MTH30 Questions : Complete Noob!

[sebDIY]

It represents an average around 30 seconds of the track.

Others exemples
Bob Sinclar: Love Generation



LMFAO: I'm sexy and I know it




David Guetta : without you




Global Deejays: Party 2 Daylight

[turbodawg]

Originally Posted by tb46

Hi turbodawg,

Post #52: "... my built subs, it sims to 94db with a f3 of 35hz..."

I just went by your previous volume and SPL remarks, it doesn't matter much one way or the other, the big difference is in the enclosure technologies. Anyway, I'll attach another 'quick and dirty' SPL comparison, and the Hornresp export files if you want to do any more work with this. As you will see, it is not even easy to say "at Xmax" or "at rated Pmax(?)".

Regards,

Hi, I've taken the models you provided and adjusted them for xmax cone displacement in the passband:

MTH30, B&C 12CL76, 6mm xmax, $162, 7 lb driver
15" Dayton HO (ported, 2.5 ft3, 39.5hz tune), 12mm xmax, $172, 24 lb driver

As you can see, the xmax limited output potential is quite comparable. Yes, I understand that the dayton is seeing over the rated RMS power in this scenario, but that doesn't matter unless you want to play test tones at xmax. The ported dayton extends to a bit below 40hz, a solid 15 hz lower than the MTH30, while having a much more pleasant response curve.

Sure, there are more capable drivers that could be used in the MTH30 to increase output, but there are also plenty of 20+ mm xmax 15's around for more money. And there's no way to get the mth30 to do anything below 50hz.

I've also attached a sim of the HO 15" at the actual 35hz tuning that mine ended up at, which extends to about a 34hz f3, while losing less than 1db sensitivity vs. the original model.

[Brian Steele]

Originally Posted by turbodawg

you can see, the xmax limited output potential is quite comparable.

If only it were so simple

Vented alignments and THs behaving very differently when the volume is turned up. Vented alignments tend to suffer from compression at higher volumes (the output from the vent is non-linear unless the vent is very large, and at higher volume levels the output from the vent is less than you'd expect it to be). Furthermore, you're running the vented alignment at a higher power level, which means that you're going to have more power compression due to voice coil heating as well.

In the case of the TH, the whole box is just one big massive vent, so vent non-linearity is much less of an issue. Secondly, you're running it at a lower power level than the vented alignment to get the same output, so power compression is less of an issue. And finally with most THs, the driver is mounted magnet out, resulting in the magnet sitting in an area of maximum velocity (at or near the mouth), so it's going to get a bit more cooling than a driver in a vented alignment, where the magnet is basically sitting in the box, uncooled by any air flow.

The end result of this is that, even if the predicted output levels at Xmax between the two alignments may be similar, the predicted peak output of the TH is likely going to be a lot closer to reality than the predicted peak output of the vented alignment, and by several dBs too.

[Brian Steele]

Originally Posted by sebDIY

It represents an average around 30 seconds of the track.

Others exemples
Bob Sinclar: Love Generation



LMFAO: I'm sexy and I know it



David Guetta : without you


Global Deejays: Party 2 Daylight

All of these suggest that you need an F3 below 50 Hz in order to reproduce these tracks as they are recorded. Any higher than that (and specifically so with one of the higher-order alignments where the FR drops like a stone below Fb), and you're losing a good part of that bass.

[David McBean]

Originally Posted by ghettosynth

What I find interesting is that hornresp designs are indifferent to cabinet dimensions

Hi ghettosynth,

Would it be possible for you to post the details of a design example where changing the dimensions makes no difference to the predicted results? Thanks.

Kind regards,

David

[turbodawg]

Originally Posted by Brian Steele

If only it were so simple

Vented alignments and THs behaving very differently when the volume is turned up. Vented alignments tend to suffer from compression at higher volumes (the output from the vent is non-linear unless the vent is very large, and at higher volume levels the output from the vent is less than you'd expect it to be). Furthermore, you're running the vented alignment at a higher power level, which means that you're going to have more power compression due to voice coil heating as well.

In the case of the TH, the whole box is just one big massive vent, so vent non-linearity is much less of an issue. Secondly, you're running it at a lower power level than the vented alignment to get the same output, so power compression is less of an issue. And finally with most THs, the driver is mounted magnet out, resulting in the magnet sitting in an area of maximum velocity (at or near the mouth), so it's going to get a bit more cooling than a driver in a vented alignment, where the magnet is basically sitting in the box, uncooled by any air flow.

