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top in the to do list... (Read 5085 times)
Adrian D.
Ex Member



top in the to do list...
01/16/07 at 15:16:23
 
yep, an imperial. no more tiny woofers, tiny boxes, no more 10's, 12', or even 15's this time. i want to go all the way. standard imperial + one 18  :-*
i'm planning on only one 18 because it's cheaper than 2 15's and will make best use of my amp (500w@8ohm). oh, and i'm going to use mach5 subs. time to support the smaller companies.
t/s paramaters for the 18. 8ohm version  ::)
can it perform right in the imperial ?
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footstony
Ex Member



Re: top in the to do list...
Reply #1 - 01/16/07 at 22:00:30
 
Hi Adrian,

The 18 sounds like a beast.  I think the Fs is a bit too low for the imperial.  Maybe you could try a scaled UP version to get the horn frequency down to 30Hz if you wish to use this driver. It would need to be 1.3 times bigger.

Regards Philip
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Adrian D.
Ex Member



Re: top in the to do list...
Reply #2 - 01/17/07 at 12:13:36
 
chitfudge. 1.3 times bigger  :-[
so i take it the imp is a 40-ish hz horn ?
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gexter
Ex Member



Re: top in the to do list...
Reply #3 - 01/17/07 at 13:20:55
 
Yep

But I thought the round about number was a tuning of 35 and give or take from there,

But it dounds like fun ! Having an Imp is a dream come true. Well thats  what I think anyway.

All the others are, hmmmm, well,  just speakers Smiley
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Adrian D.
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Re: top in the to do list...
Reply #4 - 01/17/07 at 14:11:03
 
not sure it will work in my room (150 sq feet), but i have lots of space outside  :-*
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Braggi
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Re: top in the to do list...
Reply #5 - 01/17/07 at 16:37:35
 
It will work great in your room, but you'll have to go outside and walk 150 feet from your house to hear the bass.   Smiley  

Your neighbors will love it because your whole house will become a resonator.

About the Fs and the 18" driver ... I think enough people have successfully tried woofers with Fs below 40 to show that isn't an absolute issue.

Could anyone chime in here who has used an 18" driver or 15s with Fs below 40 hz to reassure future builders as to what problems, if any, can be expected? Dank, didn't you use 18s?
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Valiant_ap6
Ex Member



Re: top in the to do list...
Reply #6 - 01/18/07 at 00:02:57
 
Hiya all,

I recall Braggi's advice regarding using drivers with a low Fs in my own Imperial build, and this mating of driver to horn is of concern and decoupling is a fact, but only becomes a problem if the driver (as in all cases) is pushed to hard, or the amp clips. These are the specs of the 15" drivers Jake and I used in our project...
Rated Input      100W RMS
Max Input         150 W
Impedence @ 300Hz   8ohm
Response         28 - 1.5kHz
Resonant Frequency  28Htz
Sensitivity 1w-1m     94 dB
Voice Coil Diam   49.5mm
QMS    4.29
QES     0.44
QTS     0.4
VAS     314.6L
Nett weight  5kg    approx 11 lb
Magnet weight  1.4kg   approx 3+ lb

They were brilliant, driven by a 185w rms per channel amp, elctro crossover point (variable, 12dB roll off) of 300 - 2000Hz. Sounded best around the 1000Hz mark.

These drivers are the only ones available off the shelf here in West Oz. Others maybe  more expensive or overseas items.

Still, for my next pair, I'll have a look around for some decent pa 15's, I'd love to hear the Imperial in full range mode.

Cheers

Gaz.


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Braggi
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Re: top in the to do list...
Reply #7 - 01/18/07 at 04:31:24
 
This is good information, Gaz. Thanks for posting.

So can you describe the moment of "unloading?" I'm most interested in the details of what happens when the amp is being pushed, but still not clipping. Amp clipping would sound so nasty at that SPL, I wouldn't want you to test it out now (or in the future).   Smiley  Please just speak from the experiences you've had.

Thanks.
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gexter
Ex Member



Re: top in the to do list...
Reply #8 - 01/18/07 at 13:27:21
 
I have experienced the " decoupling" with my modified Imp My driver had a Fs of 25, but the horn was a Imp that was a shorter Jensen design. In was a corner design without corners is the most accurate description.
This effect was only when it was driven harder than I listened to it normally. It was one of my builds that I still regret dismantling.

