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Old 28th January 2013, 03:06 PM   #71
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Quote:
Originally Posted by Brian Steele View Post
Once I saw that they substituted a PR alignment for a vented alignment, I stopped reading. The test is IMO invalid - they should have tested a realistic vented alignment with that driver, i.e. one where they either had to implement a massive vent to reduce compression effects (which would have in turn increased the box size), or one where vent size is compromized (as is the case with the majority of built vented alignments).

And even if you have managed to deal with vent compression effects, there's still the compression caused by the increase in voice coil resistance due to heat to be dealt with.
I didn't quote that as a discussion of ported designs, they are not discussing ports.......I quoted it as a discussion of some possible failings of tapped horns, as they are not as perfect as you make them out to be.

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Sealed and Tapped Horn Decay

The TH system does very well over the majority of the range but at 55Hz and 100Hz there is a distinct and very noticeable ringing of energy seen.
Quote:
Looking at the magnitude of the TH system's compression, it is a little bit of a mixed bag. Above 40Hz it has the least compression. From 25-35Hz it has the most and also right at its effective tuning. Actually, all of the systems fared pretty well at the output levels used and were below 3dB of compression effectively.
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In the second graph, the output level of the sealed and PR systems is dropped 8dB lower to the 105dB nominal sweep level and is compared with the TH distortion at its 119dB nominal sweep level. (Remember that the DTS-10 cab uses two drivers here which gives it a 6dB advantage.) The tapped horn maintains better overall distortion levels from 12-33Hz and 65-100Hz even when being driven 8dB harder. This is an interesting comparison because all of the systems are being asked for equivalent SPL levels and frequency response everywhere across the 10-103Hz range. You have to wonder what audible differences there would be between these systems? Is the extra-clean, deep bass of the TH obvious? What about the spike at 42Hz and the jaggedness at 50 and 63Hz?
There are plenty of good resources out there on how to properly design a bass reflex port, and when done properly they mitigate the issues you bring up.
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Old 28th January 2013, 03:20 PM   #72
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Originally Posted by sebDIY
Another point: you can not reproduce music at high levels with a flat response curve. It's common to push sub level for 12 dB or higher.
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Originally Posted by Brian Steele View Post
Is this true? I don't really do PA, but that sounds a bit "bass-heavy" IMO, particularly considering that our sensitivity to bass is higher at louder levels (according to the old FM curves).
A bottom boost of 6 to as much as 20 dB is common for DJs and even some live PA rigs.

This "loudness contour" can be accomplished easier by turning up the sub level or using an equalizer and turning down the frequencies above and boosting those below the point where a "haystack" is desired.

As far as the rest of sebDIY's comments regarding the Mackie HD1502, it's "theoretical" output level is something like 10 dB higher than the measured response of 125 dB.
I actually measured 125 dB of good sounding low frequency output from the box.
It surprised me, at the time I remember thinking "this box could be used without a sub for small gigs".

At any rate, I don't care what anyone here does, but the choice is fairly simple.
If one wants poppy bass 55 Hz and up, the small THs work well, if you want low bass extending to around 35 Hz in a similar size package, use long throw low Fs drivers in a BR box using large ports.

That said, I find one large cabinet easier to move than two small cabinets, and one larger TH cabinet can give the LF extension lacking in the small TH and the upper punch lacking in the BR box.

Art
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Old 28th January 2013, 03:44 PM   #73
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Quote:
Originally Posted by weltersys View Post
At any rate, I don't care what anyone here does, but the choice is fairly simple.
If one wants poppy bass 55 Hz and up, the small THs work well, if you want low bass extending to around 35 Hz in a similar size package, use long throw low Fs drivers in a BR box using large ports.

That said, I find one large cabinet easier to move than two small cabinets, and one larger TH cabinet can give the LF extension lacking in the small TH and the upper punch lacking in the BR box.

Art
Furthermore, I just happened to come across this design for a 21" ported PA sub that remains somewhat portable. This another good example of a properly designed bass-reflex box.

