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Imperials?? i want some!! (Read 8976 times)
JimP
Ex Member



Re: Imperials?? i want some!!
Reply #15 - 10/27/05 at 22:15:45
 
Braggi,

I haven't built an 18" imperial, but  I suspect that the most likely sound you'd hear when a driver gets unloaded at low frequencies is the voice coil hitting bottom.  And that's an ugly sound!  

The Fs is the speaker's natural resonant frequency.  Below that the speaker doesn't couple to the air very well and will have large excursions without an air volume to load it, either from a box or horn.  Below about 40 hz the imperial horn doesn't load the woofer so you could get really drastic excursion.  I suspect the back chamber actually helps reduce this effect allowing the imperial to unload more gracefully than a base-reflex or OB.  I've driven my imperial with 20Hz tones and not bottomed the woofers but they do fly and the output is not tremendous although present.  At 30 Hz or above the imperials will have good output and really strong at 40Hz and up.

I hope this is useful info.  And here's to NEVER hearing the death rattle of your woofers unloading!!

Jim P
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skeamah1516
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Re: Imperials?? i want some!!
Reply #16 - 10/27/05 at 22:49:39
 
i know exactly what you mean about speakers flying without making any sound. ive had alot of experience with speakers, in cars, in house, alsorts. i would hate to build such a mighty enclosure which cant alow my speakers to hit those thundery lows.

im still going to build the imperials regardless for the chance that it goes lower than expected and for the experience and fun of building somthing myself

i live in england and i was just wondering if you guys know if id still be able to buy the plans, i havnt tryed yet but i will soon, oh and do you think a circular saw will be decent enough to make nice straight cuts? im going to buy one if so and ive already got a jigsaw Cheesy Kiss
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gexter
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Re: Imperials?? i want some!!
Reply #17 - 10/28/05 at 03:24:53
 
you could but you might want to make guides jigs etc to make better cuts.
you can't do it with a jigsaw thats for sure!

I am not all that great with a circular saw free hand. I have a 12" table saw, 12" chopsaw Etc Etc.

I think 60 did all his stuff without a table saw. In my book that takes skill.

Going to go ahead huh!
Take your time and get that circular saw wish I was there!!
My new garage got vetoed so no Imperials for me!

Except that exsisting SO concept might expand to a real one unexplainably without warning as soon as I have time

Gex
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« Last Edit: 10/28/05 at 04:49:55 by gexter »  
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jj420
Ex Member



Re: Imperials?? i want some!!
Reply #18 - 10/28/05 at 16:18:32
 
disclaimer;  Horns are out of my range of definitive knowledge, but heres what ii have to suggest...

If you want the horn to go lower, then my basic understanding is that you need to make it longer and have a wider mouth...

in theory, if you scaled the Imp UP to yeild a longer horn, and a wider mouth, you would lower the tuning of the horn. So, if you went with 84" of height, and scaled the width out to maintain the porportion of the enclosure, your horn would have a lower cuttoff, and be harder to unload because of the lack of program content below about 30hz.

the downside of this is that the things are already monstrous, and heavy, this would only get worse.

you would definitley want a guide, or jig, if you plan on using only a circular saw.  Get a blade with finer teeth and take it SLOW and everything should come out fine if you keep it square.  If you spend the extra cash on a higher powered saw, you will also have an easier time, trust me i just helped a friend finish a DIY box, all he had was a cheap saw, with a wimpy motor, and it just aggravated us to no end.
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Adrian D.
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Re: Imperials?? i want some!!
Reply #19 - 10/30/05 at 09:09:26
 
i built both my dbs from precut particle board. i ordered the material and the shop cut it to my specs. it was dirtcheap and VERY easy.
the imperial has lots of panels and it will be harder to work out a cut list but with a bit of maths involved, you can do it  ;).
if you're buying the material (btw, what are you building it from? ) then ask the people at the shop if they can cut it. the shop i bought the materials from cut everything for my db-12 in about 10 mins. work out the panels so that you only have to make the angled cuts.
i'll be using the same approach in building my wo
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gexter
Ex Member



Re: Imperials?? i want some!!
Reply #20 - 10/30/05 at 15:30:19
 
There are a lot of panels and braces and cuts in a Imperial.

