lobing

I want 2 x Audax TM025F1 1" tweeters per channel for my system. What is the best way to arrange them (other than a bipole or dipole) to prevent lobing? I'm wondering if I should angle them 90° toward or away from each other. Also 2 x Tangband W4-656S 4" midwoofers crossed over really high, like 6kHz.

The whole system includes: 2 x 12" Sony Xplods (hopefully not for long, I want DSQs or something); 2 x Tangband 4" midwoofers; 2 x Audax 1" tweeters; Xovers at 250Hz and 6.2kHz.
 

pinkmouse

diyAudio Moderator Emeritus
2002-04-03 7:15 pm
Rotherham, England
Hi Kilo

Ok, but the Tangbands (you should check out the frequency curves on those things!, pretty flat all the way to 15k, except for a little spike at about 7k) will be crossed over so high, will they experience lobing?

Yes! I get the feeling you haven't quite got the hang of constructive and destructive interference patterns.

Imagine, (or better still try it next time you're in the bath!) two spherical water waves meeting- when then two peaks meet you get a peak of twice the height of the individual waves, and in the troughs, twice the depth. This causes rays of high and low response to emit from a common centre between the two centres of radiation. this is called beaming or lobing.

Now, if you move the two sources closer, this lobing becomes less pronounced, ( it is still there, but the high and low peaks are spread at a wider angle from the common centre), and this is what we try to do when building speakers to give a smooth even response across a wide angle.

However, if you were sitting glued to one spot, or a long distance away from your speakers, you could use lobing to good effect. By careful design, you could focus the constructive interference towards your seat, and destructive interference towards the walls in your room, therefore reducing room reflections to a minimum, (one of the reasons for the current trendiness of line arrays in live music PA).

This will happen with any drivers sharing the same frequency response, even if they are different, say M and T, if the roll off of the crossover is slow, there will be some common frequencies produced by the two drivers, and this will cause interference to occur. This is one of the reasons why a pair of loudspeakers have a sweet spot, and people like me spend ages mucking around with placement and toe in...

However that was the easy part... The real world is not two dimensional, with circular ripples, it has three dimensions, and the waves are spherical! This means that instead of rays of high and low pressure coming from the accoustic centre, we have miss-shapen cones.

But I could waffle on about this for ages...

To get to the point I think I'm trying to make, I think you have two options

1) Just mount all your drivers as close as you possibly can to each other, and it might be worth building a test box with a removable front so you can try different arrangements.

2) Go for simplicity and just build two boxes with a single TMW arrangement, nearly halving your costs, and producing probably a better sound "out of the box" so to speak.

I personally would go for option 1, but that's because I have the time to play, the wood working skills, and I already have a fine pair of front speakers I built myself, and so I am in no rush to achieve success, but if you want to have something you built yourself NOW, to listen to music and show off to your mates, then go for option 2

I'd use a t-line stuffed till it is aperiodic...

Couldn't agree more mate... it's the way to go...
 
One more thing though, does option 2 envolve 2 of my mids per side (4 in all, maybe 4 boxes) or just one? Does "Go for simplicity and just build two boxes with a single TMW arrangement" mean 2 boxes per side, or 2 boxes total?

I don't think 1 per side will give me enough SPL. I have 120W into 2 x 12s/side, and the mids are only 4" rated like 40W each. One will maybe be able to hit only 103dB, which is not nearly enough to keep up with the subs, but with 2 per side, and 80W, 109dB, should work, so I may have to go for option 1.
 
Kilowatt said:
I don't think 1 per side will give me enough SPL.

Hmmm... maybe a midrange with higher efficency is called for then. My favorite one is the Audax PR17 -- the paper one. Easily good enuff for home use, yet gusty enuff that a pr (in horns of my own design) were fine for PA work. 120 W should be good for about 120 dB -- i used a Bryston 4B (one channel per driver) in PA work and never popped one.

dave
 

pinkmouse

diyAudio Moderator Emeritus
2002-04-03 7:15 pm
Rotherham, England
excellent description! You should pick that up and drop it in the Wiki.

Cheers, I will when I get my brain in gear to find out out how it works!!!

One more thing though, does option 2 envolve 2 of my mids per side (4 in all, maybe 4 boxes) or just one? Does "Go for simplicity and just build two boxes with a single TMW arrangement" mean 2 boxes per side, or 2 boxes total?

Well, you could try a MTMW or TMMW arrangement, or as Dave said, try another mid, but if you're not desparate to get running speakers I would still try option 2, and just play around till it sounds good, ( just keep the drivers as close as possible).

As for power allocation per frequency I was once told a quick rule of thumb by a guy who designed systems for Turbosound- for three way systems take the power for the higher driver, double it and add another half, so for a three way driver with 10w on top, mid would be 25w, and bass 75w

Just found this link about driver and box coupling- may not be much use, but it looks good, but I can't check it properly as my registration hasn't come through yet.

http://www.meyersound.com/products/software/mapp/online/
 
pinkmouse said:
Cheers, I will when I get my brain in gear to find out out how it works!!!

Copy your post. Go to Wiki. Find the page you want to add your text too. Click edit this page. Paste your post in. Clean it up a bit if you want. Save.

