Review: Carolina Audio SM4 Folded Transmission Line Speaker

Category: Speakers

My interest in speaker driver loading and related performance led to an evaluation of Transmission Line Speaker Enclosures, most specifically the recent new model SM4 now available at . This course of action is in large part attributable to the 12-30-03 Report
1072830036, by Bombaywalla with useful clarifications and follow-up comments by others. I recently began a search for background information to initiate an upgrade on my home music system and found the seemingly dated review by Bombaywalla to be informative, inspirational, and totally relevant to my current endeavors, in fact still very accurate. Like many, over the years I had obtained custom built multi driver speakers while perusing a broad frequency range, rich, and fully blown sound production entertainment system. Given my passion for organ music, included in my now dismantled system were 4 towers with 12 inch drivers serving as stereo subwoofers. While indeed I had achieved high quality broad frequency range sound production I felt as though modifications could be accomplished to improve the ability of the system to properly "image" the program material and perhaps add some spunk to the "timbre" in sound thus creating more of a sense of "realism" such as experienced in live productions. Our relocation to the North Carolina Cost provided the opportunity to start from scratch.

The "side-by-side" multi system evaluation in Atlanta caught my attention and was significant since immediate feedback on multiple speaker-amplifier combinations along with keen and selective listening ears, provided much in the way of insight. Now having enjoyed the sounds at Carolina Audio, I feel as though I can add little to improve on Bombaywalla's description of the sound as being "unbelievable". I am paying homage to his findings and the fact that others pointed out the availability in additional manufactured broad frequency range drivers. Moreover, I would also like to add to the discussion by elaborating a bit on what I perceive to be the significance of the Transmission Line Speaker enclosures which I believe play a major role in the success of many now popular single driver speaker systems. I decided to peruse investigation on Transmission Line Speaker Enclosures based on the single drivers in preparation for my pending system replacement. Having studied other related findings on the internet and made several telephone calls, I scheduled a visit to the South Carolina location based on consideration for logistics and convenience, however, the major influence was the consensus in recommendations sighting Carolina Audio as producing première transmission line technology. While I am relating this experience as a vehicle to discuss the effectiveness of the transmission Line Enclosures this is not to suggest exclusion of other manufactured brand speaker systems.

My visit to Carolina Audio was intense however very enjoyable since we listened to many sound tracts on multiple CD's. Initially getting over the jaw dropping experience of such great sound required a bit of time for transition on my part. I then proceeded to select sound tracts with passages containing a variety in sounds known to be soft and loud, and low pitched and high pitched, to evaluate the audio response and bandwidth. I immediately was impressed by the imaging in sound production (wide stage) and the natural realistic sound emulations. I very quickly found myself beginning to focus on the ability to identify the location for musicians and vocalists. I experienced the tingling in my stomach one often experiences at live music productions although no subwoofers were in place to produce the really low 16 to 35 HZ tones.

Later in the afternoon I capitalized on my first real experience with transmission enclosures through the ability to listen to, to touch and ask many detailed questions and get honest, candid, and understandable answers, which indeed was a treat. I began to properly and more thoroughly comprehend the term "loading a driver" as being a non-simplistic however important aspect in great sound production. While being hesitant to delve into the complexities of technical explanations I will relate my impressions on Transmission Line enclosures. In a nut shell I discovered that the transmission line works on the principle of the wavelength in sound and functions to "support" (load) the moving driver cone with a "cushion" of air that is not static but "dynamic" and carefully tuned to stabilize the production of resulting sound into a more linear and even flowing pure tone output. This concept is often misunderstood and underappreciated such as in my case before this informative visit. I suspect that this audio phenomenon might hint in part to the rationale for such strong and positive reactions to the Transmission Line Speakers (some in the report were not transmission lines) by Bombaywalla and others. I had the pleasure of observing this phenomena on multiple speakers including the new Carolina Audio "SM4" 4 inch enclosure based on the MARK Audio ALPAIR-10 Driver.

My current appreciation and understanding for the fulfilling sounds produced by these single driver enclosures is very similar to the ability of a small dainty violin to produce humbling subsonic bass overtones simply by combining a sound source and properly designed and "tuned resonant chamber". My belief is that the truly representative dynamics possible with audio drivers can be enabled effectively through the use of properly tuned speaker enclosures such as the audio transmission line. While this is not to suggest the transmission line as being the only style to properly load a speaker, it sure succeeded in getting my attention. Among the sound tracks were selections from Fredrick Fennells' Pomp & Pipes which includes several passages with tones down to the 16 Hz frequency range. I found it to be amazing that the drivers never really broke up and continued to function although many dB's down in sound level and not necessarily appreciably discernable. My point is not really the sound quality at this frequency but the inherent stability exhibited by the 4 inch speaker driver cone. Unlike the Carolina demonstration system, one might normally filter out or channel such frequencies to a sub-bass system.

A more thorough understanding on the transmission line enclosure relates to the manufacturing process and the builder. The term artists comes to mind as with the famous and passionate violin makers. Ronald Thackeray "Ronnie" owner and operator, not only relates with all of the buzz words like resonance, open air frequency, and the like, he has obviously became proficient in constructional materials, the effect of finishing's and coverings, the impact of system geometry etc., and has an excellent taste for styles in speaker design. I learned that while basic construction produced the critical speaker system framework the real work is the fine tuning and system refinement to produce the truly natural and compelling sound. This to me is the stage that separates many speaker systems. With the speaker system being the final and ultimate gateway in sound production, the functionality for all other component selections can fall pray to the inability to translate the high quality signals into fine sound productions.

I am totally convinced that careful examination in the successful single driver speakers will reveal the presence of unique and successful methods to accomplish precise and dynamic speaker driver loading. To draw conclusion in this report however, I elect to resort to another important fact of life in one's willingness to pay for real quality products. In this instance I made an on the spot and unilateral decision to take home the SM4' s as an interim approach to my next entertainment system. Now speaking candidly and truthfully, I feel very confident and qualified to independently make such decisions in procurement, "as long as my wife agrees and is happy with the results"!!!!! Upon returning home I cobbled together a system with my electronics from the dismantled components and put on one of her favorite musical groups. Her reactions were immediate and very positive. In fact during our 25 plus years in marriage I have seldom if ever been as successful in pleasing the "better half" with one of my projects. Moreover, our house is built around a kitchen-family combination great room on the order of 450 square feet in floor space with a 20 foot ceiling feeding into other rooms at the second floor level. My major concern was the ability of the SM4s' to sound pleasing and adequately serve the larger area. My biggest surprise was to stand in the rear room balcony and peer down on a very apparent projected sound stage effect.

My plans for the final system will take into account a method to extend the bottom end frequencies to the lower organ notes. I believe that this will require more than plugging in a subwoofer module. A Transmission Line Enclosure is required to complement the full bodied sound of the SM4s'. I should reemphasize that the Transmission Line Enclosures produces a wide bandwidth and extremely flat full bodied frequency spectrum with incredibly natural sounding timbre. Also as important is the lack of noticeable harmonic hi-fi like sounds, the program productions are extreamly clean. Any modification to drivers in the final system will be planned with care and methodically checked for proper phasing and timing with the SM4s'. All test configurations will be measured against the SM4s' running stand alone to assure proper image projection and timbre. It goes without saying that Transmission Line Enclosures will be a mainstay in the replacement system.

Associated gear
Belles XLM Stereo Preamplifier, Soundcraftsmen Model PM860 Power Amplifier,Sony CD Player

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