Four 8 inch drivers waiting to be used

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cm961

cm961

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Joined 2001
#1
I came across 4 decent looking 8 inch drivers. They were pulled from sealed enclosures of about 2.3 cubic feet. I measured them to have about the following specs:

Re = 3 ohms (why so low?)
Fs = 37.7 Hz (not too bad...)
Qms = 3.01
Qes = 0.46
Qts = 0.4
Vas = 70.69 Litres

Question A: Did I measure these parameters right? I used the method listed by Ron Elliot (ESP Audio). I tried to be careful, but I used a number of different DMMs, and they were all slightly different. I trusted the Simpson model with true RMS.

Question B: I'd like to find a way to make these 4 drivers go really low. They aren't high powered (probably about 30W each) but I have four of them. I'm willing to try an experimental design. Any suggestions?

Thanks guys!

Pete
 
#2
Not to insult you by pointing out the obvious, but impedence is frequency dependent -- its not one number-- and often if you simply measure the impedence with a meter you end up with a rather lower value than that given by the manufacturer. I would guess these are 4 ohm speakers, perhaps meant to be used in series, or simply meant for low power amplification.

If you know the make and model of the speaker they were pulled from, it might help identify them, which might make it easier to identify the best application for them.

Of course, when I get unknown speakers, I take that as permission to use them in any demented application I can think of...
 
#3
Yeah I know all about impedance (3rd year electrical engineer) but what I meant was that the value of Re is low... But I guess thats normal. Anyways, thanks for the reply. I shall continue to research interesting designs.

Pete
 
#4
Yeah, this speaker is actually better if used in a vented box. You can make a very efficent but large EBS enclosure for them. If you series-parallel them to get a nominal 4 ohm load, you will have a spekaer that is around 96dB sensativity acording to Unibox. Because of this, x-max and power considerations shouldnt be that big of a deal, since you have such high sensativity.

Put them in a 400L box tuned to about 25-28Hz. This should get you really really low and loud. This is pretty much your only option to get very low. Sealed pretty much dosnt get you anywhere.

-Paul Hilgeman
 
#6
As a rule of thumb, Re is about 25% lower than the rated impedance of a speaker. That is because a speaker, throughout it's frequency range, always has an impedance somewhat greater than Re. In a ported box, at Fb the impedance will be 1 or 2 ohms above Re-and that is usually the lowest impedance point of the speaker.

Actually, most speakers rated 4 ohms have an Re a tiny bit less than 3 ohms, just as most speakers rated 8 ohms have an Re slightly under 6 ohms.
 
#7
cm961 said:
I came across 4 decent looking 8 inch drivers. They were pulled from sealed enclosures of about 2.3 cubic feet. I measured them to have about the following specs:

Qts = 0.4
Vas = 70.69 Litres
Click to expand...

Actually, the Thiele-Small parameters from these enclosures do not gibe with the sealed enclosures they were pulled from. These speakers, put in a sealed box of 2.3 cu in, (65 liters) will yield a final Qtc of 0.57.

While some DIYers aim for a Qtc of 0.5, the fact is that the output is 6 dB down at Fc, which is a lot to give for great transient response.

Traditionally, for decades, most manufacturers go for a Qtc between 0.7, (3 dB down at Fc), and 1.0, (0 dB down at Fc).

It is possible the manufacturer wanted the best transient response, but I think it is unlikely. Either the manufacturer did something unusual with his box volume, the speaker run changed and the manufacturer didn't quibble, or the measurement might be a little bit off.
 
#8
Re: Re: Four 8 inch drivers waiting to be used

kelticwizard said:
While some DIYers aim for a Qtc of 0.5, the fact is that the output is 6 dB down at Fc, which is a lot to give for great transient response.
Click to expand...

If you factor in room lift it really isn't... i usually try to aim for a low Q and look at the F10 as the indicator of bass extension.

BTW Q = 0.57 is a Bessel alignment IIRC.

dave
 
#9
I am unfamiliar with Bessel alignments for sealed boxes. I thought Thiele invented the term "alignment" for ported boxes.

Regardless, I get the impression that these 8's were taken from a normal, commercially produced box that is not really, really, expensive, or else cm961 would be telling us the brand, etc.

A closed box with a Qtc. of .57 is something audiophiles do, because they likely have a big amp to make up for lack of output down to Fc. Most commercially produced systems, except possibly for some expensive audiophile ones, go for a Qtc. of 0.7 or slightly higher.

I am not saying it is impossible that the manufacturer designed it this way, just saying I find it unusual, given it's likely price range.
 
#10
kelticwizard said:
I am not saying it is impossible that the manufacturer designed it this way, just saying I find it unusual, given it's likely price range.
Click to expand...

It is indeed unusual for an inexpensive box to have such low Q. For 1 it wouldn't have the peaky bass that gives the false impression of lowbass, and second, it takes quite a bit more wood to build a box with a Q that low. On the other hand it could be big just so it can be big, or it may be a 4x8" PA column (i recently pulled 4 nice FR Marsland out of such a box.

dave
 
#11
I also have four 8" drivers of unknown origin (well, they are called "Titan Boeing 160 W subwoofers"🙂 ) that came out of a 2.3 cu ft -ported- box. The cabinets are white-van specials called "Paragon Pro Series", bought in a moment of great, and embarassing weakness. Maybe they are the same? Or the same principle, at least. They do sound some boomy in that box, lemme tell ya 🙂
 
#14
I thought Thiele invented the term "alignment" for ported boxes.
Click to expand...

True, but T/S has had such an impact on speaker cab design that many of the historical definitions have either disappeared or been 'converted' to T/S. For instance, critically damped used to mean what today we call max flat (Qt = 0.707), and overdamped is now called critically damped (Qt = 0.5), with anything below this point being considered overdamped, and above underdamped.

WRT vented, technically, since they have two roll off slopes they can't can't have a Qt rating other than where F3 = Fb (Qt = 0.707), but folks like me who rarely use this alignment have gotten into the habit of assigning them a Qb based on their roll off slope between F3-Fb to define the degree of their EBS or aperiodic alignment.

Unfortunately this can potentially cause mass quantities of confusion in a discussion, like 'fast' bass does, if not defined from the get-go.

Since they're nominally 4ohms, DaveP10's TL suggestion, or ML-TL (my fave in most cases) if they're going to be used in four individual speakers, otherwise Paul's multi-driver sub looks like a plan.

GM
 
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