Follow Steves advice on assembly. I could not believe how many pieces there were to my little corner cab. and not like a little stack of pieces and two 36x36like the WO.
my tallest was 50"  the widest was 36" It is much bigger than a 18 HWK and requires more planning and thought. Then its smooth sailing once everything is cut as Adrian was saying. Looking at the lumber yard cut everything is a huge time saver. I cut my own because at the time they were careless in the cuts in some other stuff I had them cut.

I have a 14 x 14 work area and it was too tight for me and had to shut down my other projects when cutting and assembling the IMP.

Make sure you have as much room as possible. Make a list of everything you might need right down to sandpaper so you can work right through. I have a several steel squares and T squares as well as a heavy steel metre stick. I thought I had some big clamps until I started on it. I resorted to lots of 25Lb plates and strapping a Squares to it so it would not shift while it dryed.

It made me look at my wood working aspect of my shop. Most of my stuff is electronic and automotive.
ON the flip side it was just time once everything was planned out

I would build two Imperial fullsize in a new york minute if I had a place for them.

Gex
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Braggi
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Re: Imperials?? i want some!!
Reply #21 - 11/02/05 at 03:10:54
 
[quote author=JimP  link=1130329857/15#15 date=1130447745]
The Fs is the speaker's natural resonant frequency.  Below that the speaker doesn't couple to the air very well and will have large excursions without an air volume to load it, either from a box or horn.  Below about 40 hz the imperial horn doesn't load the woofer so you could get really drastic excursion.  I suspect the back chamber actually helps reduce this effect allowing the imperial to unload more gracefully than a base-reflex or OB.  I've driven my imperial with 20Hz tones and not bottomed the woofers but they do fly and the output is not tremendous although present.  At 30 Hz or above the imperials will have good output and really strong at 40Hz and up.

I hope this is useful info.  And here's to NEVER hearing the death rattle of your woofers unloading!!

Jim P
[/quote]

Yes, thanks for sharing. However, the warning with the Imperial is that drivers with an Fs lower than 40 will unload below 40Hz. Does this mean that drivers with Fs above 40 won't be unloaded below 40 Hz? it's a bit confusing to this dull boy. <Hand me another glass of wine. There now, thanks.>

The reason I specifically asked about 18" drivers is that I've never seen one with an Fs above 40.

-Jeff
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gexter
Ex Member



Re: Imperials?? i want some!!
Reply #22 - 11/02/05 at 04:01:18
 
I think I'll pass on this one with just a guess as I leave.
I think If a driver has a Fs higher than 40 it can unload lower in the Imperial at 35Hz lets say because of the horn.

can somebody yea or ney that and tell me why

Gex
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DirtDawg
Ex Member



Re: Imperials?? i want some!!
Reply #23 - 11/02/05 at 10:29:38
 
I know it's hard to imagine, but the Imperial reacts more like a BR cab in the lowest frequencies. It has a volume of air tuned by a vent with a KILLER Tapered/Horn Loaded port. Unloading the drivers is related more to the volume and vent than the Fs of the driver. The danger is in having a driver unloaded at or near the resonance frequency. It seems, from what I've read here, that the Stock Decware Imperial is tuned to the 30Hz to 40Hz range and if you use a driver with a lower Fs it will not be supported at the critical resonance frequency. Any driver will be unloaded below the tuning of any vented box, but if the Fs falls in the tuning range, the control will be ideal. Higher Fs will also be loaded well, but a lower Fs than the box is tuned for is dangerous to the driver, even more so if it has to try to reproduce that range of freqs.

