It’s 2005 and the phone rings. Luthier and guitar tech Sammy Sanchez, on the road with singer-songwriter James Taylor, is on the other end. “JT has some cent offsets he’d like to program into his Peterson StroboStomp” says Sammy, “Is there a way to do that?”
There was a way, and that’s how the ACU Sweetener for acoustic guitars came into being. Apparently, JT sometimes played small coffee house gigs where having a guitar tech was not an option. Retuning his guitar himself between capo changes – while still managing to chat with the audience before segueing into the next tune – was often impossible, so he had to figure out a set of tuning offsets that progressively flattened the open strings as they went up in gauge and down in pitch. When a capo was applied, the strings were then pulled into tune, instead of beyond and ending up sharp, as they would when tuning without the Sweetener. Strangely, the flatness of the strings was not noticeable when playing without a capo.
Another welcome side-effect was that first position chords like G and D did not clash anymore like they sometimes did when tuned “unsweetened” in Equal temperament. This set of offsets resulted in the ACU Sweetener. Its first appearance as a factory preset was in the Peterson StroboFlipTM tuner and it’s been standard in every Peterson tuner since.
We’ve since heard from people who own valuable vintage acoustic guitars with intonation flaws who say that ACU is all they use when tuning, regardless of capo use. Having an alternative to Equal temperament as a preset is faster than tweaking by ear and more consistent. Since major guitar surgery is sometimes detrimental to the value of a vintage acoustic, the ACU Sweetener makes it possible to enjoy previously unplayable guitars that would otherwise languish in their cases. The guitar pictured above is a one hundred year-old parlor guitar. Its bridge is slightly off true and its saddle is an uncompensated length of fret wire, yet it plays much better in ACU than in standard Equal Temperament.
Current Peterson tuners featuring the ACU preset include: