Brines Acoustics

Fun with BOFU

Last Modified 9/16/10

One of the more popular DIY full-range drivers is the 8" Pioneer B20FU20-51FW, lovingly known as "BOFU" While is not the world's greatest driver, it is a competent driver that is reasonably forgiving about cabinet size and types. At ~$40 a piece, this is a bargain and a great driver for the DIY'er just starting to play with single driver speakers. I have not personally played with the BOFU before because it makes no sense to put a $40 driver in a $1500 cabinet. I do make plans available at modest cost, but to date, publishing plans has been an afterthought from generating commercial cabinets. But then recently BOFU went on sale for $19 a piece. I couldn't resist, so I bought four BOFU's just to play with. Eventually I will generate some plans for the DIY'er.

What is BOFU?

I opened the box upon arrival and found that I had a case of four drivers in their original box from China. The four drivers were wrapped as a unit in polyethylene sheet, each driver wrapped in a piece of cardboard and a cardboard divider between the individual drivers. Very impressive at this price point.

The BOFU has a stamped frame that is rather thin, with the outer edge rolled up and a ~1/4in paper gasket on the the front surface. Typical construction for low cost drivers. The cone is typical black paper with decoupling rings molded in. The whizzer cone and dust cap appear to be formed form the same piece of paper. The surround is gray foam. The magnet is a chunky piece of ceramic material and it has a bumped back plate. Construction is clean without a lot of excess glue floating around.

I broke in the four drivers for an hour and then did the basic T/S measurements. These are the numbers I got:

Re 6.567
Fs 40.24
Qem 0.496
Qms 2.723
Qts 0..428
Qem 0.496
Le 0.496
Vas 67.82
BL 7.149

Testing Setup

Being disinclined to spend a lot of time and effort on woodwork for prototypes, I used existing cabinets. The standard for comparison is the Tang Band W8-1772 loaded into FB-20 cabinets. This is the prototype of the TT-20 speaker that is slightly bigger. However, the 1772 works nicely in this cabinet. The proper cabinet supplies a bit more bass extension, but sounds the same throughout  the majority of the frequency range. The BOFU is loaded into FT-2000 cabinets, which are an almost perfect fit. No filtering is applied to either cabinet -- no BAC, no zobel.

Measurements are taken with the Smith & Larson Woofer Tester Pro, using a Beringer EMC-8000 microphone that is calibrated into the single digits. If the WTPro has a singular failing, it is that the ADC/DAC is set to 44.1kHz, so the upper limit of all measurements is 20kHz. Listening sessions are run in a 15'x25' room, the speakers being placed centered on the long wall  3' from the wall and 8' apart. Real Time Analysis is taken 1/2m from the center of the driver and then scaled to the calculated SPL @ 1000Hz. The amplifier use for the comparisons, unless noted is a Yamaha receiver.

Initial Impressions

Out of the box, but with the one hour break-in for T/S testing, the BOFU's sounded bassy, warm and harsh. I let them run for some 50 hours, pretty much by themselves and at a higher SPL than I normally can stand. At this point, the harshness has gone away. The BOFU's sound bassy and warm. The best word I can think of is "fat". Compared to the 1772's, The BOFU's have much stronger bass. This is partly due to the lower tuning of the cabinet, but the extra firmness in the 80-100Hz range is all due to the driver. The midrange is a little flat, though. The BOFU's lack of detail compared to the 1772's is immediately evident. But then, we are comparing a $40 driver to a $200 driver. One would expect to get something for the money. I do feel that those who listen to a lot of rock will appreciate the extra heaviness.  Here is the frequency response of BOFU in the FT-2000 Cabinet:

BOFU FR

From the graph it is obvious that the  BOFU would sound warm and lack detail.  It is also obvious why the conventional wisdom is that this  driver needs a helper tweeter.  At 10o off axis, a normal toe-in for a single driver speaker, the BOFU rolls of rapidly after 5kHz. This why the driver is reported to be smooth -- all of that sibilance from female singers who swallow their mic's is between 5-7kHz and BOFU simply doesn't reproduce it.

Damping the Basket

One of the standard tweaks, particularly for drivers with stamped baskets is to damp the basket by applying something to the basket legs. The idea is the damping stops any mechanical ringing in the basket. Among the products used are duct seal and Mortite (a window sealant). I chose modeling clay.  Modeling clay is very dense, sticks well and will not harden. I applied 1/4" of clay to the basket legs and rim, and faired the magnet into the basket legs. Results? Nothing! I could not hear a difference and I could not see a difference in the frequency response.

Clay Filled

Sorry for the crappy graph. I'll change it out later.

Why? Well, lets look into what is happening physically. If you set the driver on its magnet and tap the rim, you will hear ringing. The basket is shaped like and acts like a bell. So lets slap a bunch of goop on the basket and the ringing will go away, right? Wrong. The thing is that once you bolt the basket down on the baffle, the basket cannot ring in bell mode because the boundary (the rim of the basket) cannot move. So, all of that goop is doing nothing but looking neat. The same goes for the fairing around the magnet. There just isn't enough turbulence to generate noise that will prorogate through the cone.

There is another vibration mode that my need to be addressed. As the cone moves in and out, it applies force to the basket.  That force is applied through the magnet and moves the basket in the opposite direction as the cone. In theory, this subtracts from the movement of the cone and reduces the output of the driver. Of course, the movement of the basket is constrained by the mass of the baffle and the rest of the cabinet. That movement is miniscule, but it is also possible that the basket itself could flex back and forth. The basket of the BOFU is very thin. This might be a problem. I don't know. Other designers will place a brace behind the driver to brace the magnet against the back of the cabinet. I may give this a try somewhere along the line.

Mod Podge™

Mod Podge™ is a PVA glue that is intended to be used as a protectant and sealer for such things as cardboard puzzles. This has become a standard tweak for paper drivers. I thinned the product 50/50 with water and applied two coats to the main cone ans whizzer cone. Here is the result:

Mod Podge

What's next?

I am going to apply a couple of thin coats of Mod Podge (PVA glue) to this pair of drivers. I will then redo the frequency response graphs, break them in again and do another comparison with the TB 1772's.

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