Midrange Porting

What are the negatives of porting a midrange. Using Bassbox Pro the response curve of most midrange speakers is very rounded (with no flat response area) with a lot of roll off especially on the bottom end. The 8" mids I will be using will be crossed over at 250hz on the low end. The response curve shows they will be down about 2.5db at 250hz. When I plot a ported design with a Vb of .35 cubic ft. and a Fb of 103hz I get a flat response from 140 to 640hz. Not only does the ported design look more linear, but it would also allow the mids to run cooler, and since this system will be operated at extreme levels cooler is an advantage.
I am trying to avoid any electrical response enhancments.
24db/oct crossover will be used

The mids are 18 Sound 8NMB420 8" drivers
95 dB SPL 1W / 1m average sensitivity
51mm (2 in) Interleaved Sandwich Voice coil (ISV)
280 W AES power handling
External neodymium magnet assembly
Single Demodulating Ring (SDR) for lower distortion
Weather protected cone and plates for outdoor usage
Nominal Diameter 200mm (8 in)
Rated Impedance 8 Ohm
AES Power 280 W
Program Power 400 W
Peak Power 800 W
Sensitivity 95 dB
Frequency Range 60 ÷ 5500 Hz
Power Compression @-10dB 0,8 dB
Power Compression @-3dB 1,7 dB
Power Compression @Full Power 2,2 dB
Max Recomm. Frequency 2500 Hz
Recomm. Enclosure Volume 10 ÷ 40 lt. (0,35 ÷ 1,41 cuft)
Minimum Impedance 5,9Ohm at 25°C
Max Peak To Peak Excursion 19 mm (0,7 in)
Voice Coil Diameter 51 mm (2 in)
Voice Coil Winding Material aluminum
Suspension M-roll, Polycotton
Cone Curvilinear, Treated paper
THIELE SMALL PARAMETERS
Fs 61 Hz
Re 5 Ohm
Sd 0,022 sq.mt. (34,1 sq.in.)
Qms 4
Qes 0,31
Qts 0,28
Vas 33 lt. (1,2cuft)
Mms 14,9 gr. (0.033 lb)
BL 10 Tm
Linear Mathematical Xmax ± 5,75 mm (±0,23 in)
Le (1kHz) 0,35 mH
Ref. Efficiency [email protected] (half space) 95,6dB
 
Going ported might well be useful in your case.

Its worth loading your model into Hornresp, as this will give you an idea of what resonances the port(s) will introduce higher up in the frequency range.
If the nasties higher up are minimal, I see no reason not to go ported: the low Qts seems to want it that way, anyway.

If you're gonna be hammering this, Rod Elliott recommends using two ports, one above and one below the driver - that'd allow some convection cooling.

Chris
 
What are the negatives of porting a midrange.
The negative of porting a midrange is the phase inversion in the frequency range where the output of the port is the primary output.

Using a 24 dB per octave electrical crossover, the affected area is usually only around 1/3 octave, so the lack of phase alignment (and "punch")in the acoustic crossover region is traded for the added SPL (efficiency) the port provides.

I used to use Electro Voice vented midrange speakers before I was understood the concept of phase, they sounded pretty darn good!

Of course, everything sounded better to my ears 35 years ago, when my ears were fresh, and didn't know why something should not sound good...

Art
 

pcb121055

Member
2008-09-01 5:17 pm
The port is in phase with the woofer at and somewhat above the tuning frequency. you need to not be active below that point as they do diverge below that and power handling drops. take a look at the excursion vs frequency graph in your modeling program. as long as you have a steep enough filter to keep you out of the danger zone, it should work well. the sealed enclosure is said to have a better transient response so if you didn't need the response extension/efficiency benefits of the vented box, sealed might be a better choice. just my opinion, of course.