Ok, I will share with you some ideas that I have in my head how to deal with horn decay asymmetry. They are in a way the further development of the Haralanov’s idea of acoustic diode – the brilliant concept from my point of view.
Before I will go into explanations I need to again to insist that what I am talking about, the decay asymmetry is not the same as too long decay of improperly integrated horn. The whole idea is that horn shall operate in the way that you shall not be able to be bother with whatever decay it has. I absolutely inset that my midbass horn made to sound properly and no one listening my installation would admit that the decay is relatively longer then attack. In fact as I type this post I have a local audio guy in my room who is here for many hours and I will just for fan question him if he feel the too long decay from anywhere in my room. I absolutely convinced that he will not confirm that he hear anything destructive.
The answer is that I am not just a guy who cut wood, solder tubes and connect wire but I am creating Sound and the longer decay that I was impaling about has factored in the total configuration and calibration of my installation. Still as the person who dealt with all little idiosyncrasies of my playback tuning I do admit that I observed the decay asymmetry in midbass horn.
So, the conversation is more theoretical or rather conceptual but I very much might not be so as if a good solution is found then it might open doors for something else.
The Haralanov idea of acoustic diode is powerful but a bit incorrect. We do not need a diode but we need some kind semi-diode that would be fully transparent in one direction and to be semi-transparent in other. Why so? From all that I know about horns there is nothing in them that would cause the asymmetrical decay. Sure, horns by nature are band-pass resonators with own ringing but the square wave ringing might be easily moderated by driver damping. So, where the asymmetry of decal might come from? The most probably it comes, as I proposed above from the fact that horn mouth acts as very sensitive microphone that pick up re-entry of bass from the room. Since horn condense the re-entered sound it might create a contra-pressure to driver (that I think is negligible for midbass horn) and the most important it create the “wondering re-entered bass”. For sure it is hugely delayed only God knows exactly how it affect Sound but a large even ¼ size 40 horn are large with a lot of room for re-entered bass to wonder around.
Now, let to put the Haralanov idea of acoustic semi-diode is use. Pretend we drive a signal from generator across the horn, read it with microphone in the listening position and output to scope. Very soon we will be able to recognize what would be distortions difference between transient signals and slow signal. We do not care about any distortions that are there and cares ONLY about delta between sustain waves and pulses. We can easy to extract the amplitude and the bandwidth of that delta. Let pretend that it will be let say extra 2dB at 70Hz-100Hz for 40Hz horn for a given room. Now we do something kinky. We have ULF channels that are sitting on opposite side in time-aligned position to midbass. The ULF channels sue 20Hz lowpass sharp filter. So, what we do it taking with another filter 2dB at 70Hz-100Hz from the main signal and inject them to ULF channels in opposite phase but after the ULD own lowpass filter. So, now we have the ULF channels that serve double purpose – pump the ULF as main duty and to cancel the midbass “wondering re-entered bass” by injecting the opposite phase error.
If you feel that it is not bold enough then let take the concept further. Pretend we have at the mouth of the midbass horn a small mono-poll driver, shooting insider of the horn (toward to the throat) the back-phase of the asymmetrical decay error. I named this little speaker Re-Entry Corrector. This Re-Entry Corrector would not need any calibration – just slide the Re-Entry Corrector filtration and level (it would be perfect to use a digital crossover) and tune the back-phase inspection until a sustain wave and transient wave would have the same passing across the horn for a given room.
This is a very different thinking about the midbass horn. Many other options are available. We can put a pressure sensor in back chamber or a microphone at listening position and from that extrapolate and predict the need from re-entry. Do not forget that we do not do “room correction”. All that we are trying to do is just to slightly touch one very narrow zone of spectra that might come boomy from bass re-entry. I think it very much might be 1-2 db at very marrow width.
What I described are not the blueprint what to do but juts the inspiration how the things might be done.
Romy The Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche