Meanwhile, this paragraph from the paper: "A more effective but expensive option is to optimise the impedance match between the driver and the acoustic load. I

Meanwhile, this paragraph from the paper: "A more effective but expensive option is to optimise the impedance match between the driver and the acoustic load. In 1940 Terman [4] patented a sound- absorbing apparatus that absorbs the backside radiation from a loudspeaker using a horn. As depicted in Figure 2, the principle is to continuously adapt the impedance from the back of the driver to the end of the cabinet in order to prevent any reflection. Despite its effectiveness, better than a simple pipe as used in the KEF Carlton, this technology suffers from several drawbacks in addition to the cost and a limited control of the wadding acoustical properties. The most obvious is the size: to work correctly, horns should be very long." Turned into the Nautaloss: https://www.diyaudio.com/community/threads/the-nautaloss-ref-monitor.247598/ https://www.diyaudio.com/community/threads/the-nautaloss-ref... which is small (because the horn is curled up), cheap (because it's made of foamcore board), and really good at absorbing high frequencies (foamcore material properties). Key paragraph from the writeup: "This speaker can be constructed from exactly 1 sheet of $1 foam core stock and the Vifa driver costs $10. These are not the most efficient drivers but at 85 dB for 2.83V input, they are not bad for near field use. The frequency response flatness, the low THD, the linear phase variations, and the near ideal impulse response make them perhaps one of the best values for a near field "monitor" from 200 Hz to 20 kHz." And of course, ideas from the 1940s are no longer covered by patents.