Real Car Audio

[XMAX-1]

Anyone vaguely familiar with the car audio market knows how important it is to have great bass in a vehicle. Now with the proliferation of home theater setups we are seeing the rising prominence of compact high output subwoofers. I'm an "old timer," so naturally lot of people have asked me who cooked up this technology. The easy answer is that the "micro" subwoofer is the evolutionary product of numerous innovative and creative people. The first application was autosound. The car interior is relatively small, especially if space is to be left over for passengers and bags; but as the appetite for intense bass is large in that arena it made sense that it should happen there first.
While I cannot take credit for these subwoofer contributions, I was at the scene of a number of these breakthroughs. So here is an abbreviated history of deep bass, woofers and subwoofers that delivered more wallop in less space than their less capable competition.

To begin with, anyone can jam a large woofer into a small box. Reproducing satisfactory deep bass from that enclosure is another story. As a woofer moves inwards the air in the enclosure compresses, acting like a spring. This stiff spring shifts the in-box fundamental resonance upward and rolls off the bass. The compressed air wants to escape and looks for some form of pressure release. Without any release, you can have buckling of the cone and surround.

Over the last 50 years various researchers have looked for ways to get more bang for their buck out of the small woofer box, using unique speakers, bass reflex venting and other box-tuning techniques, trick amplifier servo feedback circuits, and signal processing.

In 1954, Acoustic Research (AR) introduced the AR-1W, which packed a long throw (high excursion) woofer into a relatively compact sealed enclosure. Bass extended down to almost 20 Hz without any equalization. A few years later AR introduced a full range home stereo speaker (AR-3) based on this design. More recently AR went retro and reintroduced a modern day version, the AR303. Stereo phonograph records followed by multiplex stereo FM radio became a reality during the late 1950s and audio turned from a do-it-yourself construction hobby to commercially available way to play back music. This success was dependent on finding a way to unobtrusively place not a single but a pair of speakers into the living room without divorce. Amplifier power was scarce in the 1950's and gigantic highly efficient JBL and Altec speaker systems derived from movie theaters sound systems had been the solution. Amplifier power, like the Dynaco's 60 watter and the McIntosh MC275 (dual 75 watt) were the big guns at the time.

AR had patented acoustic suspension and another popular brand, KLH, became one of their licensees. Electro Voice fought the patent and eventually had it invalidated. It turned out that Harry Olsen of RCA labs had described a tweeter that used acoustic suspension in a paper on RCA's LC1A studio monitor back in 1948.

In the early 1970s Jensen had a talented engineer and mathematician by the name of James Novak who came up with equations for getting the bass reflex speaker under control. Using these techniques Jensen introduced a range of compact speakers that had much of the bass of the AR, but higher efficiency. Other researchers, such as Theile, Small, Ashley, and Benson continued with Novak's ideas and moved vented speaker design from wisdom and witchcraft to a science. The computer simulation programs used by autosound installers today are based on this body of work.

Getting a bass reflex duct long enough and large enough in diameter so the vent does not create wind noises is a big problem when you are trying to get really deep bass. To keep the air velocity low, the duct can end up larger than the enclosure! Harry Olsen came to the rescue, publishing a paper on his concept of a passive radiator as a vent substitute. Instead of a deep vent or port, a diaphragm with a suspension could be used. The mass of the diaphragm would be equal to the mass of the column of air in a long vent.

JBL, in the late 1960s offered a range of passive radiators that do-it-yourselfers could install in pre-engineered enclosure designs. Tuning of JBL's passive radiators was by mass loading -- the user was instructed to bolt some specified number of heavy steel washers to the dust cap.

In 1968 Paul Weathers, an inventor of many audio products introduced the Compass Triphonic system. The Triphonic introduced a common subwoofer and a pair of very compact satellite speakers. Although common subwoofers had been used previously, the Triphonic was the forerunner of today's modern home theater subwoofer/ satellite products. Ten years later Triad resurrected the concept using the tiny satellites and compact subwoofers.

Gene Cerwinsky's Cerwin Vega made many compact deep bass high excursion subwoofers, but most were pro audio monitors for clubs and concert sound. One of their early high excursion subwoofers for home stereo was made for the famous Infinity Servo-Static, introduced over 30 years ago. It used a common subwoofer with a servo-control coil and its own power amplifier. The full range speakers were a pair of 6' high dipole electrostatic panels.

Soon afterward, Cerwin Vega introduced their "stroker" subwoofers for high output applications, later stokers followed for home and car use. Cerwin Vega's red large multiplayer foam surrounds, enormous spiders, and long excursion voice coils continued to push the envelope for sheer acoustic output, power handling and deep bass. A talented engineer, Henry Goldansky and Walter Hoffman later joined Cerwin Vega and helped further develop pro and autosound products.

In the early 1970s through the early 1980s I had a speaker company (GLI) that manufactured club speaker systems. Some of our woofers were sourced from Cerwin Vega, and one of our designs had a passive radiator whose foam surrounds were getting beaten to death. The Cerwin Vega woofer foam surrounds were surviving just fine; and I found out that many of their designs used multiple layers of foam laminated together. This would provide the strength and the structural integrity to withstand the pressures in the enclosure without buckling. In the early 1980s Electro Voice also had to resort to toughened surround techniques in their Manifold concert sound subwoofers in which four big woofers were crammed into a tiny box no larger than the volume of the woofer chassis themselves. The pressures inside were intense, not to mention the sound pressures outside in the seating! Aside from the trick enclosures, the woofers provided super high excursion.

