So... what *disadvantages* can waveguides have?

[dfuller]

Obviously we know of the advantages of a waveguide - improves dispersion matching between tweeter and woofer, increases sensitivity around the crossover, and so on - but what disadvantages do they have other than somewhat more complex engineering?

 

[q3cpma]

No inherent disadvantage; I was going to say "necessarily narrower dispersion", but waveguides can actually widen dispersion at their highest functioning frequency, as Revel's lenses or Seas' DXT actually show.

 

[levimax]

Obviously we know of the advantages of a waveguide - improves dispersion matching between tweeter and woofer, increases sensitivity around the crossover, and so on - but what disadvantages do they have other than somewhat more complex engineering?

Here is a link to a comment from Dennis Murphy that some wave guides cause "coloration"..... https://www.avsforum.com/threads/philharmonic-audio-dennis-murphy.1348949/page-286. From what I have read there are good wave guides and bad wave guides and as always with speakers it depends how everything works together.... blanket statements are not accurate or helpful.

 

[oivavoi]

There have been some good discussions of this is in the past as well. See these threads for example (one short one super long):
https://www.audiosciencereview.com/...can-tweeter-waveguides-add-colouration.11594/
https://www.audiosciencereview.com/forum/index.php?threads/horn-speakers-is-it-me-or.9633/

Briefly, from my layman understanding, all waveguides create what is referred to as higher-order modes (HOMs). Every time someone tries to explain that it gets too technical for me, I get a headache and I just want to have a beer. One simple way of saying it is this: The ideal waveguide would just "guide" the soundwaves in a certain direction, nothing more nothing less. But waveguides don't function in this ideal way, even though some do much better than others. The "walls" and the mouth of the waveguide may also create interferences, soundwaves at certain frequencies start bouncing around with each other and crashing into each other very early on after the propagation of the soundwaves from the driver.

There have not been many studies on this and whether - or under what conditions - it may be audible. And there are also trade-offs, flat baffles also have an effect on the sound, dispersion may be unruly, etc. Some waveguides probably have fairly little of the higher-order modes thingy. As I've understood it, the basic physics of it is not controversial, it's rather a question of how much and when it becomes audible.

I tend to think that I can perceive a certain waveguide coloration in several waveguide or horn speakers. I'm not sure, it may only be bias. I also really like to listen to big horns though. But for me, listening to flat-baffle speakers in the near-field well away from room boundaries (where dispersion doesn't matter that much) seems to me to be less coloured than listening to most waveguide speakers. Would I say the same thing in a blind test? I have no idea.

Anyway I'm just a non-technical layman trying to make sense of technical loudspeaker issues which are way above my paygrade. Please correct me if I'm wrong.

 

[pozz]

I think main thing to note is that calling waveguide engineering complex is underselling it. From what I understand the market has a number of ready to buy waveguides and some rules of thumb for accepted designs, and some modelling software. A lot of people and companies use the build-then-measure approach to test simulations, there always being some disparities. Creating a custom waveguide requires more than a little work and knowledge. I don't think there is anything like a waveguide optimizer available that will suggest the correct shape for a specific build and specific response. It's still trial and error, with more error unless you have the resources of a large firm. Or enough formal physics in you to solve the underlying wave equation.

 

[LTig]

I'm sure both Genelec and Neumann have enough knowledge on hand to design waveguides despite being relatively small firms. Just look at their 3-way models with interfering waveguides for mid and tweeter.

I'm also sure they both work a lot with math and simulation before testing a real world model.

 

[pozz]

I'm sure both Genelec and Neumann have enough knowledge on hand to design waveguides despite being relatively small firms.

In that sense, as in many others, they stand out.

 

[thewas]

I remember the senior engineer of Neumann monitors telling us at a workshop how they used to hand built almost 100 waveguides prototypes measuring, optimising and iterating till they were satisfied just for one loudspeaker some decades ago.
Luckily those days are long gone and nowadays even some excellent DIY hobbyists have access to some modelling and numerical simulation and optimisation tools with stunning results like for example http://hannover-hardcore.de/infinity_classics/!!!/Aries M Dokumentation.pdf

 

[andreasmaaan]

@oivavoi has summed up the potential problems very nicely. Essentially, the main pitfalls result from diffraction effects at transition points along the waveguide, typically the mouth and (especially) the throat.

