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IZZI · Posted: Tue Oct 11, 2005 4:48 pm Post subject:

Buzzes, hums, hiss, pops, crackles and other sonic debris are the bane of home and project studios everywhere. These undesirable noises have a number of distinct causes, and thankfully, most can be reduced or eliminated at the source of the problem. A few can only be controlled with suppression devices. We're going to look at some of the ways hums and buzzes can intrude on your studio and I'll give you some ideas for dealing with it..

Crossing Audio and AC Cables

A common source of hum is generated when audio cables are placed too close to AC cables, outlets and switches. This includes running audio cables too close to the wall, as AC cabling exists behind the walls to connect each outlet in the studio. AC cables, power strips and external power supplies are a primary source of hum, because they emit large magnetic fields that can be picked up quite easily. To combat this, audio cables are normally shielded with woven copper wires (screening wires) to deflect any stray electromagnetic fields that may be coming from AC power cables. However the screens are not perfect, and the effectiveness of the screening can vary with the quality of the audio cable and the connectors.

In order to reduce hum from this situation, it's very important to route your audio cables away from (i.e.. not touching) the AC cables from your gear, as well as power-hungry elements such as beefy power amplifiers and external power supplies. The rule of thumb is, when an audio cable has to cross the path of an AC cable, make sure it crosses at a right angle, which minimizes the pickup of this type of hum.

It's also possible you have a computer monitor, television, or guitar head amp generating some hum through a nearby audio cable. You can test for this by simply cutting the device off. If you find it's causing a problem, you'll have to route the cables a different way.

Also, don't skimp and buy the cheapest audio cables you can find. Look for cables with woven copper screening (not the inferior lapped variety), all-metal connectors, and use the shortest length of cable needed to connect two pieces of equipment. The shorter the cable length, the less probability of picking up AC interference.

Stacked and Racked

Stacking different pieces of equipment on top of each other, or racking them together without a gap, may cause a problem due to one unit's AC power transformer being located in a spot that induces hum in the other unit. You'll be able to detect this quickly when initially assembling your studio gear, but if your studio is already wired, try turning off equipment just around the unit that is humming. If the humming stops, you can move the unit that's causing the problem to another location.

Usually, equipment encased in steel provides maximum containment of magnetic fields, whereas plastic and aluminum cases provide almost none. If some of your gear is steel-cased and other units not, try to rearrange it so your plastic or aluminum-cased devices are adjacent to your steel-cased equipment. The other option is to leave a single-space gap in the rack between two pieces of gear. You only need to do this, however, if you know you have a problem.

Ground Loops

An even more common source of hum is the infamous ground loop, also known as an earth loop. In a lot of ways, they are the most tricky to chase down and solve. Once you've applied most of the common-sense measures to reduce hum, any hum remaining will usually be due to a ground loop. It's possible to completely eliminate ground loops if you take the necessary steps, but it involves a systematic process of eliminating problems one by one.

Ground loops occur because most modern equipment is fitted with three-prong AC plugs. The third prong on the plug connects the chassis of your gear to AC ground, which ensures that your body cannot become the ground path for AC current. However when two pieces of equipment both have three-prong plugs and are connected together with cable, the shielding on the cable is also responsible for grounding, and a ground loop is possible. This is because if a piece of gear has two paths to ground (one, through it's own AC cable; two, through the audio cable connected to another unit, and through that unit's AC cable) a loop of current is formed that can act like an antenna, through which hum can be induced. You can even pick up radio interference this way.

Most ground loop problems can be solved by plugging all of your studio gear into a single grounded AC outlet. But by doing this, it's possible to overload the AC outlet, so you've got to be sure the AC source is properly rated to handle all the gear you have plugged into it.

The only way of being sure you have a potential ground loop problem is to listen carefully for a slightly edgy hum as you are assembling, wiring and cabling your system. If you have your gear powered up as you are monitoring for hum after each audio connection, you can quickly determine the source of the problem. Move the unit that seems to be causing the problem and try it again. You won't have to worry about battery operated gear or gear with two-prong adapters, as they do not contribute to ground loop problems.

