singing in the rain
Feb 1, 2001 12:00 PM,
BY NATHANIEL HECHT, S&VC EDITOR
There have been many clever and thoughtful methods devised fordealing with speakers that are permanently placed in weather’sway.
The most important thing to remember is that if it can rust, itwill rust.
OPTIONS FOR WEATHERPROOF SPEAKER SYSTEMS
LET’S FACE IT, THE ELEMENTS WREAK havoc on most objects wemortals build and expose to them. Speakers are the pieces of audioequipment most often exposed to the elements, especially in themedentertainment venues where water is involved. We have all seendemonstrations of amazing transducers at tradeshows that actuallyfunction while being immersed in water. Inevitably though, from carfinishes to tombstones, nothing will ever stand up to deteriorationafter the slow ravages of weather and time.
Just a cursory look at poorly maintained wooden structures inany city will reveal what happens after relentless exposure to theelements. The most we can hope for is an arrested state of decay.And in this area, there have been many clever and thoughtfulmethods devised for dealing with speakers that are permanentlyplaced in weather’s way.
The first step is to study the materials we are dealing with inthe products that are available, and then determine their abilityto resist the elements. This article will focus on weather exposureonly. Rigging of these systems is a complex task and will not bediscussed within the scope of this article. Please refer to themanufacturer who built a particular product for specific rigginginformation and hardware needs as it relates to weather-relatedinstallations.
SYSTEM FAILURE MODES
Speaker systems installed outdoors offer a number of potentialweather-related failure modes, the most obvious one being theenclosure itself. After all, if water gets into your cabinet, itusually doesn’t have any way to get out. Water entering or exitingthe cabinet will more than likely damage sensitive parts. The vastmajority of speaker cabinets manufactured these days are made outof some sort of wood product, so we will begin our discussion withwood.
Wood is relatively inexpensive, easily machined and ultimatelyquite good structurally and acoustically, which is why it is, byfar, the most popular material for building speaker systems.Speaker system manufacturers use many grades of lumber andcomposite wood products for their enclosures, ranging fromexpensive multiply, void-free birch or marine-grade plywood torelatively inexpensive oriented strand board and medium densityfiberboard. It is widely understood that, although they areexcellent acoustically, the non-plywood materials are not asstructurally sound when exposed to moisture. It is the particulatenature of the material and the glues that are used to bind theseparticles together that can be undermined by water exposure.Repeated exposure to high humidity (whether natural or man-made)can cause an enclosure to literally come apart at the seams.
It is important when specifying a speaker system for these typesof conditions that the material of the enclosure be properlyidentified. Many excellent speaker systems are manufactured withOSB and MDF materials these days, so it can not be stressed enoughto always check with the manufacturer if there is a question aboutthe material hiding just under the pretty outer covering of theenclosure.
There are other dangers inherent in the wooden enclosure thatmay require careful inspection, in particular the way the box isassembled in manufacturing. Cabinets that are internally bracedwith attachments between sidewalls, top, back and baffle are thebest suited for resisting the ravages of water exposure. Cabinetsusing fasteners such as screws will fare better in the elementsthan those attached with staples and glue. When water seeps intothe joints between these different parts of the enclosure, it caneventually undermine the glues that hold the pieces together; andif the cabinet material is not structurally sound, water can alsoweaken the wood at the bond. Continued weakening could cause acabinet failure and would definitely result in a loss of bassresponse (there will be gaps allowing air to escape that wouldotherwise exit through the tuned port designed for optimum responsefrom the woofer).
Another problem with weakening glues at these criticalboundaries in the cabinet is that many of today’s speaker cabinetsare accurately cut from one piece of wood using CNC routertechnology and then “folded” into place. Glue is thenapplied before a staple gun secures the edges. While theseenclosures are fine for portable sound applications and many fixedposition installations, they can spell disaster after repeatedwater exposure. Since these cabinets are often not secured withfasteners between top, sides and bottom, they can unravel when theglue or staples fail. This kind of cabinet design can certainly bemodified at very little additional cost and thus made considerablysafer, but the best advice is: Avoid using cabinets of this designin the specification process if your application involves moistureexposure.
