I need to design/build a portable bass speaker for a semi-acoustic double bass. Clearly small size and good bass performance don't usually go together but this is only for practice and for playing in small acoustic bands. I'd be interested to hear of people's experiences and whether there are any good designs (or easy to use design guides) out there.
Dave
How small ? Why do you need to design / build rather than buy ? Cost ? Unavailability ? What is wrong with a bass amp (as used by electric bass guitars) ?
However, for small size and good bass, an active design - like a good subwoofer - would do the job. You may also need another speaker for the mid range. Best for small size is perhaps an active folded horn design. The small BOSE hifi use this and apparently are incredible for the size (not that I've ever heard one).
Simon.
Small size and bass means a tradeoff, no matter what approach. Bass reflex gives you somewhat resonant bass, stretched out over time. Folded horn gives you a very peaky frequency response, typical of ghetto blasters. Fan subwoofers I dont have experience of.
NT
Right. any sort of bass reflex design will only be as good as the cabinet size. and you won't get decent bass down below about 100hz with a small one.
Best bet is simple sealed box and the biggest most powerful 12" or 15" or even 18" driver you can afford, and drive it with lots of power.
If its for bass only, no need to tweeter it, but a decent make of fairly stiff cone is better for an acoustic bass than a guitar speaker - get a PA speaker rather.
Not sure what exists tehse days, but JBL and Electrovoice were the bees knees when I was in that game.
Here is a possible site
Mm. No. Folded horn is no good at all for real bass. Transmission line can work BUT there is a huge time delay. Its fine for a sustained organ note, sounds rubbish with a plucked bass string.
Really as its an acoustic bass, you wont be able to amplify much without feedback anyway. So its best to go for a real flat response - sealed box is best - and drive it with plenty of power.
Its far beter to have a gently falling bass response with no peaks, than try and screw more out - in any case you can bass boost the amp signal provided the power handling is there to compensate,.
IIRC correctly bottom E is 40HZ..? You will get something at that frequency, just not a lot.
How small - ASAP ;-) Easily portable by a SWMBO. The cost of decent amps is around £600+ Guitar amps are not usually designed for a pure acoustic sound, or so it appears. Also, a DBass goes down to 40Hz or so. I played around with our KEF sub-woofer and it just didn't sound right - too "boomy". I thought I'd read that horns had an odd frequency response(?). The idea of it being "active" is interesting - do you mean this in the sense of there being a feedback loop? If so, how do the varying room effects get filtered out?
Eminence Basslite as a driver - either 10" or 12" - but now I've got too much choice ;-) I get the idea that the bass response of a ported cab is slightly better than a sealed one, but can't find anything to explain how to do the necessary sums.
Correct. I used to design em. :-) But BASS guitar amps are much better :-)
No 'top' on that. If it was a typical styrene foam cone..also probably electronically removed top..
You are right. Bass horns are crap. Mid and treble horns however..are..fabulous if done RIGHT.
Big problem with horns is high pressures at the throat - wood is unsuitable - it flexes and colours the sound..concrete horns are excellent if you have space for a 30 foot one. Old corbelled chimney with the speaker where the pot was etc :-)
No, that in general means an integrated amp and speaker, possibly with inbuilt electronic crossover and adjusted frequency response to match the speaker limitations.
There are three aspects to do what yu want.
1/. Loudspeaker drive unit. Unless you go with a crossover and mid range, you need to go up to about 1Khz for an acoustic bass, especially if its being bowed. That's a tough call for a 12" and upwards unit. You cant do it on a foam coned unit like a kef..that's why cones on speakers are ridged concentrically - at higher frequencies the centre part of the cone becomes a smaller speaker moving somewhat independently..but the downside of a speaker like that is that it tends to be coloured..the cone resonances are audible. Fortunately most bass units will go up to 500-800Hz or so, so you only need a bit of extra to cope with more.2/. Cabinet. Here for acoustic bass work and small size only infinite baffle (sealed box) works properly. Build that out of TWO layers of 3/4" ply lapped together, and stuff with rockwool.
