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Multi-Way Conventional loudspeakers with crossovers

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Old 20th October 2012, 08:28 PM   #8261
nuconz is offline nuconz  United States
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Do you remember when that was?
71 or 72 it seems?
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Old 20th October 2012, 09:15 PM   #8262
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Ah, was it the one in the old Millhopper shopping center? NW corner of town? I used to hang out there circa 1973-76. Great shop.
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Old 20th October 2012, 11:27 PM   #8263
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Genius Designer Jean Hiraga's Altec 604E MLTL speaker uses a down-firing port that is slot-loaded into the integral base plinth.

Jeanís description sounds like Onken type narrow slot ports that exit around the bottom perimeter 4-sides. A 3-side exit might be better for against the wall designs.

Wide bandwidth woofers(1,400Hz on Altec 604E coax, 700Hz on Lynn's Altec 416E) generate significant midrange frequency SPL that must be absorbed by TL stuffing as in MLTL, or stuffing around generic ports. Allowing even modest SPL midrange woofer energy to exit a bass reflex port will be audible.

For my MLTL cabinets I use 2Ē Owens 703 fiberglass panels on all 3-sides inside the cabinet in order to keep cabinet reflextions off the the rear cone, plus formaldehyde-free loose fiberglass batting TL stuffing. I also put a 1.5" quarter round radius on all front edges for diffraction control. 1.5" is the largest bit available for a normal router.

I plan to try Gene Hiraga's narrow slotted base-plinth port ideas on my next MLTL and would appreciate any feedback on this port technology.
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Old 22nd October 2012, 12:26 AM   #8264
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I can't claim much originality with the resistive-vent idea. The first place I saw a rigorous analysis of it was Richard Small's doctoral thesis - that was back in the mid-Seventies when I took a class from Dr. Ashley.

It was early days for Theile/Small theory, and resistive-vents were not part of the class material. I saw the big red hardcover book on Ashley's desk, and couldn't resist taking a look - chapters from Small's thesis were reprinted in those first AES Journal articles. Unfortunately, the resistive-vent chapter was not reprinted in the AES Journal, and is still hard to find, some forty years later.

I asked Ashley why the AES omitted that chapter, and the reason was interesting - RV enclosures are not as space-efficient as a vented enclosure, and have no obvious benefits over a closed-box. A brief scan of the chapter did indicate, as I suspected it might, that RV enclosures are not as sensitive to driver variations as the closed-box and vented-box equivalents.

A vented enclosure, as we all know these days, is a 4th-order highpass filter (Theile's great discovery), but like all 4th-order filters, is pretty sensitive to tuning conditions (this is true of opamp active filters as well). The damping supplied by the amplifier, and reflected through the BL product of the driver, has a critical effect on filter tuning. Unfortunately, amplifiers may exhibit nonlinear damping factors, and BL product is definitely not linear (well, L is a constant, but B can vary quite a lot).

By using a large resistive term in the electroacoustic highpass filter, not as much reliance on amplifier damping factor is needed. A resistive-vent enclosure, although not technically 3rd-order highpass filter, behaves like one in the passband of interest. At a very low frequency (less than 1~5 Hz), it reverts to a 4th-order system, although more heavily damped than a classical vented enclosure. From a designer's perspective, what's of interest is smooth variation in the group-delay characteristic, and low sensitivity to amplifier misbehavior and dynamic driver variations.

There's also another, more subtle benefit of resistive-vent enclosures. They're a bit more convenient for midbass drivers, since the heavily damped vent is not as prone to organ-pipe modes in the 400 to 800 Hz region, which are the bane of standard vented-cabinet designs. This is an area where the tricks used in TL enclosures come in handy - folding the path of the lossy vent, etc.

Resistive-vent enclosures have broader tuning that a vented enclosure; the output from a standard vent is typically 1/3 octave or less, basically a single large peak. By contrast, what comes out of the resistive vent is an octave or more, which lets the designer be a little more creative in locating the vent. Specifically, the vent can be located close to the floor, giving the speaker a bit of additional bass extension. Doing the same thing with a standard vented alignment is a little dangerous, since the floor coupling will have fairly unpredictable effects on the box tuning.

Vent location is an overlooked item in many vented-box loudspeakers. At the frequencies where the vent is effective - again, the output is typically less than 1/3 octave wide - the vent looks like a point-source (it is many times smaller than the wavelengths coming out of it). This point source has mirror-images in the floor, rear wall, and back wall, and the phase relation of these mirror images to the physical point source can make speaker location rather touchy in many rooms. At the frequencies where the vent is active (25~50 Hz), the walls and floor are nearly perfect reflectors, and damping materials have little effect.

If the vent has broader tuning - which is typical for RV enclosures and TL systems - and the location is directly on the floor, speaker location will not be quite as sensitive to room location.

