System Failures

Dance Floor Sound Systems: Why They Fail and How to Avoid It.

By Paul Johansen. Owner Manager Stage Sound End ltd. Est. 1974.

During the eighties the author designed, built and installed over 200 sound systems in bars and night clubs right across NZ. The accumulated experience resulted in a very high level of reliability. Virtually all of the equipment used was made in NZ by his own company which was then called Sonic Sound Systems Ltd. Here are some twenty year old design techniques and construction methods that should be incorporated into a well designed audio installation. This article will provide you with the ammunition you may need to confront an incompetent or ill prepared sound equipment supplier who has left you carrying the can on a poorly designed system.

The eighties were the halcyon days for Stage Sound. We were on a roll. We did not need to look for work because it came to us and literally dropped into our lap. We could almost pick and choose the jobs we wanted to do. We were the company of choice and the word opposition had no place in our vocabulary. Looking back in hindsight I would say that it was all far too easy and the downside was that in those formative years I missed out on the marketing learning curve. When the nineties arrived and new sound companies were crawling out of the woodwork and scrambling around like vampires slurping up all the work I was left sitting there like a stunned mullet wondering where all the jobs had gone! I had simply failed to adapt to a climate that meant we were no longer the only game in town. To kill time I hired out concert PA rigs, made hundreds of CD listening posts and went on to found the audio video hard drive company Computer Music Systems Ltd. The rest is history but it was building dance floor and live band systems that I got the biggest kick from. This is how we built them and why they very seldom fell over.

Sound systems are a vital increment in any entertainment venue using a dance floor as an attraction because if it fails you will lose the bulk of your crowd and this doesn’t make for good business. Things fail because they have an inherent weakness and sound systems are no different. In a single a night a professional audio system will do more work than the average domestic system will do in a whole year. Thermal, dynamic and mechanical stresses take their toll and if the items that bear the brunt of this relentless torture are in some way inadequate they will break down sooner or later and often when you least expect it. You will find it very hard to have a contingency plan for this expensive occurrence because unless you smell something burning the sound system won't tell you it’s hemorrhaging badly until it is all too late.

The vast majority of thermal failures or their associated causes occur between the output of the mixer and the loudspeakers.

If the music source items and the mixer are of reasonable quality and are in good working order they are unlikely to fail catastrophically because they seldom suffer from work stress or thermal faults. This assumes their users have never given them a good bath using a glass of coke or smothered them in fag ash.

Operator Incompetence.

Many, but certainly not all serious failures will be associated with an operator who has little or no skill in the art of using a sound system correctly. Operators often change and so you will find it difficult to guard against the poor technique of a stand in, especially if you cannot always be there. Relying on stand-ins can be a big mistake if you do not know them or their reputation. In this game the ego often rules and louder is often considered better.

You will have some protection against tampering if you have rack protection covers over the equalisers and the electronic crossover but you cannot always be sure that someone who is hell bent on fiddling with the system will not remove the tamper proof covers and do this when you are not watching. This is one of the more common problems associated with operator abuse or incompetence. Put a sign on the covers that makes this practice a dismissal offence. In my mind it is an outrageous breach of trust and can devastate a good sound system because it may well be pushed beyond its design limits simply because it has been maladjusted by an interfering idiot.

If you have been shown by the sound contractor (and you should have been) where the system 'red lines', this point on the mixers meters should be marked with tape or twin and all your operators should be made well aware that this point should never be exceeded under any circumstances. Just how well an audio system will cope if it is regularly pushed beyond this point will depend on its inherent 'headroom'. Headroom is the systems buffer zone and is the maximum output beyond its prescribed user limits. This is your margin of safety and many systems built today unfortunately have very little of this for several rather pathetic reasons.

Let’s start at the beginning of the chain and work through each item of equipment through to the loudspeakers.