As more people start to get a handle on the Wiki, the content that does get into it will get organized, edited & updated.

dave
 
That rule of thumb won't work for my Xover freqs, I'm going by a chart at the ESP site anyway.

If, however, I lower the upper crossover fequency, I would then need less power to the mids (which is good) but, I'd need more to the tweeters. I might then have to double up on tweeters, which is bad. The tweeters are rated [email protected]/1m, which, because they're Audax, should be accurate, and they're rated at 60W RMS, but does that power rating really mean anything? I think not. I wonder how much power those can really handle well.

I hope I'm not in a lose/lose situation here, because I'd rather not buy new speakers right now.

I might have to double up on mids anyway, I think I will do a TMMW arrangement (duh) with two T-lines built into one box. And I can then expiriment wether I want (or need) one tweeter per side, 2, or 2 in a bipole. I want sound now, but I can later expiriment and get it just right.
 
Kilowatt said:
Another thing. Who all thinks I should build TLs? I know Planet10 does. How do they sound compared to sealed enclosures? It would take a lot of adjusting to get TL stuffing just right, and they'd be difficult to make. Is it worth it?

I think we were talking about TL loading the mids. You end up stuffing them until they are aperiodic so they are a lot less critical than when you are trying to use a TL to extract that last octave of bass. Here our purpose is to absorb - to suck away - the entire backwave. Behind the driver is just light stuffing and then it gets denser as you approach the terminus (out the back of the box usually).

dave
 

pinkmouse

diyAudio Moderator Emeritus
2002-04-03 7:15 pm
Rotherham, England
That rule of thumb won't work for my Xover freqs, I'm going by a chart at the ESP site anyway.

Probably a much better idea!

I might have to double up on mids anyway, I think I will do a TMMW arrangement (duh) with two T-lines built into one box. And I can then expiriment wether I want (or need) one tweeter per side, 2, or 2 in a bipole. I want sound now, but I can later expiriment and get it just right.


Two ideas spring to mind,
1) build the top and mids into an open baffle and just sit it on top of the bass box, (if you are rolling off the bottom via a crossover, you don't strictly need an enclosure to load the bass resonse)
2)find some plastic or cardboard tubes the same diameter as your drivers, the mids of which can be stuffed as TLs, then if you fix them with hotmelt glue they can be arranged and rearranged as many times as you want, again on top of your bass enclosure
 
That 2nd idea is kind of what I had in mind, but they would be 3/4" particle board, tapered and with a single bend, not cardboard or plastic, TLs have to be extremely rigid. My bass enclosure are already done and I have been using them for a long time, so the rest will just sit on top of them.

Will lobing will effect the performance of the system as percieved from another room? (i.e., will it sound ok from another room?)

Open baffle??!!!:eek: This is a hifi system we're talking about (or at least it will be when I get some better subs). I do want to get it going ASAP, but I will then continue to perfect it, and drivers without boxes just won't cut it.

You end up stuffing them until they are aperiodic so they are a lot less critical than when you are trying to use a TL to extract that last octave of bass. Here our purpose is to absorb - to suck away - the entire backwave.
So I could just build the boxes, 1/4 wavelength, with proper line area and volume, and stuff it, and I wouldn't have to go through all that precision stuffing and re-stuffing?

:) Thanks for your info!
 
pinkmouse said:
2)find some plastic or cardboard tubes the same diameter as your drivers, the mids of which can be stuffed as TLs, then if you fix them with hotmelt glue they can be arranged and rearranged as many times as you want, again on top of your bass enclosure

Tubes with a larger diameter than the mids would work better. ie 4" on a 3" driver.

dave
 

pinkmouse

diyAudio Moderator Emeritus
2002-04-03 7:15 pm
Rotherham, England
Tubes with a larger diameter than the mids would work better. ie 4" on a 3" driver.

Yes, for ideal transmission line theory, I agree, but what we are trying to do here is get the drivers as close as possible.

Maybe use both ideas and mount the drivers asymetrically in the larger tubes so they can still be mounted as near to each other as can be arranged with the stacking.

Open baffle??!!! This is a hifi system we're talking about (or at least it will be when I get some better subs). I do want to get it going ASAP, but I will then continue to perfect it, and drivers without boxes just won't cut it.

I think many people on this forum might disagree with that statement!!!

That 2nd idea is kind of what I had in mind, but they would be 3/4" particle board, tapered and with a single bend, not cardboard or plastic, TLs have to be extremely rigid.

I'm not saying use tubes for the final box, but just as an easy way of playing around with arrangements.

But, I think you will find that for mid frequencies, thick wall plastic tube could be rigid enough, especially as the length you will be using will probably be fairly short, and the only accoustic advantage that particle board has for cabinet construction is that it is cheap.
 
You will find that tubes -- plastic or cardboard (cardboard should be puzzlecoated on at least the outside) are actually stiffer than a particle board box (i have a HUGE halloween fire a-building, largely particleboard boxes that just don't warant re-using).

A circular X-section tube is inherently very stiff - one of the reasons sonotube is often used to build subs.

And what better box is there for a mid (or an open back tweeter) than an open baffle? The aperiodic TL is an attempt to approach this with a monopole.

dave