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Old 5th June 2010, 11:01 AM   #1
Arnulf is offline Arnulf  Europe
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Default Mystery drivers - what to do with them ?

I came into possession of 2 20 cm (8") bass-mid drivers. The only parameters I was able to locate are:

Z = 4R
P = 40W
Frequency range = 35 - 3000 Hz
Fs = 35 +/- 5 Hz

So ... what to do with them ? I recently finished two fullrange transmission line speakers and since drivers I used are seriously lacking in the bass department (40-100 Hz, their response levels out only at 140 Hz and above and they are pretty insensitive to begin with) I was considering building a separate woofer utilizing both abovementioned drivers in series for 8R.

I intend to use a separate amplifier for the woofer alone. I'm not into loud thumping or parties, my usual listening level is 1W per channel max. (small triode amplifier which isn't capable of outputting more than 1.5W or so) so I take 20W max. should be more than enough to get some bass from the woofer. I have a relatively small room to fill, it'll be either 6 m^2 (65 sqft) or 14 m^2 (150 sqft).

Given the lack of technical information and room contraints I was wondering if any of you guys could offer some advice on how to proceed - what kind of enclosure (sealed ?) would be best suited to unknown drivers while still keeping it as small as possible as far as outside dimensions go ? What kind of volume should I be looking at ?

Should I be considering some sort of frequency roll-off network (to get rid of any response above 150 Hz where the full-range drivers do their job just fine) ? If yes, I'd place it before the VAS in the amplifier, that's where components are still low power and cheap

Thanks for any advice, musings or just random ramblings
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Old 6th June 2010, 03:07 AM   #2
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You could go for a dual driver t-line enclosure like what I did
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Old 6th June 2010, 07:36 AM   #3
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I noticed your topic and am following it I'm not sure whether transmission line enclosure is the smallest dimensions/volume vis a vis efficiency. I hope somebody can chime in with some more pointers for newbie builder, plenty of peepers thus far
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Old 6th June 2010, 07:39 AM   #4
GM is online now GM  United States
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Well, with no specs I can only fall back on the pre-T/S nomograph published for DIYers which for a 35 Hz tuning where the vent area (Av) = driver effective cone area (Sd) specs a ~11 ft^3 net cab Vb.

Cutting it in half to ~5.5 ft^3 net is about as small as I recommend and big as it is, it's probably reasonably close since drivers with no specs tend to need large cabs due to having a high Qts (weak motor) and/or a high Vas (suspension compliance).

A ~3" diameter hole will tune it to around 32 Hz, so should be low enough to handle most/all of your music's needs. Of course you can try bigger holes if you want to tune it higher as well as sealed, using Styrofoam blocks/whatever to reduce its Vb to raise its tuning if needed.

Note that this is for one driver/cab, so if two are in the same cab it will be ~11 ft^3 net and have two vents of equal area/length. Better to make two singles though, so that you have more positioning flexibility to help average out room modes.

Yes, most definitely use a low pass to blend them to your mains.

GM
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Last edited by GM; 6th June 2010 at 07:46 AM.
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Old 6th June 2010, 10:12 PM   #5
Arnulf is offline Arnulf  Europe
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GM, thank you for heaps of information ! Could you please refer me to the document you mentioned (if it is avaliable online - what to google for ?) so I could learn more about the theory behind these figures and make some sense out of the terminology ? I'm a complete loudspeaker design newbie

I never imagined it would take that kind of volume (11 ft^3 = 311 liters) to put them into use - I guess I might as well change the plan and use a single driver. Afterall I won't be pushing it anywhere near its ratings anyway. The downside of using a single speaker is lower damping factor but I'm really really pressed to use as little room as possible.

As my fortune (?) would have it I believe I have found yet another driver of the same kind while sorting my stuff today ...
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Old 7th June 2010, 01:51 AM   #6
GM is online now GM  United States
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You're welcome!

