Technical White Papers

RT25i Bookshelf Loudspeaker Press Brief

Our aim in developing the RT25i loudspeaker was to create a bookshelf speaker system with the sonic attributes of the best examples of the genre: superior linearity, definition, and imaging, at a price well within reach of most consumers. The low frequency extension of the speaker had to be sufficient to make it satisfying as a stand-alone speaker while keeping the enclosure size compact enough to actually fit on a stand, bookshelf or hang on a wall. We also sought to design a speaker which could handle equally well the performance demands of both music and video sources.

Enclosure

The walls of the RT25i enclosure are fabricated from medium density fiberboard (MDF). The front and rear baffles are injection molded mineral-filled polymer, braced and ribbed to minimize panel resonance. The raised bezel helps to minimize grille edge diffraction. This speaker sounds better with the grille on than off. (NOTE: Please do all frequency response measurements with grille ON)

Maple, Cherry and Black real wood finishes are available and sell for about the same price as vinyl-clad models from other vendors.

The 5-1/4" mid/woofer driver in the RT25i uses Dynamic Balance technology (see "Technology section below) to minimize resonance for more accurate sound reproduction. The driver is magnetically shielded by means of a second magnet and a steel cup.

The 1" dome tweeter also uses Dynamic Balance technology for the reduction of resonance. The dome itself is "Tri-Laminate": micro-layers of stainless steel and aluminum vapor deposited on a soft polymer material. This arrangement provides the stiffness of metal with the damping characteristics of soft materials. The voice coil uses a bent former which provide 10 times the contact area with the dome than a traditional knife edge connection between the voice coil former and dome. Our laser interferometry studies indicate that this unique geometry extends high frequency response and further reduces resonance on the surface of the dome.

The tweeter dome in the RT25i is new for 2000, with a reformulated polymer base material that yields smoother response and lower odd order distortion than previous incarnations of this tweeter design.

The RT25i is crossed over at approximately 3 kHz. Both high and low pass filters are second order. A small value Mylar bypass cap across the main (non-polar electrolytic) cap in the high-pass section improves high frequency extension.

The port on the rear of the enclosure is a Power Port bass vent (see Technology section below). The two small ports on the front baffle are Acoustic Resonance Control ports that serve to flatten midrange response (see Technology section below).

The RT25i uses three proprietary Polk technologies:

• Dynamic Balance
• Power Port™
• Acoustic Resonance Control (ARC) ports

Dynamic Balance - Beginning in 1988, Polk Audio began a long-term research project in partnership with the Johns Hopkins University. The result was a full-field heterodyning laser interferometer system capable of revealing the microscopic details of entire vibrating surfaces (such as a loudspeaker cone) in real-time. This research tool enabled us to view and analyze the entire vibrating surface of a driver or tweeter. In particular, we were able to see and understand the resonance that develops on loudspeaker cones as they move. Modal resonance is a major cause of frequency response aberrations and other distortions in loudspeakers.

Our new understanding of how resonance develops on speaker components led us to develop a design technique called Dynamic Balance. Dynamic Balance uses an analysis of the entire Electro-acoustic and mechanical systems to select composite materials and geometry that reduce resonance. The resulting drive units have unusual clarity, detail and extension.

Power Portä :

The Power Port solves the fundamental problem of conventional port systems… air turbulence. Air from inside the cabinet moves through the small port at very high velocity. When it exits the port into the room, the abrupt transition from the small diameter port to the room creates turbulence at the mouth of the port. In its worst form, this turbulence creates audible "port noise", heard as a "chuffing" sound. Also, up to 3dB of bass is lost because of the turbulence.

To reduce this turbulence, the Power Port consists of a "cone" at the mouth of the port and the space in between the rear of the cabinet and the port cone. The cone promotes laminar airflow and the ever-increasing cross-sectional area of the space mimics a long flared port. The result is reduced turbulence at the mouth of the port for better bass efficiency (about a 2dB gain), and reduced distortion.

On the RT25i, the Power Port plinth doubles as a wall mount device.

Conventional rectangular box speakers often impart a "thick" or "chesty" quality to vocals and other midrange sounds. This coloration is partly a function of a resonance associated with the internal depth of the cabinet. This "box resonance" creates both frequency and time response errors that audibly degrade the midrange clarity and imaging performance of the speaker.

Polk Audio has developed a patented method for dealing with this problem using the principle of destructive interference. In the Polk Acoustic Resonance Control method the small ports on the front of the speaker are tuned to resonate at the same frequency as the cabinet's internal depth resonance. As the radiation of the ARC ports are out of phase with respect to the driver output, the offending resonant peak is suppressed.

The audible results are greater midrange clarity and detail with noticeably more natural and accurate reproduction of vocals. Imaging is also markedly improved, the speaker exhibits superior localization stability and image depth.

Click here for complete RT25i specifications