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Have a pair of these Radio Shack 40-1284a found in poorly constructed enclosures. Does anyone have the Thiele-Small parameters for them?Here's what I know. They are 5" full range, paper cone with whizzer. Magnet is 10 oz. Factory specs show free air frequency of 90 Hz, Qts of 0.8, and sensitivity of 96 db, so these are definitely different from the 1354 discussed often in the context of TQWT. No other parameters are given.
What would be the best usage for these, sealed box, vented, or horn? Would they be suitable for TQWT?
Any experience or input appreciated!
John
Follow Ups:
John,I recently picked one up for $1.97. Just too tempting to pass up. I measured the T-S parameters with a Parts Express Woofer Tester. Here is what I came up with:
Qt 0.98
Qe 1.42
Qm 3.12
Fs 103 Hz
Vas 0.15 cf
Re 6.4 ohms
Le 0.36 mH
Sensitivity 87 db/w/mThis driver was not broken in. Fs will probably drop 10 - 15 Hz, and Qt will probably decrease to the 0.8 to 0.9 range. I would consider a small aperiodic enclosure. You will still end up with a midbass peak but it will probably sound pretty good with proper positioning, like the bass from the LS 3/5a. There should do well in a small room.
Regards,
AstroSonic
Thanks, AstroSonic, this is what I needed!I am not familiar with the design rules for aperiodic enclosures. Is there any reading material on the web you would recommend? Have you made anything using your driver yet?
John
John,
There is very little information on the design of aperiodic enclosures. They have been used and/or recommended by credible companies including Scan Speak, Morel and Dynaudio. I have heard the Dynaudio and Morel designs and they were remarkable for their bass extension and definition. Also for their rather modest efficiency. The Dynaco A-25 was an aperiodic (resistive reflex) design also. Most sources are lean on actual design procedures, but allude to an emperical method that goes something like choose a box size you can live with and make an adjustable opening. Adjust the opening and the amount of flow resistance until it sounds good. I have tried a few with quite good results. I use this design when the driver T-S parameters fall outside the range for reflex and sealed boxes (Qt greater than about 0.7). My method is to model it as a leaky reflex enclosure by changing the Q of the enclosure (normally set to 7), going for maximum extension and minimum (but not flat, which is usually not possible or in some cases desirable) peaking. I tried this for the RS 1284s and found the best result was for a 0.75 cf box tuned to 63 hz, for an F3 of 67 Hz. The tuning adds very little because the box Q was reduced to 1.6 - very lossy. The box size and leaky/lossy character are pretty similar to a compact car door! No surprise. This driver was probably designed for that kind of service. The modeled performance is nearly as good in a 0.3 cf box tuned to 63 Hz (0.9-inch diameter hole with 0.75-inch length: drill a hole in 3/4-inch plywood) with a box Q of 2.0. Just add some dacron in the hole 'to taste'. The model shows about 3 db of peaking in the upper bass. Used alone (full range,) this should give a very pleasant, well balanced sound. This latter design is what I will try after the current speaker project is finished. Just a caution that I have not tried this yet and you never really know until it is built. Good luck and have fun.
AstroSonic
Thanks AstroSonic for your analysis. Recognize that I am very new at this, and have played around a little with the WinISD box model, so I have just enough knowledge to be dangerous!I entered the parameters and changed the Q to 1.6 instead of 7 to simulate aperiodic, and got an f3 of 67 Hz as you said. However, it would seem that the straight vented case (Q=7) would give significantly more bass extension, although it would have a bigger hump in the upper bass by about an addional 2 db. Maybe I'm missing something in another area, or using the model incorrectly, but wouldn't this additional bass extension be desirable?
John,
You will get more bass without the vent damping, but it will have poor definition (loud, but without impact, pitch and definition). A somewhat simplified explanation is as follows: The driver is incapable of controlling the helmholtz resonance formed by the cabinet-enclosed air (acts as a spring) and the vent (acts as an air mass). Without the vent damping you get a double peak in the bass: one in the upper bass and one in the mid-bass. The driver and vent contribute to each peak (because this is realy a coupled resonant system with the driver/amplifier providing all the energy). The driver is the dominant contributor for the upper peak while the vent is dominant for the lower peak. Without vent damping, the driver provides nearly all the damping of both the cabinet resonance and its own (formed by the cone mass and suspension/spider compliance). The driver is too underdamped (Qt is too high) to control the cabinet resonance. Therefore damping is added to the vent to damp that resonant system. It also damps the upper peak by turning some of the drivers acoustic energy into heat. The interaction of the driver and cabinet are more complex than is portrayed above, and we are fortunate to have computer programs that do a pretty good job of modeling this.If you build one of these you can try it with a full range of conditions, from open vent, through increasing amounts of damping, to packed with damping material so tightly that it is sealed. You will find this project very instructive! Enjoy.
AstroSonic
Thanks, AstroSonic for a great explanation!I have a 0.63 cu ft enclosure which needs only a baffle board and plan to do a little experimenting. Would that be big enough? Could you make a suggestion for vent size? Would you use any cabinet stuffing?
John
John,
If you use a 0.63 cf box try tuning to 54 Hz (1.5 x 1.5 inches cutout, by 3/4 inches deep). You will get a little peaking (+ 1.5 db @ 52 Hz and + 3 db @ 115 Hz) but it should sound pretty good with an F3 of about 47 hz. For a small room this could be quite a nice set of speakers. However, in most rooms if you play loud music with a lot of bass they will run out of linear excursion (Xmax). Assuming an Xmax of 2.5 mm, they will take around 6 watts, for a nearfield sound level of around 90 db down to about 50 Hz. Note that they could be played more loudly in a smaller box with a higher F3. If you try it please let me know how it turns out. Good luck and enjoy.
AstroSonic
Thanks, I'll try it!John
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