Adventures In Transmission Line Loudspeakers

I must admit I find this thread very interesting and unlike ,any of th electronic hardware threads I am not lost by it all after the first few lines of technicalities and jargon.

Going back a week or so I understood the room/walls were basically a 'pig' and couldn't get anything to work there.

Any ideas why the Castles appear to do so or have I missed a posting?

If a single driver design doesn't do it for you Graham, you could always try multi-way using different models of Mark Audio driver. Maybe you might be able to get a two-way MA combo to sound the way you want. My former 12PW and 7P combo on just a cardboard open baffle took a giant dump on the LS3/5a. The 7P did not lack treble and I am doubtful whether a dedicated tweeter would do treble better. Metal diaphragm vs glass-fibre reinforced paper diaphragm. The emittance characteristics are a little different. By 'big music', I mean loud volume with highly dynamic and heavy music. The larger emitter is more space filling when listening at a distance. Less excursion required to move the same volume of air. Less excursion would only help to reduce intermodulation distortion.

Adventures in transmission line loudspeakers started off because of me trying to get a flat response from a pair of Pluvia 7 drivers, and latterly, a pair of Alpair 7.3's.

It's been a right old trek so far, trying to decide which the deficient frequencies are, because, as good as these drivers sound, they have peaks and troughs (mainly a rather big trough) which get in the way of my kind of music.

It has also been a good education regarding pitch - I admit I am not musically trained - instead I've been exposed to well-produced music for most of my adult life, and that's the reason I can spot if an electronic design is in trouble. Doing it by frequency banding is OK in electronics because real flatness isn't difficult to obtain, whereas transducers are plagued by multiple peaks and troughs which can be much less than an octave apart.

In choosing a small full-range driver I didn't realise the trouble I'd be getting into. I'm quite perturbed at the lack of proper explanation as to why something like an Alpair 10 would change the game, and words like "big music requires a bigger driver" doesn't cut the mustard.

However, I now know why: it is the difference which can be seen in the frequency plots, plain and simple.

I've gone over this plenty of times to now know that the large dip in the 4" drivers at 400Hz is the real party pooper here. But the art of loudspeaker design is in working with what you've got, understanding it, and making it do what you want.

So back to what I wanted: a simple small monitor. The problem in levelling the 400Hz dip is that the solution isn't simple and the acoustic solution won't be small.

But first a good point...

Whoever said bass wasn't possible with tiny drivers must have been thinking about arena PA. Bass is possible with these 4" drivers, and it isn't weedy in a average/small room, plus it's easily tuned to exactly the bass frequency which turns you on. And it goes deep enough for most music (unless you're into deep church organ notes).

And another is the precise rendering of the upper mids and most of the highs. Note that it cannot outperform a top-notch tweeter.

And OK, another, they can sound really dreamy.

But, not to my ears in the small cabinet plans offered, nor in the supplied cabinet.

Firstly there is a massive suck-out from midrange downwards. But this is something all speakers suffer from to a degree because of the change from 2pi to 4pi radiation. I have now demonstrated acoustically and electrically that baffle step compensation takes place in all good speakers - this needs to be explained to the hobbyist (but isn't unless you know how to research the net).

With a 7.5" wide front the change from 2pi to 4pi space is at 608Hz, so below 608Hz the output is noticeably lower, but comes up again where room resonance takes over. My room resonates at 63Hz. Therefore there is a big gap between 608Hz and 63Hz.

Widening the baffle softens the suck-out but the result is boxy because of the room resonance and all rooms resonate to some degree.

By far the best answer IMO was to use electronic baffle-step correction, and that gave an incredible improvement by comparison. I used a 680 uH inductor paralleled with a 2.7 Ohm resistor in series with the positive lead, and the result was really nice to listen to. This gave a 3dB reduction in output from 600Hz up.

But did it work? Yes, but with qualification. It worked on most types of music, including rock, and busy orchestral. Great also for simple acoustic, but did it tick all my boxes? No.

Why? Because I use what reviewer Geoff Husband calls "killers". Killers are musical works/albums which take musical complexity to the limit, and here's an example:

King Crimson, no, not the easy listening Court of the Crimson King, but Lizard! This is progressive rock and jazz fusion taken to further extremes, rolled into one, with particularly a slow rap number with lots of musical goings on (yes, rap, in 1970, done by whites - probably Hendrix inspired).

And this is where the rather big trough I mentioned earlier makes its presence known. Jazz fusion is hard to follow at the best of times...

"... [it] can feature odd or shifting time signatures with elaborate chord progressions, melodies, and counter-melodies. Typically, these arrangements, whether simple or complex, will feature extended improvised sections that can vary in length. As with jazz, fusion often employs brass and woodwind instruments such as trumpet and saxophone as melody and soloing instruments" (taken from Wikipedia)

The most surprising thing is that the old-fashioned, forgotten, non-trendy, can't be any good, Castle Trents, grab your mind and take you through this musical journey in such a way that you don't miss any of the scenery - it paints it in absolute detail - and you come out of it at the end with a squiggly mouth and boat loads of satisfaction (who said I couldn't write in review-speak?). There was I singing around the house "happy families, one hand clap, four went by and none came back" to the bemusement of my family...

So onto the experimental speakers with the 4" drivers. Where was the brass? The killer killed it. The brass was so far back in the mix and I was trying so hard to pick it out that I lost interest in the music...

But that's what it's all about, surely? Keeping the interest in the music!

Was it that I got the electronic baffle-step correction wrong? That was swiftly changed down in frequency, then up, but it just got worse. A few other daft ideas were tried to no avail.

The problem is the 400Hz dip. It just happened to coincide with the brass which made this piece work, or not work. This is why I have such a weird and contorted music collection, because what I do must work with it, or I don't eat, and neither do my engineers.

OK, these drivers are sold for the hobbyist and there are literally hundreds of happy speaker builders proud of what they made using these drivers, and good on them. It's good to be able to buy drivers for such cabinets, it's a great service to offer. But I'm bloody difficult to please and hyper-critical because monitoring my results is to do with my company's success or failure. I have no safety net - the buck stops with me. I fall by the sword. My demands are therefore high.

How to correct for the 400Hz dip? I suppose I can widen the cabinet again so the baffle-step is at 400Hz - did it before - but this time try electronic baffle-step correction too? And I most likely will, because that's me - once I'm on a mission I don't easily give up.

But, you see, this is why I believe right now that the bigger driver would be better, but not for the "big music requires a bigger driver" reasoning. That, if you don't mind me saying, is business-speak. No, it's because if you look at the frequency plot, you don't see the 400Hz dip. Will I try it? Just depends if I can get a cabinet together for it.