Usher UA721 Towers

[xsilverjag]

Just over a year ago, I picked up the Usher UA721 (in the project gallary) drivers and crossover on ebay used. I bought this kit because it was a good value and I was honestly scared to invest too much in a project that I might procrastinate or just not do a good job on the cabinet. Overall I am very happy with the speakers, they sound silky smooth and have a great low end. I knew I was more interested in doing a tower so I posted a help inquiry in the tech talk forum and Paul K graciously agreed to help and used Martin King's software to do the modeling.


Using 3/4 MDF the cabinet measures 9"W x 44.5"H x 11"D. The drivers are mounted just as they are in the project gallery. The port is located is located 4.5" above the internal bottom of the cabinet center on the front baffle panel and is a 2.5" diameter and a 7" length. The top 22" of the cabinet are filled with polyester fiber at a uniform density of 0.75 lb/ft3, requiring a total of 9-10 ounces of fiber. I used 3 window braces, one between the tweeter and upper woofer, a second one with its top 22" below the internal top of cabinet, and the third about halfway down between the middle brace and the bottom of the cabinet. The overall system tuning frequency is ~29 Hz.

I have a couple questions for you guys:

1. Since the crossovers were already built, I didnt rebuild them but the negative wires for the crossover are tied together after exiting the drivers. Is this ok?
2. Also, I didn't do the best job recessing the tweeter after paint and everything else, the tweeter rests a smidge above the baffle is this ok? If I was going to open up the hole is there a good way to do that using a rabbit bit?
3. After painting, I noticed my MDF joins were slightly showing.. Can I fix this by just going over again with bondo, re priming and repainting? Or did I miss a sealing step to prevent this?

Overall I am incredibly happy with this project, I cant imagine how much better some of the higher end 3 way DIY towers sound.

[Chris Roemer]

It's common to tie all the grounds together, at what point? - it doesn't really matter.

[bullittstang]

I have a couple questions for you guys:[/FONT][/COLOR]

1. Since the crossovers were already built, I didnt rebuild them but the negative wires for the crossover are tied together after exiting the drivers. Is this ok?
2. Also, I didn't do the best job recessing the tweeter after paint and everything else, the tweeter rests a smidge above the baffle is this ok? If I was going to open up the hole is there a good way to do that using a rabbit bit?
3. After painting, I noticed my MDF joins were slightly showing.. Can I fix this by just going over again with bondo, re priming and repainting? Or did I miss a sealing step to prevent this?

First, great job on a very nice looking build and knowing those woofers, you have a very capable speaker.

1. As Chris said, where you tie all the grounds together doesn't matter, so you are fine there
2. You can pull the tweeter out and "carefully" go around the recess with some 60-80 grit sandpaper and with some time sanding, you should be able to get a more perfect flush mount. But a small variance is not going to make any difference only aesthetics.
3. MDF tends to always show it's seems. Most guys use sealer, and some form of bondo/filler and sand flat a few times. Now that's it's painted, your only real option is to sand the seams, treat them properly with some bondo (or similar), sand flat and re-paint. If you like the look, others put a chamfer deep enough that the seem and edge of the chamfer meet, which "hides" the seem as well as anything.

I would leave them, no one is going to be that close to point it out and blemishes are the biggest complaint with any DIY... you know where all the mistakes and/or fixes are, when no one else would.

[xsilverjag]

Thanks! I didnt polish the speakers because of the seams and it is rather difficult to tell the error is there.

[jeezmo]

I know it's an old post but well done on the design. Could you share your volume calculations and tuning frequency?

[xsilverjag]

Sure below is what Paul K sent me

The internal dimensions of the cabinet I modeled are 7.5"W x 43"H x 9.5"D, making the external cabinet dimensions be, assuming all panels are 3/4" thick, 9"W x 44.5"H x 11"D. You will mount the 3 drivers on the baffle relative to the top and sides of the baffle as shown in the drawing for the stand-mount version. Internally, then, the tweeter's center is 10.25" below the top with the centers of the two woofers 5.75" above and below the tweeter's center. The tweeter's center is the TL's design center. The mass-loading port can be located on either the baffle or rear panel. The port has a 2.5" diameter and a 7" length. The top 22" of the cabinet are filled with polyester fiber at a uniform density of 0.75 lb/ft3, requiring a total of 9-10 ounces of fiber. The overall system tuning frequency is ~29 Hz. PE has a 2.5" diameter port with an adjustable length.. The center of the port is located 4.5" above the internal bottom of the cabinet. Can be on the front or back.

I will now show you a number of modeling graphs. Note that the TL modeling does not take into account baffle step losses or compensation and assumes there are none. The SPL output shown in several of the modeling graphs, therefore, will be higher than what you will actually achieve since the crossover design will compensate for baffle step losses in the lower frequencies by reducing the output appropriately in the higher frequencies.

For a 2.83-volt input (re: 1 meter)
System bass response (red line):

In the above graph F3 is 30 Hz, F6 is ~25 Hz and F10 is ~22 Hz, and the overall response is very smooth.

Woofers' output (red line) and port's output (blue line):

System impedance characteristic (red line):


When I model for maximum excursion I do so by determining how much input power is needed to drive the woofers to an excursion 15% greater than Xmax, a recommendation by Vance Dickason. These woofers have a rated Xmax of 5 mm Peak, and increasing that by 15% gives 5.75 mm Peak. This modeling program, however, uses mm RMS, and 5.75 mm Peak equals 4.1 mm RMS.

For an input of 35 watts
System bass response (red line):

Woofers' excursion (red line):

In the above graph an excursion of ~4.1 mm RMS is reached first at 41 Hz, again at 22 Hz, then exceeded at all lower frequencies.

Port air velocity as a percentage of the speed of sound:

The port's air velocity peaks at 5% of the speed of sound (17 m/s) at 22 Hz. This is considered to be a safe maximum level, and due to it being at a low frequency, it's not likely to be excited by musical content. Also considering that the system is generating ~104 dB SPL of output, there is little if any probability of port noise.

I suggest you use at least 3 "window" braces, one between the tweeter and upper woofer, a second one with its top 22" below the internal top of cabinet to act as a stuffing "stop", and the third about halfway down between the middle brace and the bottom of the cabinet. Other types of braces can be used if you prefer but be sure to plan for something at the 22" location.