Vent tuning on M2

[withTarragon]

I ordered a pair of the 2216nd woofers for my new build (2way vented cabinet with a horn tweeter). They finally arrived.

For the bass bin, two obvious candidates to model are the M2 and s-4700, since they are both vented designs and use the same 2216nd woofer.

So far my reading has uncovered the s-4700 is a 4.8 cu ft (135 liter) cabinet with a vent tuned at 32 Hz (shown in the EDS sheet posted elsewhere).

The specifics on the M2 are more elusive. It looks like the cabinet is about 10% larger (perhaps 5.2 cu ft). Folks have suggested the vent tuning might be in the mid-20 Hz region. This seems low since the free resonance of the woofer is either about 36 Hz or 39 Hz (I have seen both specs on the EDS sheet). However the comments about the vent tuning seem to be more speculative or second hand.

So my question is: has anyone measured the impedance in the region around the vent tuning (say 20-40Hz) with a RMS meter, power resistor and tone generator? Of course the DSP filters would need to set on bypass for the the measure to be meaningful.

It is actually fairly easy to measure, but requires access to an M2 cabinet which I do not have. Maybe someone has already made the measure.

Thanks for any assistance or reference.
-Tom

[baldrick]

It’s not uncommon with lower tuning than FS, org DSP settings also have a HP filter of 21hz that might indivate that tuning is aprox 21

[withTarragon]

It’s not uncommon with lower tuning than FS, org DSP settings also have a HP filter of 21hz that might indivate that tuning is aprox 21

Thanks for your input.

The rule of thumb that I had heard was that the vent tuning should not be less than about 75% of the free air resonance (IOW, about 25 Hz). Going below this, one would certainly want a high pass filter to protect the cone from a wild excursion. I also assume that tuning the vent "lower than necessary" has a penalty of decreased efficiency.

However, it is not clear what you think the vent tuning might be. Do you think it is in the low-20's? Have you seen this measure?
Thanks,
-Tom

[baldrick]

According to the Attached document it is in fact 27hz and 130l Box, but i've also read that it is 21, so I'm not sure, but my Guess is that the document is pretty accurate.

[withTarragon]

According to the Attached document it is in fact 27hz and 130l Box, but i've also read that it is 21, so I'm not sure, but my Guess is that the document is pretty accurate.

Thanks for the attachment.

I may have come across it before when I was first researching this. I am not trying to pick a fight with whoever created it, but there are some things that don't make sense. For one thing I am fairly confident that the internal 130 liter volume of an M2 is actually larger. Even with the larger horn, the M2 should have a larger internal volume than the s-4700 cabinet (the M2 is a bit wider and about 5 inches taller).

Like you, I have also seen numbers on the vent tuning that range from the low 20's to the high 20's.

I already have mostly completed the winISD modeling of the s-4700 cabinet since I have the EDS sheets on the s-4700 (including the vent tuning and geometry as well as the cabinet's net volume) and the 2216Nd woofer.

Now that I think about it, maybe I am mistaken about the net internal volume of the M2 (5.2 cu ft or 145+ liters)

Thanks,
-Tom

edit: No, the internal volume on the M2 should be about 5.2 cu ft and clearly larger than the s-4700

[Ian Mackenzie]

I agree it is odd.

I did a simulation in bass box .

After you subtract the bracing , drivers and horn then allow for fibreglass the net value is around 130 L.

Just start off bigger and reduce if you feel you need to.