Occasionally — thankfully, very occasionally — I get a call about a sound system that’s not working as expected or as it used to; other times, it’s a problem that I’ll notice when walking into a house of worship or music venue.
I recently heard from my esteemed editor about a service call he performed decades ago, where an LCRS system’s center channel speaker in a cinema suddenly sounded very muffled. Upon inspection with a flashlight, he saw a dead rat that had lodged itself into the throat of the high frequency horn, requiring a coat hanger to extricate it and thus solve the problem. Another one of his cinema clients once called him to say that the mono sound system (Altec A-7 behind the screen) wasn’t working. A quick look behind the projection screen revealed the loudspeaker was missing entirely, resulting in a loudspeaker rental ($$), followed a few weeks later by installing a new replacement speaker — this one bolted to the stage floor.
That first story reminded me of a sound system I had designed and retrofitted for a synagogue in the mid-1980’s. Months later, I got a phone call to repair the sound system from that now-very annoyed client. He was annoyed because “half of the microphones have quit working in only six months.” On my long drive to service the client I couldn’t imagine what would cause half of the sound system mics to fail in such a short amount of time. A cursory inspection didn’t reveal any obvious problems, but sure enough a few of the mics were inoperative. Troubleshooting the system led me to looking under the platform, where I quickly discovered that some unwanted-little residents had eaten through almost half of the mic cables. So, after showing the client the problem wasn’t due to faulty equipment — but rather, damage caused by rodents — I repaired the cables for a modest fee and the client contacted an exterminator. Most of my system projects were new construction, where I specified conduit for my cables. So, another note to myself: always put sound system cables in a conduit inaccessible to rodents (where they’re less likely to be eaten) — and BTW, it’s easier to replace bad cables in a conduit. Additionally, metallic conduits are said to provide additional shielding from noise.
Testing 1-2-3…
If the mic cable failures above were not so easy to see, I may have needed to put my vintage Shure mini/XLR tone generator and handset audio monitor to work. These days, there a number of basic “cable testers” on the market. The dbx CT-3 advanced cable tester can test up to 10 cable types and it also has an onboard tone generator. Also, its chassis (like others) can be split into a sender and a receiver, enabling testing for longer and installed cables. The Galaxy Audio CPTS Cricket is a two-box polarity & phase tester with built-in mic and speaker. The PT-AMG2 by RDL, is a precision analog-audio generator and level monitor, featuring a precision audio oscillator, level meter and a monitor speaker. Whirlwind’s Qbox is a small tester with built-in mic, generator and monitor amp. The Whirlwind Qbox-AES is an even more complete all-in-one testing device for troubleshooting digital AES-3, S/PDIF and analog audio signals. In AES test mode, the receive section consists of the LED sample rate indicators and the decoder that converts the digital signal into analog for the built-in speaker and phones monitor output.
Coverage is Everything
I once walked into a house of worship for the first time and then decided to move to the other side of the worship space. It was apparent that the speech intelligibility was dramatically inferior on one side of the room as compared to the other. After the service was over, I went to approach the sound operator in the control booth, at the centerline of the room. I introduced myself as a seasoned sound engineer and asked him if he ever compared the sound quality from one side of the room to the other. He asked me why he would do such a thing, so I pointed out my observation. For some reason, he and many people in the church were oblivious to the fact that one of the two HF compression drivers were non-functional. Of course, if a large sound system has a lot of drivers, it’s not such a big deal. But if half of a venue is serviced by one driver — that’s out of order — it’s a significant issue — especially for a P.A. (rather than a music system).
A similar sounding problem — with a lack of clarity/coverage — in smaller areas of a venue can be the result of active line array drivers and/or amplifiers failing. One such problem occurred with a repeat client who called to ask me to replace his failing system (provided by others). Unfortunately, the imported line arrays were not serviceable by the vendors, so both the line arrays had to be replaced. Some of the church congregation was oblivious and others were simply unmoved to help fund a replacement system. So, in desperation — as half of the active line array units had then failed — the client used masking tape to mark a big X on the grill of each of the dead units (some people hear with their eyes). His creative solution helped motivate the congregation to raise the necessary funds for a cost-effective system replacement. To see the case study on my cost-effective replacement loudspeakers, see the August 2019 issue of FRONT of HOUSE.
Line Array Misuse
Speaking of line arrays, there are lots of ways to misuse line arrays. Assuming the appropriate size and numbers of array modules are used to achieve the needed sound level, some early line arrays had too much horizontal dispersion, leading to high-level echoes off of the side walls (of rectangular venues). Speaking of echoes, line arrays can have very “long-throw” coverage to the back of large venues. We really need to watch out for echoes off the upper-rear walls of large venues; these echoes are all too commonly the most detrimental — as they have the longest delay time — especially if the rear walls are concave — focusing high intensity and long-delayed echoes back to the front of the venue!
If you’re replacing several HF drivers — while you’re at it — you might consider some more pleasant-sounding alternatives to the traditional Titanium-diaphragm drivers, that are now available in various polymer — and even carbon-fiber — formulations. These tend to have much less high-frequency ringing, thereby sounding much less harsh and more high-fidelity. Some of the best sounding mid-high frequency drivers are co-axial, with a separate diaphragm for the mid-frequencies and a second-smaller diaphragm for the extended high-frequencies.
However, if your clients want their systems to sound even louder than they actually are, you might want to continue using HF drivers with titanium diaphragms that can cause high-frequency harmonic distortion. Detecting damaged — but still working — LF drivers (woofers) is not as easy/obvious as their HF counterparts. For a very-small system — if you don’t have instrumentation to measure distortion — try playing a CD with simple program material such as speech or piano, to reveal bad/damaged woofers that are causing harmonic distortion. For large systems, identifying damaged or blown woofers may require playing only one of the woofers at a time, by muting the others.
Of course, most of our readers know and do this, but an important reminder. Before your start a show or complete a sound system installation, please devote some time to walk all the listening areas — to confirm that the system is working properly, and all of the loudspeakers are pointed in the right direction. I can’t believe I have to write this, but I’ve seen and heard of systems where some of the loudspeakers were not even pointed at the listening areas, so some loudspeakers were simply creating more reverberation, while some seats had no useful sound coverage.
Unfortunately, the truth is many clients won’t pay for system diagnostics and repair until there is an obvious problem. But larger venues — and more mission-critical clients — would be well served to do sound system inspections every year or two to keep their sound systems at the highest performance level.
David K. Kennedy, a 40-year live-sound veteran and author, operates David Kennedy Associates, consulting on the design of architectural acoustics and live-sound systems, along with contract applications engineering and market research for speaker manufacturers. Visit his web site at www.D-K-A.com.
|
|
|
RTA Made Easy
Sound system analysis gear can be a powerful tool in the right hands. Unfortunately, advanced P.A. optimization systems (such as dual-channel FFT analyzers) have a fairly steep learning curve, and sometimes a simpler rig, such as AudioControl’s DM-RTA kit provides an easy-to-use Real-Time Analyzer (RTA) alternative for many users.
The DM-RTA Pro Kit provides five of the most important tools to check sound systems: an RTA, a voltmeter, a polarity checker, SPL loudness meter and an oscilloscope, in a hard case with all of the required interconnects, CM-10 measurement microphone and an AC-BT24 Bluetooth Streamer/Programmer, for connecting to the DM-RTA using your tablet or phone. Price is $899, from www.audiocontrol.com.