Building a full size one to Steves specs I would still like to try and get more accurate data that I can say for sure.
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« Last Edit: 01/18/07 at 13:28:21 by gexter »  
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dank
Ex Member



Re: top in the to do list...
Reply #9 - 01/18/07 at 15:54:57
 
A speaker with Fs=30 and Qts=.35 will probably do ok in the standard Imperial.  Not great, but ok.  At $68 I don't think you will find a better choice.    All the 18's I've used have all been around 30 hz.  I've tried a number of Cerwin Vega 18's, an Eminence 18082, and a couple of Madison Executioner X18's.  My favorite is the Cerwin Vega XL18-4 with a Fs=28 and a Qts = .1675.  I keep mentioning the Qts because the number obtained by dividing Fs by Qts is relevant here, it relates to the speakers strength.  For a horn type enclosure you want this number to be greater than 100, the higher the better.  My Cerwin Vega favorite has a Fs/Qts of 28/.1675 = 167.  Your 8 ohm Mach 5 is 30.4/.35 = 87 which is marginal.  The high Qts scares me more than the low Fs.

I do think you will be missing out on something though if you build for a single 18.  There seems to be an interaction between the speakers that makes a combination of 2 different speakers sound better together than either one when paired with the same type.  Steve D. noticed this on the full range Imperial and recommends 2 different speakers because of it.  I have seen the same thing on my standard Imperials used as subs.  I'd at least be sure to build your Imperial so it could be easily modified in the future to add a second speaker if you ever decided to do it.  There was a lot of talk on this forum a while back about removable baffle boards which is how I did mine - and I've used that feature a couple of times.

DanK
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Adrian D.
Ex Member



Re: top in the to do list...
Reply #10 - 01/18/07 at 18:12:58
 
i'll integrate a removable baffle and play with the bracing so that i can switch between 1 and 2 speakers  :-*
i guess it will top everything i've ever heard as far as diy and purchased subs Smiley
thanks guys. really helpful info.
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Valiant_ap6
Ex Member



Re: top in the to do list...
Reply #11 - 01/19/07 at 23:07:25
 
I think decoupling behaviour was noticed when I was running the Imperials with a Luxman amp (LV-92, at a guess good for about 50 odd watts rms, correct me if Im wrong). By accident the volume was control was turned up high and the drivers were oscillating @ about 20Htz @ max amplitude. I cut the power straight away. I don't think this was actual decoupling, more like amp behaviour when driven to excess. I've never noticed the same problem when driven with my Playmaster - the 185 wpc beast. A good amp, as we all know, grips the speakers with a fist of steel. I can pass on the details of the Playmaster if you wish. For a guy whose previous soldering experience was a 10 component siren, this home built baby is a beaut.
Cheers
Gaz
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JimP
Ex Member



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Reply #12 - 01/20/07 at 15:30:51
 
Hi guys,

The problem of "unloading"  of the woofer below it's resonant frequency can be an issue, but likely only if you are using a somewhat powerful amp.  Below the horn cut-off frequency (35-40 hz) there is no load on the woofer, so if there is a signal applied, at say 25 Hz, the excursion can get extreme.  The Imperial's large chamber helps with this, or at least that's what some of our modelers have reported.

Would you hear this unloading?  Most likely you might notice some severe intermodulation distortion, especially if you are using the Imp full range.  You may not hear it, but certainly your woofers are feeling it and it could result in damage.  At a few watts, the woofer probably won't be flying enough to cause damage, but at higher power (50 watts from Gaz's example)  the woofer could flap.

A low-cut filter can help this and I use a 30Hz roll-off filter when we are using the Imp as a PA (very high power) to avoid problems.  Not an audio purist set-up, but it saves woofers.  It's not much different from the rumble filter you would use when playing vinyl.  A good cartridge can easily produce 5 Hz tones (like from warped records) and those can really wreck your day and your woofers.  Most woofers are "unloaded" at that frequency.

In practice, I have not had any noticable problems with unloading, but both of my woofers Fs are around 35Hz, so I think it's not a big issue with my set-up.

Jim P
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