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High Efficiency Speaker Asylum Need speakers that can rock with just one watt? You found da place. |
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In Reply to: Re: Horn stuff and a post back for Tom posted by Wayne Parham on July 8, 2002 at 02:21:20:
Wayne wrote:
Also, please define what you are calling "upper" and "lower" drivers in this system.
-----------Upper referrs to the loudspeaker(s) covering the frequency range above the lower drivers. This could be the compression driver(upper) and midranges(lower), but could just as well be the midranges and the bass drivers, or any two sets of drivers covering adjacent bandwidths.
-----------
>> That fixed delay is EXACTLY the same as if the driver were moved
>> back in time the same amount.At what frequency? At resonance of the driver, or midband? Because the delay shifts quite a bit through the mechanical resonance region.
>> Since the delays of all the drivers can be measured, the delays
>> can be compensated by an inverse physical distance.Assuming the midband delay is fairly constant, and assuming you only use the drivers through regions where this is true.
-----------BINGO! Now you are getting it, but before you respond to this, please read the next comments...
-----------
>> What your not getting is that except for acoustic phase which is
>> only + - 90 degrees if minimum phase, that the positions of the
>> drivers do not change with frequency, hence the static part can be
>> removed by physical displacement. Also that the electrical filter
>> IS NOT the response and phase which matters, it is the acoustic
>> response and phase, where your focussing is a step removed from
>> where the combining takes place.That's where the problem is, and it's where it has always been. One item in this matter is a fixed position or offset, and the other is a moving phase or offset. If this were not so, then a fixed position would work.
-----------Maybe Tom should have been clearer with his statement in that it is the acoustic response which we are interested in. In the acoustic response, he means the acoustic output from the system of crossover, driver and horn. Remember the driver/horn combo is a mostly minimum phase system, and the combination of the horn, and the power response of the driver form a bandpass output. The electrical filter is then adjusted to compensate for any response variations, which by definition corrects the phase of this minimum phase system. The "crossover" is not so much matching a textbook filter response as it is just seeking to make for a combined response which has linear phase. Realize the acoustic roll-off of the drivers is MUCH steeper than that of the electrical filters.
-----------
But as frequency rises, wavelength size is reduced. It's a moving target in the overlap region. Parallax between the sound sources and the listener forms a distance offset. And changing phase sent to the motors forms another moving offset.
-----------If the acoustic phase is near zero, there is no change in delay with frequency/wavelenght. This is a known benefit of a horn system over a direct radiator which operates as a constant acceloration device having a general midband 90 deg. phase lag. On axis, the same holds true of an electrostat.
-----------
A point source is a point source. It is a physical imposibility, like a magnetic monopole. You can say your device approaches the behaviour of a point source, but then single driver speaker owners often say that too. It is really not - neither case - because the sound source is not a point.
-----------It operates as a point source within it's coverage angle and within it's operating bandwidth.
-----------
Run a swept square wave through this thing. None of us passes that test, but a point source would have no trouble generating it.
-----------The system will produce it accurately as defined/limited by it's bandwidth and upper cut off.
-----------
>> The crossovers job is to flatten the response and trade off the
>> upper/lower frequencies.A bypassed attenuator. I use similar compensation. I gues you've probably noticed a side benefit is that strange (relatively) linear phase shift it generates.
-----------There ya go! With higher resolution measurements and accurate measurement of the real impedance of the compression driver it requires a bit more complicated circuit than a bypassed attenuator to work as described, but the concept and end goal are the same. The resulting mostly linear phase response is exactly what is expected from this minimum phase system. Since we could examine the midrange section as a separate conical horn with its apex equal in area to where the entry holes are located. Please see the attached picture of the inside of the horn corresponding to the other rendering you are inquiring about with the entry holes. Since the distances are acoustically small with in the 60x60 pattern of the horn, all four midranges acoustically couple. This can be modeled as a single driver loading the horn from this point out. As such, we have the same phenomenon occuring that we just went through with the compression driver. This means that with flat response comes constant phase, and fixed delay which can be mechanically compensated for by placement.
Is this making any more sense?
Mark Seaton
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Follow Ups
- Re: Horn stuff and a post back for Tom - Mark Seaton 12:53:43 07/09/02 (1)
- Re: Horn stuff and a post back for Tom - Wayne Parham 16:47:53 07/09/02 (0)
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