3/. Amp. What you will need, is bass boost below 100hz, and some treble boost above about 500hz, and generally a very high input impedance if the mic pickup is ceramic and so on, to try and compensate for the actual speaker deficiencies. A graphic equaliser may prove to be the best bet, coupled to a 100-200W power stage. You wont need a lot of id range power, or the thing will howl like a bitch on heat, but you will need to push the speaker hard sub 200Hz, where feedback is less of an issue and the speaker is falling away..A foldback monitor and a mixing desk and a 100W PA amp is close to the above :-)
But I don't know of anyone who makes this as a commercial unit.
What may work is a small 19" rack with a graphic EQ and 100W amp, and a suitable preamp in it, and the speaker built into the bottom..that's studio style gear.
Its easy enough to make a plywood box with a speaker in it and a slot for that at the top and some wood 19" rails..
That's what I would do.
Many subs are designed for adding impact on movie soundtracks etc, and not all are well suited to pure music applications. Normally they need other speakers to provide the upper base as well. (the better ones will allow the upper roll off to be controlled to match your chose main speakers as well. A combination of this, and the ability to swap the phase is often used to tune out the boomyness.
REL do some subs that are effective with music, but don't expect them to be any cheaper than decent guitar amps.
No, active usually just means a speaker with integrated amp. (much as "amp" indicates a amp with integrated speaker to many stage musos).
I would guess if you want a really small size, then you may have to look at a small ported box with integrated amp, and live with the peaky response.
They do, but that comes at a price... the response tends to be less flat anyway, coupled with the fact the load is harder for the amp to drive - so you are likely to get more noticeable peaks in response.
(How much of a problem this will be in practice given the limited range of the instrument I have no feeling for!)
If all else fails, Wilmslow audio do a calculations service:
It is, but at the expense of a worse drop lower down the curve, and a very bad lack of support for the cone at VERY low freqs.
I am assuming you want a 2ft square 'brick' and that wont get the cabinet resonance much below a couple of hundred hertz..so in any case its going to be pants sub 100hz.
Velodyne did a subwoofer with feedback from the actual cone (accelerometer or something). They worked fine until the accelerometer fell off ! Simon.
Philips Motional Feedback
Thanks to all for the info thus far.
I'm starting to think that I might be better-off with two 8" drivers because this should give me slightly better performance at the top end but have almost the same total area as a 15" for moving lots of air at the bottom end. Is this sensible?
No. A 15" speaker is nearly twice the area of two 8" drivers. Remember, the area is proportional to the square of the radius.
Aargh! I meant 12" (honestly, I really did!)
in one of the first issues in the UK, so you'd be hard pressed to find the original article, though!)
Instead of an accelerometer, they relied on the acoustic output of the speaker being proportional to its impedance. A low value resistor was connected in series with the speaker at the earthy end and the signal developed across it used to modify the feedback.
This was effectively positive feedback counteracting the negative feedback in the amplifier and the setting up instructions were to increase the feedback to the point where oscillation commenced, then back it of slightly!
A 'long-throw' speaker is essential, especially if you want any output below the resonant frequency which, from the small size you specify, you will definitely need!
I saw the prototype demonstrated at Olympia (not ideal surroundings!) using a 6½" Philips speaker and it produced commendably surprising results.
Another idea you might lile to pursue was a design in Wireless World (or whatever it was calling itself at the time) about 15 - 20 years ago.
This used an electronic equaliser custom designed to complement the response of the specific 12" speaker used - but you could use the Elektor method instead - in a small sealed enclosure. What was different about it though, was that it had an identical speaker at the rear, connected in antiphase so that both cones moved in the same direction, creating a piston effect.
A couple of weeks back I asked about bass speaker design and got some useful info; shortly afterwards I stumbled-over a used Acoustic Instruments Contra 310BA combo at a reasonable price. They get rave reviews from pro bassists and it has very good and tight bass from a relatively small box (speaker enclosure is 8" high x 15" diameter).
The bass driver weighs nearly 6kg so it would be good for SWMBO's back if I could find a lightweight driver and replace the original.
The current driver is spec'd at: 10", 4 ohm, 30-800 Hz, 2.5" voice coil
- it's labelled: 10-04, 10987, 6700050:48, G5. Can anyone suggest a lighweight replacement?
power output is almost a direct function of magnet weight.
So expect it to be crap with a lightweight unit.
Now I used to manhandle a split Hammond organ up the stairs at the Rock Cafe. That's WEIGHT.
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