Pair Monacor SP-60/4 in a stuffed port aperiodic cabinet? What size cabinet?
I'm building a portable, outdoor pa use, portable speaker to fit a bicycle rack. Winisd's ported box figure for a pair of Monacor SP-60/4 was too big and didn't seem to represent any stuffing, so either ported or stuffed port is fine, but the WinISD size is not. I also need cabinet bass emphasis to about +5 for outdoor use tuning and that should help to shrink the size a bit.
I've really no clue for cabinet size (the pair in a single cabinet). Help?
I need your suggestions.
Here's some figures for the speaker.
Impedance (Z) 4Ω
Resonant frequency (fs) 65Hz
Suspension compl. (Cms) 0.8mm/N
Moving mass (Mms) 4g
Mech. Q factor (Qms) 2.10
Electr. Q factor (Qes) 0.64
Total Q factor (Qts) 0.49
Equivalent volume (Vas) 6 l
DC resistance (Re) 3.3Ω
Force factor (BxL) 2.9Tm
Voice coil induct. (Le) 0.2mH
Voice coil diameter 25.5mm
Linear excursion (XMAX) ±2mm
Eff. cone area (Sd) 55cm2
WinISD seems to say that the pair of woofers fits a 10.5 liter box, if minimum sized, and any smaller would be much harder to tune. Is that correct?
So, my question is, would an aperiodic go a smaller and/or have more bass extension than ported?
Also driving each woofer from a series cap, of 1000u (-0db@80hz, -3db@60hz -6db@40hz), to give the TA2020 battery powered amplifier decreased load at subharmonic (running 60z at 1/2 power, 40hz at 1/4 power saves the battery) and, in theory, slightly flub the electronic dampening at lowest pitches to fit smaller box. That's a modest trick at best. Yet, in a 0.5l cardboard box, one of these little woofers is doing unexpectedly well on bass.
I'd put the series cap at line level - at speaker level it'll interact with the impedance peaks in interesting ways (usually a peak in output at each impedance peak - when one of the peaks is below port tuning, this isn't desirable).
For outdoors use, its worth avoiding a flat alignment - its difficult to get LF outdoors, so (using a trick that's been in the HiFi industry for years) use the port to create a bump in the response before rolloff - that way, it sounds like there's bass without there being any.
if im not mistaken the cap is for QB5 (I or II?) alignment. However, im not sure if the OP goal is to extend bass or increase power handling. Im guessing the former.
For optimum "boombox" tuning intended for outdoors use. You need to tune the cabinet to Q=1.207, instead of Q=0.707 as is the normal compromise used in indoors intended (home stereo) speakers. The result will be roughly same in audio quality, so pay no heed to people saying that it will have sloppy bass as that is not the case. In fact in audio quallty, you can basically say that outdoor Q/2 = indoors Q. So it's in fact even better dampened at Q=1.207 than an indoors speaker is at Q=0.707.
To figure out the optimum cabinet size, use winISD or other software and assume Q=0.707. Take the Vb (box volume) from that and divide with Pi. And take the Fb (box resosnant frequency) and multiply by square root of 2.
Indoors Q = outdoors Q / 2 (assumes that we compare speakers in a small room to semi-free field where speaker is relatively close to ground)
Vb = Vb / Pi
Fb = Fb * 2^0.5
As for using a series cap. Don't bother. Change the input cap on the amp instead.
+1 good point Saturnus
To further expand on the above.
If you plot the new Vb and Fb figures into winISD, you'll notice a 2.83dB peak (usually) somewhere in the 120-160Hz area. In psychoacoustic effect that corresponds to being perceived as flat to that frequency divided with square root of 2. So if it peaks at 144Hz, it's perceived flat to 100Hz.
In principle perceived flat means exactly that. Output without noticable loss. So it doesn't correspond to the normal -3dB point which is the frequency where output is noticeably lower but rather the start of the roll-off in regular tuning. For something that corresponds closely to a -3dB point, assume that it goes about 10% lower in frequency than the above calculation indicate as the flat frequency range.
You can use the demo version of Basta! to model the
effects of series capacitor interaction with the speaker.
When playing with the WinISD, I was going for attempting a +5 bump on the bass for outdoor tuning. That got it down to 10.5l. Could it be made smaller?
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