40-50hz out of a 10 inch driver means low efficiency.

That Volt driver has resonating frequency of 59hz and you should not tune the box below that point if you plan to push the stated power handling. 

If you must have 40-50Hz look for a 8 or 10 inch driver with rubber surround. Those are usually at 90-93db efficiency.

Aim for "boom" rather than "thud" as a bass voicing and you'll have much more success ;)

You'll have to trade efficiency vs extention. Do you want it loud or low? I think if you sim it you'll find the loss in spl (output volume) isnt worth the gain in frequency. 

If you need some sub, build a small efficient sub like a tapped horn (tham or ss15 ) with an efficient driver. 

I think if you want efficient & low you will need a separate sub driver/ cabinet.

The Volt BM251.4 - 10"has an xmax of 2.5mm (and a quick search showed it as unavailable)

So its never going to shift much air. There are some 12 drivers from B&C for instance that have good efficiency and a bigger x-max allowing them to go a bit lower.

To get an idea of how they might perform you might try a sim in a sealed box to see how the bass rolls off . Often for a sealed midrange a smaller box will give a steeper bass roll off with more efficient midrange where you need it.

Next step would be to try adding a port, This can be used to extend the bass of the mid driver without over-excursion but probably would end up being tuned higher than your target frequency, and needing a high pass filter, to attain any real efficiency. Hence the need for a separate bass driver.

The other alternative is to give up on the ideas of loud to get low & small.

A SS15 tapped horn might give you 103 db/w

A Tham might give you 100 db/w

A big reflex cab might give you 96 db/w

Most domestic sized speakers that go low might give you 86 db/w

I should add that another key constraint of the project is that it doesn't need to be super loud, around 110-115db would be quite sufficient to conserve battery power (and operate within xmax) and I plan to implement a limit at that.

So with that in mind, is there a driver you could reccommend for a reflex box which will be able to go fairly low (even 50hz?) with some semi decent efficiency? Ideally the bass driver should be able to get to 110db at around 50w?

So 110db would require the speaker to be 93db/1 watt - Plus a bit to allow for thermal compression, maybe 2 or 3 db.

So you would need the speaker to be 95-96 db/ 1 watt efficiency.

That is the sort of efficiency a 120 litre 15 reflex cab can achieve.....

BMS12S330 90lit box tuned at about 45-47hz would do that too. Wont go super low but it will be punchy just enough.

A Beyma SM212 in a 85Ltr box tuned to about 47Hz would also do that at a about half the cost of the BMS...

It should be able to reproduce just over 114dB @ 50watts with a -3dB point of 47Hz and a -10 dB of about 37Hz

The BMS 12S330 is a very capable driver It looks capable of soaking up a lot of power and providing a good low frequency response in a small box, Not so efficient though.

Could not get a decent response in Hornresp with the size and tuning suggested by Marjan

The Beyma SM212 seems just about on the money as APW suggested.

Sim shows BMS in grey and Beyma in black, in boxes with size and tuning suggested, both at 20Volts (50w @ 8 R)

So what if I were to sacrifice some loudness and some low end response to keep the box size down, somewhere within 30l max wouod be ideal. Id rather sacrifice loudness and efficiency than low end range. 

Let's call -3db at 60hz the minimum cutoff for bass response, and 30l the cab size? Is that doable? What sort of efficiency is possible?

The Beyma would reach x max at about 150w (34.6v)and 120db so a bigger amplifier would be possible. As long as the amp is efficient the increase in headroom would be useful. Peak efficiency is about 98db@ at 66hz

 Could do with a high pass filter to be safe at higher power levels - Hornresp is suggesting about

38hz with a 4th order Butterworth - Probably good for its 350W rating in real world (adding 3db for losses). Grey is 1 watt black is 150w without losses Filter shows displacement

Edit I see that even this is not good enough for you - what you want is what we all could do with a small miracle - If such a thing existed we would all be using it!

Sorry, my bad, that is volume for the double 12. Try 40 lit.

Are there any known alternatives?

Hi Marjan

The BMS is great in a 40L box it shows a bass hump with 45hz tuning and could be tuned to below 40hz

Sim shows BMS 50w 40L 40hz against Beyma 50w 90L 47hz

You can see what reducing the size does, it throws away 5db of output so needs 3 times the power - not so good on batteries.

I will =)

I was really just looking for general recommendations at the moment. But yes I will get off my ass and fire up WinISD and have a play

If anyone knows about this I would be very happy to hear of a similar driver =)

Interested to follow this build, please make sure to post updates and images, considering something similar myself. 

Can you explain the advantages you would see from that vs a port?

If I understsnd correctly that sounds akin to a boominator, which didn't really appeal to me for this project. Im going for more of a "mini standard PA speaker" type dealio with this. endeavour. So somewhat directional, high quality sound that covers most of the full range, doesnt need to be super loud, that can be paired with a second unit for stereo.

