Phase-alignment based method of designing multi-way speakers

[PRTG]

Also, I have managed to set up ARTA measurement software to work with particular soundcard in dual-channel mode without the need for separate mic preamp and sent it to ARTA for review. Meanwhile you can have look of it here.

[turbodawg]

Originally Posted by DBMandrake

If you've got a physical separation of many feet from main speakers to subwoofer it may be worth doing time correction, but you're talking about 0.5ms which is a delay equivalent to half a foot, which is extremely small for frequencies below 80Hz.

It's not just how far it is in terms of wavelength to consider, there's also the fact that our threshold for detecting group delay skyrockets at bass frequencies - we become very insensitive to timing at low frequencies. A good part of the reason why is time/frequency uncertainty. At 100Hz it takes 10ms for a single cycle of a bass note to be completed, we aren't even aware that a 100Hz bass note has begun until at least one cycle has been completed, eg 10ms later. As frequency goes down it gets even worse, 20ms for 50Hz etc. This sets fundamental limits on our ability to resolve small timing differences at low frequencies.

As for expectation effect, don't underestimate just how powerful it is. I've been doing critical listening to small changes on speakers for years, I'm fully aware of expectation effect and the ways that it can catch you out, and try my hardest to be skeptical of what I hear, and yet if I get lazy and perform sighted tests of subtle changes I still sometimes get caught out where I can hear an expected change when in fact there is no audible change at all. No matter how good a listener you think you are, everyone is susceptible to it.

Hearing isn't just a simple process of turning the sound waveform into impulses and sending them to the brain like some digital recording, it's a complex cognitive process that is constantly trying to interpret and make sense of the acoustic environment around you. A lot of what we "hear" is actually the brain interpolating and interpreting multiple senses including sight, and "guessing" to fill in missing pieces of information based upon past and expected scenarios. There is surprisingly little information (in terms of bitrate) being sent from the ear to the brain, so there's a lot of "reading between the lines" being done by the brain that we are not even aware of consciously.

That's why proper scientific listening studies, (such as those designed to measure the threshold of audibility of certain parameters) have to be done double blind and have the right procedures in place to prevent expectation effect and information from other senses from giving false results. Placebo effect in audio sadly is stronger than it is in medicine

I understand placebo effect, I've been doing home audio for 15 years and have tried plenty of tweaks that may have but probably didn't do anything.

But these time correction adjustment are completely audible and repeatable.

I understand group delay in general. The sub box itself has two ports, I will plug one or both if I want to get lower response and tighter bass. Keep in mind that the cabin gain in the suv is a huge factor. The sub is built well but is undamped and there are certainly internal vibrations in the back of the truck, especially when the sub is at full output with both ports unplugged.

However, the time correction is quite audible because of the transient response. Bass in music isn't just sine waves with harmonics. Bass from a kick drum or picking a bass or electronic music has plenty of tranisent attack, meaning it shares more with a step signal or square wave than a sine.

When a sub is time aligned, I have heard with my own ears much tighter, cleaner, and integrated bass, regardless of any imperfections and resonances in the sub. IMHO most bad muddy bass comes from a lack of time alignment.

[PRTG]

Originally Posted by turbodawg

Thanks for clearing up the stereo vs. mono point. I mainly observe soundstage shifts with the mains, as you say. Questions...

- Shift the time delay of the sub that is corner loaded, crossed at 80 hz / 1st order, from what is an audibly and distance calculated correct setting, by say, .5 ms ahead of the mains. The sub goes from being audibly invisible, to sounding out of sync, sped up. Or laggy, if you were to delay it the same amount. That's equivalent to a physical shift of about 6". Wavelengths at 80 hz are 14ft, so phase shift is minimal. I do understand that there is higher frequency distortion from the sub that may blend better with good time alignment. But why is a .5ms shift so audible here?

- Woofer to tweeter crossover is 3.5k, 1st order. If I shift on tweeter channel by .3ms, this results in a roughly 360 degree full phase shift at the crossover frequency. The positioning of the stereo image will be relatively unchanged, but it will sound "off", and not be as clear. Thoughts?

How about that: wide overlap due low filtering order may give more simultaneous cues so it is easier for the brain to spot the best possible alignment.

Originally Posted by turbodawg

- Why does 1st order sound and image much much better than 4th order?