It's in a long out-of-date book, I'll have to scan it next chance I get or better yet, buy it and get some other good info too: Amazon.com: Used and New: How to Build Speaker Enclosures

T/S specs info: TS Parameters (Thiele/Small Parameters) - Home Theater Forum and Systems - HomeTheaterShack.com

Well, as I implied, it's a worst case scenario based typical driver specs and the high output impedance (series resistance) of tube amps of the times that had to be factored in and why speaker cabs tended to be so large back then. Note too that a ~35 Hz Fs driver was typically 12-15" in size, so if you look to back then, 5.5-11 ft^3 (and larger) was common. IOW it takes 'X' amount of net cab Vb to reproduce 35 Hz at any decent amount of SPL (loudness) and the cab doesn't care how big the driver is, so it always boils down to how much bass you want Vs how big a cab you can tolerate: TA Speaker Topics: Loudspeaker Design Tradeoffs

Anyway, another option is to put both drivers in the 5.5 ft^3 cab, leave it sealed and add some stuffing to get an idea if they will tolerate a relatively small cab. If it sounds too 'one note', then add the 3" diameter vent and/or try more stuffing density.

GM
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Old 7th June 2010, 07:23 AM   #7
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by GM View Post
IOW it takes 'X' amount of net cab Vb to reproduce 35 Hz at any decent amount of SPL (loudness) and the cab doesn't care how big the driver is, so it always boils down to how much bass you want Vs how big a cab you can tolerate: TA Speaker Topics: Loudspeaker Design Tradeoffs
Thanks again ! This particular article is great, it explains the trade-offs in simple terms. Is there any reference to how low a response a system for reproduction of music (mainly rock) should have - in other words, how much of it I could trade for lower volume without sacrificing too much ? My reference is a pair of recently purchased bookshelf Klipsch RB-61 (they claim these go down to 43 Hz, I can't say I miss anything in the way of bass response with them plus I can't listen to very loud music anyway).

I've stored the PDFs of T/S theory which might come in handy at some point as well.

This stuff has prompted another question though: it would seem that sealed box type would be preferrable for unknown drivers, but would end up larger and less efficient than ported type. What does this mean realistically - would ported box that is unsuitable for the drivers in question result in lower efficiency, incorrect response or both ? If it's merely the efficiency, how would its loss compare to inherently lower efficiency of sealed box ?
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Old 7th June 2010, 02:30 PM   #8
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You're welcome!

Well, there's rock and then there's 'rock', but what matters is what's on the recording. The former (classic R&R) with rare exception is done by ~42 Hz, but there are some band albums such as Pink Floyd, Rick Wakeman that need a flat to ~27 Hz BW. The latter (everything else) can have rumbling syth tones down to at least 16 Hz on CD/DVD. So, on the face of it, you can get by with flat to ~42 Hz.

Due to Rock's typically highly compressed format though, the speaker needs more power handling capability down low compared to other music genres for a given peak SPL. Also, due to how we hear at lower average SPLs, the bass and highs need boosting with a smiley face looking EQ (AKA 'loudness' control on some receivers, pre-amps) to tonally balance it out, so woofer, tweeter power demand can be on a par with a much higher average SPL.

Correct if not tuned low (Fs or lower) and with what may seem a too large net Vb for the driver size since a sealed cab is much more tolerant of driver specs and its rising rate air spring will protect it much better than sealed down low. The trade-off of course is reduced box efficiency down low.

With only one exception AFAIK (dips due to TL pipe resonances), a vented alignment can't be lower in efficiency than a sealed one of the same net Vb until below its tuning where it rolls off faster, so in its pass-band its response would just not be optimized.

For a given net Vb/driver specs, a maximally flat vented alignment will be up to ~10 dB more efficient at Fb which equates to an extra ~10 W of power handling.

GM
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Old 7th June 2010, 11:04 PM   #9
Arnulf is offline Arnulf  Europe
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I have found a great online calculator which allows direct comparison of enclosure types with given parameters. Something to play with and lots to ponder tomorrow
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Old 9th June 2010, 02:46 AM   #10
GM is online now GM  United States
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Cute! It's pretty general, so other than sealed, I don't recommend using it to design anything, especially a BP.

GM
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