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Old 21st September 2007, 09:31 AM   #2121
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Default More Deep

Lynn ... If you said earlier I did not catch.
Your assessment of ~ 50hz is for the two 12's OB.
Do you plan on augmenting with a subwoofer?
In other words, do you think you will be satisfied with 50hz?
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Old 21st September 2007, 09:37 AM   #2122
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Yes. The plan to use a stereo pair of subwoofers continues. There's no point in trying to "stretch" the bass of an open-baffle, where excursion is such a concern.

All that radiating area does is merely compensate for the dipole loss compared to a monopole. Instead of dumping more power into the drivers with EQ, I increase the radiating area as the frequency goes lower. That's probably the one mildly original idea in this design.
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Old 21st September 2007, 10:47 AM   #2123
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Default Driver offset / slant

Lynn,

With all due respect, JohnK question wrt to baffle slant / aligning the ACs of the drivers is still unanswered.

Any take on that ?
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Old 21st September 2007, 11:10 AM   #2124
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I honestly don't think the panel slanting is that important, except for aiming directional artifacts off-axis. This includes artifacts from the driver itself, and standing-waves on the baffles - both are worst directly on-axis, and can be quite noticeably reduced with a minor 5~10 degree tilt either laterally or vertically. A vertical tilt provides an extra degree-of-freedom in lateral aiming, since you don't have to go out of your way to avoid the directly on-axis listening position.

The 6-degree slant posted earlier provides the same path-lengths from the 8 and 12-inch drivers to a listener sitting 5 meters (200 inches) away at a height of 44 inches (the top of the front baffle). The tweeter is easy enough to time-align, but the bass is just going to be the way it is - a few inches of panel slant either way isn't going to make much difference at 150 Hz.

Making the system fully linear-phase has some awkward tradeoffs in crossover slopes vs excursion control (increasing IM distortion as a result, already a serious challenge in OB systems), quite large inter-driver phase angles (B&O phase-link technology), or the choice of a fully digitally corrected system.

All this hassle accomplishes is a tidier initial impulse response with no overshoots - but it has little effect on energy storage after the initial impulse, which is caused by driver artifacts and lasts for much longer periods of time (typically 2~10 milliseconds of resonances, reflections and just plain unidentifiable time-domain clutter).

JohnK is heading in a different direction, and is using a different set of priorities - ones that are important to him, but not quite as important for me. Since we are many decades away from a "do-it-all" loudspeaker, each of us has to pick and choose what we want a loudpeaker to do, getting some things, and quietly minimizing others. Electrostats, direct-radiators, and horns all have different strengths and weaknesses, and current techology doesn't allow combining the strengths of all of them.

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Old 21st September 2007, 11:46 AM   #2125
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Quote:
Originally posted by soongsc

Well, if I had the math skills, I could probably derive it. But I could explain the physical aspects and ideas step by step if someone is willing to see if it's mathematically possible.

FYI, here is the impulse editing approach I was refering to which I was working on back in May.

http://www.musicanddesign.com/Impulse_Editing.html

My approach is that I don't need to know the room signature because I don't care about it. I want to eliminate it. I want to know what the impulse behavior is at long time and that can be gleamed from knowledge of the system's low frequency alignment.
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Old 21st September 2007, 12:21 PM   #2126
gedlee is offline gedlee  United States
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Quote:
Originally posted by john k...

FYI, here is the impulse editing approach I was refering to which I was working on back in May.

http://www.musicanddesign.com/Impulse_Editing.html

John

Thanks for that post, an interesting article. Extending the impulse response past the first refelction is an old topic and what you are doing is a variation of past attempts. As I stated before, see Praxis for a really good implimentation of the state-of-the-art in this area.

I worry about the assumptions that you are making about the directivity of the speaker at low frequencies. The accuracy of this assumption directly influences the accuracy of your result.