However, there's nothing mysterious about these effects: they can be modelled fairly well in software these days, and they can certainly be measured. As it is with all forms of colouration in loudspeakers, if it does not present as a problem in measurements, it won't be audible.

Also note, from the point of view of soundwaves propagating out from the diaphragm of a tweeter, a baffle is just a 180-degree rectangular waveguide. So, when it comes to box loudspeakers, there's no getting away from the disadvantages of waveguides, just differing degrees of problems and different tradeoffs.

 

[andreasmaaan]

I don't think there is anything like a waveguide optimizer available that will suggest the correct shape for a specific build and specific response. It's still trial and error, with more error unless you have the resources of a large firm. Or enough formal physics in you to solve the underlying wave equation.

I'm not aware of an optimiser exactly, but there is definitely modelling software that will give very accurate results (so long as accurate data is input).

 

[thewas]

I know of knowledgeable DIY people defining cost functions in the waveguide simulation software and iterating them in an external optimisation toolbox like from Matlab, so I am quite positive the procedure can at least be partially automatised.

 

[mitchco]

https://www.diyaudio.com/forums/multi-way/338806-acoustic-horn-design-easy-ath4.html

 

[LTig]

I know of knowledgeable DIY people defining cost functions in the waveguide simulation software and iterating them in an external optimisation toolbox like from Matlab, so I am quite positive the procedure can at least be partially automatised.

Correct. Neumann calls its waveguide Mathematical Modeled Dispersion Waveguide (MMD™). It's their trademark.

 

[Zvu]

In recent interview Earl Geddes at 01:03:40

Whole interview is worth your time but at this point he speaks of Marcel (Mabat from Czech Republic) who developed a software ATH4 for simulating waveguides and made it a freeware. When someone like Earl says he would use some of Marcel's designs because it is superior to what he did back in the day, if he was still building loudspeakers - it tells more about Marcel and his software accuracy.

As always, you have to know what you are doing but the math crunch is now done by that great piece of software.

 

[Duke]

The following is just my opinion.

A waveguide presents opportunities to screw up. The throat is an especially critical region, as is the mouth. If there is only a limited amount of space in which to fit the waveguide (which is always the case!), then a tradeoff has to be made between how far down the waveguide is effective AND having a large enough round-over to minimize mouth reflections.

If a small diameter midwoofer is used, and if pattern-matching in the crossover region is a goal, the resulting crossover frequency may be so high that you still get a lot of off-axis midrange energy on the midwoofer's side of the crossover point, resulting in a spectral imbalance between direct and reflected sound despite the pattern control of the waveguide over its region. Dutch & Dutch gets around this by using cardioid loading for their 8" midwoofer. I think Amphion does the same for their three-way models.

Regarding higher order modes, or reflections within the horn, my understanding is that they are present in all horns. Earl Geddes was the first to fully appreciate and articulate their implications, and to use a geometry optimized to minimize their generation, namely the Oblate Spheroid. So while the term "higher order modes" comes up in discussions of waveguides, this is actually an area where good waveguides are better than most horns. (Waveguides are a subset of horns, so all waveguides are horns, but not all horns are waveguides, by Earl Geddes' usage of the term.)

The relatively large dimensions of a waveguide (relative to a comparable direct-radiating tweeter) increases the vertical spacing between waveguide and midwoofer, which may be an issue in some applications.

I have used waveguides or waveguide-style horns in all of my home audio loudspeaker designs for the past fifteen years. I have measured most off-the-shelf waveguides or waveguide-style horns. Just like virtually everything in audio, they are not all created equal. And there are always jugglings of tradeoffs involved. Imo the advantages are generally worth pursuing, and the tradeoffs are fewer and/or easier to juggle in a large speaker versus a small one.

 

[Dennis Murphy]

The following is just my opinion.

A waveguide presents opportunities to screw up. The throat is an especially critical region, as is the mouth. If there is only a limited amount of space in which to fit the waveguide (which is always the case!), then a tradeoff has to be made between how far down the waveguide is effective AND having a large enough round-over to minimize mouth reflections.

