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|1st May 2002, 09:21 AM||#1|
Join Date: Aug 2001
Location: Bath, UK
8"/10" mini sub driver in UK
I'm after a pair of sub drivers for my computer sound system. Ideally 8" (but could possibly stretch to 10") operating in about 0.5 cuft sealed. As this is "only" for my computer, I'm not after ultra-low distortion, high cost drivers. A low F3 isn't hugely important because I'm planning to use equalisation and therefore an Xmax of at least 6mm would be nice.
The MJAcoustics Pro50 uses the kind of 8" driver I'm looking for:
I've heard this sub and it's very impressive for such a small unit (almost certainly uses equalisation).
Although not shown in these pictures, the Stands Unique Fathom sub uses the same driver:
Does anyone recognise this little gem or can suggest a suitable unit? UK availability is important to me!
|4th May 2002, 10:27 PM||#2|
Wizard of Kelts
Believe it or not, I have been paying attention to your post for the past couple of days. Unfortunately, I have not been able to post anything.
I am unfamiliar with the driver in your link. I have been going through the database of WinISD to find a suitable substitute. I found a couple, but there are problems. The Fujitsu Eclipse 86080.4 and the 8708 4 ohm have promise, but the sensitivity is listed as 84 dB, which is sufficient. However, BoxModel, which calculates sensitivity based on Thiele-Small parameters-it is usually accurate-says the sensitivity is really 80 dB @ 1W/1M. That is too low.
Then there is the Polk db 8.8, but lo and behold, that is last year's model and may not be available. And Polk's new models are a ten and a 12 only.
Gimme time, I'll keep looking.
The reason I am doing this is that I think you have an interesting challenge. The question I feel that you are really asking is: How low can you actually go in a .6 cu ft enclosure? And I think it is a question actually worth answering.
A few observations. The speaker is in a sealed box of .6 cu ft, and the amp driving it is between 50 and 80 watts. In a sealed enclosure, the speaker has to move all the air necessary to produce bass-the enclosure has no output. Forget about an 8" in a .6 cu ft. sealed enclosure having usable bass output down to 15 Hz. Whatever such a speaker can put out at 15 Hz, it will be too low to be worth anything. Try 30 Hz.
Even 30 Hz is daunting. According to a chart provided by Small, (of Thiele-Small fame), it takes 6 cubic inches of air moved to produce 112 dB @100 Hz. For every half octave you go lower, you need twice as much air moved. For every 6 dB you raise the SPL, you double the volume. For every 6 dB the SPL goes down, you cut the volume of air moved in half. Using these formulas, we arrive at the fact that at 30 Hz, a speaker has to move 33 cubic inches of air to produce a 30 Hz note @ 106 dB.
The volume of the air moved is the area of the speaker, (Sd) times the excursion from the midpoint to the position all the way in front, (Xmax). If Xmax is given as peak-to peak movement, divide that figure in half to find the real excursion.
An 8" speaker has about 32 sq inhes of area, so it has to move about an inch to produce that 30 Hz note @ 106 dB. Incidentally, your link says that the speaker in the MJ subwoofer has an excursion of about an inch, midpoint to back. Whether it ever gets to use that excursion is another question.
To me, a 30 Hz that is played at 102 dB is barely hearable. Any SPL below that, and the note will not be noticeable. I think it is realistic to concentrate on 30 Hz as a cutoff for a small box.
For a speaker powered by 100 watts to get up to 102 db, then at one watt it must be at 82 dB SPL. An SPL any lower, and a 1100 watt amp cannot drive it up to 102 dB.
Am I making sense so far?
All I am saying is that for you to get hearable bass out of your .6 cubic foot box, your UNEQUALIZED curve at one watt better show that your speaker is playing at least at 82 dB @ 30 Hz. Otherwise, it is a waste of time.
A ported or bandpass speaker, because the enclosure adds output, is much more likely to fulfill these requirements. If I find a subwoofer that puts out 82 dB @ 1W/1M in a .6 cubic foot box, I will certainly let you know. Right now, so far the only ones I have come across can fulfill that requirement can do so only in a ported or bandpass enclosure.
