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Just wanting some opinions. Is it possible without physically aligning the acoustic centers of the drivers in a two-way speaker, to achieve time alignment with a simple single cap, single inductor crossover?
Follow Ups:
My Boston A40's - only one Cap in series with tweeter "Upside Down" on a 20" stand have perfect time alignment measured through the crossover region at my listening position.
The upside down placement With tweeters below the ear did not seem to effect the sound stage.
Note- My MMG's with the standard crossover at a 45 degree tweeters in at the listening seat measured pretty dang close time alignment through the crossover - though panels are a little vague at a 2 meter distance.
Three most important things in Audio reproduction: Keep the noise levels low, the power high and the room diffuse.
...the woofers were probably designed to have a natural roll off at the required crossover frequency, so an inductor wasn't necessary. Also keep in mind that natural roll off was very possibly second order as with the tweeter as well. Placing a capacitor in series with the tweeter would increase that to 3rd order. Just because a woofer or a tweeter have only one component connected to them, they're most likely NOT first order crossovers.
Grammar: The difference between feeling your nuts, and feeling you're nuts.
Edits: 07/26/14 07/26/14
And this could me confirmed or measured by measuring the response of each driver by itself? Anyway, there is an inductor on the woofers.
I was referring to the previous poster's Boston A-40's lack of an inductor, not yours. And the only way to tell the crossover's actual acoustic order is to measure it. It's important to understand that regardless of the ELECTRICAL order of the filter, it's the ACOUSTIC result that defines the crossover.
Grammar: The difference between feeling your nuts, and feeling you're nuts.
Ah, good clarification. So a first order crossover could very well achieve second order or third order slopes with help from a cabinet design. What eludes me is what effect this has on phase.
You'll get second order from a sealed cabinet, 4th order from a ported one. It still doesn't seem like you're able to differentiate between electrical and acoustic crossover orders.
Grammar: The difference between feeling your nuts, and feeling you're nuts.
What do you get from a transmission line, 1/8th wave, which reportedly is not possible? I am not affecting some scholarly knowledge of acoustic engineering, I am trying to make some measurements and have a real look at a set of speakers that are sitting in my living space at the moment.
Let’s say, for arguments sake, that the tweeter is in a sealed box, as many are, then we have a technical marker for the designer’s claim that the design is transmission line/acoustic suspension hybrid. Beyond this, transmission line provides some of the back pressure of acoustic suspension, but is not acoustic suspension as the porting provides a momentary pressure that is dissipated through the transmission line.
What I would like to comprehend at the moment is how these design parameters influence phase.
Thanks for you input. Please keep it coming.
If you don’t mind my asking, how do you measure and how extensive is your test equipment?
I am trying to decide what measurement is worth using low end measurement methods.
FWIW, the step response look pretty good—comparing a 1M on tweeter axis vs. a listening position. It also looks almost identical to what my Vandersteens produce, and they are known to be one of few speakers to produce a good step response.
I suspect my level of error might disguise an early tweeter.
Jupiter-
Just a comment. The graph you show is not a step response, the time scale (horizontal) for a speaker's step graph would be maybe a few tens of milliseconds. And the vertical scale on that is dB (no polarity info). A good step response would drop in a straight sloped line like that only when shown on a linear scale. I'd guess that is a graph of decay of room refections, or possibly a nearfield of a very ringy speaker!
On some other discussions in this thread: an acoustic suspension woofer has a 2nd order highpass response, so can only approach time waveform coherence at frequencies much higher than the woofer's f3 cutoff. A very low Q alignment (sealed or OB) can approach linear phase but hard to get deep bass from.
Interesting. I don’t know how REW calculates impulse and step from a sweep. This is the actual step isolated in the window. The app. evidently looks for the response within this window.
I suspect you can turn the phase data on or off. Still, this is not what I see, for example, Stereophile listing as a step response.
I have used a different window to clear the noise around the wave form, but essentially, the REW measurement protocol for step response does use the same horizontal and vertical axis.
Or not. Only a step response measurement can tell you for sure. And the time alignment, indicated by both drivers blending smoothly in a single step (see published measurements of Vandersteens, for an example), will only occur on a VERY narrow vertical listening/measuring axis. Raise or lower your head (or microphone) a few centimeters, and the time alignment goes all to hell.
How audible this will be is a whole 'nother issue. Since the effect is easily measured, it can PROBABLY be heard if you train your ears to listen for and recognize it. But since the vast majority of speakers, including the very good ones, are NOT time-aligned, and this doesn't seem to bother the vast majority of audiophiles who enjoy them, it's PROBABLY not that big a deal. Just another triviality for a few obsessed designers and audiophiles to neuroticize over.
