VR 2 Measurements and Manufacturer Comments

Mr. DellaSala,

Thanks for your magazine's interest in our products, including the VR-2 hybrid transmission line design Mr. Stein reviewed. I have enlosed a Manufacturer's Comment on the review and wish to thank you and Mr. Stein for the chance to show our products to your readers.

• Non resonant sound, i.e. "no one-note bass reflex boom."

• Flattened impedance peaks, i.e. linear impedance transfer from the amp to driver.

• No ringing in the transient response.

Using a computer simulation at Cal Tech, it appeared that the folded T-line we built from the Wireless World article could be simulated by a shorter path length, IF the right woofer with suitable Theile/Small (electrical and mechanical characteristics) could be found or built. Based on experiments conducted over 2 years, we determined that the main sonic feature of a transmission line speaker is the tight, controlled bass, combined with added frequency extension not possible with a sealed box. Hollow or understuffed bass reflex cabinets enable "ringing" due to the hollow cavity resonance and lack of mechanical resistance to damp the woofer's rear wave oscillation.

Our "triple chamber hybrid" is a combination of both bass reflex design, which specifies a particular "alignment" between the woofer, enclosure, and vent, and a tuned pipe T-line. The tuned pipe design suffers from lack of efficiency and also lack of deep bass extension unless the pipe is made very large, which would "kill" it in the marketplace. As everyone knows, bass reflex speakers often sound tubby, soft, and have a prominence of excess power in the midbass region. The trick is to combine the best qualities of both speaker designs, which requires me to develop the following technology:

I designed the two VR-2 woofers with very specific and unusual Theile/Small parameters, so that the system resonance appeared at 25Hz in the short line; this was accomplished by adjusting the free air resonance of the driver (Fs), the mechanical and electrical Q's (Qes and Qms) and finally, the suspension compliance (Vas). These T/S parameters are adjusted by varying the cone weight, voice coil resistance, magnet weight, flux density in the gap, stiffness of the spider and surround edge, etc. Note that these characteristics need to be balanced to each other, and that it's difficult to accomplish without a large lab and prototype facility. Our cabinet design will NOT work with off the shelf woofers!

With the suitable woofer design, we now need to develop a specific line length, stuffing material and density, and proper exit port location and tuning, as follows:

a. Line Length: This is also intertwined with the necessary volume of air mass required to resonate at the desired frequency to achieve the target bass response. We determined that 25 Hz was the practical limit for 2 - 6.5" woofers at a reasonable volume level with low distortion. The result was the 40" tall tower with 16" depth and 8" width, with an internal air volume of 1.85 cubic feet. This air volume was determined by a computer simulation using the Theile/Small parameters of the custom woofers.

Note that a labyrinth design uses a very long line path in order to simulate an organ pipe's resonant behaviour and does not require a specific woofer design, i.e., it can operate with almost any off the shelf woofer; the penalty for this universality is the extreme size and complication, which renders it less marketable and less unique to the efficiency of size versus cost. In other words, anyone can build a textbook T-line if the size, cost, and complexity is not an issue.

b. Stuffing Material and Density: Due to the high cost of wool, along with the difficulty of suspending it uniformly in the line, we utilize Dacron polyester in sheet blanket form. Several layers are used to develop the required thickness and density, and this in turn allows us to "tune" the air space cavity behind the woofers to absorb the rear wave at the required level of resistive damping. This results in a "triple chamber" design:

1. the air space behind the upper midrange/midbass driver, which operates from 25 hz to 2200 Hz; this cavity needs to be really stuffed in order to absorb the rear wave in the midrange frequency band, for the best vocal clarity.
2. the airspace behind the lower woofer, which operates from 25 Hz to 200 Hz; this cavity is not stuffed as tightly, as we don't feed midrange frequencies to this driver. We want a higher Q of resonance at low frequencies and thus don't need as much resistive damping at this point.
3. the combined airspace of the above two chambers, along with the bottom portion of the cabinet. This is the chamber where the exit port is located, and requires even less stuffing, so that the port is not blocked. In current production, we are mechanically fixing the Dacron so that it will not block the port. 

NOTE: In my hybrid design, the exit port cannot be located at one end of the line; I experimented to find the correct location by "mapping" the response inside the cabinet by placing a small measurement mic INSIDE the cabinet at various points. Also note that the Q of the vent has been modified by experimenting with the length, diameter, and foam lining; all of these factors are simulated by computer, with final tests verified on the prototype. Also please note that none of these methods are described in text books, they are proprietary to my research over the years!

c. Kevin sent you a response measurement taken in our highly damped measurement/design room. That graph shows the entire response with an MLS measurement taken at one meter; this graph shows the typical dips and peaks caused by reflections from the boundaries. We include this graph to show the overall room response, which is quite flat!

The second graph is a nearfield response taken without the room reflections, i.e. it is anechoic. Note that this clearly shows the VR-2 going down to the 25 Hz response

VR 2 Near Field Measurements in Design Room (Click Picture to Enlarge)

VR 2 Near Field Measurements without room reflections (Click Picture to Enlarge)

 

VR 2 Impedance Plots
(Click Picture to Enlarge)

Real Time Analysis (RTA) from the Sencore SP295C Audio Analyzer ( updated 4/12/04 )

Figure 1. Quasi Nearfield (1 meter) 1/12 th Octave FFT Frequency Response

The overall frequency response of the VR-2 is uniformly flat and smooth with perhaps a slight emphasis in the highs. However, some of the bumps and dips in this response graph may be partially attributed to room acoustics especially since the room treatment is most effective in the mid bass to upper midrange area where the response is extremely smooth and flat, but de-emphasized slightly due to absorption.

Figure 2. Quasi Nearfield (1 meter) 1/12 th Octave Low FFT Frequency Response

The VR-2's exhibited impressive bass response. However, since they seemed to be a bit energenic in the lower bass region (27Hz - 40Hz), I recommend placing them a few feet away from side and back walls to avoid excessive boundary gain. It appears the VR-2's had no problem reaching down to 25Hz and then some.

Figure 3. Listening Position 1/12 th Octave FFT Frequency Response

The VR-2's appeared to integrate well within our sound room with impressive low frequency bass response, smooth midrange and upper frequencies. I initially positioned the speakers with no toe-in (as in this measurement), thinking the slightly elevated high frequencies would best be minimized. However, it was apparent that they rolled off very smoothly off axis. If a little more energy at the high frequencies is desired, I recommend slight toe-in.