Some people like a good buzz, but in the audio world, there's no such thing. With this problem, there are two likely suspects, inducted noise or improper grounding (also known as DC introduction or a ground loop). Here we are going to discuss inducted noise.
Things like data cables and high voltage lines can cause major problems with any audio installation. Here's where the real problem usually is: Improperly shielded cables. You can have an audio cable that is shielded, but this won't do an ounce of good if the cable isn't properly grounded at one of the terminated ends.
If you have installed a sound system and you hear a noise that sounds like a fax machine, or you have a 60 Hz (sometimes also a 180 or 240 Hz) hum, it is quite possible that one or more of the cables is not properly grounded. This can usually be fixed by properly grounding the cable shield or inserting a noise filter. The right way to fix it is by grounding the shield. At least one end of the shield should be connected to the negative audio conductor (typically the "sleeve" when dealing with RCA or 3.5mm line level).
If, by some crazy oversight, a cable has been installed that is not shielded, then a noise filter may be needed. These can affect frequency response, but typically not to a degree that will greatly impact speech intelligibility. Watch the cable manufacturers. There was a case recently reported to us where a client ordered 500 audio cables for a large job and not one of them was properly grounded. This caused many of the audio systems to buzz or hum, costing the installation company thousands in troubleshooting time and materials.
If you have verified that all of your cables have been properly grounded and you still have a hum or buzz, then read on to tech tip #2, you may have a different problem. |
Probably 80% or more of the time that you encounter a bad hum or buzz, it is going to be the fact that the DC has been introduced into the audio system. How does this happen? There are many different ways. Basically when two or more devices are connected to a common ground through different paths, a ground loop occurs, and you hear a hum. They are common because there is no such thing as a perfect ground.
Different resistant levels in different cables inevitably create a potential difference that will usually create a hum at one level or another. Sometimes the hum can be eradicated simply by turning the volume of the amplifier down and turning the volume of the input device up. Many times, the user will never exceed a particular volume anyway, especially in a classroom or conference room environment.
On the other hand, if the ground potential is high enough, other measures can be taken. Typically the more grounds there are in a system, the higher the chances of a ground loop issue arising. So if Possible, reduce the amount of grounding points, and if not possible, make sure all of the grounding points are well connected, perhaps by a common cable. If you have a say on the electrical requirements before an install takes place, require that the ground for the amplifier, projector, VCR/DVD, computer and any other component be on a common ground that is properly connected and terminated.
Also make sure that all of your audio cables have a properly terminated ground that is of sufficient gauge so as not to create too much resistance in any part of the system. If all else fails, insert a ground loop isolator (such as the Roemtech™ Plenum Rated Hummbuster, HMB-200) into the system. Again, this may affect the audio quality, but you would probably be hard pressed to notice a difference in most commercial installations.
Remember, if doing a large install, it is almost guaranteed you will face some audio issues with improper grounding, provide for this in your contract or in your quote to the consumer. |
