420 @19hz

[joyspring]

Sure, the Altec 420 will go flat down to sub-20 Hz... albeit only at small signal.

Its Xmax (linear excursion, difference between voice coil and magnetic cap height) and resulting Vd (volume displacement) ultimately determines maximum sound pressure vs. frequency.

What simple small-signal Thiele-Small alignment calculations (i.e. - WinISD Beta) don't impart are excursion and sound pressure limits with large-signal input. Excursion requirements increase four times for every octave lower at equivalent sound pressure.

The 420A at a relatively small (compared to modern drivers) 4 mm Xmax will have negligible dynamic headroom at 19 Hz and further, a system tuned to that f3 will most likely exceed the Xmax even within its passband at the lower end at moderate listening level. 15% over Xmax yields clearly discernable (3%) third-harmonic (octave + fifth) distortion not to mention possible driver damage as well.

For the 420, I'd recommend using a different alignment to achieve a flat passband response and higher f3, and if you do require flat frequency response down to 20 Hz, I'd recommend a different driver with a high Xmax and resulting high Vd (volume displacement).

BobR

[zelgall]

The 420 has the same Xmax as a 416 and is similar to a 515 - about .5mm less. I don't think that any of these are good subwoofer candidates but that's not the normal application.

[GM]

Greets!

Yeah, using the published specs I get a T/S max flat alignment (0.707 Qtb) in the 46-48 ft^3/~19.4 Hz Fb range depending on how much the program's math's been rounded off. 'Flat' of course being a half space condition (speaker buried flush in a large field with measurement mic suspended over it) that assumes no inductance rise, so its in-room response will be anything but flat, though a cab this big tuned this low will have considerable boost down low from room gain, ergo factoring in a 25 W power handling limit there should be no over excursion issues you mention if the specs are reasonably accurate. Indeed, if the room gain is known, the cab's volume (Vb) can be reduced to flatten its in-room response, with the Fb adjusted higher only if there's no need for such a low tuning, like if you only spin vinyl and/or cassette tape and/or oldies transferred to CD/DVD, then tuning in the ~27-35 Hz makes sense.

Yes it does, though for good transient response it will need to be big, I get ~17.8163 ft^3 for a T/S max flat sealed (0.707 Qtc), though stuffed it can be reduced somewhat, so to keep it from exceeding Xmax at rated power (25 W), a ~12 ft^3 max is recommended. Obviously, from this we see that in the much smaller cabs Altec recommended it's going to be under-damped whether in sealed or vented alignments if the specs are reasonably accurate, which they probably are based on my dim memory of how 'thumpy' they sounded to me in Santanas? Using such an acoustically small sealed cab makes sense for a manufacturer though since folks that like loud and/or have kids that do are likely to be cranked until the distortion gets objectionable, so this 'tight' air cushion helps protect the driver.

Speaker cab tuning alignments are derived from electrical filter theory to complete the audio pioneer's choice of using an electrical equivalent circuit to define a driver's electro-mechanical properties with Qtb, Qtc defining the vented, sealed alignment's total quality (Qt) factor, i.e. the Q of the driver/cab's roll off slope below its F3.

Note that vented alignments only has the one Qtb (0.707), though sometimes one is used to define the slope between the F3, Fb in custom alignments such as an EBS (extended bass shelf) like I currently have, while sealed can be any number from a theoretical 0 = DC to as high as you want to go, though normally a Qtc = 0.5 (transient perfect) up to an audibly under-damped 2.0 is as low/high as you're likely to go short of defining an over-damped system using less than 0.5 Qts drivers in an IB or a typically severely under-damped horn compression chamber.

Anyway, I'm not all that familiar with these and while I have no doubt it will perform ~as predicted if the specs are reasonably accurate (with the above caveats), I can't help but wonder if a small cab is needed to keep the mids diaphragm under control, so caveat emptor on using a really large cab. Sure would be nice to know how it performs in a large ~12 ft^3 cab tuned to ~19 Hz though.

GM

[bfish]

Originally posted by GM:
...Sure would be nice to know how it performs in a large ~12 ft^3 cab tuned to ~19 Hz though.

GM

<font size="2" face="Verdana, Helvetica, sans-serif">Valhallax, I'd personally consider GM's final comment more of a knowledge/experience-based suggestion for your 420As than the curiousity-based speculation it might appear to be.

FWIW, WinISD can yield reasonable large signal predictions if you supply any missing data in the parameters section (fill in ALL the blanks correctly), then go to the "plot" tab, and substitute a higher wattage parameter. The results aren't particularly obvious or intuitive, but if you know what to look for, can be useful in that regard.