"The successor of NS15, Yamaha NS18 was produced between 1969-1976 - given its rarity on the second hand market, apparently only a very small series. NS18 is a dipole speaker with an odd looking asymmetrical 18 x 23 inch woofer/midrange driver with some very special cone material and ALNICO magnet system. Frequencies from 5 kHz up are handled by a bullet-type horn tweeter JA-05036B, the level of which can be adjusted with a three-step attenuater. The cross-over is of a second order, and some EQ is probably applied to 1-5 kHz. NS18’s claimed frequency response is 35-20000 Hz, and the sensitivity 100 dB. This latter figure turned out to be an overestimation, 95 dB being closer to the truth (granted that measuring the sensitivity of a dipole speaker is always tricky)."
More info here: http://www.thevintageknob.org/yamaha-JA-6002.html