Data-Bass

Quote:
This custom bass reflex system was designed around the B&C SW152 driver to be used in a portable PA style system designed for sound reinforcement where abnormally low bass extension was required. Typical pro bass bins roll off below 40Hz or even 50 or 60hz in some cases. The extension is sacrificed for efficiency and output. This system was designed with an eye towards useable 25Hz extension, high output, the largest vent that could realistically be used to avoid compression and noise and still remain portable. The target tuning was 28Hz, but the proximity of the vent opening to the back wall of the cabinet lowered the effective tuning to about 25Hz instead. Still the cab exhibited very good efficiency over its effective range and exhibited powerful overall performance. Useful output to 20Hz was realized coupled with large amounts of headroom above 30Hz and very low distortion levels.
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The response has a roughly 3db peak at the vent tuning at 25hz but this is flattened out with the addition of a 25Hz high pass filter. At high output levels vent compression also reduces this peak, despite using a 10" diameter pipe wih a 1" round over on both ends. A 25hz steep high pass filter was engaged to prevent excessive cone excursion below the tuning and also flattens out the peak at tuning.

The maximum long term and short term output both are very high at 25hz and above. There is even a large amount of output at 20hz still which is below tune substantially. There was a slight tapping or over excursion sound that developed near 30-35hz at the loudest levels. There was some grill buzz evident but no noticeable vent noise at the highest output levels achieved. The compression performance for this system is also quite good even at the highest power levels used. Notable vent compression is in evidence though, plus this is an area of high power and low excursion for the driver so it is usually a double whammy for reflex systems. The distortion performance is also very good up until the final nominally 127db output sweep where the driver seemed to hit over excursion briefly near 32hz. Even at the 125db nominal sweep level the THD was below 10% everywhere above 18Hz! The THD skyrockets below the tuning which is typical of resonant systems.
Here is one of the designs that inspired me to go the ported route:

Q15 Compact 15" Bass bin - Speakerplans.com Forums - Page 1
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Old 28th January 2013, 04:38 PM   #74
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Quote:
Originally Posted by tb46 View Post
Hi ghettosynth,

Post #57: "... I want to develop intuition for how these parameters interact..."

The best and easiest way I have found to get a grip on what these boxes are doing is to learn Hornresp Hornresp it is a great free program that is very well supported. You may have found this already: Hornresp for Dum... hmm... Everyone - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com or this one: Simple Tapped Horn Tutorial using Hornresp

That way you can determine if what you are looking at are minor or major differences. Hornresp has been found to be an accurate tool to evaluate tapped horns. You have to be careful about getting the simulation input to reflect the physical layout; e.g.: if you increase the height of the THAM12 you will increase L23, and you will also be changing the cross sections of the horn path (depends on where you add the additional height), both changes have to be entered into the simulation.

Hope this helps.

Regards,
Yes, absolutely. At first I was just going to build something, but I took precisely that path, along with a tutorial somewhere on the net, to do what you are describing. You're right, of course, increasing the height requires some care in how one deals with the horn cross sections, particularly in the front of the cabinet. I was not ignoring this detail, however, it's not clear to me that some imprecision here would not be dominated by the additional height. I don't know this, I'm making this assumption based on my reading so far.

I'm not rushing out to build this, of course, I'm trying to build up my understanding first. As turbodog points out, either the THAM12 or the MTH30 built as is would add significant low end extension to my small 10" tops, but, if I could adapt either design slightly to lower f3 somewhat, they might be even more useful to me.

I just want to add, regarding the 15" tops, that for me, they are not a solution. In fact, I had initially planned to go that route and simply not bother with subs for small gigs. There are numerous problems with this approach, however, including that I didn't like any two way 15" cabinets that I listened to and three way cabinets are large and/or expensive. For my purposes I considered both the PRX635 and the Line6 cabinets but both are too heavy/large for those times when I wouldn't need even that much low end and too expensive for those times when I need a speaker in a harsh environment. Obviously I could take them, but I'd be far too concerned about them. Of course, I could purchase those in addition to the small altos, but I see no advantage to that, small subs are a much cheaper option and give me more flexibility.