Basically if one estimates the impulse response from a model of the drivers and system via T-S type of analysis then one can extend the impulse beyond the first reflection by extrapolating the data within the window using the model. Personally this is a lot of analysis and I prefer just to do a near field and far field and splice the two data sets together. Arguably this is not extremely accurate at the LFs, but I would also argue that LF accuracy is not that important.

I also don't believe in the "baffle step" as being a seperate issue from the system response. The system response includes whatever effects the enclosure has. In a highly rounded enclosure design like the Summa, a baffle step is hard to find or define.

If you are interested I can share with you what I have learned over the years about this topic, although its technical enough that off-line may be a better approach.
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Old 21st September 2007, 12:34 PM   #2127
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One of the reason I am following the path I am is that while in the past there was a lot of concern about time alignment and transient perfection between the midrange and tweeter, which I still believe is important, there seems to be a growing consensus that phase distortion introduced by the mid/woofer crossover may be the more audible problem. I can not site any specific references other than to say Martin Colloms discusses this in the 5th addition of High Performance Loudspeakers with regard to audibility of the phase distortion introduced by the high pass character of any speaker system, indicating that any high performance speaker must have extended low frequency response to avoid this. I have taken that in a natural extension to the mid/woofer crossover since the phase distortion of the mid/woofer crossover for a typical crossover (LR type for example) is similar to that which would occur if the system had a cut off at the midrange high pass corner.

For example, consider a system with a low 2nd order, Q = 0.5, 25Hz woofer alignment. If this system uses a 150 Hz LR4 crossover then the group delay at 400 Hz is in excess of 0.5 msec. and at 1 KHz the GD is still in excess of 0.08 msec. The GD of the 15 Hz crossover is added to that of the 25 Hz high pass woofer alignment.

Now with the type of crossover I am implementing in the ICTA no additional GD is introduced by the crossover at 150 Hz. As a result, the GD is already down to 0.05 msec at 400 Hz (order of magnitude reduction) and at 1k Hz there is insignificant GD (on the order of 0.01 msec). Even if the mid/tweeter x-o is not transient perfect, the LP crossover of the midrange would only introduce a relatively constant GD below the mid/tweeter x-o point. The result is very little transient distortion across the majority of the midrange band.
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Old 21st September 2007, 12:50 PM   #2128
gedlee is offline gedlee  United States
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Quote:
Originally posted by john k...
One of the reason I am following the path I am is ...
Most of the scientific studies of group delay have concluded that it is not audible - most recently Brian Moores AES paper.

At any rate I don't see the connection between your concern and the impulse response editing discussion.
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Old 21st September 2007, 03:25 PM   #2129
jlo is offline jlo  France
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Quote:
Most of the scientific studies of group delay have concluded that it is not audible
I also wanted to try it for myself so I did a small soft to listen to phase distortion here
For most standard types of filter, I haven't heard any audible effect.
The only real audible effect of group delay that I heard was when the low frequency cutoff has a high Q, then you can hear it, especially with a sawtooth signal.
Strangely, rising the group delay gives a sensation of more bass (I really don't know why).
This is only a very personnal conclusion, everybody should try, just use the soft.
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Old 21st September 2007, 05:04 PM   #2130
DDF is offline DDF  Canada
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Quote:
Originally posted by jlo

I also wanted to try it for myself so I did a small soft to listen to phase distortion here
For most standard types of filter, I haven't heard any audible effect.
The only real audible effect of group delay that I heard was when the low frequency cutoff has a high Q, then you can hear it, especially with a sawtooth signal.
Strangely, rising the group delay gives a sensation of more bass (I really don't know why).
This is only a very personnal conclusion, everybody should try, just use the soft.

Using the files here, I heard a sharpening of transient attacks when a 4th order LR phase was superimposed.
http://www.pcabx.com/technical/LR-300-3K/index.htm

There's a wealth of rigorous testing info showing the audibility of moderate group delay with headphones, but just not in room.
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