Technical White Papers
RT15i Bookshelf Loudspeaker Press Brief
Design Goals
Our aim in developing the RT15i 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 bookshelf or hang on a wall. We also sought to design a speaker which could handle equally well the disparate performance demands of both music and video sources.
System Description
Enclosure
The walls and of the RT15i enclosure are fabricated from 3/4" 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.
Natural Cherry, Maple and Black wood veneers are the available finish options. The RT15i is unusual in that they feature real wood finishes at vinyl-clad prices.
Mid/Woofer Driver
The 5-1/4" mid/woofer driver in the RT15i 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.
Tweeter
The 3/4" dome tweeter also uses Dynamic Balance technology for the reduction of resonance. The dome is polymer treated silk, the voice coil is cooled by ferro-magnetic solution.
Crossover
The RT15i is crossed over at approximately 2.5 kHz. Both high and low pass filters are first order to minimize ringing and phase shift. A small value Mylar bypass cap across the main (non-polar electrolytic) cap in the high-pass section improves high frequency extension. The tweeter and driver are in-phase.
Venting
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).
Technologies
The RT15i 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 very long flared port. The result is reduced turbulence at the mouth of the port for better bass efficiency (about a 2dB gain), as well as reduced noise and distortion.
(see Audio, May 1996; The Amazing Shrinking Subwoofer by Matthew Polk)
On the RT15i, the Power Port plinth doubles as a wall mount device.
Acoustic Resonance Control
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 port on the front of the speaker is tuned to resonate at the same frequency as the cabinet's internal depth resonance. As the radiation of this second port is 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.