Since the introduction of CDs, MiniDiscs, and now the Super Audio DVD, even the most modest DJ mobile and club system rivals the dynamic range attained by state-of-the-art recording studio equipment of just a few years ago.

Digital audio has banished sound reproduction defects such as tape hiss noise, pops and clicks, compressed dynamics and frequency response limitations of analog tape and records. Even acoustic feedback at loud levels is greatly reduced over analog records by CD players. The wider dynamics and cleaner sound of digital music sources is seductive and many DJs respond to this by using sound levels approaching or even exceeding the live performance. At elevated listening levels, especially in conjunction with the extended deep bass capability of modern audio electronics, the stress on the loudspeakers is great.

Potentially, one of the biggest problems of "the digital age" for mobiles is speaker failure. The music industry has embraced the increased performance capabilities of the digital media, with bass that extends down to the lowest octave and bass transients that challenge the headroom of the largest amplifiers. It is not just the "demo" discs that offer this level of performance, but most all of the popular music releases.

When the acoustic suspension speaker designs, so cherished by audiophiles in the 1960s, were used for reggae music in the late 1960s for "disco" basement parties, woofer burnout problems became a plague. During the 1980s, the power handling of pro speakers began to increase and, spurred on by digital audio, even more dramatic strides are now being made. So what new techniques are being used to combat speaker failure?

Speaker failure modes can be roughly grouped into categories – excursion and thermal capacity. At lower frequencies, the voice coil and speaker cone may travel too far and can be mechanically damaged. In a woofer this commonly takes the form of a ripped cone, torn suspension parts, or the voice coil deformed. In a tweeter or horn driver, the diaphragm may shatter, or the suspension may tear. When the transducer (a fancy name for any type of speaker) is used in the middle and upper range of its response, excursion becomes less of a problem, while overheating becomes the more important issue. It is really voice coil overheating that burns out speakers, not how much power is applied to the driver. So what new technologies are being used by speaker manufacturers to improve power handling and prevent speaker failure?

Excursion Limitations

The most damaging condition for a woofer is when the voice coil hits the back plate. Almost instantly, the coil is deformed and the speaker is ruined. One solution is for the speaker manufacturer to use a bumped back plate. Actually, this is an obvious common sense solution. The metal disc that you can see at the rear of the speaker driver has a simple "dent" around the pole piece (inside the speaker structure – facing the voice coil), which provides the additional clearance so the voice coil will not hit the back plate on large excursions.

Another solution to overexcursion limitations is the bass reflex enclosure. This is not a new solution, but an established technique that preceded the air suspension design. Vented, or bass reflex enclosures are popular again. Bass reflex enclosures make optimum use of the deep bass energy radiated off the back of the woofer and "tune" this energy into useful output, thereby reducing the excursion requirements of a bass speaker for a given sound level.

Subwoofers also result in reduced excursion for the main speaker’s woofers. Typically a crossover, either in the subwoofer, or an external network, rolls off the deep bass in the woofers, and sends it to the subwoofer, which has extended excursion capacity.

Electronic solutions to speaker protection have become popular in pro-audio, with sophisticated processor speakers that use limiters, thermal sensors, and micro-computer techniques to monitor the condition of the speaker system. These are used in high-end systems from Yamaha, Apogee, Renkus-Heinz, Bag End ELF, Electro-Voice and others. Even many of the less expensive semi-pro speaker systems now use protection circuits in the crossover networks like transient absorber varistors on tweeters and circuit breakers on the woofer.

Thermal Limitations

Most DJs have seen toasted voice coils pulled from some hapless woofer or tweeter. The voice coil consists of copper or aluminum wire. The wire insulation temperature limits can be as low as 150-degrees Fahrenheit or as high as 400-degrees F. Everything else being equal, the larger the diameter of the voice coil, the greater the thermal power handing capacity of the speaker. Of course, this is not always the case, and there are examples of 1.5-inch diameter voice coil woofers that can handle more power than 2-inch diameter voice coil woofers.

