Portable Battery Powered Subwoofer

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Aloha :)

I'm hoping the world wide audio-head community can help a musician in need of advice. I'm looking to build a portable sound rig that will all fit onto a custom-build bicycle trailer....but pump out as much bass as possible!

More info:

-Powered by lithium batteries...could be either 12v or 24v or 48v, whichever is more effective.

-This rig will mainly be used in the street for small-medium crowds (say max100) -The trailer is yet to be built...so it'll be made to fit speakers + instruments + electronic gear + power supply

-Music type is a mix of bass heavy electronic and acoustic, including alot of drumming...so ideally the sub would need to be effective down to at least 40hz if possible and also give a good thump.

So its crucial for the sub to be: lightweight and power efficient.

Sooo... any help with advice on:

Box Design & Materials: lightweight, not too big(at the moment Im thinking to make 2 subs to go under each speaker...so size would be say: max 20w x 24h x 24d I've found a wee bit of info on guys using foam core, laminated in fibreglass. Is this the lightest feasible material?

Whats the best box design to push out the bass to fill the open air space and maximise on the power used?

Driver:

Whats the best options for a lightweight power efficient driver that will reach the low frequency desired?

Amp:

Whats the most efficient amp type that will give the necessary power? I've read that the class T amps are super efficient. Is there such a thing as classT amps to drive a sub?

Would I be better off using 12 or 24 or 48 volts?

Wiring: My sound will be coming out of a Motu Mk4 audio interface which has multiple 1/4" TRS dc coupled outputs: 2 are L/R output, the others are balanced. What would be the best way to wire my audio to the different speakers?

Thanks for reading and any advice offered :)
 
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Music type is a mix of bass heavy electronic and acoustic, including alot of drumming...so ideally the sub would need to be effective down to at least 40hz if possible and also give a good thump.
How much space do you have available? Power efficient means that it cannot go low, if size is limited. See Hoffman's law. For a kick drum you could get away with strong 70 - 80 Hz output. Capturing the booming bass of EDM requires 50 Hz. The relation between net cabinet volume, extension and efficiency is something like:

[volume] * [lowest frequency]^3 / [efficiency] = [constant]

Giving up one octave in extension gains you 9 dB in output.
Box Design & Materials: lightweight, not too big
I've found a wee bit of info on guys using foam core, laminated in fibreglass. Is this the lightest feasible material?
For a given weight, fibreglass with a foam board spacer indeed is one the stiffest materials available which is still practical to use foy DIY. The walls however take up more volume than an equally stiff plywood enclosure. Which material you choose depends on whether cabinet volume or cabinet weight is more important. This thread might be of interest for you: DIY Lightweight Composite Cab Build | TalkBass.com


Whats the best box design to push out the bass to fill the open air space and maximise on the power used?
Ported, 6th order bandpass, tapped horn.


Whats the best options for a lightweight power efficient driver that will reach the low frequency desired?
A PA subwoofer driver with a neodymium magnet. Those aren't cheap though.


-Powered by lithium batteries...could be either 12v or 24v, whichever is more effective. Whats the most efficient amp type that will give the necessary power? I've read that the class T amps are super efficient. Is there such a thing as classT amps to drive a sub? Would I be better off using 12 or 24 volts?
Any class D amplifier will do. Class T is a variant of class D and is also fine. You might want to consider lead acid batteries to keep costs down. If ~50 W is sufficient an amplifier can run directly on the 24 V batteries. If more power is required, you'll need a step up converter.
 
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So its crucial for the sub to be: lightweight and power efficient.

Sooo... any help with advice on:

Box Design & Materials: lightweight, not too big I've found a wee bit of info on guys using foam core, laminated in fibreglass. Is this the lightest feasible material?

Whats the best box design to push out the bass to fill the open air space and maximise on the power used?
Aluminum, about 1" thick or more. Anything that flexes isn't good, and a subwoofer produces a lot of pressure. Otherwise, any other material !

Also subwoofers for cars might be good, I see they are available with very low ohmage, say 1-2 Ω, which might be harmful for the amplifier if not designed do drive very low Z loads. Usually a 4 Ω load is safe for an amplifier. As the sensitivity lowers ( 80-82 dB/W/m you can find easily) the power has to be raised for a given output SPL. Indeed most are rated > 500 W max input power.
Check the excursion ( X max) and watch for sturdy construction.
 
