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In Reply to: RE: Can a simple crossover (cap/inductor) achieve time alignment without a stepped baffle? posted by Presto on July 18, 2014 at 09:11:18
Well, I have this set of speakers in for listening, but I can’t work out how the designer has achieved the results. A frequency response chart is shown.
http://www.nsmt-loudspeakers.com/PSM-Super-Monitor
I know the drivers are custom designs, and I know some tweeters have a deeper acoustical center than others. The speakers image incredibly well, much like my Vandersteens. The sound is good, but I simply can’t make sense of the design, which bugs me. How can these be time aligned?
Are delay characteristics different between series and parallel crossovers different?
From what I read, methods for determining the acoustical center of a driver are very difficult.
I am a bit baffled. No pun intended.
Follow Ups:
not to mention, they say that that it's an acoustic suspension speaker when it's obviously a ported or TM design, so not sure that all that marketing language is accurate. anyway, hope they sound good. it's a chunk of change.
Reading up on transmission line designs, some do allow a good amount of pressure to develop, thus providing some benefits of an acoustic suspension design. I am almost certain this is what the designer is describing, although we know a true acoustic suspension needs to be a sealed box. I do find a good bit of discussion about TL designs providing substantial internal pressure, enough to effect the response of the drivers.
Having lived with this step response for 20 years:
[img]http://static.dyp.im/03FMvUlE9q/77118cb841ad8eae52acd680be3e52e8.jpg[/img]
I am left wondering if any tweeter distortion is baffle diffraction, time misalignment, or crossover.
Tom
Alignment requires the LF and HF signals arrive at the same time. The easiest way to get this is to off set the drivers in the baffle board.
If you are using an Altec 604 or 605, the drivers are fixed . So you need to time delay the Lf signals in the XO.
See my web site
www.donpatten@mac.com
Don
Or is the crossover on the tweeters in the Altec coaxials steeper and thus the output of the tweeter more delayed than that produced by the woofers?I am planning a WMW 2-way using two larger spherical enclosures for bass and one smaller sphere with a Manger (M) driver.
Leaving aside the stand issues. IF I am using 1st order Passive Line Level filters - (and any R/C/L on each driver at the 150Hz transition) -
the WR centre sphere's Manger driver's centre will need to be further back than the 'VC centre' of the two bass spheres?Correct?
Also! IF I do use 1st order filters, what is the maximum driver offset I can have at 150Hz? I know there's a rule of thumb - which depends on filter order and lambda but I am not sure what it is. IIRC it is a 1/4 of a wave-length for 1st order filters. It might be a whole lot less!?
If I can get the two bass-driver centres a lot closer than what is required, do those two distances - from the main WR driver - still need to be almost exactly the same?
But, if I go with 3rd-order line-level filters, most of these issues become less critical, yes?
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 07/23/14
Timbo
The XO delay is caused by the coils in the woofer path. More "L" more delay. Since a higher order filter has more L , the higher order filter has more delay.
Use the test signal on my web site;
slide the drivers fore & aft to tune the delay.
You will need a scope to tell when the delay is correct.
www.donpatten.com
...and the published frequency response graph has nothing to do with time alignment. One needs a step response curve (like the one Sterophile publishes for speaker reviews) to determine actual driver time alignment. The tech details do say that the crossover is first order so it MAY indeed be phase aligned. But they are NOT time aligned. Most speakers are not time aligned and many people cannot hear the difference between a time-aligned and non-aligned speaker for a variety of reasons. People are much more sensitive to frequency response anomalies than they are to time and/or phase anomalies.
For me, time and phase alignment is a must for the most natural sounding reproduction. I started with Theil's but never loved their bright high end (this was back in the 1980's and the Their model 03A). Today I am quite happy with my Vandersteen 3A Sigs which get the phase and time alignment along with a natural-sounding frequency response.
I love my Vandys, but they are old and failing, and they do not make a speaker that is ideally suited to my room. I felt the same way about the Theils.
