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Old 23rd February 2008, 12:37 AM   #1221
iand is offline iand  United Kingdom
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Reflex parameters (first post was SPL)
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Old 23rd February 2008, 08:52 AM   #1222
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G'day again Ian

>Your info about port nonlinearity is very interesting, and much more severe than I would have expected for decent-sized ports (20cm diameter per driver) -- how did you calculate these numbers? This is exactly the kind of large-signal effect that's ignored by most speaker modelling software (but obviously not yours :-)

Tom is spot on bringing up the port non linearity problem. Most modelling software will not show you just how bad the problem is. I've attached a picture of one of a pair of subs I was using in a domestic environment a few years ago. My aim was to reduce this port nonlinearity as much as possible. 250L box volume with an Fb of 22Hz. The port took up over 100L of the box volume yet still had measurable port compression. The driver used was the 18 Sound 18LW1400. This is the cleanest high output vented sub I've built. It's performance can be beaten with a much cheaper driver in a tapped horn.

>But I have to point out that most people (amateurs like me!) don't use big stacked arrays, which is why I was comparing a single tapped horn with a dual 18" reflex -- 133dB at 1m from a 320l box is enough for the likes of me :-)

A sub sitting in the corner is acoustically the same as a stack of four subs. I put mine in the corners. The tapped horns respond very well with this placement.

Cheers

William Cowan
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Old 23rd February 2008, 02:58 PM   #1223
tb46 is offline tb46  United States
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Default TH v. Bass Reflex

iand: Hi Ian, Thanks for the screenprints, that will give me something to look at. You may also try downloading the free Gadwin PrintScreen utility, it gives you additional capture features.

Actually, I feel that William Cowan and Tom Danley have already answered the question you posted, but, I'll see if I find anything useful to add (low probability).

djk: Thanks for helping with the screen capture.

cowanaudio: Post #1222: Now, that's a duct, nice woodworking.
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Old 23rd February 2008, 04:58 PM   #1224
MaVo is offline MaVo  Germany
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Quote:
Originally posted by cowanaudio
G'day MaVo

Generally 3:1 is seen as the upper limit when designing bass horns. More than this puts too much stress on the cone, which will soon fail. This rule of thumb applies with pro sound drivers in a pro sound environment, but I believe is applicable at home too. I have seen photos of drivers that have been ripped up in horns with a too high compression ratio, so this is a very real problem. I think in comparison the distortion created when using high compression ratios is a non issue. In the home the levels where this will become significant will be causing plenty of distortion in the structure of the listening room!

Cheers

William Cowan
Hi William, i thought about this and searched in the lab forum archive for information. I found a post by Tom, concerning the throat distortion in the lab and bt7. First of all, just like you said, throat distortion in a basshorn seems to be no issue, even with such extreme designs as the bt7. the problem is the one you mentioned, that the driver can get torn up.

The strain on the driver and throat distortion is proportional to the driver velocity, which Tom pointed out in this post. The equation "velocity = excursion * frequency" describes the velocity. We see, that velocity rises with frequency and excursion.

One could take the labhorn as an example for what strain is acceptable. It seems to have a compression ratio of around 3 and can be used to around 100hz - judging from the response, maybe up to 200hz - but i dont know the exact numbers. With this information together with the fomula v=x*f we can get maximum compression values for horns with different xmax and cutoff. The cone stability has obviously a place in this equation, but i think one can assume that two high quality drivers with about the same cone weight will have about the same limit here.

For example, a horn with 50hz upper cutoff, which is about half of the labhorn and the same xmax as the lab could have a compression which is twice the labhorns compression. Another example which could accept the doubled compression could be, that we want the same upper cutoff of 100hz, but limit the excursion to the half, since we only want to use it in a home environment.

Following this, i would say, if we choose the right constrains, such as low cutoff or spl, we can build horns with compression ratios of greater than 3. I have simulated a TH with a peerless xxls 830842 with a compression factor of 6. The constrain could be to limit it to frequencies lower than 50hz. This design easily scales to lower cutoffs (i would really like to hear/feel a <20hz sine wave ) by adding horn length to each horn and increasing the number of horns.
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Old 24th February 2008, 02:07 AM   #1225
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G'day MaVo

I'm not sure that velocity is the right parameter to look at if you want to get a feel for the acoustic load on the driver. Consider a 10:1 compression ratio, the driver would have a very great load on it, but the excursion and therefore the velocity would be low. Picture two drivers mounted face to face and driven with the same polarity, so they are fighting each other. The load on the driver would be very great, but the excursion would be nearly zero. You would likely destroy the drivers if you pushed them very hard.

Also remember that the LAB12 driver was specifically designed for horn loading, and as such is very robust. It's likely a driver with a lower Mms will not be as robust as the LAB12.

Be careful.

William Cowan
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Old 24th February 2008, 05:28 AM   #1226
judtoff is offline judtoff  Canada
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Quote:
Originally posted by cowanaudio
G'day MaVo

I'm not sure that velocity is the right parameter to look at if you want to get a feel for the acoustic load on the driver. Consider a 10:1 compression ratio, the driver would have a very great load on it, but the excursion and therefore the velocity would be low. Picture two drivers mounted face to face and driven with the same polarity, so they are fighting each other. The load on the driver would be very great, but the excursion would be nearly zero. You would likely destroy the drivers if you pushed them very hard.

