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Labhorn vs. Ported

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#1 of 9 OFFLINE   Matthew Will

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Posted February 25 2003 - 07:14 AM

Hello, What is the difference between a LabHorn designed enclosure and a ported design? Why is the Labhorn that much louder? Matt

#2 of 9 OFFLINE   Dustin B

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Posted February 25 2003 - 11:40 AM

Ported is bass reflex, the labhorn is well a horn. Horns can have extremely high efficiency. A Lab12 Basshorn if I remember correct has sensitivity up around 103-105dB/W/m and it keeps going up the more you use. It can also handle well over a kilowatt of power. A bass reflex design uses a second passive resonator to aid output around the tuning point (port round or square, passive radiators, may even be some other methods). You can design a ported sub to get close to the sensivity of a large basshorn, but none will match it (and it's not free, extension and size will be the trade offs).
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#3 of 9 OFFLINE   Greg_R



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Posted February 25 2003 - 11:52 AM

However, the design & construction of a ported enclosure will be significantly easier (vs. the horn).

#4 of 9 OFFLINE   Mark Seaton

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Posted February 25 2003 - 12:26 PM

First, note that the "LABhorn" name comes from the fact that this particular design was developed on the ProSoundWeb's Live Audio Board (www.live-audio.com). Our designer, Tom Danley, offered to do the computer simulation for a horn of a given cut off and size as determined in the dedicated forum. A friend of ours at Eminence Speaker offered to have a driver built up and distributed for those interested in building it and worked with Tom Danley to achieve a good match to the end goal. In many ways, this is rather akin to our 15 year old BassTech7 subwoofer, yet using a pair of more conventional 12" drivers.

Now for why it works... The exact details of how the horn works can get quite complex, so let's keep it general. In the ideal, a horn provides a better coupling between the driver(s) and the air. Looking at actual power into a speaker vs. acoustic power out, we see that for a normal speaker, like a ported box, the efficiency is quite low, on the order of 1%. This means that 99 of 100W is being lost to heat. In the case of a horn, we can ideally reach about 50% efficiency while still having ~flat response. As you can see, we just took 49 watts of that 100 and made it sound power rather than just heating up the driver more. Now, most of us understand that making bass is about "moving air" or total volume displacement. So how is it that two 12" woofers can produce more output than 4 18" drivers? The reason is because of the mechanical "matching" of the air to the driver. This makes more sense when we consider this ideal case of reaching ~50% efficiency. At this efficiency, what effectively happens is we couple the motion of the air at the throat(in front of the drivers) to that of the horn mouth. This means that 1/2" of air movement at the throat makes this large horn mouth move the same 1/2" (22" x 45" in the case of the LAB horn). Figure out how large a driver that is! Now we have this other tool we can use which is called compression loading. This means that the area of the throat is smaller than that of the driver. In the case of the LAB horn, the throat is 1/2 the size of the two 12" drivers, so it is ~equal to the area of a single 12" driver. This means that the 2 12" drivers only have to move 1/4" each to produce 1/2" of movement in the throat. Since motion in the throat is ~equal to motion at the mouth, you quickly see that even a pair of 12" drivers produce a lot of air motion. Think of the horn as a mechanical "lever" between the driver an the air. As with any lever, if you are moving a lot with a little motion, it takes more force than normal, and thus the hefty motor on the LAB 12 drivers.

So why don't we see more horn subs? Simple, this "lever" is only effective over a limited frequency range, and that frequency range is determined by both the driver parameters and the physical dimensions of the horn. With most bass horns, in order to provide loading(our lever) at low frequencies, they generally need to be at least 1/4 wavelength long. This means just to get to 40Hz, the horn needs to be over 7' long, and twice that for 20Hz. Similarly, the mouth of the horn must be large enough for a given low frequency output. The mouth of a bass horn for outdoor use must be quite large. In fact, the LAB horn is designed to pack 4-6 units side by side to provide a large enough mouth to work outdoors. Fortunately we can make use of boundaries to effectively mirror the size of the mouth. For outdoor use, we can count on the ground to provide a mirror of the mouth, which is why to achieve what is effectively a roughly square mouth, we need the "real" mouth of the horn to be twice as wide as it is high. Imagine that the other half of the horn is below the ground, just as you would see it if a mirror was placed on the ground. We can do the same with side walls indoors, cutting the required mouth in half again.

