Spica Angelus loudspeaker

I am puzzled. No, really. I know you find it hard to believe that we sacerdotes of the golden-eared persuasion could ever be perplexed, but I have been pondering the imponderables of ports. Ever since the classic work of Richard Small and Neville Thiele in the early '70s showed how the low-frequency response of any box loudspeaker can be modeled as an electrical high-pass filter of some kind, with the relevant equations and data made available to all, there would seem to be very little reason why all loudspeakers with the same extension should not sound alike (or at least very similar) below 100Hz. Yet after reviewing 20 dynamic loudspeakers (and using 24) in the same room over the last seven months, I am led to the conclusion that speakers vary as much in the quality of their mid-to-upper bass as they do in any other region. A few are dry, more are exaggerated in this region; some are detailed and "fast," most are blurred, with the upper bass "slow" (by which I mean that the weight of bass tone seems to lag behind the leading edges of the sound).

Have I found any empirical correlation between bass character and design philosophy?

Sure. The speakers which I have found to work best in the mid-to-upper bass in my room use, almost without exception, sealed-box loading for the woofer, and those which work worst are reflex (ported) designs.

Now before you all jump to your feet and accuse me of Anglophilia—after all, you all know that the English have an inherent love of small, sealed-box loudspeakers—I must point out that I have no inherent bias against reflex designs. Mathematically, a sealed-box woofer loading basically gives the speaker a second-order, high-pass filter response, with a 12dB/octave roll-off below its resonant frequency; a reflex gives it a fourth-order response, with a 24dB/octave roll-off. Given an example of each type, with identical sensitivity, cutoff frequency, and Q of resonance, they should sound pretty much equally good or bad in the bass, apart from the greater phase shift around and below the cutoff point in the reflex design, which equates to less good performance in the time domain. In fact, the reflex design could even sound better, due to the woofer cone not having to undergo as much excursion for the same LF SPLs and therefore not introducing as much distortion.

Provided that neither speaker is driven beyond its excursion capabilities (and putting to one side the matter of reinforcement of the bass response below the cutoff point by the proximity of room boundaries), the size of each woofer, or its cone material, and the size of each box will not be relevant to the quality or quantity of bass.

Now, of course, you are jumping to your feet and accusing me of spouting nonsense—everyone knows that bigger speakers give bigger and better bass. Well, sorry, fellas, that just isn't so. The bigger the box, the higher its sensitivity, and the bigger the cone the more air it can move for a given displacement, both factors endowing the speaker with a greater dynamic range at low frequencies. But all other factors being equal, all that matters when it comes to bass quality is cutoff frequency and the Q of the bass resonance.

So why do I seem to prefer the bass featured by sealed-box loudspeakers?

Answers, please, on a postcard. But in the meantime, I can only assume that it is easier for a loudspeaker designer to get a consistent bass tuning in production with a sealed box. The designer may be able to get the same target extension and Q with a prototype ported system, but the inevitable errors and tolerances in production, coupled with the inherently less-good time-domain performance, seem always to raise the Q of the reflex tuning, resulting in too much upper bass and a "slow" overall bass quality. (It is always possible, of course, that loudspeaker manufacturers know that out there in the real world, in dealers' demo rooms, too much bass with too little damping will always help sales . . . Naaah. Surely Stereophile's readers wouldn't be taken in by such blatant salesmanship?)

All of which is by way of an introduction to my review of Spica's new Angelus loudspeaker, only their second full-range model to appear in five years. The previous Spica speaker, of course, was the small two-way TC50 (reviewed in Vol.11 No.1), which has become a best-seller, offering excellent performance at a very competitive price of just $450/pair. The Angelus, dedicated by its designer John Bau to the memory of Richard Heyser (see my interview with John elsewhere in this issue), was introduced at the 1988 Chicago Summer CES, but only went into full production in the Fall.

Again a two-way design, but now floorstanding, it is significantly larger than the TC50 and is distinguished by its unique "waisted" styling, with hardly a right angle anywhere, apart from the join between the cabinet rear and the base. The lower two-thirds of the speaker cabinet consist of an elegant trapezoidal prism veneered on the front and sides. On the top of that is mounted an inverted truncated pyramid, with the baffle sloping back. The woofer is positioned near to the "waist," with the tweeter in-line above it where the baffle widens, the placement of the drivers being asymmetric and handed. Both units are surrounded by an acoustic environment of ¾"-thick felt, the small cutout for the tweeter being shaped like the transom of a boat.

In his driver choice, John Bau echoes David Wilson's WATT by going for relatively modest but well-behaved units: a version of the well-known 1" fabric-dome tweeter from Audax, and a TPX-coned 8" woofer, also from Audax. (TPX is a new plastic material said to have less inherent coloration than polypropylene.) Builders of Dick Olsher's Dahlia and Dahlia-Debra loudspeakers (see Stereophile Vol.9 No.1, Vol.10 No.4) will be familiar with this drive-unit, but John Bau modifies it for use in the Angelus. As supplied, the cone has a distinctive rod-shaped dust cap, intended to equalize the airspace either side of the voice-coil. This "rod" was found to interfere with the tweeter's operating environment by introducing a sharp discontinuity from which the sound can be reflected. Spica replaces it with a flat dust-cap made of felt.

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