The covering materials on wooden enclosures can also be a bigproblem in moist installations. Gone are the days (thankfully) whenspeaker manufacturers used animal glues to bond carpet coverings tothe enclosures. Not only do animal glues smell when they get wet,but they did not continue to stick very well when the materialbecame saturated, leading to instances of cabinet coveringsactually falling off the enclosures. Most material is now bondedwith insoluble glues, which activate under extreme pressure. Somemanufacturers seal the wood with a layer of plastic between thecarpet and the enclosure, allowing the carpet to become saturatedwithout allowing water to reach the wood or cause harm. Generallythough, wood enclosures in the past would have to be“gel-coated” to waterproof them, which almost alwayswas an after-market process.
There is no mystery to the process of gel coating: It is simplythe application of fiberglass resin to the outside surface of theenclosure to seal it from moisture. This is a messy process thatalso adds a great deal of weight (mass) to the cabinet, but whenfinished is excellent protection from weather. Systems designerswould often specify systems with this type of a finish, and itwasn’t long before most manufacturers began to provide this serviceas a custom feature.
CABINET PORTS AND GRILLES
Tuned ports can be nasty entrance points for moisture,especially in those parts of the world that experience wind-drivenhorizontal rain, or where the speaker system is oriented so thatwater can flow directly down the port. Extending the port ordiverting its angle to avoid water exposure will change the tuningfrequency of the cabinet, causing potential damage to the wooferitself. Blocking the port will ruin bass response and can alsocause woofer damage.
One solution resourceful professionals have employed over theyears is the famous “3-screen method,” where threewindow-type screens were lined up one on top of another to reducethe directly exposed area of the cabinet front to the elements. Thewater would bead off the screens and flow out of the bottom of thebox instead of into it — if it were properly designed.Aluminum or stainless steel screens are best to use for thisapplication since they offer the best resistance to rust and rot.Today, most manufacturers have adopted a modification of the3-screen method: They use two screens sandwiched around some densefoam material that is water resistant (such as polypropylene).
If a manufacturer doesn’t offer these as accessories, or if youare dealing with existing old stock, these water-condensing grillscan be easily fabricated by a custom speaker shop (or your shop) atvery little cost. Another creative material to use in thissandwich-speaker-grill design is the very finely woven, stainlesssteel mesh that is used in the throat of many compression drivers.Water tends to bead up on this material quite nicely and will flowacross it and drip down to the edge of the material, which, whenpositioned properly, will drive water to the outside of the box.Remember the old adage: An ounce of prevention is worth a pound ofcure!
WHAT ABOUT WHAT’S INSIDE?
Next, we have the actual components inside the cabinet tocontend with: woofers, drivers and horns, tweeters and passivecrossovers. Many manufacturers are building self-powered systemstoday, adding the danger of active circuits to the list of thingsthat need protection from the elements. Let’s look at each of theseelements individually.
The majority of woofer cones manufactured today are still made(at least in part) of paper, although many other combinations ofmaterials such as carbon fiber and various plastics are gainingacceptance. We all know what happens to paper when it gets wet.Most cones are fortified with other materials, such as fiberglass,to help stiffen and thicken the paper and provide a bit more waterprotection, but without more extensive measures you really have noadequate protection from constant water exposure.
The use of Varathane (polyurethane spray or paint) is oneeffective way to protect speaker cones, provided that it is usedsparingly. The problem with any material that is applied to thecone is that it adds mass, which will change the sensitivity of thedevice and can cause alterations in the frequency response. Anotherrubber type waterproofing method, made by C.P. Moyen, a well-knownadhesives company, is used by many manufacturers as a waterproofingagent. Some manufacturers provide it to their customers in smallquantities, but otherwise it is only sold in large oil drums.