I would use a braced 15mm plywood box to save on weight.

How much space can you use? How much can you spend?

If you are looking for compact you could try a Dayton Reference HO 8" driver in a port box.

The Tang Band 6.25" drivers are quite impressive for their size but the cabinets don't always come up that small but there are some tapped horn designs about that will give you best efficiency.

Any old TPA3116 should do the job at 24v but for more grunt you'll need more volts - unless anyone can suggest something!
 
Lithium batteries seem expensive when directly compared to lead-acid but properly maintained they offer over an order of magnitude more charge cycles than lead-acid and on that comparison they can be an “expensive bargain”. The weight savings goes without saying.

For real bass you need real power and it’s going to be hard to beat quality amps built for automotive use.

I don’t know big your bike trailer will be but a proper tapped horn will flat smoke any direct radiator in open space. The Danley THMini at 24” X 22.5” X 15” punches way above weight. Load one with a 2 Ohm driver to best utilize a performance car amp. That should be impossible to beat in your application.

One note, I have seen several guesses posted on the internals of the Danley TH Mini, none of which are correct.

Barry.
 
See the "Boominator" thread for TONS of info on this type of construction.

The back-to-back woofer layout is a great idea, IMO:

  • it uses the speaker frame as an essential part of the box bracing.
  • opposing drivers mean less box vibration.
...so you can build a lighter cabinet than otherwise.

---

I'm basically a +1 on most of what TBTL said, particularly this:

Giving up one octave in extension gains you 9 dB in output.

A boominator is portable and loud and goes to 80Hz. To get to 40Hz (keeping everything else equal), would require a box that is 8x bigger, which compromises portability - it might be too large for a bike trailer.

A PA subwoofer driver with a neodymium magnet. Those aren't cheap though.

Neo really is a lot lighter. Some neo woofers are cheap, but whether they are usable in your project depends on the cutoff you want.

Check out the sale items here, see if some of the neo bargains would work for you:
Speaker Clearance

You might want to consider lead acid batteries to keep costs down.

Yea. Optionally: you can use (free) old lead acid batteries for prototyping, then spend some $$ to upgrade when you build the real deal and get tired of lugging the heavy batteries.

Another idea for prototyping: build from polystyrene + truck bed liner (look it up). Probably too bulky for the final design, but might be good for a quick trial build.

EDIT: the OP seems to have vanished / done something odd to the 1st post. I guess this is a dead thread.
 
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Thanks for the input and links so far :)

The trailer is yet to be built so it will be constructed to suit whatever speakers I end up making. Im thinking something in the range of 35" wide by however long is necessary. So sub box size can be something in the ballpark of say 20"w x 24"h x 24" d

Since the music being played is bassmusic, the lower frequencies are essential...so the sub would need to get down to the 40 ballpark or lower. The Kappalite 12s look like a good option...and paired with the tuba 24 seems like a good way to get alot of badaboom that would still be small enough. But what would be the most efficient way to power this??? I'm not too sure about the whole matching numbers up thing...thats where I start doing alot of headscratching!
 
Since the music being played is bassmusic, the lower frequencies are essential...so the sub would need to get down to the 40 ballpark or lower.
That is unrealistic given your constraints (portable & battery powered) without some clever engineering.
Aim for 60hz+ and maybe look at a MaxxBass or equivalent processing to fortify the sound.
Most people don't know what real 40hz feels like, it's only a number that you can brag about but not effectively achieve - you'll get more wow factor with high impact upper bass, ie 80hz such as the Boominator's goal.

I've built a few bicycle-based battery powered rigs, and also played with big boxes, to develop the above opinion...
 
The trailer is yet to be built so it will be constructed to suit whatever speakers I end up making. Im thinking something in the range of 35" wide by however long is necessary. So sub box size can be something in the ballpark of say 20"w x 24"h x 24" d

You seem to be envisaging a standard layout trailer where ~150mm of width on each side is 'wasted' by the wheels.

I don't know how feasible other layouts are. If you could do a tadpole or delta format, or put the wheels under the load (as pictured, sort of), that'd allow the cabinet to be much wider.

