Need guidelines to "MLTL" this array

[zobsky]

If I want to use 8 - 10 of these audax midbasses in a line array, and want to get , say 60- 65 Hz, out of a straight MLTL'd array, can someone tell me
1. if the idea is feasible

2. what kind of dimensions , port I would need to use (and the math behind it, .. I don't have MJK's software.?


http://www.madisound.com/cgi-bin/ind....2873&pid=1928

I've attached a rough sketchup rendering of what my idea looks like.

[ttan98]

there are more experts there at this site:

http://audioroundtable.com/ArraySpeakers/


cheers..

[bjorno]

Hi Zobsky,

I don’t think it’s a good idea to port load this array using a single volume behind the drivers and try the MLTL concept because of the given T/S parameters and excursion capability.

The layout and size is otherwise ok but the port shown will unfortunately not work as intended.

Your picture however, is more realistic for a short far field array for the whole of the mid bass FR range and only the last half octave of the upper usable driver FR range will reach far enough to cause a near field region at a normal listening position.

If you want to port this array with success, you should first make internal divisions i.e. all drivers should have separated chambers ported on front or at the rear side.

But first; Lets speculate your baffle internal front side is 25.4 cm wide, the internal height is about 8.8 cm and the internal depth can be varied either 12 cm or 25.4 cm in depth.

The thickness of the internal dividers is 1.2 cm.

This minimizes the c-c distance between the drivers and two different volumes can be used, one of 2.33 L and one of 4.96 L i.e. any volume enlargeing is by increasing the depth.

First investigate the requirements in case for closed boxes.
If you choose the smaller volume and no port, a separate closed volume is formed with flat FR, with f–3dB and fc about 122 Hz and a Qtc = 0.707 when no dampening material is used.

With 4.96L the Qtc will be 0.577 also when not filled and the f-3dB is 126 Hz and fc about 99 Hz.

Now both volumes shows a Qtc’s value over 0.5 and if <= 0.5 a volume of more 11 L is required.

If a successful MLTL or normal designed ported speaker is the target, a volume > 11 L would be required for each driver with Qt-value of corresponding >=0.5 for a flat FR with no peaking at cut-off.

By avoiding 11 L but using 4.96 L instead with a suitable internal 5 cm diameter and 15.2 cm long port, its possible to lower the 3dB point close to your mentioned target –3dB point at least if you accept an f-3dB of 74 Hz with flat FR and no peaking and can be regarded as close enough.

This suggested ported box would peak at lower frequencies but by placing dampening material in the port the peaking can be cured, R. Small calls this method a dampened box, dampened vent system.

The guy who first evaluated this design was de Boer who called this speaker type for an aperiodic system if the internal absorption of the box is made equal to the port losses and made equal to sqrt (port acoustic mass/box compliance).

The rear driver acoustic load is then made pure resistive and this case will lower the peaking response that otherwise would destroy FR quality and is correctly aligned when the box transfer function reaches third order.

The dampening material needed for each box volume is 0.028 lb (density 0.25 lb/cu feet) and port dampening is only 0.00053 kg= 0.53 g (density 0.5 lb/cu feet).

If slightly more peak dampening is necessary (a dB at most), increase the box filling density for an inch or two and place close to the driver. Never start filling (over trim) the port as for this case, the system efficiency above the peaking will suffer and decrease more than the gain from the peak reduction.

The pass band and low frequency FR response of a short array will not solely be defined by the box/port alignment.

First, making a too short 4-inch driver array causes mid-bass FR to drop.

If using 10 4” drivers with minimum c-c distance the drop will occur at about f=1/3 x h = 328 Hz; h = array height (read Jim Griffins nflawp.pdf).

The baffle step will increase SPL at a frequency depending of the final width of the array front baffle.

When integrating the FR to a sub, this FR drop and FR step must be dealt with and is in my opinion, more troublesome to cure and impacts the in band quality more than if the –3dB point is at 75 instead of 65 Hz.

Finally I would only recommend using the closed boxes without a filter if integrated to a sub but not if the ported box variant is used.

The existing driver x-max limitation needs an at least 24 dB/octave steep filter crossing over not higher than 1.7Khz to counteract the driver break-up region that otherwise will pollute the ear most sensitive critical bands, more than necessary.

Se plots for the closed boxes: 2.33 and 4.96 L and the ported box of 4.96 L with and without dampening material in the port.

b

[zobsky]

Thanks very much, .. a lot of food for thought.

[bjorno]

Edit:

The second sentence from the bottom of my posting is screwed up.

[QUOTE]The existing driver x-max limitation needs an at least 24 dB/octave steep filter crossing…[/QUOTE

Should be:

The existing x-max limited driver needs also an at least 24-dB/octave steep filter crossing…

b