A passive radiator is used instead of a port / bass reflex so works in a similar way. Typically you will use 2x 10" passive radiators to 1 x 10" Active driver if they have a similar Xmax. (See the below youtube video for an example subwoofer with passive radiators) 

Have a little google of the benefits as you'll find better information than I can give. Here are some links to get started.

https://www.css-audio.com/single-post/2018/01/02/When-Passive-Radiators-Are-Better-Than-Ports" rel="nofollow - https://www.css-audio.com/single-post/2018/01/02/When-Passive-Radiators-Are-Better-Than-Ports

http://www.centerpointaudio.com/passiveradiators.aspx" rel="nofollow - http://www.centerpointaudio.com/passiveradiators.aspx

https://www.youtube.com/watch?v=3OH15UHXcOQ" rel="nofollow - https://www.youtube.com/watch?v=3OH15UHXcOQ

Hope this helps!

Before I do that, what are peoples thoughts on this driver from Volt, the BM 165.1

https://voltloudspeakers.co.uk/loudspeakers/bm165-1-6-5/" rel="nofollow - https://voltloudspeakers.co.uk/loudspeakers/bm165-1-6-5/

I've read a lot of good things about these small Volt drivers. This one us supposedly able to produce down to 40hz in a 20l ported enclosure.

Thoughts?

I doubt it....

If 1 mtr = 105db, then:

2 mtrs = 99db

3 mtrs = 95.4db

4 mtrs = 92db

5mtrs = 91db

in the real world you will be ragging every last ounce out of the driver so you will also have power compression to deal with, you can probably knock another 3 to 5db off those figures.

When placing the BM220.1 in a 20ltr box tuned at about 50hz (50hz gave the best results with 20Ltrs): Yes you get you an f3 of about 45Hz when looking at just the data below about 150Hz, however you have already lost 3db when compared to the baseline data above 150Hz so you are back to a sensitivity of 88db/w below 150Hz!!

Like has already been suggested: Download & install WisISD, then enter the data from the spec sheet for any drivers you are interested in (the driver may be already in the database) then try different box sizes and tuning, this will give you some idea if a driver is going to be any good for your needs.

It wont do 50Hz.

It will do 50 Hz, just not loud enough 

But Marjan is right, if you want something happening down low, 12" would be the minimum for me.

12" would be too big. I want to keep the box size below 30l.

Goddamn physics! 

I'll have to compromise further somewhere in the design, I can't decide wheter to sacrifice low end, or efficiency (and as a result, loudness).

To elaborate further on what I'm aiming for, the unit would operate primarily as a battery powered Bluetooth speaker (looking at getting a pair of Lutkevelds boards). 

I'd like it to produce a high quality relatively full range sound at low-moderate volumes, that would be it's use day to day. But with the option to operate as a compact PA speaker, at which times efficiency on battery power could become non-essential since I could arrange to bring additional batteries to power it for longer periods..

Hmmm. Getting WinISD as we speaker =)

If that really worries you, you can always build a separately powered subwoofer system and bring it out when necessary. But that will *not* be every party. 

When you Say Box size 30L are you talking External volume or internal Volume - Box minus walls drivers. ports etc?

The Lutkevelds boards are Class D but do not have DC DC converters so will not do anything like 50w

More like 7 watts RMS on a 12volt battery into a 8 ohm speaker. or 15w in to a 4 ohm driver

What, really? So what is the quoted 45w per channel referring to then??

Pros - Best efficiency (92.5db), Looks very durable and heavy duty (should be resistant to power compression?), reviews say it can be crossed around 1500-2khs with the right tweeter on a waveguide.

Cons - Slightly lower bass response than most of the other options (90db from 50 - 100hz), quite expensive

ScanSpeak Classic P21WO20
Pros - Very good bass response, flat to 50hz. Tried and tested classic driver, great sound quality based on reviews

Cons - Lower efficincy (89.5db)

SEAS - Ca22RNY
Pros - Quite good efficiency (91.5), smooth bass rolloff from 100hz. Great reviews, relatively affordable
Cons - Very light construction, looks like it may not be very durable?

Peerless - HDS-P830869
Pros - Best bass response, nice hump at 50-100 hz. Easily the cheapest
Cons - Lower efficiency (90.5db), nomex probably not be the most durable? Unsure about crossover frequency.

Any advice on the above greatly appreciated =)

Sensitivity of 91.5db

F3 of 50hz and F9 of 40hz

1st port resonance is 2979hz

Maximum rear port air velocity at 50w is 26 m/s.

Response curve shows a hump at around 70hz up to 92.5db.

http://ibb.co/n3FgNP3" rel="nofollow - https://ibb.co/n3FgNP3

Can anyone see anything fundamentally wrong with this design? The port length seems fairly short, shorter than it is long, is that right?

I would have preferred to go with a lower tuning frequency to get slightly more bass extension but since I am planning to cross over at 1500hz, I need to get the port resonance up to around double that (or greater). Is this really necessary?