So, I have two cheap decent speakers that meet my needs, alone, for many simple "gigs" where I need portable sound; adding some small (reasonably priced) subs to that, I can use those speakers for even more gigs that I would normally have to rent a truck/van for in order to haul out my larger (still small by any professional standard) setup. For these speakers, weight and size matter, and by that, I mean that I'm considering each dimension. Making a THAM12 4" taller doesn't affect my ability to carry one (modulo the additional weight), but making it 4" wider or deeper might.

The MTH30 is much larger than the THAM12, but people seem to prefer the MTH, is this largely due to ease of building, or is there something that I'm not seeing in the simulation comparisons that suggest, to me, that they perform rather similarly?

Regards,
gs
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Old 28th January 2013, 05:20 PM   #75
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Quote:
Originally Posted by weltersys View Post
At any rate, I don't care what anyone here does, but the choice is fairly simple.
If one wants poppy bass 55 Hz and up, the small THs work well, if you want low bass extending to around 35 Hz in a similar size package, use long throw low Fs drivers in a BR box using large ports.
And if one wants poppy bass and some low extension but willing to compromise on both a bit with an f3 of between 40 and 45 hz in a box that isn't much wider than a 12" driver, then my sense is that one should still choose the TH, please correct me and point me to any suitable design if I'm wrong. Is this not especially true if power is also a concern?

Perhaps a large part of my value perception is more related to how practical matters drive designs as opposed to what is actually necessary. 12" TH designs are often nice little rectangles where as BR designs are often big cubes; I suspect that this is done to maximize the relationship between surface area and volume which minimizes weight and cost; I don't actually know here, I'm just guessing. So, 12" BR designs based on one driver, EV ZX sub, etc., don't have an f3 of 35hz because the boxes aren't large enough because they're made at a size appropriate for the 12" driver. Moreover, if you're going to make the box larger, you'd still want a cube-like box so you might as well use a 15" driver?

Further, ease of construction is important to me. Isn't it the case that if I did make a more rectangular BR design that it would require more bracing than a cube? The TH design naturally braces the long dimensions of the rectangle. The bracing in the 15" design shown above appears to use irregularly shaped L sections. Ok, that's not too hard, I've got a jig-saw, but it's the sort of thing that adds time to construction and may not be as cut and dry as the bracing in a TH design.

If I'm way off base here then any links that would help clarify trade offs would be much appreciated.

Quote:
That said, I find one large cabinet easier to move than two small cabinets, and one larger TH cabinet can give the LF extension lacking in the small TH and the upper punch lacking in the BR box.
Art
I would sometimes agree with this, but for this system in particular, I want smaller cabinets because the trade off will often be one small vs one large as opposed to two small vs one large.

Regards,
gs

Last edited by ghettosynth; 28th January 2013 at 05:32 PM.
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Old 28th January 2013, 06:24 PM   #76
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Quote:
Originally Posted by ghettosynth View Post
And if one wants poppy bass and some low extension but willing to compromise on both a bit with an f3 of between 40 and 45 hz in a box that isn't much wider than a 12" driver, then my sense is that one should still choose the TH, please correct me and point me to any suitable design if I'm wrong. Is this not especially true if power is also a concern?
So, 12" BR designs based on one driver, EV ZX sub, etc., don't have an f3 of 35hz because the boxes aren't large enough because they're made at a size appropriate for the 12" driver. Moreover, if you're going to make the box larger, you'd still want a cube-like box so you might as well use a 15" driver?

If I'm way off base here then any links that would help clarify trade offs would be much appreciated.
The only link you need to clarify tradeoffs is Hoffman's Iron Law, which applies to TH as well as BR.
TH require more cabinet volume to go as low as a BR, but can be about 6 dB more sensitive.
Cost savings at the expense of size.
In general, larger drivers need larger boxes to go low, the 15" cone area sensitivity advantage over a 12" may be negated when put in a small low Fb BR.

As far as ease of build, little difference between the two small TH under consideration.
On paper the two TH cabinets perform quite similarly.
The Tham 12 is really a good compromise between small size and high output with low power.
Given the compromised LF of both designs, I'd go with the smaller of the two.

That said, a BR is easier to build, the shape matters little, and you could maximize cabinet volume specifically to fit in available space in your car, generally harder to do with a folded horn.
The larger the panels in any cabinet the more bracing required.