Some people solve ground loop problems by using a ground lifter (three-prong to two-prong adapter) on one of the units, thus breaking the AC ground and severing the loop. I have to stress that this is a very dangerous option that should not be used, because you're giving up the safety factor that the AC ground wire provides. Again, if you choose to use 3- to 2-prong AC adapters, electrocution may result.

The best (but more expensive) way to fix a persistent ground loop problem is through the use of a transformer. The job of the transformer is to ensure there is no electrical contact between two pieces of equipment, except for the audio signal. Transformers have no ground connection between the input and output connections, thus effectively breaking a ground loop.

When buying transformers for ground loop problems, it's important to realize that the more inexpensive variety may color the sound a bit due to frequency response irregularities. Good engineering and construction quality costs!

A company called Ebtech makes a relatively inexpensive transformer isolation box called the Hum Eliminator, which is two transformers in a single metal case. As of May, 1997, it sells for around seventy dollars. It may serve you well to have one around to diagnose ground loop situations. Ebtech also makes an 8-channel version for about $250. There are other companies such as Jensen Transformers that manufacture more high-end units.

Bad Power Supplies

If you've inserted transformers between two devices and you still hear hum, it's possible the power supply on the output device has gone bad. There's no way to be sure of this unless the power supply is external and you happen to have a second power supply handy. The only way to cure a problem like this is to have the power supply checked by a technician.

Bad or Improperly Wired Cables

Sometimes the audio cables themselves introduce hums and buzzes, due to defective shielding. You can check for this quickly by simply swapping cables. In addition, you wouldn't be the first person to connect an output to an output, or an input to an input, so get a good flashlight and take ten seconds to make sure you've got your equipment hooked up correctly.

Short-Burst Edgy Buzzes

If you hear a buzz that only appears for a short time and at a constant level, you may have a pulse in your AC lines, which can be caused by the switching action of fluorescent lighting, dimmer switches, window air conditioners, or a refrigerator turning on and off. The best situation is to locate your studio in a room without these items. If you must share an AC circuit with of these elements, you're going to have to install a proprietary noise suppressed AC distribution panel, which will give you a clean power supply for your studio.

When All Else Fails

The ideal solution for your humming and buzzing situation is to correct it using one of the methods above. There is no substitute for checking for these problems as you hook up and wire your studio from the ground up. It may even be worth disassembling the whole studio and starting over, checking each device as it's connected to verify the 'cleanliness' of the system as a whole.

If you really can't solve your problems any other way, it may be time to consider an audio solution to surgically remove the offensive sounds. Remember though, this is simply treating the symptoms of the problem, and you're avoiding the root cause of the problem altogether.

In the past, parametric equalizers were used to lessen the effect of hum in the mix. To remove a noticeable AC hum, you would simply set your equalizer to 60Hz, which is the frequency of AC hum, and notch out the sound with a very narrow bandwidth. However, surrounding bass frequencies would almost certainly be affected.

Today, you've got units like the Roland SN-700 Noise/Hum Eliminator, which claims to "...easily remove AC line hum, dimmer buzz, fluorescent lighting, hum, ground loop noise, fan noise, tape hiss, SMPTE bleed and more." Of course the same caveats apply to a device like this as with the parametric equalizer--you may experience a coloration of the overall sound, which may or may not be a problem for you.

Finally, you can try using a noise gate, set to gate at low levels so that background signals such as humming and buzzing are eliminated in quiet sections of the audio. The down side is that you may be removing quiet sections of your music as well. Also, when the gate is not set properly, it can be very obvious when it is finally engaged, making the hum more obvious. If you have to use a gate, try to limit its use to a single channel or two instead of the entire mix, so it's side effects are not as easily noticed.

Source : http://www.guitar9.com/studionine7.html
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