The first device following the mixer is the graphic equaliser and the way this has been set up is very important. These things are a collection of volume slider pots that control all the different audio frequencies. These devices don’t get very hot and they don’t have to work hard but they are like steroids and excessive use can blow out a systems muscles very easily. Excessive low frequency boost can be very stressful to sub woofers or to any low frequency component in your speaker system. It is a good idea to roll off steeply all frequencies below 40 Hz. This is done by pulling down the slide pots below this frequency to the bottom of their travel. Doing this acts like a filter to low frequencies and it stops them reaching and subsequently stretching the suspension components in the woofers. It can also reduce the likelihood of noise complaints so you get the double whammy by doing this. A small dip at the 2 KHz point is also a good thing because this can sweeten the sound and take some of the pressure of the horn diaphragm at the important crossover frequency making the system less prone to feedback damage which is a common cause of failure in these items. Avoid any heavy boosting at any frequency and as an extra precaution at the very high end, it is a good idea to roll off or pull down the top slide pot as this can protect the system against radio frequency instability which can be caused by a faulty mixer input cable. RF can be very damaging to both amplifiers and high frequency speaker components alike. You can't hear these supersonic frequencies so they can melt things and you won't know it’s happening.

The next item is the electronic crossover unit and this device's settings are also critical. These things also run cool and rarely fail but their settings are crucial to reliability so listen up. Assuming you have (hopefully) been sold a three way system which will have three power amplifiers, bass, middle and high, you will need to be aware that the crossover point between the midrange speaker and the horn driver is very important. If this critical frequency has been deliberately set to low or has been altered without your knowledge it can easily result in the permanent demise of the horn diaphragms. This crossover frequency is generally between 1 KHz and 2 KHz and should never be set lower than this because it will allow damaging low frequencies to reach the amplifier and overpower the horn at frequencies that they are not designed to cope with reliably. If this setting is to high no damage will result but the sound quality will deteriorate accordingly as the mid range speaker attempts to reproduce frequencies that it cannot do very efficiently.

Three Way versus Two Way System Design.

It is well worthwhile at this juncture to explain why a three way sound system is so much better in terms of reliability and sound quality than a two way system. Three way systems use three power amplifiers and two way system obviously only use two. The flown or satellite speakers generally have two components and these are usually a large cone speaker and a horn. A two way system has a passive crossover built into the speaker enclosure and apart from the obvious need to provide a crossover point it is also required to drop the level of the highly efficient horn component so that it produces the same level of loudness as the less energetic cone speaker. The problem with this is that the resisters that do this can get extremely hot as they dissipate the energy that would otherwise be fed to the horn driver and they can overheat and burn out if the audio system is ever overdriven. I know of one club that had a speaker enclosure actually catch fire due to this. This tends to be a major cause of failure in two way systems that are pushed hard. Three way systems do not need a passive crossover as this is done by the three way electronic version at low level before the signal reaches the power amplifiers. This means no pad resistors are necessary. Three way systems as a rule also sound far superior at high volume levels because the extra power amplifier extends the dynamic range and greatly reduces the stresses associated with clipping or distortion. If the low frequency amplifier is clipped or distorted by over driving this distortion does not affect either the midrange or the horn amplifier which will remain clean. In my opinion the three way system is the only way to go with dance floor systems. However not all sound contractors seem to understand why, or appreciate all the technical benefits.  Perhaps they find that repairing two way systems when they fail is a good way of making money.

The final electronic device is the power amplifier. These are the engine room of the sound system and so must be very reliable indeed. They can and often do get stinking hot. The main reason power amplifiers fail is overheating and this occurs for two reasons. Poor primary or secondary cooling or a sad combination of both inadequacies.

Primary cooling is part of an amplifiers inherent design so there is not much you can do about this one. Modern power amplifiers have their heat sinks buried deep inside them and they are often rather small and rely on an inbuilt fan to keep them cool. If this fan clogs up with dried dust as they so often do, the fan will sometimes jam and or, fail completely. If this happens the amp should thermally shut down and its goodnight nurse or at least until it is cleaned out. Modern power amps need cleaning internally every six months or so to ensure this catastrophic event does not occur. This means pulling the amp out of the rack and removing the lid in order to do it properly. A paintbrush and vacuum cleaner make good tools for this very messy job. Care should be taken not to hit the internal components with the brush or the cleaner’s nozzle when you do this task or you may break something off. The amplifiers we built during the eighties had very large heat sinks mounted on the outside of the chassis and they ran reliably even if the fan stopped, so dust was never a problem for us. Our amps virtually never failed. Not so for many amplifiers built these days and you will probably have no idea about the quality of the internal construction of what you are buying either. Very few power amplifiers made today have heat sinks mounted on the outside of the chassis for economic reasons. Externally cooled power amps are very expensive to produce, are often larger and can weigh a lot more as well. Thus a great design concept is relegated to the archives of history just to save a few lousy bucks. I could never understand this, to me it seemed insane.

