In order for this article to be understood, we must start at the beginning. Sound travels through space at about 720 miles per hour based on altitude, temperature and humidity...so this is an average figure. It's also widely called the “sound barrier' because this is where sonic booms come from.
We also know that sound does not travel in a vacuum. But how many are aware that sound travels through water, through solid surfaces such as sheetrock (drywall), through hard woods, glass, and other materials at far different speeds?
Here are just a few examples of the speed of sound propagated through different materials:
Medium and Velocity(ft/s)
Air (20°C) Air (40°C) 125 164
Aluminum, longitudinal wave 10200 - 21060
Brick13700
Concrete 10500 - 11800
Cork 1200 - 1700
Glass13000
Granite 19635
Hardwood 13000
Iron 19635
Lucite8790
Rubber 130 – 492
Butyl Rubber6039
Steel20000
Steel, stainless12045
Titanium 20031
Water4700
Wood (hard)13000
Wood10820 - 11810
Conversions:
1 m/s = 3.6 km/h
3.6 km/h = 196.85 ft/min
196.85 ft/min = 3.28 ft/s
3.28 ft/s = 2.237 mph
As you can readily see, there is greater than an order-of-magnitude between the speed of sound through glass and aluminum when compared to air. Rubber, which has long been known as a dampening material, transmits sound at almost one-sixth the speed of sound in air. So we're dealing with widely disparate speeds that bring sound to the listener. One of the most interesting, to me, is the difference between plain old rubber and butyl rubber. The ratio is about 100:1, thought the materials could easily be mistaken for one another.
And look at how fast sound moves thorough wood! More than a thousand times faster than through air! Something not to be ignored when you're planning how to install – or tweak – the optimal sound system for your sanctuary. Of all the materials shown (and we aren't holding any special ones back) only air and rubber are the slowest. That's why you may have seen professionals, or audiophile hobbyists, try to de-couple their loudspeakers from floors, especially hardware materials using cones, or a variety of other methods.
What does all this mean? Simple. It means that the speed of sound propagated through normal building materials such as wood, glass, and others is far greater than through the atmosphere. This can, and will, cause significant time smear of the audible experience that each member of the congregation hears, depending on the building materials and where that member is seated.
Though the sound that's transmitted by vibrations through solid materials will rarely be at the same level as the direct sound that the loudspeaker is propagating through the air, it still plays a factor in the overall response and is important to be aware of. A large, vibration-prone loudspeaker bolted firmly to an aging hardwood panel will have a far greater influence in the overall sound quality, than that of a small, tightly constructed model that is providing front fill or delay fill at, often, quite low levels.
What can be done? One possibility is to plaster the wall of the sanctuary with enough absorbent material to minimize sound conduction through the building materials. Expensive and rarely practical. Moreover, it would reduce the desirable reverberant field that makes a sanctuary, especially a larger one, feel bigger and more angelic than your TV room at home. But too much is too much.
Plan “B:” Loudspeaker Isolation
Another possibility that works very well is to mechanically isolate the loudspeakers from contact with the walls, ceiling, and floor surfaces. This is especially valid for subwoofers, which are typically placed on the floor, producing a significant amount of energy. It's also true for ceiling-mounted speakers, usually the low profile type, that are commonly used in sanctuaries with low ceilings.
Suspended loudspeaker arrays, or individual loudspeakers that are flown from the ceiling, do not generally cause the same issues as those that are firmly mounted because they are largely isolated by the suspension hardware. The opposite of this is loudspeakers that are mounted with U-brackets or other hardware that will transmit their vibratory response to the building.
One more factor, an important one, is the nature of loudspeakers' construction. A loosely assembled loudspeaker with weak, even rattling panels, will certainly induce more parasitic energy into the building than a tightly constructed one. At one point in my career as a loudspeaker builder (Apogee Sound Inc.) we would spray the insides of the enclosures with a thick, gooey dampening material. The cost was outrageous, the curing time lengthy, and the toxins significant. We found more benign ways of causing the same, or similar, effect without toxifying our environment.
Solutions
Fortunately, there are commercial products available that offset these transmission discrepancies. I use the word ‘offset' rather than ‘cure' because some measure of mechanical coupling will always occur, even with suspended loudspeakers.
The new crop of commercial isolation mounts are built of various types of materials with elastomeric properties that provide the isolation along with the requisite steel or aluminum frames to manage the weight of the loudspeaker, or loudspeaker array. Be cautious though; most isolators are intended for free standing loudspeakers, rather than those that are firmly attached to the ceiling structure. Check carefully with the manufacturer (not the dealer) to be sure that they can handle the same weight load in expansion as they can under compression.
One such brand that is very promising is Iso Acoustics, out of Markham, Canada. These are people who take isolation very seriously, providing innovate designs that really work. A short audition will confirm that. Of course, there are other products to choose among, but this is one that I have direct experience with.
Safety, always being the number priority, means that you should look at the load bearing specs of any isolation system quite carefully. You don't want to improve your sound system's sonic quality only to have an isolation mount cause a loudspeaker to fall onto a church member. This also entails considering life cycle of the product, it's flammability range, it's ability to absorb energy, especially lateral waves from a seismic event, and not just what a great job it will do for you fresh out of the box.
And of course, as with all rigging, one must always (emphasis added) use safety cables that will handle the load if the primary support of the suspended object fails for any reason.
Here's a short list of possible failure modes:
Material failure. It may take years, but it definitely could occur, especially when elastomeric materials are involved.
Seismic activity. We've seen some seemingly intelligent designs that allow great flexibility when gravity is pulling the load straight downwards. Add an earthquake into the mix, and a few aftershocks and the rigid threaded rods or other hardware that looked so solid could easily snap off faster than a wishbone --- but with far more consequence.
Support Failure. The beam that looked so formidable during installation, that you drilled into and set your anchor points might have decayed in places that cannot be seen. Moreover, many buildings from about 1960 and later use glue laminated structural beams (glue lams for short) that can be seriously comprised when holes are drilled through them. That 150 lb. loudspeaker attached to a massive beam might be all it takes, under the worst probable conditions (snow on the roof or pooled water) to break into more than one piece. The right way is to envelope the beam with a sheet metal bracket that transfers its load to the top of the beam, not into a cut-away bore hole.
So enough about safety, except to say that you need to hire a professional mechanical engineer or theatrical rigger.
What to except by isolating your loudspeakers.
Loudspeaker isolation won't reduce architectural reverberation, at least not significantly. It's likely not to even be measurable, except with extremely sensitive equipment. But it will reduce time smear. The amount of perceived improvement will be a function of how much of a problem the mechanical coupling caused to begin with. What I've experienced in a number of instances is that clarity improves, usually quite noticeably, and issues like feedback that was once very hard to ‘dial-out' with EQ, will also disappear, or at least become much more manageable and linear.
I'm a strong proponent of using parametric EQ and a high-resolution analyzer to help solve many acoustic problems, but that said, it's always best to solve them at the electro-mechanical level first, if at all possible.
Almost every top-shelf recording engineer has realized that decoupling the monitor loudspeakers from the meter bridge pays off in spades. Now it's time to expand this concept to whole room systems.