I have in fact played with a design which counts on these "mirrors" to allow a large, but managable sized box to provide horn loading down to below ~30Hz when placed properly in a room. Having around 140dB peak capability from a single box in a closed room is quite an experience indeed. If you search the DIY forum you will find a thread where I described how one or two boxes can be put to good use in a home.

What should also be noted is that there are other benefits to a well designed horn beyond just loudness. The loading of a horn not only makes for a more efficient system, but also makes for a more accurate system over its range of loading. A typical sub will by nature smear the sound with respect to time. Effectively the system delays lower frequencies more than higher frequencies. As we approach 50% efficiency, this phenomenon changes, and more of the frequencies eminate with at the same time. If we were taking measurements, this can be seen most directly in a measurement of group delay. Looking at a measurement of phase can tell us the same thing, yet is less intuitive as flat is only "flat" if the response is near zero. If you see a reviewer examining a square wave through something, the qualities of the resultant square wave is tightly dependent on the phase/group delay characteristics.

Hopefully this helps more than it confuses! Posted Image
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#5 of 9 OFFLINE   Michael R Price

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Posted February 25 2003 - 12:40 PM

Thanks for the very informative post Mark. For some reason, the speakers I really want are always too big.

If you could recall off-hand, what kind of extension could one expect with a pair stacked in the corner of a medium size room? I know a pair's normally good to around 30Hz, but it would be interesting if one could somehow load the horns to reach to some of the infrasonics in movies.

I do remember that thread about the use of these in rooms, but couldn't seem to find it. You see, this is the kind of thing I would love to have somewhere out there in the future and I can't help but think about it now. Posted Image Thanks.

#6 of 9 OFFLINE   Mark Seaton

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Posted February 25 2003 - 06:01 PM

Hi Michael,

Go back and search the DIY forum here at HTF and search for Prosound Web, or just search for my posts. You will come across it eventually. Should only be a couple months back.

As for two stacked in a corner, the idea is to essentially couple to the height of the room. By placing the mouth firing *into* the corner, spaced 28" from the corner, you effectively extend the horn by a large amount. It is a somewhat difficult situation to model, but I would expect SERIOUS output to ~28-25Hz. Given some room gain and other effects, the roll off could be somewhat gradual, but I can't say for sure. One nice thing is that even after the horn begins to unload it is still a sealed box and allows for a good bit of EQ. I would bet money that two stacked in a corner could be EQ'd to still produce some pretty serious 20Hz output. You have the benefit of the drivers barely moving from 30Hz up, so you have some extra excursion to play with. Adire's DPL-12 might give a little more in this application as well.

Something wacky like this may one day see the extreme HT/ studio market, albeit targeted at a somewhat limited bunch of nuts like myself. At least I can say I've met a few even further obsessed.Posted Image
Mark Seaton
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#7 of 9 OFFLINE   Chris Tsutsui

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Posted February 25 2003 - 07:54 PM

Stacking subs and corner loading can benefit ported and sealed subs as well. I think horns get that extra spl advantage over sealed and ported though. I think the biggest drawback for horns is the space they take up. Bass reflex can aid in low frequencies simply by lengthening a port. Horn designs also takes careful design and craftsmanship, and generally lots of space. I could only imagine the benefits of the horns sensitivity with subwoofers as I've heard how horns can actually sound great when I auditioned Avantgardes. Why not also ask about TL, Line array, cluster, bandpass, isobarik, and dipoles. hehe, I never knew how many ways there are to move air until I discovered this forum.

#8 of 9 OFFLINE   Michael R Price

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Posted February 25 2003 - 10:09 PM

Thanks Mark. It turns out what I was looking for was the second page of the original LAB thread.

#9 of 9 OFFLINE   Mark Seaton

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Posted February 26 2003 - 04:54 AM

[quote] Stacking subs and corner loading can benefit ported and sealed subs as well. I think horns get that extra spl advantage over sealed and ported though. [quote]

Hi Chris,

There is a distinct difference in these cases, where a Boundary Compliant™ horn can be effectively made larger and longer through careful design. This is much more than just the increased intensity due to the spacial confinements. Boundary loading may increase the observed output of a ported or sealed subwoofer, but it will not extend the low cut off or change the driver loading (as identified by a change in impedance).
Mark Seaton
Seaton Sound, Inc.

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