Secondary cooling however is something you can actually control and there are a few important things you can do to improve the situation substantially. Amplifiers mounted in a rack should be spaced with a 1U gap between them to allow hot air to escape easily and not transmit heat to each other through conduction. This also makes them far easier to get out in a hurry if you ever need to replace one urgently. A fan should also be mounted inside the rack to blow cool air through it and the rack should not be pushed hard up against the wall behind it. Do not use carpet inside the booth because carpet generates dust.

The temperature inside a DJ booth on a hot summer night can easily exceed 30 degrees centigrade so good cooling is very important. Air conditioning the booth is also a very good move unless you like saunas.

The final items to be discussed are obviously the loudspeakers and these are usually the most likely devices to fail as the result of abuse or poor system design. This may been seen by some as a controversial statement but there are two types of loudspeakers. Those commonly available at retail shop level and those supplied or built by specialist sound companies. Professional loudspeakers will not generally stand a normal retail margin as they become prohibitively expensive and therefore are not often seen in retail shops. Their turnover rate also tends to be too slow for most retailers so they tend to be very reluctant to stock them. This will generally mean you won’t be sold them, so you are the looser.

The main problem when purchasing loudspeakers is that you are most unlikely to be shown what is inside the cabinet. This could be likened to buying a four-cylinder car when you were under the impression that you were buying a V8. Power ratings can be very misleading and it is difficult to know what you are actually getting without having a look at the components inside. Unfortunately to the untrained eye even this may not mean very much. Small magnets generally mean low power, as do small voice coils. Pressed frames rather than castings are also a sign of poor quality. Heavy enclosures are a good sign you are buying a good product, unless they are made of custom board (which is very heavy) and not plywood. Plastic moulded enclosures may look nice but they do not often sound as good as timber ones because they flex and are generally regarded in the trade as a cheap compromise. Speaker boxes that do not have an internal crossover are known as 'bi-amped' and are generally very good quality especially if they are one of the big name brands.  The following brands are among the names that are seldom associated with the words ‘cheap and nasty.'

Electro voice, B and C, JBL, RCF, Radian, Meyer, EAW, Alter Lansing, PAS and TAD.

Sub woofers need careful assessment as well and their size, number and power potential should be selected so they are in proportion to the size of the venue. There are no absolute rules here but experience has taught me two important things. They perform much better when mounted in a corner and horn loading is the most reliable enclosure type as far as the prevention of serious speaker wear and tear is concerned. Eighteen-inch woofers are capable of moving more air than fifteens but they do take up slightly more space. A good sub woofer has a 1000-watt RMS power rating and the cone should have an excursion factor of at least 10mm in both directions.  They should also have a magnet vent, a huge magnet, a cast frame, a very stiff cone and a 100mm voice coil. These things all contribute good node points in my not so humble opinion.

 This causes the cone's surround (or suspension) to stretch which results in the cone not cantering correctly and causing rattling or rubbing. The only cure for this kind of damage is a complete recon and this can be very costly. The other common cause of speaker shutdown is Fabian wire breakage. These are the braided wires that join the speaker's voice coil to the input terminals that are mounted on its frame. They are about 50 mm long and they vibrate hundreds of thousands of times in an hour. If they are stretched to far they will eventually stress and break. This can be usually avoided by realistic equaliser settings.  Don’t boost the really low bass frequencies and do not over power the subwoofers with power amps that are far too big!  If you have a DJ who is constantly overdriving the bass you can turn it down at the crossover by removing the cover, re-setting the bass output volume control and then replacing the cover again. You can also cut the bass back on the graphic equaliser but don’t forget to replace the cover.

