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Old 12th July 2008, 02:53 PM   #21
Jay_WJ is offline Jay_WJ  United States
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Quote:
Originally posted by sreten
Anything in common with the Scanspeak 18W8545K as claimed
by the "reputation" ? (The PL18 is a 18W8545K on the cheap),
technically no. In fact what is claimed in this "reputation" is
more accurately true of the Usher 8945P.
Right, sreten. Some people believe that the Usher drivers are simply cheap clones of the Scan-Speak classic drivers. This is not true. Usher people did not copy the SS design. They designed motors on their own. Particularly, the 8945P's motor is very different from the SS 8545's. And the 8945P measures better than the 8545 in nonlinear distortions. Even the Usher 8945A, which "looks" similar to the 8545, is different, as Zaph mentions:

"This woofer (8945A) was considered by many to be a clone of the Scan Speak 8545. The cone and surround are similar but there are some differences internally."
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Old 12th July 2008, 06:01 PM   #22
Jay_WJ is offline Jay_WJ  United States
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Trey,

As I said above, checking on drive units' distortion measurements will help you make an easier decision. The following are Zaph's harmonic distortion measurements of the Usher 8945P and two other drivers that have been used in many existing designs for their good cost-to-performance ratio:


Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.


Let's first compare 8945P and RS180. The RS180 is a very cost-effective 7" driver. Its distortions in a frequency range (below 1.8 kHz) in which it is normally used are relatively low compared to other inexpensive 7" drivers. But when compared to the 8945P, it turns out that the Usher is in a totally different league. Focus on acoustically obtrusive 3rd and 5th order harmonic distortions on the above graphs (indicated by blue and grey lines). On average, the 8945P's odd-order distortion levels are 15 to 20 dB lower than the RS180's in the range of 200 Hz to 1.5 kHz. Remember that a driver's harmonic distortions cannot be corrected by a crossover, that is, no matter how well the crossover is designed.

Now to the CA15RLY, due to Zaph's different measurement settings between 5.5" and 6.5" drivers, we need to subtract 4 to 5 dB from the CA15RLY distortion levels. But even if we do so, you'll see that the CA15RLY's odd-order harmonic distortion levels are even slightly higher than the RS180's in the same range as above mentioned.

This shows how low-distortion the 8945P is. Actually, as I already said in my above post, the quality of the Usher 8945P is top notch regardless of price. In a 2-way system, the midbass driver's quality is much more important than the tweeter's because a frequency range it covers is the range where most fundamental tones in music are played. But when a midbass driver is this good, we want to match it with a same low-distortion tweeter.

Also, note that in your situation of speaker placement in the room, you'll definitely need a reduced BSC crossover. Many existing designs don't have this option unless the designer is willing to modify the crossover for you.
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Old 13th July 2008, 12:26 PM   #23
Calvin is offline Calvin  Germany
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Hi,

I wonder if the measurent of the RS180 is correct though. The Dayton´s fundamental´s curve stays close to 0dB, while the others -as it would be expected- show a HP-Response. This leads to a difference of up to 25dB at the lowest freqs between the Usher and the Dayton.
Besides in the freq-range of interest above 40Hz the differences are small and should be well below the treshold of recognition anyway.

jauu
Calvin
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Old 13th July 2008, 02:02 PM   #24
Jay_WJ is offline Jay_WJ  United States
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Quote:
Originally posted by Calvin
Hi,

I wonder if the measurent of the RS180 is correct though. The Dayton´s fundamental´s curve stays close to 0dB, while the others -as it would be expected- show a HP-Response. This leads to a difference of up to 25dB at the lowest freqs between the Usher and the Dayton.
Besides in the freq-range of interest above 40Hz the differences are small and should be well below the treshold of recognition anyway.

jauu
Calvin
The shape of fundamental curves in Zaph's "old" measurements is simply an artifact that didn't affect the distortion measurements. Read Zaph's description in his test page. You'll see the same pattern in all his old measurements regardless of driver Qts.
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Old 13th July 2008, 06:52 PM   #25
Calvin is offline Calvin  Germany
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Hi,

ok, maybe its an artifakt (sure that this artifakt isn´t included in the distortion too?)
Anyway, the questions remains: "Do these differences matter?"
According to what can be read in another thread about harmonic distortion it is claimed to be a rather ´useless at all´ parameter.
Comparing with other scientifical reports dealing with recognition tresholds of distortion these tresholds are already high with stationary signals (sinewave) and rather unrecognizable with natural instrument sounds (stochastic signals with varying amplitudes and phases that already contain multiple harmonics). The values are so high that most of the drivers would have work under overload conditions.