Last edited by Lynn Olson; 22nd October 2012 at 12:52 AM.
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Old 22nd October 2012, 12:40 AM   #8265
soongsc is offline soongsc  Taiwan
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All ports have a resistive nature. You could still design a port to be pretty resistive, it just won't be as easy to built.
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Old 22nd October 2012, 12:54 AM   #8266
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Lynn ... I found this in UK.answers, "Adding resistance to the air flow in the duct lowers the Q of the port and increases the resistive component of the impedance at the box tuning frequency, f.sub.B. The result will be increased woofer motion, i.e. over-excursion at f.sub.B, which undermines one of the primary advantages of the vented-box loudspeaker enclosure design." The "over-excursion" bothers me, would you be kind enough to translate? Zene
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Old 22nd October 2012, 04:56 AM   #8267
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Compared to vented box, sealed or resisted-vent box gives you larger driver excursion in certain conditions. But larger is not necessary to be "over".
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Old 22nd October 2012, 07:20 AM   #8268
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Originally Posted by Zene Gillette View Post
Lynn ... I found this in UK.answers, "Adding resistance to the air flow in the duct lowers the Q of the port and increases the resistive component of the impedance at the box tuning frequency, f.sub.B. The result will be increased woofer motion, i.e. over-excursion at f.sub.B, which undermines one of the primary advantages of the vented-box loudspeaker enclosure design." The "over-excursion" bothers me, would you be kind enough to translate? Zene
The "excursion control" of a conventional Theile/Small vented box is both a blessing and a curse. First off, excursion is only significantly reduced at the box frequency - in fact, right at the box frequency, in a low-loss enclosure and a slippery vent, woofer excursion can nearly drop to zero. All of the output, of course, is going out the vent, while the cone is nearly silent.

But this is a narrowband effect. An octave above the box frequency, excursion reduction is slight - very close to the closed-box equivalent - and an octave below the box frequency, excursion is very great, much more than the closed-box equivalent. An octave below the box frequency, the woofer is pretty much in free air, and there is little to limit excursion except for the fairly minor resistance of the spider and surround.

In a closed box, the air-spring of the enclosed air is in series with the woofer compliance, so woofer excursions in the below-cutoff region are much better controlled.

These are the inevitable tradeoffs of closed vs vented enclosures. You win in one area, and lose in others.

A resistive-vent system is simply intermediate in behavior between the two. No reason to get lost in the weeds pursuing the impossible dream of a "non-resonant" or "aperiodic" enclosure. The dream is impossible because all loudspeakers, regardless of operating principles, are both highpass and lowpass filters. In plain English, they do not have infinite bandwidth, and cannot have infinite bandwidth.

Since the loudspeaker must have a highpass function - the response doesn't go down to DC - that implies electroacoustic reactive filter elements. They cannot be avoided.

However - we can go a little deeper and ask how stable are these elements? What are they actually made of, and how stable are they under dynamic (musical) conditions? That's a more interesting question.

We can pin down what isn't going to change: the mass of the woofer, the volume of air in the box (although lossy filling can shift this a bit), and the length of wire on the voice coil (the L of BL product). What does change is the B term (which then changes driver Q and efficiency), suspension compliance (this can be quite nonlinear and exhibit hysteresis effects), and changes in amplifier damping factor as Class AB stages pass through the zero crossing, or momentary slew or voltage-clip events, when the feedback loop fails to correct the amplifier.

Since a resistive-vent system is simply an intermediate alignment between closed and vented box, we can make the intentionally added resistance of the vent a dominant part of the highpass filter, instead of a loss term we're trying to minimize. This makes the shift in driver Q and shifts in amplifier damping factor less critical to the electroacoustic highpass filter we'd like to synthesize. By contrast, a classical vented-box system is quite sensitive to shifts in driver Q and variations in amplifier damping factor.

I should add that choices like this frequently appear in amplifier design. If there are no real resistive terms in say, a driver stage, and the loads are all reactive or determined by things like current sources/sinks, the amplifier can enter odd states where the behavior becomes unpredictable. That's when "latching" or "sticking" events happen, or outright failure due to transient overcurrents. A small bleeder resistor in the right place can keep things from getting out of hand, and have the nice side effect of making sure the amplifier stays in the "textbook" range of operation.

It's a little dangerous to assume that XYZ interface component will always behave as theory says it should. Theile/Small theory implicitly assumes linear behavior from the driver, and near-zero (and linear) source impedance from the amplifier. The theory starts to break down when the driver goes nonlinear, or the amplifier exhibits nonlinear source impedance.

This is why I prefer to design low-Q crossovers that accept a range of source impedances and still stay within limits, and bass alignments that tolerate driver variation and a (limited) range of source impedances.

Last edited by Lynn Olson; 22nd October 2012 at 07:39 AM.
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Old 22nd October 2012, 09:02 AM   #8269
AuroraB is offline AuroraB  Norway
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I've been following this thread off and on since it started, as I was originally interested in building the Ariel, - which for several reasons did not happen...
Now, it seems I've fallen back to old habits, and is looking for a new project....

To the point, though...
most people will probably associate 'resistive vent' with something like the old Scanspeak vents with fiber wool stuffing to make up for the resistance.....
Somehow this discussion make sme think of something undefined "else" ---?
What kind of box volume, and what sort of vent is referred to here, for the 416s?
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Old 22nd October 2012, 09:46 AM   #8270
soongsc is offline soongsc  Taiwan
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I think Lynn has a good feel for Q control.
It seems the kind of port also depends on driver characteristics. A port design that flattens the impedance as much as possible is really providing good control of the driver, allowing it to perform more optimally. Of course if the driver has short throw, then you exceed the limits quite easily, but that is where you really want to be careful how much peak SPL you are looking to get out of the driver at what frequency.

You can also try to design an aperiodic enclosure the wrong way, and end up with really dead music. The issue is applying appropriate resistance to the port while avoiding too much load on the driver. You also want the design to be tolerant to tolerance of the drivers over a period.
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Last edited by soongsc; 22nd October 2012 at 09:53 AM.
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