Understanding Zu-Griewe, The Griewe Gap, and Setting Up Your Speakers.
Most Zu loudspeakers are not sealed systems. They incorporate an acoustic technology that can be thought of as an acoustic impedance matching system that helps the Zu full range driver adapt to the room or space the speaker is placed in. It also quiets the cabinet. We call this passive room adapter "Zu-Griewe," after its concept progenitor, the late Ron Griewe and the Zu Audio adaptation for loudspeakers. For the technically-curious, details follow, but for everyone else, there are just a few principles to grasp, in order to set up your Zu speakers for best sound.
Today, the Zu-Griewe driver-to-room interface is through the bottom of the Zu speaker. Best use of a Zu speaker with Griewe requires an evenly-spaced gap between the bottom of the speaker and the floor. This is the Griewe Gap. Getting this gap right affects the performance of the speaker well into the midrange, but the consequences of getting it right or wrong are most obviously heard in bass performance. There are two floor scenarios to consider -- hard floors and rugs or carpet.
1/ Hard-surface bare floor, whether wood, tile, concrete, linoleum, vinyl or other types: The Griewe Gap setting is easy in this case. Adjust the four threaded stub feet in the base of your Zu speaker to level it, and attain a Griewe Gap of 3/8 inch, or 10mm all the way around. That reference gap will usually work perfectly. However, depending on your amplifier and a wide variety of room traits, you may adjust the bass character to your preferences by making incremental increases or decreases to the Griewe gap. It only takes small changes to the gap to hear audible consequences. Lowering the speaker to a gap height of 1/4 inch or 6-7mm, for example, tightens up bass that sounds too soft, flabby, fat or bloated at 3/8 inch. Raising the speaker to a gap of 1/2 inch or 12-13mm fills out bass that sounds too lean at 3/8 inch. You can trust your ears and try even taller or shorter gap heights. You can't harm the speaker in any way by experimenting. Strictly from a performance standpoint however, do not eliminate the Griewe gap by resting the speaker with its bottom surface flat against the floor. Conversely, if you raise the speaker for a gap of more than 1-1/2 inches or 38mm, you also begin losing the benefits of the Zu-Griewe scheme.
2/ Carpeted floor or rugs: When you don't have a bare, hard floor to precisely measure the Griewe Gap, setup is more complicated. Instead of uniform air velocity throughout the Zu-Griewe system, carpet, rugs, pads, underlayment, etc. add differences in material density affecting air motion non-uniformly. The pile of the carpet or rug is also a variable. Where exactly is the floor, from a Zu-Griewe air molecule perspective, to measure from? You have to experiment. On thinner rugs with minimal padding underneath, you can simply adjust the stub feet for a starting gap of 5/16 inch or 7-8mm. Most smartphones are about 7-8mm sans case. On thicker carpet with substantial padding you will need to use the optional carpet spikes to pierce through the soft layers and mechanically ground the speaker to the floor. Similarly, start with the same reference gap, as best you can measure it, and tune by ear up or down from there.
If you find working with a thick or high-pile carpet is too difficult, you also have the option of tilting the speakers back, slightly. This is a less-perfect use of the Zu-Griewe principle but it still yields a good result. To do this rest the rear bottom edge of the speaker cabinet on the rug or carpet, and use carpet spikes on the two front corners to set a front-edge gap height double the normal reference gap, so 5/8" inch or 15-16mm. To optimize your sound, raise or lower the front edge of the speaker cabinet to your sonic preferences.
If you are handy and ambitious, you can also set a precise Griewe Gap by building a pair of speaker platforms. Customers have used tile, butcher block, and plywood as a base. Install threaded inserts into the four corners of the platform, for your preferred size of carpet spikes, and level the platform close to the floor. Place each speaker on a platform and precisely set the Griewe Gap per the hard floor instructions above. For Zu-Griewe to work in this fashion, the platform must have dimensions at least the same as the base of the Zu speaker. But the platform may be made larger than the speaker's footprint, and doing so will further aid performance by the Zu-Griewe scheme. The advantages of a larger platform over carpet diminish rapidly at dimensions extending more than one foot or 305mm beyond the speaker's footprint, all the way around.
