Velodyne ULD-18 & ULD-15 subwoofers
In a short five years, Velodyne has aggressively marketed five subwoofers spanning a price range of $645 (the VA1012) to $2595 (ULD-18), and distributed the products widely. Velodynes can be purchased both in very high-end audio stores, such as Lyric High-Fi or Definitive Audio, and in chain stores in the Washington, DC area. Their dealers number 240, and exceed the total dealerships for the seven subwoofers reviewed in past issues of this magazine by Dick Olsher (Vol.12 No.1), J. Gordon Holt and John Atkinson (Vol.11 No.4), or Martin Colloms (Vol.10 No.2). They have been successful in selling a product that has long been considered a well-heeled audiophile's accessory. As I shall point out, the ULD-18's performance more than justifies this success.
The Last Crusade: Is Subwoofing Possible?
The lowest frequencies of the musical spectrum are filled with much of its raw energy. The double-bass, the pipe organ, and the bass-drum produce tremendous emotional power. The subwoofer's raison d'être is to enhance the performance of the main loudspeaker by adding accurate, low-distortion sound in the 20100Hz region. Planar speakers (including electrostatics) have difficulty generating much output in the low-bass region without doubling or distorting. Panel systems, particularly small electrostatics, are limited in diaphragm excursion, and cannot move a great deal of air. In theory, the subwoofer should improve a general-purpose loudspeaker's performance by removing a major source of distortion from its range; freed from the constraints of producing low bass notes, they can play louder with greater dynamics. No wonder the most expensive loudspeakers come with large bass-driver towers.
Harry Partch, modern composer and inventor of musical instruments, put it best. The occasion was a recorded commentary of his building of musical instruments to generate profound bass. It was difficult for him to "share" the impact of these instruments via records. He lamented the general lack of "adequate playback equipment" which could convey the power of his huge, 6' bass marimba, the "Marimba Eroica" (Delusion of a Fury; A Ritual of Dream and Delusion, Columbia MS-30576, side 1). Partch quips: "In the right room acoustically, the Eroica is felt through the feet, against the belly, and, if one sits on the floor, it ripples through his bottom. It is very difficult to put on tape, and especially on records with any fidelity. Adequate playback equipment is absolutely essential, which means that the poorer generally are not privileged to experience a rippling through their backsides by an art form."
But cost has always been one of the tickets of admission to serious subwoofing. Unfortunately, the best subwoofers and their supporting amplifiers and crossovers are very pricey. The Janis W-1, a 15" unit, now lists for $795 without crossover or amplifier. Sound Lab makes a very exotic electrostatic subwoofer, the B-1, which lists for $6500 mono, $10,000 stereo, and uses panels that are approximately 7' by 4' by 4"! The Entec, another crossover/amp/servo systems approach like Velodyne's, costs between $1995 and $20,000/pair, depending upon the number of drivers. The Maggie/Crosby-Quad System, a noncommercial hybrid heard in Madrigal's Consumer Electronics Show display, uses the woofer section of a $3800 Magneplanar Tympani IV system. Stacked against these, the Velodyne ULD-18's "complete" $2595 price tag does appear more reasonable.
Besides cost, distortion has been the other Achilles heel. Subwoofers, like many dynamic drivers, are prone to large signal-distortion products in amounts greater than 10%, tracking the signal only as well as their voice-coil drives, elastic suspensions, and cabinet designs (acoustic suspension, transmission line, etc.) will allow. Dick Olsher, in Vol.12 No.1, found that these distortion products had disastrous effects on "finesse in bass response," which he defined as "speed, tightness, pitch definition, and resolution of bass detail." He dreamed of a subwoofer that would give him both "quantity" of low bass while also maintaining its "quality." This supposed tradeoff refers to subwoofer distortion, which increases rapidly as signal level increases. Sometimes, the distortion can be so objectionable that one can feel slightly ill (footnote 1).
Why did this problem occur if the subwoofer generates sound in an area where the human ear is notoriously insensitive? As it turns out, our ears are very sensitive not to the subwoofer's main signal, but to the unit's harmonic distortion products. A 30Hz marimba beat is felt as much as heard; its third harmonic, 90Hz, is definitely heard, however, and small traces will easily color male voices. If the woofer has 5% Total Harmonic Distortion (THD) at 30Hz, it means that a 90dB spl 30Hz note would produce a 64dB spl 90Hz note. Since the human ear is 10 times as sensitive to the 90Hz note, the sensitivity of the human ear unduly weighs the sound of the 90Hz distortion over the fundamental marimba sound (footnote 2).
Footnote 1: Larry Archibald excused himself and left an audition of an otherwise outstanding system at the recent June CES. He objected to a recording of low bass, in this case the Saint Sa;uens Symphony 3 ("Organ Symphony"). Apparently, deep bass (in this case 32Hz organ notes) can make him feel quite uncomfortable. Chuck Turigliatto, national sales manager of Velodyne, comments that he can feel bass "descending down his body" as the frequency goes down from 25Hz to 7Hz, and the 10Hz notes do induce quite a queasy feeling. I used a signal generator to drive the ULD-18 at 20Hz, and adjusted the peak-to-peak excursion of the cone to about ¼" (as Hall suggested). I placed my ear close to the woofer cone (not recommended!). I could see the cone moving, and felt a pressure in the air around my body. As my left ear got within 8" of the driver's center cap, I experienced definite sharp pain, even though I could not hear the 20Hz note! This means that I was encountering 120dB SPL sound levels, although was unable to hear any higher frequencies, indicating (to me) that the THD was well below 3%.
Footnote 2: Hall suggested an interesting test to illustrate my ear's sensitivity curve. I turned the ULD-18's enclosure over so the driver was facing upward, and poured a few grains of salt on the woofer cone. I again drove the subwoofer with a 20Hz signal, set the cone to 1/4" peak-to-peak excursion (using a Q-tip, a bit of Scotch tape, and a ruler), and listened. I could easily hear the grains of salt dancing on the cone, but was deaf to the fundamental 20Hz signal, although it was many dB higher in level!