STEADY STATE LOUD SPEAKER MEASUREMENTS 143 



transmitted through this position may be constant. Response 

 measurements with the condenser transmitter at any one position can 

 therefore mean very little under such conditions. 



To our knowledge it is practically possible only by working outdoors 

 under very particular conditions to obtain a medium sufficiently free 

 from reflections to make suitable response measurements at all fre- 

 quencies with the condenser transmitter located at any one position. 

 By using a room with all dimensions very large compared to the 

 distance between the condenser transmitter and the loud speaker 

 (which distance is determined by the size of the loud speaker and the 

 highest measuring frequency as discussed above) and covering the 

 walls with sound absorbing material, it is possible to reduce the 

 reflected energy at the transmitter position to a small value over a 

 considerable frequency range but any practical method of reducing the 

 reflected sound to a negligible value at all frequencies of interest in loud 

 speaker measurements is as yet not available. 



In a plane standing wave system the energy density at points of 



maximum pressure or minimum velocity is equal to — :, , where p is the 



pc- 



r.m.s. pressure at these points. The locations of these maximum 



pressure points change with frequency but if the position of the 



condenser transmitter in loud speaker response measurements is 



changed at each measuring frequency to a maximum pressure point 



within a suitable region the indicated pressure will be a measure of the 



energy transmitted through this region. The measured response 



would then be approximately the same as would be measured in a 



reflectionless medium except for a magnitude diff"erence due to the 



addition of the reflected energy. Such a procedure for loud speaker 



response measurements indoors would thus be suitable if it were not 



for the fact that the standing wave system in the room is usually of a 



very complicated configuration in three dimensions instead of being 



simple. The probability of being able to locate a position in any 



desired region of the sound field of a loud speaker where the pressures of 



each of the standing waves which may traverse this position are a 



maximum, is obviously remote. 



A method of response measurement making use of the mean square 



pressure instead of the maximum value is more practicable. With a 



single frequency sinusoidal sound source, the pressure-space distribution 



of each of the standing waves in a room is likewise sinusoidal. The 



maximum r.m.s. pressure squared of each standing wave would then be 



twice the mean r.m.s. pressure squared over a half wave-length or any 



multiple of a half wave-length ; also approximately twice the mean over 



