Saturday, December 15, 2007
Top Ten Fixes for Bad Sound - Kent Morris
Frequency, crossover, tired woofers or dusty chaos and cheap cables behind the console-we've got the solution. Respected audio consultant and known Baptist rebel Bill Thrasher was asked the best way to fix the acoustics of a particularly bad sanctuary design. His reply was, "Bulldoze it and start over." While mere mortals cannot demand the removal of entire structures, there are some things within the realm of possibility that are quite effective. Gleaned from the real world, here are the top ten audio system fixes:
1. Properly set the system gain structure.
Nothing is worse than having a great sound system with the knobs turned the wrong way. The idea of gain structure is confusing because the controls can be set in any position and some semblance of sound will emanate from the speakers. There are many valid methods, but one that works well in the church environment is unity gain, where each piece is set to pass a nominal level that grants enough voltage to provide a clean signal while retaining sufficient headroom to prevent clipping.
For outboard equipment, (equalizers, delays, reverbs, etc.) pushing the bypass switch puts the unit into a unity gain state, so achieving unity gain is a simple matter of comparing the audio level when the unit is engaged versus when it is bypassed. If the engaged level is lower than the bypassed level, raising the gain control to compensate will result in a unity gain setting. On the mixing console, the individual channel gain (also termed trim or preamp) should be brought up (with the main channel fader control at its zero position and the board EQ at nominal) until the console meter reads zero. Power amplifiers can be set with a CD playing and the amp "volume" controls left in the fully counterclockwise position. Rotate the controls up in level until the audio is appropriate for the room. By not bringing the amp controls fully up, the system will exhibit less noise while providing plenty of signal amplification for the room. A biblical example of unity gain is found in a paraphrased version of I Timothy 6:6-8, when Paul admonishes, "Strong signal with headroom is great gain. For we brought no boost into the gear and we can take no boost out of it. But if we have unity and no noise, we will be content with that."
2. Find and reduce the influence of resonant frequencies.
Every enclosed space vibrates sympathetically at certain frequencies. Especially problematic in square rooms or rooms with common dimensional denominators (e.g. 20'W x 30'L x 10'H), these frequencies mask other frequencies and tend to ring in sound systems. Measure the room to determine the possible problem areas using wavelengths as a guide. To illustrate: a decrease of an octave equals a halving of frequency, so 500Hz is twice as long as 1KHz and 250HZ is twice as long as 500Hz. Since 100Hz is approximately ten feet long, the room example above will probably exhibit anomalies at 50Hz, 100Hz, and 200Hz, with other harmonic issues possible. The best course is to bring in a professional who uses SMAART PRO or another program capable of separating the room response from the system response via time domain data.
For those on a budget, Bill Johnson has developed a "poor man's" method that may prove effective. Taking an Audio Tool Box (www.terrasonde.com) or at least a Real Time Analyzer (www.rane.com) with pink noise generator, run the noise through the system and, using the measurement mic in a common seating area, stop the noise and watch for the last frequency to decay. That frequency is the most powerful in the room and may be a target for acoustic treatment.
3. Set the active crossover correctly.
The most prevalent mistake is to set the crossover point between the subwoofers and the main speakers too high. Most subs respond poorly to energy above 100Hz, so setting the crossover at 90Hz may clean up the mix. If possible, a steep cutoff should be engaged below the tuning point of the sub, typically around 40Hz or so. Also, it may work to skip some frequencies between 150Hz and 300Hz if the room emphasizes them on its own. For instance, EAW's AS series enclosures sound dramatically better when energy in the 160Hz region is removed with a crossover.
4. Clean up the rat's nest behind the console and amp rack.
Technicians seldom win Better Homes and Gardens awards for cleanliness. Lurking in the mess behind the board are poorly terminated RF cables spewing interference, dreaded Radio Shack cables passing noise, and a doughnut from last Christmas. Clean it up! Coiled cables can emit trouble as well, so it is best to Lazy-S them around or, better yet, reterminate with the correct length.
5. Use the best mics on the most important inputs.
A common church sight is the $59 catalog special microphone on the soloist while the bassoon receives a $400 Austrian condenser. With apologies to bassoonists, the key is to present the lyrics in the best manner possible, so put the best mics where they will do the most good.
6. Roll off low frequencies.
Bass energy is omnidirectional and bleeds into everything on stage. Reducing the input and amplification of low frequencies is essential to a good mix. Engage the low cut (also termed high pass) filters on every channel except kick drum, bass guitar, synth, and cello. Use variable high pass controls to dial in the low cut just below the lowest fundamental note of the input. For example, take out energy below 200Hz for a soprano soloist. Roll off everything below 100Hz in the monitor mixes and run the subwoofers via a post-fade auxiliary send on the console.
7. Apply acoustic solutions to acoustic problems.
Standing waves (regenerated energy from parallel boundaries) can never be taken out with a graphic equalizer. Bring in a professional to determine what is needed and follow the recommendations exactly. A good site to learn the basics is www.Acoustics101.com.
8. Avoid wireless mics whenever possible.
The only advantage wireless mics bring to the table is freedom of movement. Otherwise, their wired counterparts win in terms of sonic quality, reliability, and cost. If the pastor demands a wireless, then buy the best one available and restrict its use to the pastor. Inexpensive wireless systems will fail quickly, have high repair costs, and prevent the simultaneous use of multiple systems in many environments.
9. Replace fatigued woofers and compression drivers.
While a new speaker system is usually out of the question, tuning up the old one makes a great deal of sense. If the speaker is from a major manufacturer, chances are it can be brought up-to-date with minimal expense. For instance, many old aluminum compression driver diaphragms may be replaced with ferro-fluid cooled titanium models for about $150. Some companies, such as Peavey and McCauley, even make field replaceable baskets for their woofers, saving the time and expense inherent in professional reckoning procedures.
Brighten up the effects and use dynamics controls.
In nature, reverb decays first in the higher frequencies, retaining the low frequency information for a longer period. In church mixing, it is often advantageous to do the inverse -- roll off the lows and let the highs come through. The idea is to use less reverb than normal in order to retain articulation and intelligibility. By returning the reverb's "wet" signal through an open channel, the board EQ can be used to take out energy below 200Hz. No high frequency boost is needed since the absence of the lows makes the highs appear stronger and the mix more polished. Then, adding some light (10 threshold, 3:1 ratio, 50mS attack and .5S release) compression to the vocals will male them appear stronger without fear of transients exceeding the limits of the system or the parishioners' ears.
Kent Morris is a veteran media system designer and Seminars4Worship instructor focused on the ministry aspects of technology. .
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