If a small diameter midwoofer is used, and if pattern-matching in the crossover region is a goal, the resulting crossover frequency may be so high that you still get a lot of off-axis midrange energy on the midwoofer's side of the crossover point, resulting in a spectral imbalance between direct and reflected sound despite the pattern control of the waveguide over its region. Dutch & Dutch gets around this by using cardioid loading for their 8" midwoofer. I think Amphion does the same for their three-way models.

Regarding higher order modes, or reflections within the horn, my understanding is that they are present in all horns. Earl Geddes was the fully appreciate their implications, and to use a geometry optimized to minimize their generation, namely the Oblate Spheroid. So while the term comes up in discussions of waveguides, this is actually an area where good waveguides are better than most horns. (Waveguides are a subset of horns, so all waveguides are horns, but not all horns are waveguides, by Earl Geddes' usage of the term.)

I have used waveguides or waveguide-style horns in all of my home audio loudspeaker designs for the past fifteen years. I have measured most off-the-shelf waveguides or waveguide-style horns. Just like virtually everything in audio, they are not all created equal. And there are always jugglings of tradeoffs involved. Imo the advantages are generally worth pursuing, and the tradeoffs are fewer and/or easier to juggle in a large speaker versus a small one.

That has certainly been my experience. If you really know what you're doing, waveguides to have important advantages, as anyone evaluating a typical Revel product would agree. . But as you say, they're tricky, and even skilled engineers like a certain British chap can go wrong if they don't have enough space in which to work. Building a wave guide into a standard tweeter flange is particularly risky. One good example is the Pioneer BS22 with its fairly aggressive wave-guided tweeter. It measures fairly well, particularly off-axis, and the crossover is better than in most budget speakers., but there's a murky quality to the lower treble that ruined the speaker for me. I switched in a conventional tweeter and diddled with the crossover, and the resulting sound is much cleaner even though there's a typical off-axis flare around 3 kHz. My takeaway from that experience was that some wave guides are worse than the disease, and the disease (lack of constant directivity) isn't nearly as bad as many would profess. I also had problems with the tweeter waveguide on the first generation ELAC Debut series, and the much larger wave guide (or maybe it's a non-wave guide horn?) on the Geddes Abbey.

 

[Zvu]

Gedlee Abbey had a waveguide. Difference from Summa was just diameter. Earl didn't make anything less than a waveguide for Gedlee speakers.

@Dennis Murphy How did you narrow it down to waveguide problem in that pioneer speaker ? For that to be possible, you'd have to use the exact same tweeter, just without a waveguide. Is the original Pioneer tweeter equipped with ferrofluid and what is the replacement tweeter you've found to sound better than stock ?

 

[FrankF]

Wow! I'm way off , thought they were just to prevent tweeter damage. Nothing worse than a pushed in metal dome.

 

[Duke]

I also had problems with... the much larger wave guide (or maybe it's a non-wave guide horn?) on the Geddes Abbey.

If you are comfortable doing so, could you elaborate?

 

[Dennis Murphy]

Gedlee Abbey had a waveguide. Difference from Summa was just diameter. Earl didn't make anything less than a waveguide for Gedlee speakers.

@Dennis Murphy How did you narrow it down to waveguide problem in that pioneer speaker ? For that to be possible, you'd have to use the exact same tweeter, just without a waveguide. Is the original Pioneer tweeter equipped with ferrofluid and what is the replacement tweeter you've found to sound better than stock ?

My first guess was that the crossover simply lacked adequate baffle step compensation. So I redesigned the circuit to flatten everything out, and took another listen. Same problem. Then I thought there might be a distortion issue, but THD was quite acceptable at the levels I was hearing a problem. That's when I decided to toss the tweeter. I didn't try using it without the waveguide because I wouldn't know how to do that--it's integral to the tweeter flange. This is the mod that Amir tested after reviewing the stock BS22. I've sold a bajillion of them, and brought it back for a brief spin after Amir's review, but it's now officially retired. Too much work. Oh--the replacement tweeter was the venerable and super cheap Vifa (now Peerless) BC25TG15-04