OK, Not So Hard, My Speakers are Set Up! Now I Want to Know More.
Zu-Griewe technology is used in the majority of Zu’s loudspeakers. Its concepts can be used in any loudspeaker where there are internal velocity changes. The technology concept was introduced in Zu’s very first loudspeaker, Druid. The Zu-Griewe technology is a multi-octave impedance modifying acoustic model that can be applied to any acoustic system with alternating velocities; electro-acoustic, electromechanical, internal combustion engines, and so on. It is developed exclusively by the late Ron Griewe and Sean Casey. The original concept is Ron Griewe’s, a motorcycle legend that had a nice bit of engineering insight as he sat inching through Los Angeles traffic one afternoon. (More about Ron Griewe below.)
Let’s outline a few basic principles expressed in the technology and contrast them with bass reflex (ported) designs, as well as transmission lines, pipes and horns. A bass reflex loudspeaker uses a simple Helmholtz resonator to augment lower frequencies via the air “spring” in the cabinet and the “mass” in the port, tuned to add amplitude response, and control cone motion. A Helmholtz resonator consists of a rigid-walled cavity (the volume) with a neck (“port”) with an area and length. The fluid (air) moves as a unit within the port tube to provide the mass element and the acoustic pressure within the loudspeaker box provides the stiffness or the spring element; the resistive element is provided by the opening that radiates the simple source sound. All Helmholtz resonators contain these basic elements, and all create sinusoidal waveforms but cannot recreate the complex and dynamic waveforms typical of music.
Zu-Griewe technology is expressed, on a fundamental level, like that of a waveguide with expanding acoustic cross section and terminated and driven at one end. Propagation within the Zu loudspeaker is mostly planer and standing waves are not as stimulated as in other similarly sized designs. Development of acoustic models accounting for drivers introduced dynamic variables within horns, pipes, and transmission lines that revealed areas of non-planer propagation. This turbulence (noise) is wavelength-relational and proportional to amplitude. Zu-Griewe can reduce many of these problems.
Applied to loudspeakers, this new technology reduces noise in varying degrees over the majority of the audible bandwidth. These new ideas in acoustic impedance transform function, and the designs realized are proprietary. The basic idea however is acoustic impedance matching of the high Z of the cone to the low acoustic Z of the room; reduction of non-planer propagation in more than just a single octave; reduction of internal cabinet standing waves.
Ron “Ogre” Griewe 1939–2005
The original concept as applied to exhaust systems for internal combustion reciprocating engines, is the brainchild of the late great Ron “Ogre” Griewe—two wheels and a fist full of throttle. Sean had the pleasure of working and riding with Ron back in the nineties at ATK motorcycles.
Ron was a well-known and well-respected character and professional in the motorcycle world. Known in his younger days for desert racing, always running a bit too close to the edge, but always with a grin and a cigar. Later, Ron made a significant contribution to bikers, street and dirt, in his writings, projects and leadership at Cycle World magazine. While there he rose to Editor-In-Chief, a position he held for 16 years. But the Ron that Sean knew was really an inventor and engineer, a guy that didn’t get his knowledge from a can. For him, leaving Cycle World—a biker's dream job getting paid to ride all kinds of bikes all over the globe—to drag ATK America out of being a custom one-off dirt bikes shop and into a real production-based American made dirt bike factory made perfect sense. Yeah, it was a while back when Ron and Sean tore it up together and only a season long, but Sean will never forget the time spent riding, talking and working with him. Ron turned 60 the month before Sean returned to audio; Ron was one fast mother, forget about age. Seriously fast, technical single track or super fast fire road. Riding the deserts of Southern California? Hit the Husky Monument.
https://www.latimes.com/business/autos/la-fi-hy-c1-husky-memorial-20160730-snap-story.html