A try at MTM in MLTL configuration with HiVi D6.8

[Forsman]

Hi

I would appreciate some feedback on my latest thoughts on a MLTL with Hivi D6.8 and SEAS 27TDFC.

I have been inspired by zaphaudio.com where he is combining a standard dome tweeter with a simple waveguide from MCM.

My idea is to use the same tweeter and waveguide and combine them with a pair of HiVi D6.8 in an MTM network.
I think I will aim for a crossover frequency at approximately 1.9 kHz. At that frequency I get a wavelength 7.1” witch the spacing between each driver shall fall bellow to avoid comb filter effects.

The cabinet is a straight mass loaded transmission line with a cros section of about 1.6*sd and 44” length. The port is 4” wide and 5” long placed close to bottom of the speaker.

I have simulated with MJKs worksheets and with all room reflexions and get this in room response:





I am thinking of changing it though to 40” length and a sd of about 2, witch gives the same output but with a shorter box.

Do you think this could be a descent speaker or am I on the wrong path?

Regards
/Forsman

[GM]

Some folks would find the ~4th order roll-off this high up in frequency too under-damped ('boomy'/'loose') sounding so consider a T/S max flat alignment for a single driver, then add the second driver and tune it to ~flatten the lower impedance peak to see what this much better damped alignment looks like in-room at the listening position.

GM

[Forsman]

Hi GM

Many people on this forum and others recommend me not to tune the cabinet to low. The reason stated is that the bass risk to become loose and soft. Personally as the armature I am I don’t understand this properly. The transient response of a MLTL tuned to 45 or 30 Hz are to me almost identical.

Here is another shot, tuned a little lower.

Hight: 44”
Cross section area: 2*sd
Center of drivers from top: 11”
Port center from top: 40”
Port diameter: 3”
Port length: 5”
Stuffing: 0.3 lb/ft3 in the upper 2/3 of the pipe

Anechoic frequency response:





Do you think it is better to let it roll off lower like this?

/Forsman

[taj]

What a funny coincidence. I'm currently working with the exact same two drivers (D6.8 and 27TDFC) on a bookshelf size speaker. This is just a quick and simple vented 2-way though, nothing elaborate. I haven't measured anything yet though.

I thought the coincidence was notable considering how many drivers exist and the number of combinations that are possible.

..Todd

[Forsman]

Originally Posted by GM

consider a T/S max flat alignment for a single driver, then add the second driver and tune it to ~flatten the lower impedance peak to see what this much better damped alignment looks like in-room at the listening position.

When I´m reading this section again I realize that I dont understand properly.

Could you please elaborate on this matter a bit more? I would apprisiate it very much.

Originally Posted by taj

What a funny coincidence. I'm currently working with the exact same two drivers (D6.8 and 27TDFC) on a bookshelf size speaker. This is just a quick and simple vented 2-way though, nothing elaborate. I haven't measured anything yet though.

I thought the coincidence was notable considering how many drivers exist and the number of combinations that are possible.

..Todd

True

What Xover point and order have you chosen?

Regards
/Fredrik

[GM]

Originally Posted by Forsman

Many people on this forum and others recommend me not to tune the cabinet to low. The reason stated is that the bass risk to become loose and soft.

The transient response of a MLTL tuned to 45 or 30 Hz are to me almost identical.

Do you think it is better to let it roll off lower like this?

Greets!

Correct, me included since I normally recommend not going below ~0.707x Fs or ~38.8 Hz in this case (Fs with high output impedance amps) for simple reflexes (BR) since Fs is kind of an acoustic fulcrum point.

MLTLs that are acoustically long/large enough to significantly reduce a calculated length enough to increase the recommended BR vent size OTOH can be tuned down to this size since the cab proper is effectively acting as a large vent extension. As effective Qts, Vb increases and room gain is factored in, IME it's not uncommon to tune an octave or more below Fs and why I always recommend making the biggest cab that can be tolerated just to have as much tuning flexibility as practical.

WRT your specific alignment, it's mildly under-damped, so either adding some series resistance and/or more stuffing will perceptably 'tighten it up'. This sort of alignment tends to only work well though when well away from any boundaries in a room large enough not to have any appreciable gain in the speaker's pass-band.

GM

[GM]

Take your design and change the driver specs to emulate two drivers and compare them to see what performance differences it makes, then adjust the vent length as required to further lower the lower impedance peak.

With 2x Sd and 0.5x the suspension compliance, it has much better acoustical damping, i.e. a way to get some of the benefits of compression horn loading without all the added build complexity/bulk. A byproduct is a roll off slope that usually blends with a room's acoustics much better than the max flat alignment.

GM

[Forsman]

Thanks for your feed back GM!

I see that you have used the Fs published at zaphaudio.com. The official figure is 43 Hz and the measured figure on zaphs site is 55 Hz. (55*0.707=38.88).

To be able to simulate two drivers in the same enclosure I have modified the TS according to MJK paper Modeling Two Drivers In One Enclosure. The previous sims are also made that way.

I have increased the amount of stuffing to 0.5 lb/ft3 and its placed uniformly in the hole cab. The vent is now 7 in long. All other dimensions are the same.


Here is the same cab as above but 6 in shorter.


Even though it is shorter it still goes very deep in the bass.

And this is what happens when I reduce the cross section area to Sd*1.6 (237 cm2 * 1.6).


The last sim as .pdf.

Is this any better?

Regards
/Forsman

[GM]

Yes and no. I mean you shouldn't need to add any stuffing and it should wind up being a more rounded off response.

GM

[Forsman]

- ok, then I need to do some re thinking and read your previous posts even more careful to see what I´ve haven’t comprehend.

/Forsman