From the designer:
“With regard to time coherency, the speakers are time coherent. Many stepped baffle loudspeakers are not time coherent. We use only a single air core copper coil and a single capacitor (a first-order or zero-order crossover) and unlike most loudspeakers both the tweeter and woofer are wired in the same positive polarity (most designs flip the polarity of the tweeter to gain better coherence). This indicates that the drivers are acoustically matched and the cabinet matches the parameters of the drivers very well. Keep in mind complex crossovers are meant to correct aberrations in the loudspeaker response and that you cannot get a flat frequency response with a first order, positive polarity design unless the drivers are time coherent.”
So the claim is clearly being made, and the relevancy of the frequency response in relation to the time coherence is mentioned. From what I read, time alignment is difficult because methods to determine the acoustic center of the driver are difficult and don’t apply across the frequency range.
You should probably check out the Gallo Ref. 3.1s and 3.5s. They are time/phase aligned and are virtually crossover-less. There are some great deals on them for $2500 and less on A'gon....
-RW-
I do find the language and thought process interesting which makes me wonder if the designer information matters. Remember the designer will talk up their design.
In the verbiage:
"and unlike most loudspeakers both the tweeter and woofer are wired in the same positive polarity (most designs flip the polarity of the tweeter to gain better coherence)."
I have not seen simple cap/coil designs (pseudo first order) flip tweeter phase. That is typically done when using a 2nd order electrical using a cap and coil on each driver.
Maybe everyone is just over thinking what this speaker is especially considering the price.
PeterZ
Only TRUE second-order designs, with both drivers -6dB at crossover frequency, and rolling off at 12dB/octave for at least an octave above and below that frequency, do that. Utterly unnecessary with first, third, or fourth-order crossovers.
Remember, we're talking acoustic crossover here, with slopes being the sum of the electrical transfer function of the filters and the natural unfiltered response curves of the drivers. Unless there is a VERY broad, flat overlap of the unfiltered driver responses, like four octaves, true first or second order acoustic slopes are really difficult to attain.
Ah, well maybe so. As a book designer living on fairly tight cash flow, the price is not small to me. I also chose to audition these based on the design goals. I think a measurement of step response will show what is really going on.
The acoustic suspension/transmission line bit struck me as odd from the beginning, but I assume the design uses a box with a small vent—small enough that the woofer gets some pressurized support, but less than an actual tuned, sealed box.
My biggest issue is one of not wanting to own a speaker sold on false pretenses. That means giving the designer several chances to respond to questions AND doing what measurement I can to satisfy myself.
For acoustic suspension, the box must be AIR Tight. There is no such thing as a small port. A TRUE acoustic suspension woofer wouldn't begin to work properly with any size opening. And to imply there is any relationship between acoustic suspension and transmission line is totally ignorant and says the writer either doesn't know what he is writing about or is spewing plain BS.
They claim they have a time-coherent (transient perfect) loudspeaker with a cap and coil for an MTM design with no baffle or driver offset. "Cap and coil" is a 1st order electric crossover. To get a first order acoustic response with a 1st order electrical crossover using just driver matching is, in my mind, quite a feat.But, according to theory you can't have a *true* transient accurate crossover unless you have a transient accurate acoustic response *and* drivers have their acoustic centers lined up. There are also other types of crossovers which can yield a transient accurate response, such as the B&O filler driver approach, or subtractive delays designs, or with group-delay correction in the DSP realm. In the DSP realm, anything is possible...
It's a simple matter of seeing this speaker's step response.
I'm skeptical, but if this speaker does have a transient accurate step response, I'd love to figure out how the heck they pulled it off. They may be guilty of taking a liberty with word usage "time coherent". A phase coherent speaker does not necessarily have constant group delay. The LR4 acoustic response is phase coherent but not time coherent, for example.
These terms sometimes mean different things to different people, depending on who you ask - and what you ask...
Cheers,
Presto
Edits: 07/18/14
I am skeptical as well. If all the claims are true, it IS something unusual. I am just looking for any rational idea.
I will ask again for an impedance curve and also a step response graph.
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