Electro Voice first introduced Doug Button's brilliant EVX1500 and 1800 which offered 2"+ peak to peak excursion and even larger clearances to eliminate damage during over excursion. These woofers had many innovations and not all these techniques were fully ready for field use. Soon afterwards EV backed off and introduced a less radical product series, the EVX 150 and 180, which are still in production and doing yeoman's duty today in clubs and concert sound.

It was not just the ability to withstand buckling and flapping from internal box pressures that needed to be addressed when surrounds were reformulated, but also distortion reduction. In the 1970s the British Broadcasting agency, the BBC, conducted a great deal of research on speaker distortion (they were on a quest for the ultimate monitoring speaker) and they found that at lower sound levels distortion would rise due to the lack of pressure in the enclosure; and that at medium levels the increased pressure within the enclosure would keep the surround taut enough and reduce harmonic distortion. I figured that if enough pressure (and sound level) were to build up the distortion would eventually increase. There were a few interesting surrounds designed to counteract buckling forces, such as a V cross-section corrugated surround from Hitachi while Celestion produced a dimpled rubber design, both in the 1970s.

JL Audio applied the techniques of AR and Cerwin Vega and others for the car audio environment. It was big woofers with heavy cones and high excursion in tiny spaces in cars that put JL on the map. JL moved the spider further away from the frame and increased the spider diameter in order to increase the maximum possible excursion of their subwoofers. The cones were reinforced and other innovations were implemented to prevent the cones from crunching due to the high pressures within the enclosure. Obviously this equation worked well for JL, as they have become an established market leader.

Around the mid-1980s Earthquake (San Francisco, CA) started designing and buying speakers from Sammi Sound in Korea (I was working with Sammi at the time as part of technology transfer program of the Korean Gov't). Earthquake wanted big surrounds and stacked magnets and large diameter spiders to out-do the competition. Since Sammi tooled and fabricated all their own cones, surrounds, frames, and spiders, with guidance from Earthquake, this was easy. For the surround, instead of laminating layers of foam, we just used thicker and higher performance foam. It was not just the thickness that was growing, but also the width and height of the foam rolls to enable longer and more linear excursion. Eventually Earthquake applied these techniques to their line of compact high output home theater subwoofers. Another autosound subwoofer pioneer was SoundStream, which offered some extreme excursion drivers like the 1993 SS-10R woofer (see picture). In a test report in Car Audio & Electronics (June, '94)) the famous Peter Mitchell tested several boxes under one cubic feet and moved massive distances.

Velodyne started up in the early 1980s and was my first consulting job when I moved to the West Coast. The company's first effort was a home theater subwoofer that used servo control to enable a big 18" woofer to have extended bass in an enclosure not much bigger than the woofer itself (the ULD-1. Dave Hall, the founder and inventor of the High Gain/Ultra Low Distortion servo system based his design on an extreme variant of an acoustic system where the enclosure was far too small for the woofer. In this case, it was an 18" woofer that was used in an enclosure about half the optimal internal volume; and the servo system corrected for both the bass response and some of the driver distortion.

Acoustic suspension was first embraced by the audio community as a path to more linear reproduction (low distortion) compared to bass reflex. This is true except when extremely small enclosures with larger woofers with very high excursion, where the air becomes non-linear. The Velodyne ULD-18 of 1985 reached these extremes of acoustic output and intense air pressure within the enclosure and a sensor was integrated onto the voice coil to monitor the voice coil position -- where is it was and where it should be -- and correct the error "on-the-fly." The ULD 18 distortion was a tiny fraction of the competition, and later models (including passive radiator versions) established the home subwoofer market, as well as Velodyne as a leader in the home theater subwoofer business. To deal with the need for higher power without excessive heat (fan noise is a no-no in consumer applications) Velodyne began working with switching amplifiers in the late 1980s.

A wave of bandpass enclosures hit the market in the 1990s, although the complexity of construction, less than tight impulse response, not to mention patent issues, have dampened enthusiasm for this approach.

About five years ago Bob Carver's Sunfire introduced a really compact home theater subwoofer that put out a lot of bass. It quickly became very popular. The subcontractor for the woofer was Ground Zero (although the OEM group operated under a different name) and one of their engineers was Walter Hoffman. I mentioned earlier on that he had worked at Cerwin Vega for a number of years and had extensive experience in compact car audio subwoofer design techniques; and no doubt this came in very handy. Ground Zero soon offered a variation on the driver used in Sunfire's design, and the large surround/super long voice coil subwoofer race shifted to high gear -- along with a bunch of lawsuits, among these manufacturers. Bob Carver has filed several claims with the patent and trademark office -- this is the source of much agitation in both autosound and home theater subwoofer businesses. There are many claims in Mr. Carver's patent(s) and perhaps this is an oversimplification, but it would seem that any further upward scaling of voice coil length, surround size and stiffness, and excursion in general are now the property of Mr. Carver.

No doubt, it is all in the court's hands, but I think there is enough history to ensure an accurate decision. The reader can rightly infer that I feel the car audio and pro-audio crowd have been the primary contributors to the latest innovations that were used in the home theater market.

Thanks to all the dedicated audio guys mentioned here (and other unheralded engineers), today's subwoofer designer has the foundation for achieving good results using tight volume enclosures with high excursion woofers in home theater and autosound applications.



By Mike Klasco
CA&E MAgazine
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