducing the mass of the vibrating system at the high frequencies. Two or 

 more separate loud speaker mechanisms may be used, each designed to 

 reproduce a certain portion of the range. Multiple cones driven by a 

 single voice coil may be arranged so that the mass of the system decreases 

 at the high frequencies. The voice coil may be sectionalized to decrease 

 the mass and inductance at the high frequencies and thereby increase the 

 high frequency range. Multiple coils and multiple cones combined into 



111 



