This article answers your sound questions. We’ll find out how to distinguish good and bad speakers, and the steps to reducing noise and hum from a PA.
Topics: sound engineering
This article is based on some emails I have received. Please feel free to send me your specific questions, although I will refrain from making brand-name recommendations.
Q: How can I tell if used speakers are any good?
A: Always do a listening test with recorded material. In general, PA speakers will either work or they won’t, and a quick listening test will be sufficient. However, there are some extra items to check that can save you hassles later, or allow you to knock down the price.
First, check that the cabinet is strong. It may look pristine, or bedraggled, but if the box is not rigid, it won’t sound right. This may not be obvious during a casual sound test, because the changes to the acoustic resonances may not be exposed by what you play through them. Pick up the speaker, or lean on it, and verify that the corners are strong and none of the panels are cracked.
Next, check the woofer. If it is made of a paper material, check that there is no water damage. If the cone is made of a polymer/plastic, then it is likely to last a very long time. If the center dome is pushed in, don’t worry, it just looks bad. Then check that the coil is moving freely. To do this, place your hands on either side of the woofer, with your thumb on the speaker cone. Place your ear as close to the cone as you can, and gently push the cone in a little. Try to push both side evenly. Listen for any scratching that might be the coil rubbing on the magnet. Try this on your home stereo first so that you have an idea of what an undamaged speaker should sound like. If you are convinced that the coil is rubbing, avoid this speaker because the coil wires will eventually wear through and break.
Next, check the port and padding. If there is a port or vent, check that it is not damaged. Some speakers have tubes, others do not. Also check for any padding (looks like house insulation or packing foam). If there is some present, check for water damage or bugs (yikes!). So long as no permanent harm has been done, this type of damage is OK.
Last, check the wiring. If you can, remove the jack panel and observe that all of the wires are undamaged. Usually, these wires are loose and perhaps disorganized, but so long as there are no temporary splices, it should be OK. Make sure that the panel jacks themselves are tightened. Check for fuses. There should not be any fuses for the woofer, but it is OK to have one for the horn or tweeters.
Q: What is the best way to reduce hum and noise in our PA?
A: Here are some simple steps to take. but remember that noise is sneaky, it gets into the system in some really weird ways.
Step 1: Buy an outlet tester. Just because an outlet has 3 holes, doesn’t mean that they are all wired. Proper grounding is important for two reasons. First is safety; ungrounded systems can cause minor (and major!) shocks. Second is hum; hum can really ruin an otherwise excellent sound system. If there is no “earth ground”, don’t use that outlet. If you are at a bar, and none of the outlets are grounded, suggest that the building inspector be contacted (no joke). (As an aside, I don’t know what the standards are for non-US systems)
Step 2: Plug all your PA electronics and amps into the same outlet. Assuming that you have a reasonable bar PA sound system, you won’t draw more than 5 amps (figure 1 amp = 100 watts) for all of your gear, so plug it all into one outlet strip. If you like to keep your power amps near your speakers (a good idea, but not crucial), then it is probably OK to plug them in near the stage, and plug in your mix board, etc wherever you set up. (If you do get a bit of hum, try running an extension cord from the amps to the mixer, just don’t lay it too close to the snake or mic cables.)
Step 3: Avoid the lighting outlets. Lights draw an incredible amount of power, and dimmers create an enormous amount of electronic noise. If you hear a high-pitched buzzing like a mosquito, and if it changes as the lights are dimmed, then you are sharing a circuit with the lighting system. If you have to, run a long cord to the other end of the building just to get away from it.
Step 4: Always use the correct cables. Don’t use speaker cables for microphones or guitars, and vice versa. They may all have ¼ jacks, but they are not the same. Also, use 3-pin XLRs for your mics, and don’t forget the shielding on the cables.