In the distant past, 40 years ago, the Japanese company Sony released the APM (Acurate Pistonic Motion) loudspeaker series, using technologies that many considered the technologies of the future. Having measured the tweeter of the Sony APM-33 speakers, let's go back from today's future to the past to see if they were really that cool.
Above are the measurements of the on-axis (in between the tweeter and the woofer) axial frequency response at a distance of 1 meter at a voltage of 2.83 Volts. The response smoothing is 1/6 octave.
The THD measurements at voltage levels 4 - 5.6 Volt were carried out with High Pass Filter "on" (2nd order Butterworth type, 200 Hz cutoff, HPF2-200)