The acoustic radiation field close to the speaker or some other acoustic source.
"Wow! Very impressed with my first experience with your company. I had purchased a Laserdisc from eBay of the legendary harpsichordist Scott Ross, then considered a few businesses that do conversion. Yours was the only one not to say 'sorry, pal, we don't do PAL' (in fact, I found your website saying you welcome both NTSC and PAL Laserdiscs). ..."