Pearless, Sea or Vifa ???

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#86
These are things I still wonder about, like for instance when calculating a filter: do you have to calculate with the average impedance or do you need to take the value (from the impedance response graph) that the driver has on the x-over freq.? (example: an 8 ohm woofer can have an impedance at 3khz of 20 ohms, so do you calculate with 8 ohms or 20 ohms? I don't know for sure...).

Most of my experience is out of listening to hundreds of commercial speakers on a switchpanel...(A/B test..)
I'm not a calculator artist....my ears never lie though...

But I think there are a lot of acoustical factors to take into account that also influence Q... But Q is very audible, so I think it's more important to know which direction is easiest to tweak to (like the enclosure size, you can't make it larger very easy, but you can make it smaller pretty easy so it's probably best to start on the large side...)

These things will probably show well enough in a proto...
 
#88
Hi,

What do you think of using a 2th order Linkwitz-Riley at 2200Hz. That way, I'm 2 octaves over the tweeter Fs(550) and a little more then 1 octave under the point where the drive SPL start to decrease(5000Hz) ? I plan to use a tweeter L-pad and a EQ for the driver to remove the impedance spike around 4kHz and 5kHz.

Vifa driver specs
http://www.madisound.com/pdf/vifa/pl18wo09-08e.pdf

SEAS tweeter specs
http://www.madisound.com/pdf/seas/H1149.pdf

Thanks,

REALFLEO
 
#89
Hi Realfleo,

nice you linked to the driver specs, but I don't understand nobody else is responding...I had trouble with my computer, had incredible luck with the repair. My computer didn't want to start, PSU was dead...now despite the fact that everybody disadvises to repair a compu-PSU I replaced a suspicious capacitor (the top had swollen and it was near the obvious overheated PCB area, so I thought let's give it a shot). The original cap was 2200uF 10V and I had a 2200uF 16V in store...
After replacement it went on immediately! Yippeeehhhh!!! :worship:

But to get to the point, 2.2K seems like an excellent starting point to me, choose the cap for tweet filter on the large side is my guess and the coil also because you can unwind it a bit...the larger values will result in lower x-over freq.(also for the PL18) it also helpsa bit to get rid of the slight peak in the response curve of the PL18, crossing over slightly on the low side will help... Since the tweeter is going to produce a large part of the curve you might not want to build it in an external (small) enclosure, but I would experiment with this in the proto, maybe add a few pieces of wood to enlarge it to compare the difference...

I'm not sure if the impedance peak needs to be straightened out, it's pretty far out of the freq. area...But if you can measure it you can try to flatten it out nicely and create a nice even load for your amplifier...

I presume you will use fixed resistors for the L-pad, I would not recommend using nasty potmeters...(if you want adjustability I would recommend a stepped switch and create a few levels with fixed resistors)....

12dB/octave Li-Ri will probably do the trick well enough...(I prefer to use the lowest possible order in speaker filters, with the lowest possible component count in the high power signal path...)

Welcome again offcourse...
 
#91
Thanks for your posts v-bro.

I designed my crossover and I'll try to post it before wednesday.

I re-mesured Q last weekend with another technique (http://sound.westhost.com/tsp.htm) and I mesured Fs around 39Hz and Qts of .40 instead of .35 so I'll use this Qts for building my cabinet. Maybe it's due to break-in...

I want to design my x-over before cabinet to include the x-over resistance into my calculation of Qts.

I was sure you had a problem with your computer or internet connection cuz usually you post couple of times per day 🙂

Also, I found a nice damping material in a car stereo store near my place. It's call dynamat(http://www.dynamat.com/). I'll test this material with my proto.

Thanks,

REALFLEO
 
#92
Let me guess, 6.8 or 10 for the tweeter cap and L will be 0.6? Two resistors one 5 ohms series 20 parallel? Just some wild guesses.....🙂

Great links...if not only for me, it's great for the apparently interested other readers...😉

Only couldn't find the exact damping material you proposed...

Good idea to calculate Q before creating the cabinet, the closer the pre-calculation the easier it will be to tune it to good results...

I've also been very busy with birthdays and work and christmass stuff lately, but I'll be 'round...
and keep
:smash: ing...😀
 
#94
Looks very good for panel damping, allthough there are constructional ways to prevent resonances (like mounting the driver by fixing the magnet to press the driver (with foam or rubber sealant strips) against the baffle. This is known to work like a charm....

You will probably still have to add foam or stuffing to reduce air turbulences within the cabinet...but most cabinets only get this, most manufacturers don't care much about cabinet resonances.....