Most professional woofers have cast aluminum frames, but someframes are stamped from cold-rolled steel and can be subject torust. Other rusting problems are posed by the machined metal backand top plates of the device. There is really very little one cando about those parts after market, since they are close toleranceparts and are difficult to treat without dismantling the devicecompletely. In modern manufacturing, most of these devices are puttogether with pressure-sensitive glues, making dismantling themimpossible without permanently damaging the device. The bestsolution in protecting these sensitive surfaces is coating themwith some other rust-resistant material — such as chromium— during manufacture. Most manufacturers do not provide thisfeature as a matter of course due to expense, but some may offer iton a custom basis for OEM purchase. This type of custom work oftenrequires tooling costs and large orders, so don’t expect to findthis kind of service from your local speaker parts manufacturer ifyou are only building a few systems.
Hardware is another big issue, from connectors and fasteners tointernal bracing. The most important thing to remember when itcomes to hardware is that if it can rust, it will rust. Stick tostainless steel or aluminum hardware for systems in moistenvironments.
Voice-Coils, Diaphragms and Surrounds. Voice-coils, whetherattached to woofer formers or diaphragm kits in compression driversand tweeters, are designed to pass current. For that reason, theyabhor water. If water gets into the gap of any device, a number ofdifferent problems can happen. One damaging situation iscondensation. Water will condense on metal parts due to temperaturechanges and ambient humidity wherever the system is installed. Whenwater is present in the gap due to condensation, it is usuallydriven off by moderate amounts of heat generated in the coil itselfduring regular operation. The critical step to avoid damage in thissituation is to warm up a system that is suspect with verylow-volume, full-range music for half an hour or more. This warm-upperiod will allow the water to be driven off slowly, causing thesmallest amount of damage to the device. The worst problems occurwhen a system that has condensation in the gap is hit with a lot ofenergy quickly. When hit with a lot of power, the water in the gapis effectively flash boiled and quickly turns to steam. The steamseeps up into the former and undermines the glue that is used tokeep the former and coil together resulting in the eventualunraveling of the coil and bubbling of the former material. Sincemany low-powered devices are often used in outdoor speaker systemsthat have paper formers (as opposed toKapton™ or other plastic materials)they are even more susceptible to this type of failure.
Diaphragm material in compression drivers and tweeters can alsobe damaged by moisture, but it is less common. The most effectivematerial in resisting moisture is impregnated cloth, also known asphenolic material. Phenolic diaphragms are extremely robust and forthat reason are used in high-power, high-output devices such aspowerful midrange applications and (believe it or not) foghorns.The limiting factor with phenolic material is bandwidth, since theyusually run out of steam around 7 to 8 kHz. Other common materialslike aluminum and titanium are mostly impervious to moisture andare often coated for added protection; but they can become reactivewith contaminants in rainwater that may eventually induce afailure.
Surrounds and spiders are also not immune from water problems.Foam surrounds are the most vulnerable. Foam is a fantasticmaterial for low-frequency woofers, allowing for more excursion,which, when combined with a properly designed cabinet, increaseslow-frequency output. There have been many advances in thetreatment of foam from the old days when smog and UV radiationwould deteriorate foam surrounds, but water is still a killer tomodern foam surrounds. Most paper cone speakers designed to beprotected from the elements use treated paper surrounds instead.Spiders (the stabilizing materials that affix the bottom of thecone to the frame of the speaker) are usually very durable and arenot often the sole cause of failures due to water. The fact is thatthere are just too many other things that can go wrong before thespiders give out when moisture is involved.
Passive Crossovers and Connectors.
Electronics and water simply don’t mix. Water can be aninsulator but is more often a conductor, and when it gets into theprinted circuit board of your passive network, it will cause shortcircuiting and often permanent damage to parts on the board.Humidity and condensation are probably the two biggest hazards topassive electronics, but direct exposure to rainwater, eitherthrough a cabinet port or gap in the connector, is equallydangerous. The slow-warming-up process discussed above is oftenenough to drive off humidity and condensation build-up on or arounda passive crossover preventing any major damage. Since the portholeis the biggest entryway for water into the cabinet, properlyprotecting the port is the key to minimizing internal waterdamage.