Probably not a good approach to tip an MTM design. They need to be firing straight on. And yes, my Vandersteens do produce a nice step response. I am currently attempting to measure these, but software issues are making it difficult.
If anyone knows a good how-to for getting a decent step response measurement, please post a link.
Should looked at the link. Tipping those sure wouldn't help! Only a stepped baffle would time-align those, like Dunlavy/Duntech used.
The Arta software (see link) can measure impulse and step responses, along with frequency response and other parameters. You will need compatible computer sound card and a good calibrated mic.
Hi
A friend wrote a nice article about this from the engineering standpoint and this might be helpful.
Refer to fig 12 on phase, the upper fig is time coherent and preserves the input waveshape.
It appears to be a single driver, while the lower one is that of 2 way speaker with a 4th order crossover with the drivers acoustic centers aligned but even so, is not phase coherent / cannot preserve the input waveshape as the upper and lower sources are separated in time due to the crossover phase shift.
One might recognize then that that situation is the condition produced by an ‘all pass” filter where just like the crossover, the low part is behind the upper part.
All named filter shapes above first order have that same effect on phase and produce 90 degrees rotation per order. A 4th order filter produces a 360 degree phase rotation going from well above to well below crossover. The lf signal is behind the hf signal as it emerges from the crossover but requires that the drivers have the acoustic centers be “time aligned” ironically even though the resultant acoustic output is not and has the all pass filter phase property.
To make a first order crossover work, the assumption is the upper and lower drivers have ruler flat magnitude and phase because the Sum is the mag and phase of the drivers added to that of the filters. The problem is that this is very hard on drivers and isn’t practical when higher outputs are needed.
http://www.prosoundweb.com/article/do_you_speak_geek_the_unique_language_of_audio_analysis/
Hope that helps
Tom Danley
Danley Sound Labs
Presto's answer is good.
In general I find reading their web site difficult. They speak of combining acoustic suspension and transmission line. I assume this means one woofer is a transmission line and the other is acoustic suspension. Strange combination and I couldn't find out why they do it.
They speak about acoustic suspension and transmission lines performing in a similar way. Bud Fried(IMF and Fried speakers) must be rolling in his grave. He was a leading line exponent and hated closed boxes. And I suspect they don't know the difference between closed box and acoustic suspension. There are differences.
First, you do need to align acoustic centers. This is done with sloped baffles, stepped baffles or delays if the crossover is digital (DSP).
Second, the simple "1st order electric" crossover you've described - this is a common misconception. The crossover needed for a true 1st order ACOUSTIC response is not going to be the same crossover for a simple 1st order ELECTRICAL transfer function (cap and coil). This is because with shallow 1st order slopes, drivers have natural frequency and phase variations that occur well past the intended crossover points. (In other words, the acoustic response - amplitude and phase - of a given driver is in play at least 2 octaves beyond the crossover point).
If you look at the crossover of a true 1st order acoustic design it will be far more complex than a single cap and single inductor. This is because amplitude and impedance equalization circuits are required to get the drivers acoustically "flat" enough to meet the 1st order acoustic response.
This means the designer needs drivers with specialized characteristics that suit a 1st order acoustic design. A big one, for example, is power handling of the tweeter in such a design.
Cheers,
Presto
Hello everyone,
I am lucky to find this thread as this topic greatly interests me now (I am thinking over building something for the first time).
I am thinking about a FAST (3 inch fullranger + 8 inch woofer) two-way, with crossover frequency of 300 Hz. For crossover topology I will probably choose first order passive line-level. Based on what I read, this could allow me to achieve the 'point-sourceness' of a loudspeaker. However, based on the physical dimensions of drivers, the pathlength difference between drivers could be something like 10-15 cm. Will that preclude me from achieving transient-perfect response?
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.
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.
"Phase alignment" and "time alignment" are two different things.
You can have two drivers which are, for example, 360 degrees out of time with each other (at some frequency), but are in phase at that frequency.
Ok, that's all I know.
:)
Using a first order crossover will provide phase alignment but in order to get time alignment, the drivers must be physically aligned properly.
Very clear answer. So this speaker can not be time aligned?
http://www.nsmt-loudspeakers.com/PSM-Super-Monitor
Look at commercially available time-aligned, phase coherent designs (e.g. the old Thiel line, Vandersteen and some others) and you will see that the cabinet designs are such that the drivers themselves are aligned at their acoustic centers. This requires either a sloping baffle or a stepped baffle. The speakers you reference cannot be time aligned in their current cabinet.
I have Vandersteens. Thanks for your response.
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