I'd say it depends on application of first order and type and element precision of 4-th order filter.

Generally lower orders have better transient response than that of higher order filters.

One of the articles that explains it.

Originally Posted by turbodawg

Also, I should also mention that is in an SUV, with a eclipse CD8053 head unit (dsp, preamp only), 3 vintage class a/b amps, custom door pods, morel/peerless front stage, 15" sub in the back in very well built 4 cube box. Sounds better than some $10K+ home stereo's I've heard. LOL.

Later you mention that you have ported sub. Ported design is high-order acoustic filter itself. If you combine it with high order active filter summed transient response could be suboptimal enough to be heard. By having just first order you probably compensate for that.

I have it the opposite way for my car sub: H-frame where construction is T-frame and trunk floor adds the missing plane of H. Actively crossed at 80 Hz with two sequential 12 db/oct filters (head units + sub amps).

[PRTG]

Finished Toole's book, lots of general enlightenment and barking towards industry.

My wish was that the following topics would be covered in more detail (hopefully in next edition):

- Bipoles/dipoles as front speakers and impact on direct/reflected signal balance;
- The need for psycho-acoustic SPL alignment as function of frequency of diffuse soundfield (reflected signal) in respect to direct signal mentioned by Zwicker/Fastl wasn't mentioned at all;
- Role of speaker microdynamics (detail resolution capability when operating at low SPL) and associated blind tests of sensitive vs unsensitive speakers;
- more info on how horn/waveguide types and sizes or avoidance of using them impacts far field borderline distance;
- preference blind tests in IEC listening room of controlled directivity (smooth rollup of index) vs constant directivity vs reversed directivity (smooth rolldown of index) pattern speakers.

What gave me reassurance was:

- encouragement to use contemporary types of horns/waveguides without fear of coloration (which I still actually doubt);
- statement that coherent phase matters in achieving higher fidelity, though effects are subtle, also reference to tests performed by other scientist in this field, still in very limiting way;
- statement that resonances are to be fought as much as possible (nothing new here, still topic is related to my method);
- faint bow towards assumption that THD/IMD are less important than well controlled directivity pattern.

I also have over-thought the tests to make the matter more simple: I'll be focusing on directivity patterns only and compare:
- electrically damped vs undamped tweeter alone and along with midbass driver with active and passive filters crossed close to Fr of the tweeter;
- electrically aligned Z curves of tweeter/midbass (visually Z-tuned Zobel added to midbass) vs misaligned.

Toole's book was good read indeed. Coincidentally while still reading I've got a chance to build a sound for custom Home Theater room (everything is connected in this world, right?). The book has helped me a lot with this tasks and had already paid off. By now I'm close to finishing it. Speakers set contains 6 full-range drivers in each of three speakers built as curved/shaded/truncated line source dipoles as the front drivers and three same type drivers as four closed-type wide dispersion tripole speakers on the sides plus 4 subwoofers built into ceiling following 25%-from-sides recommendation. This explains a bit of the silence here in case you were following.

[DavidL]

To be honest if no one has PM'd you then interest has died out

[dumptruck]

Interest has died out because nothing was built. I just read this whole thread, for some reason (well, mostly looking for good links related to something I'm doing). Some good discussion, but kind of silly, really. The initial post of a method/recipe was certainly debatable, but it quickly turned to a "let's build something" thread, and the basic goal was straightforward and common enough to not require any debate in that context. But, five months and not much of anything except arguing about basic concepts (and some not-so-basic). If the OP had some drivers and started some measurement and simulation, there would be something to talk about here.

There is one little thing that I think could be debated pre-raw-driver-measurements, though: the proposal to use a dipole design. That's adding an unwelcome layer of complexity to the whole thing. A simple 2-way with wide-band drivers and without worrying about reaching frequency extremes seems like the logical candidate to me.

[lduarte1973]

i was hoping for some building by now

[PRTG]

Alright,

Seems I have to apologise for having done less than was expected from community during the period. But hey, did I mention any milestones?

Last four months I was a bit busy building speaker set for small indie cinema in Riga:

IMG_2376_small.jpg

If you visit the city I hope you give it a try.

Building hybrid dipole/bipole speakers and testing effects of matching speaker electrical phase curves and physical alignment - coming soon!