But the voice coil does not function alone – it’s part of a system. The voice coil assembly is attached to a cone (or domed diaphragm in a dome tweeter). The coil is wound on a former, which is typically made of Kapton, Nomex, or aluminum (more on this later). The coil is centered in a magnetic circuit, which, if optimally designed, can effectively pull the heat off the coil.

The most important insight to be gained from this article is that it is not the power that you put into the speaker that burns it out, only the inability of the speaker to get rid of the heat. Woofers are only 5-percent efficient, at best, so 100 watts of
audio input ends up as 5 acoustic watts (sound) and 95 watts of heat.

To get the heat off the voice coil, various heat paths exist. The voice coil former can be thermally conductive, such as an aluminum bobbin. Aluminum effectively pulls the heat off the coil. Black anodized aluminum is a more effective radiator than natural aluminum. Aluminum bobbins are not everyone’s favorite materials, as the electrical conductivity results in eddy currents, which increase harmonic distortion and voice coil inductance. The eddy currents also create a rocking force on the coil as it moves. Kapton, an aerospace polyimide film material, is one of the most popular bobbin materials for high performance speakers. Kapton offers high strength even at intense temperatures, low weight, and none of the eddy current related problems of aluminum, but is not thermally conductive. DuPont, the manufacturer of Kapton, also offers Kapton MTB, specifically formulated for high performance speakers. Kapton MTB is thermally conductive and black, so heat is both pulled from the coil.

Anyone that has worked on thermally insulating their home has learned that air is the best insulator. Unfortunately, the air gap between the voice coil and the top plate, and the air gap between the bobbin and the pole piece provides an almost insurmountable barrier for the heat to leave the coil and bobbin. If the coil temperature is 350-degrees F, then the top plate is probably 150-degrees F. Most woofers have a vent down the pole piece, so the dust cup will pump air and transfer heat off the pole piece. At the same time, air is also pushed past the coil, pulling heat off. JBL has introduced a vented gap cooling system in their big speakers, which is a sophisticated extension of this approach.

A very effective and increasingly popular solution is to reduce the thermal resistance between the coil assembly and the magnetic system by replacing the air in the gap with a thermally conductive fluid. Ferrofluids, developed by NASA and licensed to Ferrofluidics Corporation, are magnetic fluids that are thermally conductive. The magnetic fluid is held in the gap by the speaker’s intense magnetic field. The thermal resistance of ferrofluids is four times lower than the air it replaces.

Ferrofluids have other benefits, such as a liquid bearing effect that keeps the coil centered, as well as being a lubricant, so coil rubs are less damaging to the wire insulation. While ferrofluids have been used in speakers for well over a decade, stable operation at elevated temperatures and woofer grade ferrofluids have only been commercialized during the last two years. Peak power handling of ferrofluid-treated woofers typically increases by a factor of 4 (at frequencies above the excursion limited range). For years Community, MTX, Meyer Sound and Apogee have used ferrofluids in woofers.

Actually, improving the heat dissipation capabilities of loudspeakers has other important benefits, besides just reducing coil burnout. As a speaker coil heats up, its impedance rises. This results in the speaker drawing less current and the signal level dropping. The DJ responds by bringing up the level, and the loudspeaker responds by suffering from increased power compression. Secondary, less catastrophic phenomena accompany power compression. If a passive crossover network is being used, then the speaker’s impedance rise will also interact with the crossover point, with a shift of an octave not being unusual. Speakers undergoing power compression also have a drop in their top end response, so the sound quality, not just the level, is degraded. The thermal cycling greatly fatigues the glue joints and other materials, and the heating causes expansion of the coil and increased likelihood of the coil scraping the top plate of the magnetic system.

DJs will continue to strive for increasing and dramatic music presentations. Loudspeaker manufacturers are responding by engineering driver components that integrate advanced materials and techniques to handle reproduction of both the extended frequency response, as well as increased dynamic range. Advanced Kapton polyimide films, ferrofluids, and other material technologies, once reserved for aerospace applications, have moved into professional speakers.

If you have any questions for Mike Klasco, please write to DJ Times, 25 Willowdale Ave., Port Washington, New York, 11050, fax 516-944-8372 or e-mail djtimes@testa.com.