Product: TQWP loudspeaker
Manufacturer: BD-Designs- Holland
Cost: 275 $/Euro
Reviewer: Geoff Husband
Reviewed: August 2001

Introduction

In the past couple of years I've tried to have a look at some of the more unusual kit loudspeakers that are available to the home constructor. The savings that can be made are considerable and rely on pretty minimal woodworking skills. They also allow access to unusual designs not usually seen in the commercial world, the IPL's transmission Lines, the PMT "tubes" and now a TQWP from BD-designs.

What's a TQWP?

A Tapered Quarter Wave Pipe (or sometimes Tube - hence TQWT). Back in the '30's that pioneer of hi-fi Mr P.Voigt designed the Voigt Pipe. The idea was that putting a drive unit half way down a pipe you'd get a bass resonance that would boost bass output. The 'quarter wave' refers to the length of the pipe, which is a quarter of the wavelength of the frequency you are trying to re-enforce - sort of like an organ pipe or blowing over the end of a bottle. By tapering the pipe the re-enforcement is 'spread' over a wider range of frequencies and it also has some 'horn' characteristic. In a TQWP the tapered pipe is folded in half, but the theory is the same, it just makes a more conventional looking cabinet than a 6 foot high tapered pipe... For more theory go to the Single Driver Website which is a mine of interesting information.

But as always things aren't quite that simple as with the re-enforcement comes a suck-out at a higher frequency, then another re-enforcement - then suck-out, resulting in a comb filter effect of a lumpy bass response. The taper helps, as does putting the driver half way down the line, as does judicious use of damping materials and of course the port at the end of the pipe has an effect.
You're starting to get the picture aren't you? Yup TQWT's are much like transmission lines and horns - there's some basic theory and a hell of a lot of fiddling and tuning and rebuilding to get their compromises working together.

-So why bother? Well they are very efficient and so produce speakers suited for use with low powered valve amps but they're a lot smaller and easier to build than horns. They're also ideal for use with full range drivers, something which brings it's own advantages - no crossover, good phase relationships, a point source, less doppler effect (see Loth-x Polaris review) and in some cases a lower cost.

The "BD designs" small TQWT

And here we have one of the cheapest... For your 275 e you get four Fostex/ACR 103 drivers (yes four...) sufficient damping felt, a pair of ports and two binding posts. So really it's not so much a 'kit' as a set of basic parts and some plans to build the speakers. I might add that the cost is about what you'd pay for the components - the design is essentially free which is how kits should be priced - IPL do this but others certainly don't - I think they make enough by selling the drivers etc without fleecing us for a box design...

The Build

Well the plans work out - though I screwed up by buying 18mm MDF and then after it was all cut out I discovered I'd been sold 20mm by mistake :-# . I cut out all the bits and glued them together using wooden pegs to aid location - thankfully after a bit of work with the surform it did fit...

All but one side are put on and then the wadding added (see pic). I then glued it all up, sanded and finished the cabinets then cut the driver/binding post holes. This worked well but required a lot of hoovering to remove the sawdust.

Overall the build was pretty easy even given the basic nature of the plans - just drawings, no instructions - have a look at the BD website for the full details.

Measured performance

The speakers use two drivers each, one firing forward the other firing upwards and backwards at about 45 degrees, add a port firing from the bass and you'll understand that 1 metre results would be pretty meaningless so I kept to room response from my listening position. This produced some surprises. Overall the midrange was commendably flat, with a shelf 3 dbl up from 1 to 2.5 khz. Treble was extended to 10khz but then dropped away somewhat - not surprising considering the lack of a whizzer cone, though there was some output up to 20khz.

The bass went down surprisingly low for a small floorstander using 4 inch drivers, with good output at 30 hz followed by a (partially room induced) suck-out at 50 hz, and then another, not room induced, at 100 hz - hints of the 'comb effect'. The idea of the rear driver is quite clever, because most full range drivers have a response peak in the upper midrange the rear firing driver will bolster the lower end which is unidirectional whilst adding less upper-mid of which only some will bounce off the rear wall. It's also supposed to add 'scale' to the sound - certainly the flattish midrange show it to be a worthwhile addition.

One of the main "raison d'etre's" of the speaker was delivered with ease. I can't measure absolute efficiency but the Loth-x ANT-1 300b single-ended amp needed only the 8 watts available to produce serious sound pressure levels in my 6.5*4.5m room. This was a relief as I'd been disappointed by this aspect of the PMT's performance. The impedence of 4 ohms didn't seem to pose any problems either so here, to my knowledge, is the cheapest speaker suitable for the task.

Listening

Much of this was done with the aforementioned 300b amp. Though this cost around 15 times the speaker kit it was a reasonable match as for those with aspirations for a 300b amp but without the space for a pair of monster horns might well buy the TQWT despite it's low cost.

The Bad News...

This sounds resolutely mid forward, and quite hard with it, much more than the measurements suggest. In fact whilst doing the 1/3 octave curve I noted that the mid tones sounded harder and nastier than they measured. This hardness was especially noticeable on female vocal - The Pretenders 'Last of the Independents' being very hard on the ears, Tracy Chapman lacked her normal warmth.

This rather dominated the overall scene, but as time went by the problem eased considerably. Small light cones need time and after a couple of week's use the result was much more acceptable, partly due to acclimatisation, but the characteristic remained.
BD were a bit horrified I'd used MDF "they'll sound hard" so I have to put my hand up on that one. I was aware of the problem but I was trying to keep costs down for two reasons - one I wanted something cheap for our readers, and two I had no intention of keeping the speakers so wanted to keep my own out of pocket expenses as low as possible:-) Building them with Birch ply will almost certainly produce a better balanced speaker but at a much higher cost...

The Good News...

After the run-in period the sound became much more to my liking and allowed other positive aspects to shine through. Treble was pretty well extended for a single driver, some of the sheen was missing from the cymbal in 'Take Five' but it wasn't disastrous. Bass was a pleasant surprise, although one or two bass notes went AWOL the overall result was a surprisingly warm yet fast bass. The bass holes were picked up most by walking bass lines such as in Simply Red's 'Sad Old Red' but didn't intrude too much, it was something that on most records you'd have to listen for though the hole at 100hz did make the overall sound a little dry.

With a 'point source', no crossover screwing phase and a narrow baffle you'd expect good imaging and this they delivered. Stage dept on the aforementioned Pretenders album being very good with a wide spread.

And then there's that other thing... The coherence that single driver speakers seem to have. It's tough to describe but easy to spot. When things get busy they seem to hang on to individual threads better than comparable multi driver speakers. Although the response anomalies do intrude at times, it's this coherance that draws you into the music and allows you to forget the rough edges. BD did say "It's not a high-end speaker" but there are hints of what the real big boys like the Loth-x Polaris can do, and so though cheap they are not out of place with an expensive amp.

On that point I'd like to make one thing plain. If you have a cheap transistor amp - forget it. The TQWT's major on midrange clarity and my experience with them on the end of a good tranny integrated were not pleasurable or extended...

Conclusion

Well here it is folks - it's taken a bit of finding but if you've a valve amp, vintage included (or even recommended), then this is about the cheapest way to make them sing. The review reads much like the one for the Loth-x BS-1 but the BD's have considerably more bass response, don't need a stand and cost a fair bit less, though you'll have to build them:-)

An Idea (not mine)

If you fancy doing the math and the fiddling involved the BD kit gives you all you need to do an interesting experiment. Build not one but two TQWT's in the one box. Use the same front/back driver combination but tune the two tubes so they cancel out each others 'nulls' in the frequency plot. Castle amongst others, have done this to good effect and as you've got the drivers...?