Nonlinear Distortion is reported to be recognizable from >1% on using sinewave signals (IM-distortion, again with music the treshold is far higher).
Consequently IM distortion is quoted to be a more sensitive parameter than harmonic distortion measurements.

jauu
Calvin
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Old 13th July 2008, 07:05 PM   #26
Jay_WJ is offline Jay_WJ  United States
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Mark K has written up a very balanced view of this issue at his home page. But alas, he accidently deleted all this valuable info!

Anyway, in sum, non-linear distortions (including simple single-tone generated harmonics or multi-tone IM distortions) won't be an issue for well-designed low-distortion systems or drive units. But what's a well-designed system or drive unit? Normally, for a home system with limited size (e.g. equal to or less than 7-8" midbass drivers) and limited xmax, non-linear distortions matter. In my personal experience, I was abel to hear difference in midrange distortion levels between the RS180 and the 8945P during various music passages, even when I tried a low 1.5 kHz xover, higher order LP filter for the RS180 to mitigate its breakup-induced distortions.
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Old 14th July 2008, 02:33 AM   #27
mlwebb is offline mlwebb  United States
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Default MTM bookshelf

In my opinion the skill of the speaker/crossover design is far more important than "high-end" drivers. I am very happy with the Jon Marsh dayton/seas MTMs I built - do not discount the RS-180s because they are inexpensive.
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Old 14th July 2008, 02:45 AM   #28
Jay_WJ is offline Jay_WJ  United States
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Default Re: MTM bookshelf

Quote:
Originally posted by mlwebb
In my opinion the skill of the speaker/crossover design is far more important than "high-end" drivers. I am very happy with the Jon Marsh dayton/seas MTMs I built - do not discount the RS-180s because they are inexpensive.
Of course, the RS180 is excellent for the price. But it's not the best, lowest-distortion driver money can buy. At a higher volume level, its distortion characteristic (a bit higher tall order harmonics excited by midrange tones in the range of 300 Hz to 1 kHz shown in Zaph's measurements) is clearly audible when compared to lower distortion drivers (e.g., Scan-Speak Revelator or in my case Usher 8945P). Note that distortions in this range cannot be corrected by a crossover, that is, no matter how well a crossover is designed. In my experiement, which I performed when I designed my RS180/27TDFC MT crossover, I used a 4th order electrical filter with a notch filter (just like Jon Marsh's C-E type) with a 1.5 kHz xover point for the RS180.
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Old 14th July 2008, 08:32 AM   #29
sreten is online now sreten  United Kingdom
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Quote:
Originally posted by Calvin
Hi,

Anyway, the questions remains: "Do these differences matter?"
According to what can be read in another thread about harmonic
distortion it is claimed to be a rather ´useless at all´ parameter.

Nonlinear Distortion is reported to be recognizable from >1% on
using sinewave signals (IM-distortion, again with music the
treshold is far higher).
Consequently IM distortion is quoted to be a more sensitive
parameter than harmonic distortion measurements.

jauu
Calvin
Hi,

Well IMO a "useless at all" opinion taken to non-logical extremes.

Remember that distortion measurements are symtoms, very much
incomplete, so too much cannot be read into them, but then again
reading too little is an even worse travesty.

The distortion papers at Klippel.de are well worth a read. Especially
the analysis of symtoms and how they relate to various distortion
mechanisms. Low odd harmonic distortion is a very good thing ...

(And very much related to IM ...)

/sreten.
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Old 14th July 2008, 09:20 PM   #30
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Jay_WJ - I sent you an email asking for some help. Sorry to post this here but I can't send one via the site since I am new so I used the one in your webpage.

Thanks! Back your regularly scheduled program.
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