Frugal sub with old drivers - possible?

[38eighteen]

Hi,

I am pretty new to DIY. A few years ago I built a small 2-way pair from a kit (my own sealed box design) and am almost certainly going to build one of Peter Daniel's LM3875 kits for myself for Christmas.

But, I have these old speakers that I bought about 20 years ago. They were cheap at the time so I can't imagine that the components are any good, but I'd appreciate any advice.

The woofers in these old speakers are Foster 300L05 12".





The problem is that I have no idea what the specs are for these. The sealed box they were originally in is 2.2cuft (I am metric, but I know most readers here are not). My understanding from the relatively small sealed box they were originally in, the obviously small magnet and the low power rating (20W at 8ohms if you can't make it out on the image) mean that they are not suitable for a "normal" ported sub.

I guess I have 2 main questions:

1. is it possible that I can make anything worthwhile from these? Note that I am looking at this more as an interesting project rather than a way to get a cheap sub.

2. what can I do about the fact that I don't know the specs? I have read about how to measure everything in Vance Dickason's book but I don't have the equipment. I think I can follow his instructions using a software signal generator on my PC and a couple of multi-meters? Will any results I get without the correct equipment be worthless? Can I just make some assumptions based on the original box size?

Thanks,

Brian

[PeteMcK]

looks like an OB candidate to me...

[planet10]

Is it a 12" (or a 10")?

I have some similar Foster 12s with larger magnets and the Q is about 0.5, so with a smaller magnet the Q should be higher, so OB is a likely best solution.

dave

[38eighteen]

It is 12". I should have also mentioned that I was planning to use both drivers to build a single sub.

Since posting my question I found this page about measurement. I think I can follow this. Anyone have any experience with this method?


Off to do some research on Open Baffle speaker design... Any pointers?

Thanks,

Brian

[PeteMcK]

at 20W they're probably not going to be able handle sub duty too well... better for mid-bass.
You really need to establish Fs (their resonant freq).

OBs - get as high Q drivers with as low Fs as you can;
as many as possible - maximum surface area,
and as big a baffle as your room or significant other will let you get away with...Check out the Visaton 'No Box' design.
http://www.visaton.com/en/bauvorschl...xbb/index.html
(There are other ways of doing it, but they're generally more complex/expensive)

So get down to Bunnings and grab a couple of sheets of 1200x600 ply, cut a hole & you're in business...

[38eighteen]

OK - I'm confused. All material I can find about Open Baffle suggests that it is for midrange. I'm looking to build a sub out of them.

Is suggesting OB a polite way of saying they are no good for anything else?? :-)

I'm going to measure the T/S parameters using the method described on diysubwoofers.org and do some modelling.

Thanks,

Brian

[mashaffer]

No OB subs are out there alright. In general OB is sort of a small (but very bright) subset of the audio hobbyist world. If you google "open baffle sub" yielded almost 150 hits so there is information out there.

The reason people are thinking OB is that from the pictures it looks like a driver that will have parameters more appropriate for that configuration. Midrange is easier to do on OB than bass because the wavelength is such that there is little cancellation but bass can be done. The low power capability will limit the loudness as bass require more grunt to produce the same volume level. However in a small room it might work fine if the other parameters are right.

Once you get the fs and Qts measured for these things folks here will have a better idea of what the various options might yield. If your requirements aren't too great and you are looking for a learning project it is quite possibly worth doing.

Alternately you can build a baffle with small side wings and top plate and feed it some test tones to see what you get. If you get good output down to bass guitar frequencies (40s) then you might be able to do a standard OB type sub of sorts. If not then a ripole (another good thing to search for here and on the web) might get you a little lower.

mike

[planet10]

Originally posted by 38eighteen
I found this page about measurement. I think I can follow this. Anyone have any experience with this method?

You will need a solid SS amp with good LF performance or the rails will sag and compromise the measurement. I went thru 3 or 4 amps before finding one suitable for this.

dave

[planet10]

Originally posted by 38eighteen
[B]OK - I'm confused. All material I can find about Open Baffle suggests that it is for midrange. I'm looking to build a sub out of them.

Here is what i designed for mine (but i was going to use 4 12s per side). A ripole (a specific designation for a folded OB)

dave

[38eighteen]

OK, I went and bought myself a multimeter and did some measurements/calculations using the procedure on diysubwoofers.org.

		Free Air	Box
Re		7.10	7.40
Rs		10.70	7.70
Vs		0.80	1.45
Is		0.04	0.10
Fs		47.40	113.00
Vm		0.18	0.41
Im		0.02	0.05
Rm		37.66	19.46
Ir		0.03	0.07
r0		2.72	1.80
Vr		0.29	0.55
Fl		36.90	72.00
Fh		59.40	144.00
Fs (check)	46.82	101.82
		
Fs (mean)	47.11	107.41
		
Qms		3.45	
Qes		0.80	
Qts		0.65	
		
Vas		80.75

I'd appreciate any feedback on these numbers. For example, are there things there that make it obvious that I did things wrong? Is the gap between the measured and calculated resonant frequencies (particularly in the enclosure) normal from the sophistication of my tools?

I plugged these numbers into WinISD and played for a while. I was amazed how flat and low (F3 of about 25Hz!) these would go if I build a sufficiently large box - 389.8l in this case :-) Is there any chance they would actually perform like that if I built the box??

I have thought of a use for them that might make more sense however. I'd like to build a bed head for my son's bed and incorporate an amplifier and some speakers. My current modelling suggests that if I combine a pair of FE103Es with one of these drivers in a 4th order bandpass that I should be able to fit at the end of the bed and make look like a bookshelf I will get something like this:


No need for any electrical cross over as the limits of the drivers and the tuning of the band pass take care of things.

Does this sound like a reasonable project?

Thanks,

Brian