MLTL questions

[Kercher]

My latest speakers are 604-8k drivers in Altec 612a cabinets. They have a flat response down to 40Hz, but then start to roll off. I was thinking of getting a nice sub, or maybe a high quality EQ, to shore up the low end. Then, some recent emails with Gortnipper have got me thinking about MLTL cabinets.

I tried running a simulation for these as a vented box. The MLTL design is a 10cuft box tuned at 33Hz. This is similar to the 6-moons design. I compared my current 612a cabinet (tuned at 40 Hz) and a standard Altec 620 cabinet tuned at 26Hz. Below are the results.

You can see my 612 cabinet is pretty flat down to 40Hz, but then really rolls off. The 620 cabinet can go lower, but the response starts to sag below 100Hz. I can tune the 620 cabinet higher in frequency for a flatter response, but that shifts the roll-off higher in freq too.

My first question is, what does adding the quarter wavelength effect into the MLTL cabinet do to the frequency response at the low end?

Also, a MLTL cabinet design is sensitive to the cabinet length, driver position relative to the line end, and the cross sectional area. Is it possible to build a MLTL corner cabinet if it meets these same criteria? I like the idea of a wide front baffle and how a corner cab could couple with the room.

comparison: http://farm3.static.flickr.com/2706/...c5f06cbd_o.gif

Thanks,
Dan

[Panomaniac]

Flat to 40Hz is nothing to sneeze at! I mean if it's really flat. That puts you 3dB down in the 30s. There really isn't much music down below that. Some room tone, yeah. Or pipe organ.

What do you feel you're missing down that low?

[Kercher]

Good question. Maybe I'm just obsessing over things. When I'm building amps and such I like to design a frequency response that goes two octaves below the the lowest note I want to hear. Probably too much to ask for with speakers.
Dan

[GM]

Yes, you can make MLTL corner cabs.

While you can use a simple box program to sim/compare basic non tapered pipe alignments (I use BoxPlot), programs such as MJK's MathCad software or AkAbak are required to factor in the effects of 1/4 WL resonances and stuffing on the response and vent size/length.

The basic non tapered pipe alignments really only mass loads the vent, i.e. affects vent size/length, increasing driver damping.

Consider an Onken or Jensen Ultraflex alignment. It has a vent system that ideally has a summed area = Sd which causes the vents to be long, severely limiting which drivers will work in it. A MLTL variant OTOH will have a shorter vent length to achieve the same high damping/acoustic efficiency around/at Fb. Factor in that it's easier to design/build and has more tuning flexibility combined with superior reproduction accuracy due to its lack of vent harmonics comb filtering with the driver's mids output and the choice has been clear for me for over 46 years now, ever since an ex-Bell Labs/W.E. EE exposed me to Harry Olson's RCA tall reflex (MLTL) cab for his new 15" 'FR' drivers (missing a major marketing coup!), though for some this results in too 'dry'/'sterile' a presentation.

AFA response goes then, the lower it's tuned, the higher F3 will be and the quicker it will run out of Xmax above Fb, same as a reflex.

WRT cab size, vent tuning, ideally you want it to audibly blend to the room down to at least its Fb, then use some form of baffle step compensation to adjust its mids-up BW to be flat WRT to whatever the in-room efficiency turns out to be, so the less room help and/or the higher the desired system efficiency, the more cab efficiency required if no sub system is used.

Bottom line, the 604 is a low Qts driver designed for mating to a high output impedance system, so depending on the system efficiency required, either a significant amount of some form of boost or shelving EQ will be required to flatten it out if a ~flat in-room response to its Fs is desired.

For whatever reason(s), most folks don't want to make the effort to map their room's response, so all I can recommend is to make the largest cab tolerable since it will have the most tuning flexibility and use a combination of damping, vent length and BSC to hopefully get an acceptable trade-off between size and ~flat in-room BW.

GM

[Kercher]

Hi GM,
Thank you for that excellent reply. I'm curious about your last comment "most folks don't want to make the effort to map their room's response, so all I can recommend is to make the largest cab tolerable". Is there any other cabinet design you would recommend for a 604 if room mapping and EQ is used?
Dan

[GM]

Greets!

Well, it would be some form of TL if a large BLH wasn't an option, just not necessarily a basic zero taper (straight) one. I personally prefer tapered pipes when a large vent is required since they load it even more.

GM

[Altec Best]

What do you feel you're missing down that low?

The answer: VIBRATION the music you can feel but can't hear (House Shake,Window Rattling) something a couple good subs give you.The kinda music that helps get your feet moving