The Sound of Silence

Jan 1, 2004 12:00 PM, By Oliver Masciarotte

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Last month, I discussed an Open Source DAW, so this month, let's take a look at some hardware aspects of that DIY DAW by taking what is a free computer — or nearly so — and shutting up everything that creates so much darn acoustical noise. In a word or three: a silent PC. Heads up all you underpaid musicians and engineers! It's fairly easy to find an older computer for free, or less than $50 at least, that someone has in their closet gathering dust. The price is right, but you probably wouldn't want it sitting next to you when you're working, because the noise it makes will ruin your day.

DEALING WITH NOISE AT THE SOURCE


There are actually two things to consider in this whole effort to quiet down a garden-variety PC, one being electrical and the other acoustical. Let's start with electrical or RFI noise within the box making its way into your sound card. Suffice to say that you want both directions of analog conversion outside the case, otherwise you're asking for a less-than-ideal noise floor in your signal paths. Granted, with careful grounding and shielding, it's possible to convince an analog converter to make nice in such a hostile environment, but why fight it? Better to simply take an electrical digital audio signal — either AES/EBU Type I or Type II unbalanced, MADI, USB or FireWire — out of the computer and convert it externally. Notice I didn't say optical. Plastic Optical Fiber (POF) is high-loss and thus jitter-prone, so unless cable lengths are kept short, it's crap. Once the data's outside the case, convert it to analog using the best that your budget can afford and — boom! — you're there, wherever “there” is for you.

As to RFI/EMI escaping the box, one has to assume that the entire case is conductive and grounded, and that all air gaps in the case are gasketed. Though I can't imagine not having one already included inside your PC, an inline filter on the incoming AC goes a long way at keeping the power cord — all three prongs intact, please — from becoming a noise-radiating antenna.

TURNING DOWN THE HEAT


Okay, so we've got the radio spectrum noise dealt with, now let's take a look at the three horsemen of the acoustical apocalypse: motors, turbulence-induced noise and mechanical resonances. The Number One problem is motors. They cause the other two problems, so if we could eliminate them entirely, we'd be way ahead of the game. This is possible to do as long as the high-current drawing parts, like the CPU, are cooled in some unconventional manner.

Cooling a CPU the old-school way entails increasing the surface area of a passive heat sink, which is great if you're not using a modern hot-running Intel part. Another approach is water cooling, quite common in over-clocking circles, while a third method, waste heat removal, is to heat pipes; both require coupling to oversized heat sinks, a viable, if not visually pleasing and space-saving choice. Heat pipes distribute the unwanted heat to the furthest reaches of the heat sink and, from there, convection transfers the heat to the surrounding air. A fourth option, which seems to have fallen somewhat out of favor due to efficiency and design limitations, not to mention reliability issues, is to use yet more electrical current to run a Peltier (solid-state cooling) device, usually in conjunction with water cooling or heat pipes. Despite the drawbacks of earlier designs, modern Peltier coolers lend themselves nicely to a silent PC outcome. By the way, water cooling is basically an active alternative to passive heat pipes that uses forced cool water instead of a volatile working fluid. See the “Pedant” sidebar for more on cooling devices.

Let's assume that we've cooled down the CPU, graphic card, disk drive and any other toasty thing in the case. How about reducing the waste heat itself rather than simply (re)moving it? Power factor correction in the power supply is one answer. Without getting into the gory details of power factor correction and having everyone's eye balls roll back in their head from dweeb overload, better-quality power supplies incorporate additional electronics that greatly improve the unit's efficiency. This means less waste heat to remove from the case, so look out for that feature if your budget permits.

On to turbulence: If you must incorporate a fan, run it as slowly as possible to reduce chaotic air flow. Smoothly flowing air makes little or no noise as long as the velocity isn't too high. Most fans make a racket because they create turbulent flow on their blades or through the surrounding housing. Thermostatically controlled fans reduce the blade speed unless more cooling is needed and sophisticated blade and venturi housing designs smooth the air flow, reducing a fan's operating noise. As an example, Apple chose an inexpensive, proven forced-air cooling design for its new G5 Tower, but also chose to employ nine independent, thermostatically controlled fans in four separate chambers with a perforated metal front and rear fascia to keep everything cool and quiet. Each fan runs at optimal speed and the large 35-percent perforated surface area ensures low turbulence while preventing RFI leakage.

We're closing in on our ephemeral goal, but wait — I haven't talked about resonances. Whether it's cheap sheet-metal cases or thin metal grilles, “ya gets whats ya pay for” and what you get are buzz and rattle. There are many materials that damp vibration, and they all work by adding mass to the offending part. Whether it's strategically placed lumps of Mortite or carefully applied sheets of Dynamat, you can bet that adding a pound of prevention is worth far more than an ounce of cure.

OUT OF SIGHT, OUT OF MIND?


We've gotten rid of every source of noise that we can, but somehow, it's still not quiet enough. Once you've exhausted all other options, your only choices are to fall back onto the old tried-and-true: Stick the whole 'puter in an enclosure or remote it with a KVM, a device that remotely provides keyboard, monitor and mouse functions to one or more computers. Recent innovations in enclosure design from AcoustiLock include heat pipes to transport heat from the noisy and hot interior to the quiet and cool exterior.

Other than that, a basic acoustic labyrinth concept, either actively cooled or passive, remains the most effective and inexpensive choice. It was June of 2000 when I last visited the subject of KVMs. These days, the latest thang is an embedded Web server with IP connectivity, which replaces the old hard-wired style with an Ethernet connection and any Web browser. It is simple and cost-effective if you already have a run of unshielded twisted pair.

That's about all for this month's “Bitstream.” I hope you find this little survey useful and that this new year brings renewed health, happiness and prosperity to you all. See you next month!


Between a bevy of phone consults and a bout of GPS madness, Omas took time to soak in the unalloyed mayhem of Sergio Leone's The Good, the Bad and the Ugly and the bright, sophisticated bop of McCoy Tyner's The Real McCoy. Drop by www.seneschal.net for links on this month's subject.

PEDANT IN A BOX


Heat Pipe:

a sealed tube that contains a wick and a liquid that vaporizes at a relatively low temperature. If you heat up one end of the pipe, then the liquid at that end vaporizes, which dries out the wick. Cool liquid moves — wicks, actually — into the hot end of the tube while the newly created vapor condenses back into liquid at the cold end, replenishing the supply of liquid for the wick to suck up, and the whole process goes 'round and 'round. The result is that heat is drawn away from the hot end, raising the temperature of the cold end, all without any external power requirements, save the heat source itself.

Peltier Device: named after Monsieur Jean Charles Athanase Peltier, who demonstrated the thermoelectric effect that bears his name way back in 1834. Peltier devices are solid-state, thermoelectric heat pumps that, in response to an applied electrical current, move heat from one physical side of the device (the “cold” side) to the other (the “hot” side) in direct proportion to the current applied. If the polarity is reversed, then the “pumping” changes direction.

Modern Peltier coolers use such exotic semiconductors as bismuth telluride doped with selenium and antimony. The Sharper Image, that bastion of questionable exotica, sells a “trim, discreet” personal “climate-control system” using a Peltier device. Now that's dope!
Omas






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