As a DIY you can also go with a lower tuning and use better drivers than the typical PA subs aimed at people that don't care about LF, they just want to get the kick drum as loud as the bass player for as little money as they can get by with, a good chunk of the money spent going to convince them that some tiny cabinet with a crappy driver will somehow produce awesome bass.
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Old 28th January 2013, 09:27 PM   #77
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Quote:
Originally Posted by weltersys View Post
The Tham 12 is really a good compromise between small size and high output with low power.
If going with the THAM 12, I suggest paying a lot of attention to the bracing in the first fold. Don't do like what I did with my THAM12 lookalike and think you can get away with making the external panel removable and mount the driver through the hole. Two layers of 3/4 ply AND metal bracing inside and that panel STILL vibrates, enough to toss one of my tops off of it when I turned the bass up during the last "war volume" test.

Which reminds me of another advantage to THs that don't show up in the sims - all that wonderful free side-panel bracing that comes as a result of folding a horn into a box .

I wonder why MLTLs are not being considered? They're basically a cross between TL and vented, and offer some of the advantages of both alignments. I'm considering one now for my PA310, which on paper seems to offer similar LF extension to my TH with the same driver, but in a smaller box and a slight decrease in efficiency. The major plus to the new design is I'll be able to get both the MLTL and my tops into the back of my car .
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Old 28th January 2013, 09:27 PM   #78
PASC is offline PASC  Brazil
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Hi!

Simple. Make some dust.
Make one of each.
Put some elected drivers innit, measure it, and finally hear music thru it.

Regards,
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Old 28th January 2013, 11:24 PM   #79
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Quote:
Originally Posted by Brian Steele View Post
Once I saw that they substituted a PR alignment for a vented alignment, I stopped reading. The test is IMO invalid - they should have tested a realistic vented alignment with that driver, i.e. one where they either had to implement a massive vent to reduce compression effects (which would have in turn increased the box size), or one where vent size is compromized (as is the case with the majority of built vented alignments).

And even if you have managed to deal with vent compression effects, there's still the compression caused by the increase in voice coil resistance due to heat to be dealt with.
I wanted to avoid building another box just for this one test. Sue me! All 4 of my bass reflex test enclosures are PR so I can vary the tuning from about 15-45Hz. You are correct that there would certainly be vent compression if using a port. That little 12" could really use a 6" aero port. It would have been impractical to tune so low with a vent in that small of an enclosure as well. Still assuming that the bass reflex is an ideal scenario with a very large flared vent it is an interesting comparison.

Truthfully it would be more interesting to do this comparison with cabs having a more typical alignment with a 30Hz tuning. I could do it with my 21sw152's with the cabs that I have but it is a lot of work and a PITA.

Another interesting comparison on the other end of the spectrum would be various alignments of the same external volume and low corner using the same driver.
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Old 28th January 2013, 11:45 PM   #80
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Originally Posted by Josh Ricci View Post
I wanted to avoid building another box just for this one test. Sue me! All 4 of my bass reflex test enclosures are PR so I can vary the tuning from about 15-45Hz. You are correct that there would certainly be vent compression if using a port. That little 12" could really use a 6" aero port. It would have been impractical to tune so low with a vent in that small of an enclosure as well.
LOL. You could replace the PRs with MDF disks cut to the same diameter and screwed into place. Then use one of those disks to host the vent. If it's too long, mount it externally


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Originally Posted by Josh Ricci View Post
Still assuming that the bass reflex is an ideal scenario with a very large flared vent it is an interesting comparison.
One of the interesting studies I've seen suggests the use of a vent that looks like the nozzle for rocket in order to reduce compression effects. Basically taking the concept of flaring almost to the extreme.


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Originally Posted by Josh Ricci View Post
Another interesting comparison on the other end of the spectrum would be various alignments of the same external volume and low corner using the same driver.
Might be difficult to get that in practice. To get a flat response in different alignments with the same driver may require different box volumes, and trying to achieve the same target F3 is going to make it even more difficult.

I guess you could try a TH compared to a BR with the same Fb where the vent for the latter is sized such that the total box volume works out to the that of the TH, but I'm going to bet that's still going to be a bit difficult to actually achieve.
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