Sound system information

I've been learning quite a bit from talking with professional sound engineers. Here are some of the things I've learned so far:

• When mic-ing a choir, make sure the microphones are 3 feet out in front of the front row of the choir and 1 foot above the heads of the first row.
• Take note of the location of your speakers and the angle at which they send out sound. Make sure that you are sending sound towards your "audience." Also note how the speakers interact. For example, if you have a large room with a speaker on each side, if you sit closer to one speaker than the other, you will hear a delayed sound image from the speaker that is furthest away from you. If you can position the speakers so they don't interact with one another (their "angles of service" don't overlap each other).
• Make sure you try and place your speakers between the audience and where your microphones will be. This helps eliminate a load of feedback problems (I know, I know - common sense).
• Typically speakers should be above your audience to get the treble sounds to cover the entire area.
• There is roughly a 3:1 ratio between the length of the room your speakers must cover and the height at which the speakers must be. For example, if you have a platform where you have your microphones at one end of the hall and then you place your speakers in front of that place, measure from the front of the speakers to the back of the hall. Next divide that measurement by 3 and that gives you the vertical measurement from the floor to the bottom of where the speaker should be. If your hall is 60 feet long, the bottom of the speakers should be 20 feet in the air.

Here's a method for using white noise to balance your speaker output in a small room:

Once you have your source material ready, the next key element to check is your speaker placement. Having your five speakers and subwoofer in a room is a start, but for proper placement, you really need to measure the sound levels at the prime listening location. Most A/V receivers, equalizers, and preamps have the ability to generate "white noise." This is a rushing sound, similar to that of a waterfall. It is comprised of equal sound levels of all frequency ranges. To properly set the sound level, you need to start generating white noise, one speaker at a time. You'll also need a sound level meter. The sound level meter should be placed in the prime listening position, and you should raise the volume for the sound channel in question until the meter reads 75dB. Do this for each channel individually, including the subwoofer. When all 5 speakers and the subwoofer output the same amount of sound, your system will sound better than ever.

Notes about feedback:

Acoustic feedback occurs when the amplified sound from any loudspeaker reenters the sound system through any open microphone and is amplified again and again and again.

What can make feedback problems worse?

• Placing loudspeakers too close to microphones.
• Too many open microphones.
• Boosting tone controls indiscriminately.
• Room surfaces that are hard and reflective such as glass, marble, wood.

What to do if feedback occurs before the sound system is loud enough?

• Request that the talker speak louder into the microphone.
• Reduce the distance from the talker to the microphone. Each time this distance is halved, the sound system output will increase by 6dB.
• Reduce the number of open microphones. Each time this number is halved, the sound system output can be increased by 3dB.
• Move the loudspeaker farther away from the microphone. Each time this distance is doubled, the sound system output can be increased by 6dB.
• Move the loudspeaker closer to the listener. Each time this distance is halved, the sound system output will increase by 6dB.
• Use an equalizer/feedback reducer to cut the frequency bands in which the feedback occurs. The sound system output will typically increase 3 to 9dB.

NOTE: Do not rely solely on an equalizer/feedback reducer to provide sufficient additional output in a sound system where the microphones and loudspeakers are too close together. The result will be unsatisfactory.

What's the difference between parallel and bridged mono modes when connecting amp and speakers?

The parallel setting duplicates the effect of cross-patching the two inputs, using an internal switch to save the hassle of an extra cable. The two input jacks are connected together, so that a signal on either jack now appears in both channels (therefore only a single signal should be connected). Each channel's gain control regulates its volume as usual, and separate speakers connect to each channel as usual. When using this mode, DO NOT combine the output of both channels into a single speaker; this is likely to damage the amp. The parallel mode is useful when you want to send the same signal to both channels of an amplifier but with the ability to set the volume controls for each channel separately.

The bridged mono combines the two channels of an amplifier into one channel with twice the power at double the impedance. This setting is useful when you want to deliver the entire power of the amp to a single 8 or 4 ohm speaker load. Amplifiers cannot drive a 2 ohm load in this configuration as each channel must share the single load. This means each channel only "sees" 1 ohm of a 2 ohm load which is too close to being a short. 2 ohm loads are NOT recommended as this loads each channel at only 1 ohm each. Consult the amplifiers owner's manual for the proper gain control settings.

Return to Electronic Projects page

Return to HOME, Dorothy!

Created on 3/31/98 / Last modified on 01/31/01
by Drew Hickman / drew@hickmans.net