Also remember that the LAB12 driver was specifically designed for horn loading, and as such is very robust. It's likely a driver with a lower Mms will not be as robust as the LAB12.

Be careful.

William Cowan
Agreed completely.

You could imagine that the driver has to "lift"(move, etc) all of the air in the horn. That is a substantial amount of mass! I know thats not how it works exactly, but I think its a good analogy to consider.

Or maybe that it is essentially mechanically(using air) linking the mouth of the horn to the drivers cone. If the mouth has a huge surface area compared to the throat, then there will be a large amount of resistance to the cones movement.

Hmm...well at least thats how I view it.
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Old 24th February 2008, 10:53 AM   #1227
MaVo is offline MaVo  Germany
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Hi William, i think i assumed a constant horn load on the driver over all frequencies, so to speak an ideal horn with a flat frequency response. As the horn unloads the driver in the low frequencies and becomes acoustically invisible, the strain will be even lower.

Lets say we have this perfect horn and a 10:1 compression ratio. Imagine you push the cone to its xmax limit with your hand and hold it there. This will not strain the driver. Now imagine making the compression ratio higher to maybe 50:1 and and doing the same. Nothing will strain the driver, as long as you push it slowly enough. This is a simulation of a near dc signal. With such a big compression ration, imagine pushing the cone in and out periodically. The strain might increase a little, which you could feel as a force, which opposes your pushing, as the frequency rises.

Here we have an example were one can see that the force on the driver rises with frequency, excursion and compression ratio. One could make a pseudo equation like "strain = excursion * frequency * compression ratio". Because frequency * excursion = velocity, we can write "strain = velocity * compression ratio". Now lets take the drivers construction into account. My assumption has to be that the drivers robustness is proportional to its cone mass. Then you can write "strain = velocity * compression ratio / mass".

Your example of two same polarity drivers works very well with my pseudo equation. The compression ratio for such an example would be very high, thus you have to keep the other factors (excursion and frequency) low, to prevent damage. If you have very robust drivers, you could increase velocity a little, as it can stand more strain.


Hi judtoff, please imagine, that the driver has to lift all the air in the horn very slowly. This will be no problem. Imagine, it has to do this very fast, which will be a problem. Imagin you build a heavier cone, now it can stand a heavier load. This is were i take my pseudo equation from.
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Old 24th February 2008, 11:23 AM   #1228
AndrewT is offline AndrewT  Scotland
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If it helps.
The dynamic mass of the air load must increase as the horn becomes more efficient.

Look at a couple of examples.
reflex efficiency ~2% implying that 98% of the input is converted to heat in the VC and cables.
Horn efficiency ~50% implying the same heat conversion in the VC but all of the remainder of the input is converted via the horn air load into acoustic output.

Would it be right to assume that the horn air load is 50/2 (100-50%/100-98%) more massy than the reflex?
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Old 24th February 2008, 01:45 PM   #1229
MaVo is offline MaVo  Germany
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Good question AndrewT. I hope someone with a little more knowledge can help us here, as all i do is guesswork.

My approach was simply to take a very efficient horn like the labsub and take its parameters as a limiter for other designs, since i think that Tom has made it as efficient as possible, leaving little room for improvement. Then i tried to find a relation how i could transfer this limit into other designs. This is where the equation comes from, which i posted earlier.

My reasoning is this: If the lab12 in a labhorn can take the strain of 13mm excursion at 100hz, then it could (if the xmax would be bigger) also accept a 26mm excursion of 50hz, which would equate into the same amount of stress on its structure. For example, if the labhorn would be limted to 50hz upper cutoff, it could accept a doubled compression ratio.

I dont know it this is correct, but i doubt that a rule of thump (for example a maximum of 3,5 ratio for basshorns) is accurate under all circumstances, especially since what we do here is not really PA woofer realm. I think that in reality one can trade different factors, which i would like to identify. In the moment, i think the factors are frequency, excursion, compression factor and cone stability. You and William mentioned the air load on the driver, generated due to the efficiency of the horn. I dont know how to take this into the equation.
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Old 24th February 2008, 03:28 PM   #1230
kstrain is offline kstrain  Scotland
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Default Pressure/Force

A few ideas floating around the topic of maximum compression ratio.

The cone can fail when the unbalanced force on it becomes too large.

Two scenarios come to mind:

a) pistonic: when the limit is the strength of the weakest ring of the cone (probably fails around an arc then tears outwards (seems quite common from pictures of cone fractures)

b) modal: when an air mode (or at higher frequency a cone mode) leads to too-great shear about a nodal line (probably less common, requiring an "error" in design - but I can think of one or two ways it could be accentuated).

AkAbak can tell us the overall force (or equivalently pressure) on the cone. That should be good enough to hint at potential problems. A few (not very systematic) tests show that, in slightly different THs and looking around 30-50 Hz, the force can vary drammatically for very little change in the efficiency.
I'm not sure where to expect failure, but >>100N forces starts to get a bit scary, I guess.

That - with some experience and trusting AkAbak- should cover a). Probably b) is a relatively minor problem except in a particularly poor design (unless I'm missing some obvious case).

If I had an AkAbak Lab-sub model I'd make some comparisons, but I don't - I agree that is a good reference point.

Ken
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