Confused by Specs (Small Subwoofers)

[Surtsey]

I'm considering adding some sub-bass to my friend's restaurant system. In truth it's just a small cafe. Low power reinforcement is all that's required. It's a cafe, not a car-boot, or a night-club. I want to use 2 x 8" (vented) drivers.
Can somebody tell me why these cheap Monacors are not a good choice?

https://cdn.shopify.com/s/files/1/0809/2387/files/sph-210.pdf?5823

 

[stv]

They are not soo cheap, as far as I saw.
They are a bit old-fashioned, spec-wise, with low xmax.
Low qts will lead to a small enclosure (around 15 liter for max flat tuning, good) but also a rather high f3 (57 Hz).

Edit:
you could compare data and price to SB 20PFCR-30, about double Xmax and half the price. or slightly smaller, cheaper 16PFCR25.
Or maybe dayton SD-215A or DCS-205.
all those need a bit bigger enclosures but go down lower and have more xmax.

 

[head_unit]

It's a cafe, not a car-boot, or a night-club.

😆
Why 2x8"? Not arguing, just wondering.
How big a box(es) do you plan? Will this sub be by the existing speakers or what is the physical layout? What ARE the existing speakers? And can you highpass those somehow?

 

[Surtsey]

Because 2 x 8" are easily hidden. In this instance I'm forming a 33 litre cabinet in the bottom shelf of a free-standing book case. Currently there are 2 8" speakers in the suspended ceiling which will be high-passed (500hz).
It must be a low-power system. Cafes have a lot of things like crockery, cutlery, and glassware which rattle at high volumes.

 

[Brian Steele]

Edit:
you could compare data and price to SB 20PFCR-30, about double Xmax and half the price. or slightly smaller, cheaper 16PFCR25.
Or maybe dayton SD-215A or DCS-205.
all those need a bit bigger enclosures but go down lower and have more xmax.

I can vouch for the DCS-205. It's a nice driver. Of course, the OP can just go with one of these instead... https://www.daytonaudio.com/product/1122/rss210ho-8-8-reference-ho-subwoofer-8-ohm