With that in mind, it should be possible to modify the chamber volume and vent circuit in order to support any driver. Extensive modeling and testing would be necesary to guarantee good results. If you just want to whip out (like it's some minor undertaking) some Imps, you're better off using drivers that have been proven in the past.
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« Last Edit: 11/02/05 at 10:53:12 by DirtDawg »  
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Adrian D.
Ex Member



Re: Imperials?? i want some!!
Reply #24 - 11/02/05 at 15:43:52
 
if all this unloading appears, then why does steve say on the site that they had a 28dB gain @ 28.5hz ? are freq this low achievable without unloading ?
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DirtDawg
Ex Member



Re: Imperials?? i want some!!
Reply #25 - 11/02/05 at 16:37:41
 
"... achievable ... ?"

Absolutely, since unloading doesn't happen all at once. Your driver doesn't just reach the edge of a cliff an fall off. At the tuning frequency, whatever the actual value is, there will be a useable range on both sides of the "cutoff".

One beneficial effect of using the horn shape for a port is that the actual cutoff rate (in dB/octave) is less steep, meaning it will resonate across a wider range, instead of just a narrow band centered around the actual tuning freq. Another benefit which we are all interested in, is that whatever energy reaches the horn, after loading the chamber, will be better coupled to the environment (amplified).

When considering reflex ports of all kinds, try not to think in terms of a few frequencies, but rather in octaves. Depending on (many) design factors, some ports roll off at slightly more than 6dB/octave, which is the natural driver rolloff in an infinite baffle, while others rolloff much faster, even 24dB/octave because the driver output and the port output are approaching 180 degrees out of phase with each other. In that case you will have a port which, instead of suppressing excursions, actually helps to cause extreme excursions.

Grin
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Hornlover
Ex Member



Re: Imperials?? i want some!!
Reply #26 - 11/30/05 at 21:48:09
 
I still dont understand why so many folks believe that you need a driver with an Fs above 40Hz. This isnt the case. The primary driver Jensen recommended for the Imperial was the P15LL (part of the KT-31 3-way kit), and it has an Fs of 37Hz. I have used other drivers with an Fs in that range, such as the Selenium WPU1505 (36Hz) and have had great performance. The only place I have seen reference to an Fs above 40Hz was in a variant Jensen published of the Imperial, which used a different back chamber, and was smaller than the full sized version.
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Corey
Ex Member



Re: Imperials?? i want some!!
Reply #27 - 11/30/05 at 22:55:36
 
Hornlover,

I really don't have a clue about the chamber differences between the two but keep in mind the Decware Imperial is different than the Jensen and the Jensen facts you stated may or may not apply.

Corey
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stvcmty
Ex Member



Re: Imperials?? i want some!!
Reply #28 - 11/30/05 at 23:29:17
 
John F has done a lot of modeling of the Decware imperial, and when a driver with a FS below 40Hz is used, there is a peak between 80 and 90Hz.

Jim P had some funny peaks that he had to tame with his crossover and graphic EQ.

Stv
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JimP
Ex Member



Re: Imperials?? i want some!!
Reply #29 - 12/01/05 at 01:13:20
 
Stv,

I'm in the middle of teaching my unit on sound and resonance so all of my equipment and the Imperials are set up in my classroom.  I will try to do some frequency analysis of the speakers.  One thing i would like to try is a FFT analysis of the spectrum for white (or pink) noise.  We have a computer-based system that can do a frequency analysis up to about 11kHz.  For the Imperial I would be happy with up to 1kHz.  I just have to find the time, but now seems like the best opportunity.  I should have the resolution to pick up any big dips of peaks in the response although room effects could be a problem.  I'll mic it close to the horn mouth.

If I can get this done I'll report back.  Maybe by this weekend.  I'm curious to know how much output there really is below 40hz.

By the way, 2 Imperials in a classroom is a guaranteed conversation starter with the kids!  And the neighbors might start complaining - we were doing sine sweeps today so the kids could get a feel for what 20-20,000 Hz really means.

Later,

Jim P
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