I should also point out that a speaker in a ported or bandpass box can produce 102 dB @ 30 Hz while using MUCH less excursion than the sealed box. An 8" sealed box woofer must travel 1" forward and 1" back to produce that note at that volume. An 8" in a ported box can travel only .25" to produce it. A bandpass box can travel about .5" to produce it.
So your choice of drivers is enhanced if you go bandpass, ported, or passive radiator, (ported's first cousin).
|4th May 2002, 11:00 PM||#3|
Wizard of Kelts
Just for comparison, here is a graph compaing the output of an Fujitsu Eclipse 8708 4 ohm in a bandpass and closed box, both .6 cu ft.
The bandpass arrangement-.2 cu ft in front of the woofer, .4 behind it, tuned to 42 Hz, will result in higher output.
Bandpass is in purple, closed box is in green.
Fujitsue claums 84 dB sensitivity at 1 watt for this woofer, BoxModel measures it as 80 dB. Even if it only 80 dB, it will put out 100 dB at 30 Hz with a 100 watt amp. If it actually is 84 dB sensitive, then it will produce 104 dB at 30 Hz-close to what we need. In the closed box, it is not even close.
Fujitsu is a giant Japanese company, and their products should be available in England. Some of their woofers are very very expensive, but I don't think the 8708 is top of the line. I also don't know if the 8708 is a current model.
I just used the 8708 to show what might be possible. I think over 100 dB zst 30 Hz with a 100 watt amp is incredible performance out of a .6 cubic foot box, and probably significantly better than than the performance of the MJ subwoofer.
PS: The port can be replaced with a folat passive radiator, which will not take up room in the front chamber. More about that later.
|7th May 2002, 01:48 PM||#4|
Join Date: Aug 2001
Location: Bath, UK
Great work Mr Kelticwizard, Sir! I must admit that bandpass is something I didn't consider - but it might be fun to try (I've yet to design and build a bandpass box!). The main reason I was considering going sealed is the ease at which equalisation, using the Linkwitz Transform, can be applied.
Your figure of 102dB at 30Hz as being right on the hearing threshold is probably quite accurate, but I'm not sure if I'll even be aiming that low. To give you some background I'm currently using a single 6.5" Audax driver (HT170K0) in about 1cu ft ported tuned to 40Hz-50Hz to give some low-ish bass to complement my satellites. Some pictures of the complete system pics here:
Although I call it a sub, it's only a woofer really as it rolls off at around 40-45Hz, IIRC. The "sub" is surprisingly good at moderate sound levels but runs out of puff quite quickly. The bass boost I've applied means that the driver is reaching it's 3mm of Xmax at my normal listening level so any extra drive causes distortion, although it's not really noticeable until excursion approches 5mm.
It's only really used for games so I don't want to spend a lot on drivers but some newer games, like Jedi Knight 2 and Return to Castle Wolfenstein, have some very respectable bass! One promising driver I have found is from RCM Akoustik's Alcone line of aluminium sandwich cones:
The low Qts figure of the 8 ohm, 8 inch (AC8-SW8) is obviously more suited to very small ported applications so this driver in an EBS alignment should meet my requirements. A 20 liter EBS box tuned to 35Hz gives a mid-shelf f3 of around 32Hz but with efficiency 5dB less than mid-band, giving 85dB. When I get home tonight I'll try modelling this 8" driver in a bandpass enclosure. The 10" 8ohm driver seems much more suitable to sealed use with 17 litres giving an f3 of 63Hz.
The Fujitsu drivers you mention certainly seem interesting. I haven't seen them in any of the catalogues I have but Fujitsu's website markets them as Autosound drivers and the companies I use for drivers are audiophile only (nothing wrong with Autosound drivers of course, could be ideal as they're usually optimised for small enclosurers). I'll have a look round the web for UK dealers.
I'll let you know how I get on. Thanks for all your help.
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