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Technologies > The RSLA System
Allan So The RSLA System - Simple, Rugged and Cost Effective

Article by Allan Sohl

Allan Sohl has been with PAS for almost 25 years and is credited with developing Time Offset Correction™ as well as designing PAS speaker systems.


A bit of history

Photo 1. A 10 cabinet array
- Photo Gallery -
PAS has been producing coaxial systems since the early 80’s. These early systems used low frequency drivers with 3 inch voice coils and one inch compression drivers. They were passive only and contained relatively complex dividing networks that incorporated time correction (see TOC™ sidebar) and equalization. These systems were known for their accuracy in reproducing transient material such as percussion and piano. They were also known for smooth response when the listener is off axis or not directly in front. In the mid 80’s, PAS began producing active systems as well. With the introduction of a four inch voice coil, two inch throat coaxial driver in 1989, PAS began to increase it’s focus on higher performance systems.

Time Offset Correction™ electronically moves the low frequency device so that it
radiates from the same plane as the high frequency device. Thus the mid-high component is not only coaxial, it could be said to be coplanar. The results of this are twofold:

Transient accuracy. The high frequency and low frequency components of a sound are combined with the correct temporal relationship and the correct polarity. Acoustic guitar, piano and percussion benefit greatly from this.

Consistent off-axis response. Since both LF and HF drivers appear to radiate not only from the same point in space, but from the same plane, there is no angular error to cause increasing amounts of comb filtering as the listener gets further off-axis.

"We need a full range, single box system that can be used without subwoofers."

Single coaxial systems are fine for studio monitors, floor wedges or in conjunction with subwoofers, but our customers were asking for something with enough low end to be used without them. The solution was to add a supplemental 15" driver to which the high frequencies are rolled off. Using simple low pass filter prevents the lobing that would occur if it were simply in parallel with the low frequency section of the coaxial.

The version of this cabinet that uses a two inch compression driver (the RS-2.2) was introduced in 1995. It is only two inches wider than the two 15” drivers it contained and only five inches taller. Since one of the requirements was that it be tuned below 40 Hz, the cabinet had to be 24” deep. Though its shape was initially cosmetic, the width of the back along with the angle and the bracing of the sides results in much less radiation to the rear than a rectangular shape.

When large systems are made with these cabinets, the angle needed between them can be five degrees or less. Considerable high frequency interference will occur unless the horns are rotated 90 degrees. So effective is this reduction in interference that an array of eight cabinets produces very smooth response across the included angle of all the cabinets. However, the response outside of that included angle is reduced. This is exactly what occurs in a line array.

Figure 1 shows the horizontal off-axis response of an RSLA cabinet in 10 degree increments to 50 degrees. These are ground plane measurements done at 10 meters. (The level shown is not corrected for distance.) Clearly the response is consistent to 30 degrees for a total of 60 degrees. We are not judging this by looking at the -6 dB points. Instead, we simply recognize that the response deteriorates beyond 30 degrees.
FIGURE 1. Horizontal off-axis response of an RSLA cabinet to 50 degrees. Enlarge
Using this to our advantage...

Photo 2. RSLA arrays
rig quickly
- Rigging the system -
Since there was overwhelming evidence that suspending RS cabinets as a line array would result in greatly improved performance, PAS has developed a system to do this. Instead of a complex system of welded tubing and pivots, cams or hinges, the RSLA cabinet uses sections of aluminum alloy track attached to internal steel plates with structural screws and captive nuts. These parts are located in channels cut into the 25 mm Baltic Birch cabinet ends using a CNC router. A connecting plate is used between cabinets. (See Photo 3.) This plate attaches to the cabinets with quick release ball-lock pins and contains holes for suspension, pivoting and fixing the angle between the cabinets.

Since all of the system weight is carried by the structure used for suspension, arrays of up 16 cabinets may be used without requiring that the top cabinet carry the full weight of the rest.

Coaxial Loudspeakers:
Why use a horn? Most manufacturers don't.
Of the manufacturers of coaxial loudspeakers, PAS is one of the few that uses a horn for the high frequencies. Most use the diaphragm of the low frequency section as a "wave guide". Although this has the advantage of having no discontinuity at the front of the wave guide, it has some significant disadvantages.

Speakers of this type have a very broad circular pattern. This is fine for a ceiling speaker but it allows the high frequencies to go everywhere, including where it is not needed or wanted. Much power is wasted because of this.

This wasted power results in lower apparent sensitivity. A coaxial of this type with a three inch high frequency diaphragm may have a sensitivity of 105 or 106 dB (1w/1m). The same compression driver when used on a 90 x 40 degree horn will be at least 3 dB greater. When used on a 60 x 40 degree horn it will be at least 3 dB greater still. The result is 6 dB greater SPL for the same distortion level. Saying this another way: 6 dB greater headroom.

The Important Questions...

PHOTO 3. Connecting plate in place, showing ball-lock pins and holes for angle adjustment.
PHOTO 4. Rigging case in which the connecting plates and pins are stored when being transported.
How does the RSLA sound?
We think that the RSLA system sounds like a big RS-2.2. This cabinet along with the RS-1.2 (which uses a passive dividing network) are our most popular systems and make up a significant percentage of PAS' sales. We were very impressed with its performance as a line array system.

Is the RSLA system really a line array?
At low frequencies where directivity control would be difficult without large horns, the RSLA system has the impact which one would expect from a line array.

At higher frequencies where the coaxial horn is used, the question is really: does it do what it is supposed to do? Users who had previously needed cabinets for downfill have found that those extra cabinets are not necessary when they use an RSLA system. At 200 feet the high end is clear and "in your face" and has much the same balance as at half that distance.

When measured outdoors without subs, the average level 50 feet
behind a four cabinet array is 20 dB below the average level 50 feet in

Is an RSLA system really cost effective?
The RSLA is a biamplified cabinet. It becomes a triamplified system when subwoofers are used. Only two-thirds as many amplifiers are needed than for many systems. For example, three, two channel amplifiers are required for eight cabinets. Two of these should be capable of at least 1600 watts into 2 ohms per channel. The third need only be capable of 400 watts into 2 ohms per channel. Although a 2" compression driver rarely sees more than 5 to 10 watts (6 to 9 volts rms.), it is best to have at least 10 dB of head room.

Since the RSLA is a two-way cabinet, only four conductor cables are needed (rather than six or eight conductors with other systems).

An RSLA cabinet is 17 1/2" high, 24" deep, 36" wide and weighs 143 pounds. It is manufactured from 15 mm Baltic Birch except for the ends which are 25 mm. It is supplied with a castor plate that is secured to the cabinet with linch pins.

As we mentioned earlier the RSLA does not use a complex system of welded tubing and pivots, cams or hinges. Each array requires the frame from which it is suspended, the top connecting plates with their hardware and side connecting plates with their ball-lock pins.

We at PAS believe that the RSLA System is more than a cost effective alternative. It is a serious contender at a reasonable price.
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