DIY THX speaker system

[PWatts]

Hi guys,
I've noticed that there are no info on the web/diyAudio for people building THX-inspired speakers. True, they aren't musical which may deter many people, but look at the scenario:

1) You have large floorstanding speakers for the home cinema. I've found that, unless your main speakers are in the league of the biggest like the Wilson Grand Slamms, playing bass-heavy movies at THX-spec volume with the speakers set to "large" is enough to cause the woofers to bottom out. Even if that doesn't happen, the top end often becomes harsh as the tweeter is overdriven - of course with big 3/4ways this is less of a problem. In my experience, setting big speakers to "small" completely destroys the soundstage and the slam you otherwise had.

2) You have smaller speakers and a sub. Setting the speaker size to "small" on the processor may cure the bass problem, but often integration is weak (as the speakers mostly are ported, they create havoc with the phase response) and the top end still suffers from the excessive powerhandling.

A proper THX-style speaker system is the answer, especially if you have a THX-certified processor/receiver. The basic guidelines for THX front speakers are a sealed box, Q=0.707 with F3 at 80Hz. The THX processor has a 2nd order Butterworth highpass at 80Hz and 4th order LR lowpass at 80Hz for the sub, thereby creating a full 4th-order electro-acoustic crossover. To address the powerhandling issue, most of them also have more than one tweeter.

Anyone who has heard a proper THX Ultra home cinema system (Like M&K) would admit that it really sounds awesome. Because of the nature of home cinema and the compression used on the source material, there is no real need to use expensive drivers like Scan-Speak, Focal or Seas Excel. The Seas Standard series should be more than sufficient.

Does anyone have any ideas with possible driver combinations/layouts etc? Crossover design is of lesser importance (as long as it is possible), and would probably be digital during design process and eventually rebuilt passively. I was thinking of say 2x 5.5" woofers and 2x 25mm or 3x 199mm tweeters per speaker.

[David Gatti]

I really don't see the need for multiple tweeters, especially if you're crossing over to a 5.5" at say 3-4kHz. One HQ tweeter is preferable IMHO.

[Madmike2]

Also THX does not specify sealed enclosures. It has to meet distortion specs at a given SPL and a certain measurement for FR curve for the speakers. Other then that i only read that manifesto once and lost interest when it got into electronics certification.

[simon5]

You could use a high efficiency tweeter, or a horn loaded tweeter, or a compression driver... no need for multiple tweeters.

Then, yeah multiple midranges and multiple large woofers would be nice.

[ShinOBIWAN]

My advice would be to forget trying to build THX speakers. Lucas has test labs that subject products sent in certifcation. There's specific guidlines for THD, SPL, directivity, frequency response and dispersion among others. Without having your speaker sent in for testing you have no way of knowing the outcome.

Its fine to aspire to THX standards but you'll never actually know if it hit the criteria. So any DIY speakers that claims to be THX is erroneous at best.

I agree with you that M&K are quite compelling for HT if rather rough handed with music but I've auditioned quite a few studio monitors(Mackie, Dynaudio, Genelec, M&K, Adam & Quested) and you can do better than M&K when comparing designs in similar price brackets.

Its very rare that I actually would spend money on commercial speakers, virtually all what I do is DIY so what I say next is high praise.

I bought a pair of Mackie HR626's simply because they are quite astounding quality for the money. You'd have a very tough time beating these speakers with DIY when you consider that £900UK buys you speakers, active crossovers and bi-amplification.
Many folks on here love DIY including me but its pointless to try and beat that for the money when you consider amplifcation also.

The Mackie HR626's are THX PM3 certified which means they are mastering grade monitors intended to be used for actually mixing and creating the soundtracks we hear in movies. I believe the Matrix Trilogy was created entirely with the HR626's.

I'd go with these if I was in your position rather than try to create an unknown for a similar cost.

BTW: The Mackies are superb with music too, not at all harsh like the M&K can get.

[PWatts]

I'm well aware of the THX licensing process and that the exact criteria is classified. My idea was not to try and and create a speaker for production (i.e. licensed etc), but rather one that complies to some of the THX criteria.