Connectors can be a bigger problem though, especiallyunprotected ¼ý jacks. Water will eventually rust andweaken the tines of ¼ý phone jacks. Since manymanufacturers still use these connectors, and many rental companiesare loath to switch over their cabinets due to expense andcompatibility issues, this failure can be a real nuisance. Thereare often two of these connectors on the back of speaker cabinets,which means that often, unless the speakers are being used inseries, one connector will be left unplugged and open to theelements. Another fact to bear in mind is that the ¼ýconnector that is plugged with a ¼ý phone jack is notsealed to the elements either.
Some cabinets have jacks equipped with small covers to protectthe opening, but in an attempt to decrease costs, these have almostcompletely disappeared from current speaker models. The realsolution to this problem is to replace ¼ý jacks withSpeakon connectors or other positive-locking, water-imperviousconnectors. The moisture problem is quickly eliminated with thisstep (to say nothing about the upgrade from a ¼ý jack,which is incapable of passing higher amounts of current betweenmodern amps to modern speakers).
For those who persist with ¼ý jacks, one trick is touse a plastic dummy plug on the other connector to block up thehole. Be careful to study the crossover network design, as some ofthem have switching jacks that allow for direct connection tointernal components by bypassing the passive crossover. This can beeasily discovered in listening tests with the plastic plug inplace, as one or more components will not be passing signal. Butthe advantage of using other types of positive-locking speakerconnectors such as the Speakon is that you don’t have to plug theother jack when it is not in use to avoid water damage because theconnectors themselves are self-sealing.
Self-Powered (Active) Systems.
Self-powered speaker systems use their own internal poweramplifiers, which means there is higher voltage present. Standard110-volt power and water don’t mix very well at all. If there isany indication that a speaker cabinet’s integrity has beenbreached, power down the system before touching it. Generally,manufacturers of such systems have placed safeguards in theirdesign to account for all weather use if that option has beenselected in the ordering and specifying process, but do not takethis for granted! Always research the applicability of aself-powered system for water exposure applications directly withthe applications department or product manager. Only then will yoube truly informed about the safety issues involved.
Surprisingly, in conditions of high humidity and condensation,the internally mounted amplifiers are really a big benefit. Heatgenerated from the amplifier itself will help to drive off themoisture that can condense on the metal parts inside the enclosure.In high humidity situations, this added heat can really make adifference in prolonging the life of the speaker components withinthe cabinet.
CHOOSE WHAT’S RIGHT
Deciding on what product to use for your particular applicationdepends on many different factors. Price is often the mostimportant consideration since there is always a budget to consider.Expected product life span in the installation needs to bediscussed with your client. No one should expect that a speaker ina harsh-weather environment will last forever. In fact, most peopleconsider a 5-year life as the realistic limit for products in theseapplications. If there is an existing product in an installationthat must be made water resistant, make a cost estimate comparing awaterproofing overhaul to just scrapping the current system andupgrading to a new weather-resistant product.
Within the array of products available for weather use, you mustdecide if the most expensive system is the best system for theexpected life span, or if a less expensive product can be specifieddue to the expected duty cycle. And unless your client couldn’tcare less about the sonic properties of the system, sound qualitymust be evaluated. While most products are acceptable, the perfectproduct for the application may not be the best-sounding product,so it’s a good idea to audition the products in your client’s pricerange before placing that purchase order.
Nathaniel Hecht is the editor for S&VC. He can be contactedthrough the magazine’s Web site at www.svconline.com.
Exotic Materials Inside and Out
ONE WAY AROUND THE PROBLEMS that wood, paper and foam have whenthey encounter water is to use other, more resilient materials.Now, foam and paper for speaker surrounds have started to give wayto advanced plastics and rubber designed for water resistance. Mostrecently, composite materials of carbon fiber have been used forforming cones, compression driver diaphragms and even entirespeaker cabinets.
Early plastic enclosures broke down and became discolored withexposure to UV light. The cold made some so brittle that theycracked in winter weather. We can thank advances in materialstechnology for the current crop of solutions in cabinet design.
Additives to the plastic have controlled the UV-exposureproblems that caused discoloring and brittleness, and plastics suchas polypropylene have been formulated to resist breakdown andreduce the problems with outgasing (the slow release of fumes fromthe plastic over time that can cause paint finishes to fail). Thetwo most common manufacturing techniques for plastic enclosures areinjection molding and rotomolding.