Could you even omit the trailer? Could you simply mount 200mm pneumatic castor wheels and a hitch point directly onto the speaker cabinet?

I've built a few bicycle-based battery powered rigs, and also played with big boxes, to develop the above opinion...

Grand. Do you have pictures of these anywhere? I've been curious about making a trailer. I currently use panniers for everything, but there's a limit to how much they can haul.
 

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I'm terrible at documenting my work... here's a pic of the bike mocked up with speakers, it looks much neater without the battery, amp and wiring mess.
Yuba Mundo "longtail" with some plywood panels on the rack and sideboards to give big flat surfaces with strap mounting points.
Subs are approximately 55 litres each with Eminence Deltalite 2512 tuned to ~70 hz.
Tops are Klipsch 8" with dome tweeter.
Usually powered by a Pioneer car amp with a big 80AH SLA battery. I have a Maxxbass 103 in the signal chain, which helps give some "oomph" to the low end. This is enough to push the subs to distortion, so they're the limiting factor right now.

It has been used for a few Bike Rave events (search for "Bike Rave Sydney" on facebook etc) and was one of 3 bikes built as a float for this years Mardi Gras.

Next version of this rig will have slightly bigger subs, maybe 70~80L, tuned to 60hz.
The tops outperform the bass, as expected, so I'll focus on the low end first before changing anything else.

I'd love to build a big trailer, maybe 900x600mm, but that'd really weigh down the bike. Gotta make sure it's still practical to get around, and incrementally push the limits of bike and rider.
 

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@hollowboy...I definitely need a trailer...its more than just speakers Im hauling...also instruments and audio production gear. Going underneath with the wheels would put the trailer too high....and it needs good size wheels to take the weight and have low roll-resistance...Im thinking 20" wheels. Some things Im not sure of trailer wise is how best to add suspension to the wheels to keep my gear from getting damaged??? Also would I be best with only two wheels...or better off having a third wheel in front like the pic you posted??? I imagine 3 wheels would add to the roll-resistance...but on the otherhand it would make it alot more stable as well as free standing without the bike.


@bitsmasher...good tip about maxxbass. I'll keep that in mind. Love the pic! Im curious how much run time you get with your rig with that particular battery?
 
it needs good size wheels to take the weight and have low roll-resistance...Im thinking 20" wheels. Some things Im not sure of trailer wise is how best to add suspension to the wheels to keep my gear from getting damaged
Many commercial trailers use <20". I've never had a bike trailer, but I know skateboards and trolleys are super useful for moving heavy furniture at a ~walking pace, and they have very small wheels :)

How fast are you expecting to go when burdened with a sub, instruments and other gear? If you could add 50 hours units of build time to save 10 minutes of riding time, would it be worth it?

To keep the build achievable / simple, can you avoid adding suspension - just put anything delicate on the bike, and only use the trailer for the sub?

Alternatively, if you are seriously looking at a complex (expensive) build with suspension, maybe you should also consider an electric assist motor, like the first article in the following link. It is essentially an open-source build: there's a page of instructions and blueprints to let you DIY any part of it, so you could probably do it cheaply with an ebay sourced motor.

Cargo cycles

The 3rd article (if you follow the links) has been developed into a silly but quite functional demo unit, that shows how ludicrously far you can scale up the concept.
 
IF you are going to need tons of power (down ton 40Hz will need 8X power needed to reach 80Hz) you might consider adding a small gasoline generator to your rig.
NO battery even approaches the energy density of gasoline (yet).

have you ever had a 50-300w say speakers via class-d amp running from battery with a power meter? I found that most of the time, ya using a lot less power then ya might think cus of how music varies, is my guess. meaning the load is not constant. Unlike, say powering a led work light. which is constant. thats harder i find. flashing and fading light helps for longer run time.

fossil fuel generators are very inefficient. most of that power density of fuel is wasted. i think engines/generators are only around 16% efficient! + weight, and ££ cost of fuel and planet pollution.

lithium batterys may not be as expensive as people think if ya find the right shop to by from and read up about BMS protection, charging, storage, etc etc
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rc lipo packs from hobby king i found to be poor. a cell died in mine. there not designed to last as someone far more knowledge then me, tried to warn me.

lead acid vs lithium ElectroDacus
 
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