Quote Originally Posted by Steve Schell View Post
His theory is that Higher Order Modes (HOM) are very problematic. He regards HOMs as any wavefront that bounces off the horn walls and creates a delayed wave launch, much like edge diffraction on a flat baffle.
Hi,

An old thread found while looking for PT waveguides. Many of You might not exactly imagine what the HOM hype is all about.

Dr. Geddes did a real great job on horn calculations, I mean it. From that he derived a suspicion regarding Higher Order Modes, HOM for short (it is a trade mark for male underwear here in Europe, beware of copyright issues!).

His claim is that HOM has ever since degraded waveguide sound until his own special one was commercially available. The only investigation regarding audibility of HOM has not shown the effect of HOM directly. It is hold as its signature that HOM introduces undulations on the amplitude and phase response. From a certain degree of contamination as to say this is quite audible. So far so good. That we knew before already.

If HOM does not become perceptable by superstition we have to deal with the generated sound field. The only effect is linear distortion so far, namely phase and amplitude aberrations. We know how much of it is critical. Why not forget about HOM then? It's just an other source of the above mentioned linear distortion. As is diffraction at edges (enclosure, horn rim, finite horn length even with the Geddes type), interferences from driver internals, crossovers and a lot more.

Look at the graphs and You might discuss the whole total of linear distortion. Is it critical or not? With most of the more honorable systems one would find that it is not.

HOM for itsself is prone to misconception. The lowest order mode - in contradiction to the higher ones - of wave propagation in a horn is some kind of arbritary:
- with Websters horn theory the lowest order is a plane wave
- with tractrix it is a spherical wave
- with Geddes it is something else

A waveguide is virtually HOM free only when the lowest oder mode is THE mathematically exact solution of wave equation with given boudary conditions.

Geddes is right in saying that his oblate something is one that solves the wave equation exactly. But it does only with infinite horn length with that exactness - as far as I understand the whole deal. With that given even Geddes type waveguides have its hassle with HOM.
Additionally one should not forget that the driver itsself introduces HOM. It is questionable how far a waveguide reacts on that, could it attenuate that - a long way to go ... . As Geddes' waveguide has to deal with HOM anyhow, from what do we tell that Geddes' oblate something is better than prevous designs regarding - as shown above - arbitrary defined HOM?

Look at the graphs and decide from that. Considering HOM as a source of something mystique "bad" is misleading. HOM isn't to well defined and its effect on hearing delight hasn't been shown in to bright light by now.

buy

ehm: If Geddes' waveguide do measure good, they are good. But the same is true for all the others. As expenses with Geddes' solution do not explode as with some fairy tales in audio You may decide by taste first, omitting any HOMology. It seems his SUMMA is an honorable offspring of the 4430 but with a smoother response. in't it?.