I ran into some interesting material at work a while ago (believe I allready posted this in this thread...). If you first stick a thin layer of foam to the insides and stick the dynamat stuff on top, finish it off by adding another thick layer of stuffing (always best to anchor it to something...) you get something about the same.....(I got to use it to silence a generator in a small van (a journal van with dishes on top and lots of equipment needing to be self sufficient in the field) it silenced it almost completely(only two meters away!!)).

my :2c: again...
 
#98
To make a long story short, I built a prototype according to params that I mesured myself:

Qes: 0.46
Qms: 3.125
Qts: 0.4

I computed the volume and vent size with a software named AJ Vented Designer
http://www.ajdesigner.com/speaker/index.php

I also confirmed ouput of this software with formulas from Loudspeaker Design Cookbook and some other sites and they were really close.

What the software told me:
Vb=21.5421lts
Fb=40.1713

To this volume, I had 20% plus space occupied by the vent and braces.

So the volume I used for the prototype is 27.3535lts.

Last weekend, I produced a impedance curve and tried to mesure box losses with this procedure:
http://users.ece.gatech.edu/~mleach/ece4445/downloads/ventedbox.pdf

I obtained -0.12 !!! 😱

I recalculated many time and I got the same Ql then I realized something: maybe my Q params were wrong so I recalculated box volume with Vifa's params:

Qes=0.4
Qms=2.46
Qts=0.34

The 0.06 difference on Qts gave me a Vb=12.15.

So I made my box twice as big as it should be...

There is something really strange in this. According to LDC it is not unusual for box losses to be lower than 7 and I should increase the box size to fix it ... but it's already 2X bigger. It's a nonsense

This is where I am now, asking myself if I should build another prototype with this new volume...

I'd like to understand why my mesure of Q's were so offset...

Should I forget the strategy of oversize the box as suggest by LDC and fill it with a solid filler to ajust box losses and just try it the right size ?

What you think ?

Thanks,

REALFLEO
 
#99
http://www.partsexpress.com/projectshowcase/nebula/nebula.html

But perhaps you can close the holes of one bracing and fill the compartment with sand for extra damping....

When I lick my finger and put it in the air I think any volume for a ported box for the PL18 between 8 and 17 liters will do fine and can be tuned further with the portlength/diameter. A lot can be done with the right amount and placement of the stuffing.

For a closed design about 5 to 8 liters will do fine.

Basically all the cabinet does is handle the back wave, so a triangular or spherical cabinet sounds different from a rectangular one. Just look at wave patterns in water, a lot of this can be compared to acoustic wave behaviour. A closed design started upon the idea of eliminating the back wave which is basically anti-sound (reverse phase) and a speaker would produce low frequencies yippee! 😀

A ported system started upon the idea that if the backwave should leave through a hole in the cabinet it would flip phase and amplify the lower regions....smart! 😉

About 20 years ago I built my first loudspeaker, it was a closed cabinet design with a way too large (12") woofer and two speakers taken from a port. radio/cassette as midranges (bad idea... 😀 ) and Philips dome tweeters. I basically licked the finger and glued 12 panels together.

Later I bought Elektuur 'loudspeaker special' magazines and decided to build a horn from MDF (the mcFarlow exciter) it was cheap, produced a whole lot of sound for the buck and the designing was prefabbed...

Than came the books and design software, and I finished my studies in EE.

The thing is that it can become enormously complicated to try and calculate every little electrical, acoustical and mechanical influence in a system. You can learn more from experimenting, trial and error, and measuring what happens (or listening well to what happens).
And it will still be complicated enough...And there will always be this person telling you to go for a whole other approach.:cannotbe:

Nowadays most of the times I basically look at a few designs with the driver I'm going to use, and use some online calculators (most of them work very well and produce similar results) and mix in a bit of experience in the design (construction/material/shape) of the cabinet. Than build (a couple of) proto's (unless it's 'love at first audition' 😛).

Like I'm now thinking of a sea urchin shaped enclosure and all spikes are small TL shapes....that should handle the back wave nicely... Or a sphere in a sphere, the driver in the centre sphere and a hole in the outer sphere larger than the driver....the two spheres held together by distance blocks...could be a TL or a BR...🙂

A nice shape is where it comes down to IMO, the volume of the content can result in very similar sound and measurements over a certain (wider) range of volume....

I do follow manufacturers data mostly because I think they want you to get the best results with their product, but I use them and calculate things just as basic guidelines to the eventual design. Along the design process it will become clear what measures deliver audible differences and the design basically is tailored to my preferences. So I can't just say do this or that....

Cheers!
 
#100
Nice post thanks v-Bro.

It's a good idea to use sand to ajust my box volume. I guess that'll give me a good idea of how a smaller box will sound.

I think I'll try different volume and I'll build another prototype after some listening tests.

I noticed there is not enought bass with my oversized box...
Can you explain it ?

Thanks,

REALFLEO
 
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