This process involves the injection of heated, viscous plasticbetween the forms of an inner and outer mold. After cooling, theproduct is removed from the 2-piece mold. Since molding and coolingare done quickly, great care must be taken in the manufacture ofthese products to reduce flow stress areas in the mold where theplastic may have a harder time fully penetrating, such as arounddetailed areas and at the edges or corners of the speakerenclosure. Care must also be taken to reduce and eliminate airbubbles in the molding process, since they can result in thin spotsthat can crack or break when stressed. Since this care in theprocess takes time, which translates to money, it is often the moreexpensive products that are the best built when it comes toinjection-molded cabinets.
You may have noticed that cabinets manufactured this way arelimited in the size that can be produced. The chief reason for thisis that the cabinet wall thickness is limited by the material.Since a rigid wall is necessary to avoid cabinet resonance, allboxes must be internally braced. If the cabinet size is too largeand/or not properly braced, then the box will resonate duringnormal operation and detract from program quality. Knowing thequality level of manufacture for the product you choose for animportant application may require listening tests and evendismantling the cabinets for a closer inspection.
Rotomolding has been around in various forms since the mid-1950swhere it began life as a process to produce flight cases. Today,products from coolers to picnic tables — to speaker systems— are made this way. The process begins with a mold or toolthat is designed to accept a “charge,” which is theterm used for the powdered plastic that is injected into the mold.The charged mold is then passed through an oven followed by acooling chamber. After cooling, the product is ejected from themold. Rotomolding is time consuming but allows the plastic time tofully anneal, which strengthens the cabinet. Since the mold iscompletely filled by the charge, there is very little chance of airbubbles and the flow stress problems that can occur in injectionmolding, and cabinets can be manufactured with greater thickness inareas requiring more strength. Since greater wall thickness can beachieved, internal bracing can be eliminated in place ofconvolutions designed into the surface of the mold to createrigidity and reduce resonance. As you’d expect from all thebenefits and the longer manufacturing times, rotomolded cabinetsare often considerably more expensive than injection-moldedcabinets.
Carbon fiber is the newest addition to the array of materialsused for water-resistant cabinets, and it seems to have all theadvantages. Sheets of carbon fiber material can be formed andpressed under heat into cabinet shaped molds. Carbon fiber has theunique property of being extremely strong and durable, yet verylight weight. This combination makes the material perfect forportable and permanently installed systems, as long as price is noobject. With reduced resonance, more rigid cabinet designs andreduced system weight, riggable speaker products made from carbonfiber require less hardware. At present, these kinds of cabinetsare in limited production, but they are available and, dependingupon the application, can be worth the expense.
This old standby has not been lost on speaker system designers.Fiberglass boat hulls withstand temperature fluctuations, exposureto seawater and continuous exposure to the sun. Of all thematerials mentioned here, fiberglass is still the most commonlyused and the most resilient material for preparing speaker cabinetsfor the elements. It provides the high mechanical strength forstructural support and resonance dampening. The added aspect ofbeing moldable into essential shapes in horns, baffles andwaveguides makes it equal in almost every way to the benefits ofinjection or rotomolded cabinets, with the only exception being thesheer weight of the system.
THE UNIQUE PROPERTIES OF FERROFLUID
Ferrofluid is a material that is used in many speaker componentsto dissipate heat in the gap, resulting in greater efficiency fromthe component. Although considered controversial by some,ferrofluid has a unique property where water is involved: Itspresence in the gap does not allow moisture to infiltrate.Ferrofluid is a viscous liquid that has magnetic material suspendedin it and actually hangs in the gap of a speaker product due to theinherent magnetic field. It is most commonly used in tweeters andsome midrange components but should also be considered forprotecting the gap in woofers from high humidity, temperaturechanges, fog, steam or salt (as in seaside, pool, spa and saunainstallations). The Ferrofluidics Corporation can provide morespecific data on individual systems, but be sure if you do add thematerial to the gap that you do not void the manufacturer’swarranty.