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June 2010 Ask the Pro: Speakers

Here are some frequently asked questions by churches regarding speaker systems.

How do I choose the right amplifier power for my speaker system?
Ideally, you should pick an amplifier that can deliver power equal to twice the speaker's continuous IEC power rating. This means that a speaker with a "nominal impedance" of 8 ohms and a continuous IEC power rating of 350 watts will require an amplifier that can produce 700 watts into an 8 ohm load. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 ohms.

A quality professional loudspeaker can handle transient peaks in excess of its rated power if the amplifier can deliver those peaks without distortion. Using an amp with some extra "headroom" will help assure that only clean, undistorted power gets to your speakers. Some professional amplifiers are designed so they have additional headroom. These amps can cleanly reproduce transient peaks that exceed the amplifier's rated power. In this case, select a model with an output power rating equal to the continuous IEC power rating of the speaker.

If budget restraints or legacy equipment force you to use an amplifier with less power, extreme care should be taken to see that the amplifier is not driven into clipping. It may surprise you to learn that low power can result in damage to your speaker or system.

What makes speakers fail?
Transducers can fail due to excessive mechanical stress or excessive thermal stress. One form of mechanical stress would be a mic stand through the speaker cone.

The speaker surround and spider, known collectively as the "suspension," must be flexible in order to accommodate the excursion of the cone or diaphragm. At the same time, they have to keep the cone or diaphragm from tipping or becoming "de-centered". Suspensions can fail due to environmental factors such as exposure to heat, UV rays, or humidity. It is also possible for the adhesives that attach the suspension to the cone and the speaker basket to fail or to be improperly applied at the time of manufacture.

The separation of tinsel leads is also a common failure. Conducting the electrical signal from the speaker terminals to the voice coil is the job of the tinsel leads. The leads must be light, flexible, and immune from breakage due to fatigue. At the same time, the leads need to be able to carry the full rated current load of the speaker.

What is Signal Burn?
Signal burn is a failure mode where the voice coil is burnt across its entire width, indicating uniform voice coil travel with respect to the stationary magnet structure. Such a burn pattern is not indicative of amplifier malfunction but instead is due to excessive signal or program level. The cause is simply trying to get more from the speaker than it is capable of delivering.

What is DC Burn?
DC (direct-current) burn is a failure mode where the voice coil is burnt only at one end. This is an indication that it has been traveling in one direction more than the other. Since the transfer of heat is from the voice coil to the adjacent magnet and metal parts, the voice coil will be burnt on the end that stays the farthest away from the top plate.

Woofers: When a DC burn pattern appears on the voice coil of a woofer, the problem will be due to a fault in the associated electronic equipment. Most likely, the power amplifier has leaky or shorted transistors that are allowing its internal power supply voltages to be applied directly to the loudspeaker or loudspeaker system.

Midrange and Tweeter: When a DC burn pattern is observed on the voice coil of such devices, it does not always mean that the amplifier is faulty. In systems with passive crossovers, mid and high frequency drivers are protected from DC by the cross-over. The most likely cause of DC-like burns is an overdriven amplifier.

When an amplifier receives an input signal capable of driving it beyond its power rating, the result is clipping. This means that the negative and positive peaks of the amplifier's output.

Should I fuse my speakers?
A fuse may blow with a signal that would not damage the speaker, but it can also pass a signal that can damage the speaker. To protect your system, use adequate, clean amplifier power and watch for amplifier clipping. Add a limiter to your system to electronically limit any potentially damaging transients.

What is power compression?
Speaker voice coils are made of copper or aluminum. As these voice coils increase in temperature during normal operation, the DC resistance of the voice coil increases. Greater voice coil resistance means less power transfer from the amplifier. As a result, the speaker will not play as loud when it's "warmed up" as it did when it was "cold." Some speakers may exhibit 3 to 6 dB of power compression. This means that power compression can have the same effect as taking away half of your PA!

How do I select the correct wire gauge for my speakers?
Selection of the appropriate wire gauge is important to system operation. A cable that's too "light" will result in amplifier power being wasted due to the series resistance of the cable. It will also result in the loss of low-frequency performance due to a degraded damping factor. On the other hand, a cable that is too "heavy" is unnecessarily awkward and costly.

This information is courtesy of JBL Professional, www.jblpro.com.

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