The end result of this is that, even if the predicted output levels at Xmax between the two alignments may be similar, the predicted peak output of the TH is likely going to be a lot closer to reality than the predicted peak output of the vented alignment, and by several dBs too.

Tapped horns still have to flow air just like any other box and do suffer compression. Most of them don't make their sim's either. Real world, when you start putting one or more 180 degree folds into a duct, with no rounding of the turn, you're going to see some very noticeable flow losses once things get going. Not to mention the cone distortions, wonky phase response/group delay/ringing, and time correction to your mains needed to set one up properly.

Data-Bass

There are plenty of poor ported boxes out there. If you take a 15" and put it in a box with a single 3" diameter port, of course you're going to have compression issues around tuning. The JBL white paper and various other sources go into considerable detail about how to size and roundover ports properly and how to counteract compression with frequency response by creating a slight peak at tuning.

The subs I have built use a slot port 15"x2.5", which is equivalent to a 7" round port in section area (not as optimal in flow but close). It also has rounded inlets and outlets, with a 90 degree turn at the end with a 45 deg slat. Bracing has also been optimised to be parallel to the flow of air, there is virtually nothing restricting the flow getting to the port. Running 35hz (actual tuning) test tones at about 2/3rd's max output, port noise (and hence compression) is minimal and there is quite a large amount of air being moved. Trust me, there is plenty of air being moved to cool the woofer motor, which also happens to use a large aluminum cone.

If you really want a proper and optimized ported box, you have to go look at competion car audio. They build extremely stout boxes that are designed for massive power, long excursion drivers, and huge air flow at relatively high tunings. They have numerous tricks optimize air flow which are not normally done in pro or home audio.

[ghettosynth]

Originally Posted by David McBean

Hi ghettosynth,

Would it be possible for you to post the details of a design example where changing the dimensions makes no difference to the predicted results? Thanks.

Kind regards,

David

Hi David,

Out of context my comment probably appears to make no sense, of course hornresp designs consider the dimensions of the cabinet, however, only as a consequence of the dimensions of the unfolded horn. In other words, the cabinet dimensions proper that are not a function of the unfolded horn dimensions are of no concern to the model. Please see my comment a few pages ago regarding the THAM12 design. If one increases the height of the cabinet, holding all other areas constant, then one only changes a single variable in the hornresp design. Experimenting with this it seems that I gain some low end extension by increasing cabinet height allowing for a longer horn, but get diminishing returns beyond 3 to 4 inches. My question was not whether or not this is true, it is, rather, it's whether or not I was overlooking anything else. In other words, it appears to me that my modification would adapt the design more to my liking without compromising any aspect of the fold.

Regards,
gs

[Brian Steele]

Originally Posted by turbodawg

Data-Bass

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.

[turbodawg]

Originally Posted by sebDIY

Sorry, but most hits of house I have to play have nothing below 50Hz. Here is an exemple with spectral content frequency of Black Eye Peas I've got a feeling :



As you can see, there's nothing to reproduce below 50Hz. Even you have a 30Hz PA system, it's change nothing!!! So a 50Hz -3dB is perfect for this type of music

If I worked in a club, I would prefer a 40 or 30Hz PA system, because acoustic is supposed to be good, and music isn't just over compressed commercial tracks with nothing below 50Hz. But mobile DJ is another work, and in this way, I repeat again, MTH-30 does the perfect job for this, and I don't think it's a coincidence if the MTH-30 is one or if not the most used DIY bass cabinet by mobile DJ in europe.
!

I've recently listened to a good amount of pop dance music on my 15" HO's configured several different ways:

- 26hz 8th order high pass
- Ports plugged and eq'd to get usable response to 20hz.
- Various other lower order high pass, 25 to 30 hz.

There is most certainly intentional content below 40hz in about half the current pop dance music out there, and in most house remixes. And even below 30hz in some cases. I found that using 8th order high pass allowed the least impact to the rolloff of the low bass vs. lower order filters which had a noticeable impact to the 30-40 hz range. I get really noticable low end grunt out of them, much moreso than any common PA.

Of course plugged and eq'd it dug deeper at the expense of headroom.

I can imagine that a MTH30 would be very effective with a pair of 10" or 12" mains that have very limited bass extension below 100hz.

[tb46]

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,