Bottom line is however that many people who enjoy listening at high volume levels feel cheated by being forced to set their floorstanding towers (unless they're of the big Legacy's or JMLab's maybe) to "small" in order to prevent damaging their woofers. If I'm going to have to highpass my main speakers, the easiest is to build a pair that would integrate well with the THX spec - it doesn't have to be exact THX, but merely a monitor with has good powerhandling, controlled vertical dispersion and rolloff characteristics.

Even though the spec doesn't call for a sealed box per se, I've yet to see a vented/TL/bandpass/aperiodic/dipole THX speaker (subs excluded of course). Small sealed boxes is the norm. The 80Hz F3 is often deviated from, but usually no more than 10Hz or so. The only exception I know of is Boston's (discontinued) VR35 which had an F3 of 65Hz.

If crossed over high enough a single high-quality tweeter (the Seas Millennium automatically comes to mind) should indeed do the job, but there's e a reason why M&K, B&W et al use 2 or 3 tweeters per speaker on their THX series. Firstly it's because I've found even good tweeters to start sounding harsh (like my own Scan-Speak 9500's, crossed 3rd order at 3kHz) at extreme SPL levels, and more importantly to control the directivity for the desired limited vertical dispersion. The mentioned Boston VR35's used a single tweeter, but vertically flanked by two 2" upper midranges for example.

It's for that reason that I've been toying with the idea of something like 2x Seas L18RNX/P and 2x 27TDC's per speaker - maybe in a MTTM, or perhaps M&K style with 2 vertical woofers and the tweeters also vertical at their side.

Naturally the chances of creating something that would actually pass the THX test if submitted is fairly slim, but I believe with sufficient measurement and tweaking one could come fairly close.

As to buying commercial THX speakers: I've long stopped going that route, and the Mackies mentioned are way out of my budget anyway.

[simon5]

As I said, with a single compression driver or a horn loaded tweeter, you can still use a single tweeter.

For example a single compression driver can exceed 130 dB easily.

Maybe you could use a ribbon tweeter also.

[PWatts]

Simon5, yes a compression driver would easily do the trick regarding power handling, but for the controlled vertical dispersion multiple (2-3) tweeters are needed. The gained powerhandling is a side-benefit of it, but most THX speakers use the multiple tweeters for the directivity.

[mgoedeke]

Ever checked out the directivity plots on a waveguide? I can tell you that a good 1" exit compresion driver on a CD horn/waveguide employed properly can result in very, very good results and have much better controlled directivity than almost any dome tweeter based system could hope to achieve.

A good high-efficiency paper cone mid-bass (there are numerous options) will redefine your idea of dynamics. I don't know of too many hi-fi type drivers that would land a spot in my home-theatre, especially not if it were halfway big.

I use a P.Audio WN-12S below a RCF H100 horn + compression driver (active using the Behringer DCX 2496) and for HT there is nothng you'll see anywhere in any hi-fi boutique that would touch it for clean dynamics. Sound is a damn sight better that it has any right to, but the design has a couple of compromizes (it doubles up as small PA top cab and thus weight is a concern and it's BR-ports have a few issues still to be addressed).

[ShinOBIWAN]

Originally posted by PWatts
Bottom line is however that many people who enjoy listening at high volume levels feel cheated by being forced to set their floorstanding towers (unless they're of the big Legacy's or JMLab's maybe) to "small" in order to prevent damaging their woofers. If I'm going to have to highpass my main speakers, the easiest is to build a pair that would integrate well with the THX spec - it doesn't have to be exact THX, but merely a monitor with has good powerhandling, controlled vertical dispersion and rolloff characteristics.

I very much doubt they feel cheated when viewed against the benefits. Performance is often best when using the speakers set to small (80hz low pass by THX standards). Indeed THX requires limited bandwidth channels for all the front, center and rear speakers.

Fact is one point source for LFE is better than 3. Its a simple matter of placement, get it wrong and you'll be looking at suckouts, peaks, phase errors etc.
So for many the easy and most reliable option in a home environment is just have one point source for LFE ie, the sub and cross this to the rest.

Its audibly superior in 9 out of 10 setups. Sure you can get a system that uses large throughout to sound well but its considerable effort.

I'm very surprised that the THX Ultra2 specs haven't called for EQ or room correction to mandatory on subs. Thereby requiring say a +/-3dB in room response from 20hz - 80hz for the best gear that falls within the Ultra2 spec.