A device called a 'hard line limiter' can be placed in between the bass output of the electronic crossover and the input of the bass amplifier. This can be set to act like an automatic brake that comes on at a certain speed. Once the bass level reaches a maximum pre set point the limiter cuts in and it will not allow an increase in volume no matter how far the DJ tries to push the level up. These cost upwards of $400 but can be a good addition and cheap insurance if set up correctly.

Distortion or a lack of clarity is a good indication that the system is being thrashed. It will only cope with so much of this treatment. The points at which it will fail are not really predictable, but believe me it will eventually fail. Nothing surer!

Many catastrophic failures occur as the direct result of what is known as thermal recycling. This is the term used to describe a component that gets stinking hot and then cools again on a regular basis. The end is nigh when stress fractures (as a result of thermal recycling) occur. These are predominant in output transistors, power transformers, crossover pads, loudspeaker and horn driver voice coils that are constantly overheated. If not thermally stressed these items can all last for many years trouble free and be very wallet friendly.

The composite or mid / high speakers may also fail if the crossover settings are incorrect, i.e. too low. It is also very important to fuse protect the horn drivers. The best way to do this is to mount the fuse holders in a rack panel on the front of the amplifier rack so you can change them without having to climb a ladder. Remember to bi pass the ones in the back of the speaker box. Feedback will blow the fuses but usually not the horn diaphragms. Very cheap insurance! Fuses cost a dollar each. Horn diaphragms cost $150 or more each. If four two inch horn diaphragms blow it will cost you $1000. This is when you get to wish you took this article seriously. 1.5 amp for a 1" driver and a 2 amp fuse for a 2" driver. Don’t forget now!

Humidity causes moisture build-up and moisture is the main cause of corrosion. Sometimes corrosion causes speaker connectors to go open circuit. We used screw terminal blocks rather than in line connectors on our speaker boxes. They never failed.

We have a number of other techniques that we developed over our years in the spotlight but they fall into the category of trade secrets so please understand my reluctance to reveal everything in a free to air article. The above collection of sensible precautions and techniques should form an invaluable checklist of things that will arm you well against an unscrupulous or incompetent sound contractor with the objective of bolstering your overhead.

I certainly hope this information will assist the owners of substantial audio systems to get much better reliability from them.

Don't be duped into replacing a faulty system when it may well be repairable for a fraction of the price of a new one. If you hear comments along the following lines: Aw, I think she's stuffed mate, time to ditch it I’d say. And "Yeah, it’s all pretty dodgy, nothing really worth saving here mate."  Now watch while your trusted contractor relieves you of $25,000 or more as he replaces what may be a perfectly viable sound system with a new system that may not even sound as good as the one he enticed you to throw out. We know this is happening so don’t get caught especially if your system is less than 10 years old. Get a second opinion from someone who possesses some integrity and is not motivated by sheer greed.

The last really big system we built was for the giant Coliseum Nightclub in Manual, which was owned by Ken and Marianne Ingles. This was the largest nightclub in NZ at the time accommodating over 1200 people. Our system ran for at least three years and during this period we were called out at night only once. The operator for the session had accidentally left the Band / DJ selector switch set in the wrong position. He was a stand in!

I must confess I had a very strong ulterior motive for wanting to build reliable sound systems. I genuinely resented being dragged out late at night and I hated swimming through the clouds of marijuana fumes to get to the amp rack so I learned very quickly how to avoid this unwelcome intrusion into my spare time. To me the money earned by doing repairs did not justify the inconvenience of getting dragged out at night and I also saw it as a form of failure. I got no satisfaction from invoicing clients for work done on sound systems that I felt should not have let them down in the first place. Any sound company possessed with a similar attitude will know there is no substitute for integrity, quality and dedication. This philosophy has its own rewards in the form of repeat work. Over the years we had some clients we built three or more different audio systems for.

For any further information on this or other blog topics please call Paul Johansen on Ph: 444 8776

 


Stage Sound Enterprises

Stage Sound Enterprises Ltd
Unit 4-77 Porana Road, Glenfield, North Shore, New Zealand
Phone: 09 444 8776 Email: info@soundman.co.nz
Opening Hours: Mon-Fri 9 AM to 6 PM