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RE: Electro Voice 12TRX's need a new cabinet

Brandt

The 3dB down point spec was EV's attempt to define the realistic lower limit of the bass response of the driver in a specified vented box volume. This particular one was from the "How to Build an Electro-Voice Component Speaker System. From the ground up." booklet from the early 70's which featured the 12TRXC. EV was aware of the fact that they were then competing with other manufacturers who spec'd everything as "20 to 20K Hz" response. What this meant, in most cases back then, was that if you put your ear against the woofer with a 20 Hz signal playing at maximum power, then you may be able to detect some sound coming out of the woofer. EV was notable for trying to put specifications in a realistic perspective.

Open baffles are indeed easy to build, as you just need to cut a hole in a piece of plywood. BTW the 3dB down point for a flat baffle can be approximated as follows: F=4560/W, where W= the baffle width in inches and F= the 3 dB down point. This is simple enough if you use a square or round baffle, but the art of it comes into placing the driver in an asymmetric position on a rectangular baffle for instance, where several -3dB down points have to be integrated. This is not my department (I'm a horn guy), but I've heard some very surprisingly good open baffle designs. Anecdotal experiences from others can help you here. However, for really low bass the open baffle starts to get really big quickly as the above calculations will indicate: an 8 ft. square baffle will get you a -3 dB point at 47 Hz. A box is a much more space efficient way to go for most people.

The 12TRXC is a coaxial driver with a T35 horn tweeter mounted in the middle of an SP12C woofer. There are 2 kinds of box speakers: 1. The closed, or sealed, box where the acoustic springiness of the air confined inside the box is a major factor in complimenting the bass performance of the driver; 2. The vented, or ported, or bass reflex box where a vent, or tube, or rectangular tunnel is introduced into the box to resonate with the box volume at a frequency which extends the bass response lower than it would be in free air. In former times these two types became associated with (1.) an "east coast" sound (which was typified by closed box speakers which some considered had "tighter" bass which was considered better for classical music), and (2.) the bass reflex "west coast/JBL" sound which others felt had more powerful bass in the region which complimented the electric bass guitar, but was "looser", and had a steeper bass roll off even though it may have gone lower. In the present time this has all become a bit vague, and the choice of the cab type is more an issue of the intent of the designer limited by the box size. Anyway, to evaluate a driver for an optimal application, the following factor is useful: Effective Bandwidth Product (EBP). So Fs/Qes=EBP in Hz. A result of around 50 suggests a sealed box is appropriate; a result around 100 suggests a bass reflex box; results above 120+ suggest a horn application. Some drivers overlap these regions of course.

The EV Aristocrat is a rear loaded folded horn, in that the front of the woofer is a direct radiator, but the rear of the woofer (which is not enclosed in a box as the above examples) is actually a folded horn inside of a box which is intended to exhaust into a corner which will complete it's mouth, as the corner can act as a large horn mouth. A horn presents an acoustically resistant "load" to the driver. This is all much in the same way as if you cut the last 1/4 or so off of the length of a straight horn, and point it into a corner. An Aristocrat cab will certainly work if it's not in a corner position, but the bass will roll off at a higher frequency. Closed box and bass reflex configurations can also have their bass response affected by their proximity to a corner too, for better or worse.

For the 12TRXC EV recommended a box with a minimum of 2.93 cu. ft. (with a 57 Hz -3dB down point) up to a maximum of 23.3 cu. ft. (for a 23 Hz -3 dB), with a 5.85 cu ft. cab having the flattest response overall (all of these cabs having their appropriate tuned vent dimensions of course). So evaluate your Dad's cabs with this in mind. If you have minimal woodworking tools you may get by just modifying them. At least try them and see how they sound.

Good luck from the Electro-Voice Brotherhood

Paul


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  • RE: Electro Voice 12TRX's need a new cabinet - Paul Eizik 21:58:35 06/20/12 (0)

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