Basics of Loudspeaker Design & Technology (Part II)

In Uncategorizedby tfwm

One of the questions most commonly asked of loudspeaker manufacturers concerns matching power amplifiers to loudspeakers. We’ve found that the majority of loudspeaker failures encountered by our service department are the result of improper power matching.

It might seem logical to use a smaller amplifier as a safety measure; after all, it stands to reason that there won’t be enough power to damage the loudspeaker. Yet we see many more loudspeaker failures due to under powering than any other cause. When the amplifier exceeds its maximum voltage capability, it can “clip” the peaks off the electrical signal it is supplying.

Once the amplifier starts clipping, the average (or RMS) level rises, which can be extremely damaging to the loudspeaker. In layman’s terms, for example, your 200-watt amplifier can approach delivery of 800 watts of power, but it will be volatile and heavily distorted, and thus potentially quite harmful to the loudspeaker.
However, using an amplifier that is much more powerful than the loudspeaker’s rating can do just as much damage. Supplying the speaker with more power than it can handle will often result in thermal (heat) and/or mechanical damage.

Basically, any amplifier with a 20Hz – 20kHz power rating that matches other specifications of a given loudspeaker will safely drive that speaker at full output under most conditions. Therefore it’s vital to understand the meaning and impact of the most important loudspeaker specifications as they relate to amplifiers.

First, there’s power handling capacity, usually defined by terms such as “Continuous,” “Program” and “Peak”. Continuous is the level of long-term average power accepted by the loudspeaker; Program is based upon a test signal that simulates a “real-world” program signal; and Peak refers to the maximum amount of short-term power the loudspeaker will handle.

Based on these terms, then, what is the proper amplifier choice to capably and safely drive a loudspeaker rated at 100 watts continuous, 220 watts program, and 400 watts peak? The best choice would be an amplifier delivering about 200 (continuous) watts per channel. Plenty of power for normal full-range operation while also capable of meeting short-term peak demands without going into clip.

It should be noted that although an amplifier delivering 200 watts per channel is appropriate for use with a full-range loudspeaker rated at 220 watts program, subwoofer enclosures tend to require a bit more power to generate their lower frequencies. In other words, a single mono subwoofer rated at 200 watts should be paired with an amp offering absolutely no less than 200 watts. In fact, a good rule of thumb when dealing with subwoofers is to employ an amp rated somewhere in the vicinity of 20 percent higher than the subwoofer. With that in mind, a 250-watt amplifier would be ideal in this application.

Another factor to be aware of is the impedance load that the amplifier is being required to drive. Impedance is the resistance to the flow of the audio signal by the loudspeaker. Measured in ohms, it is usually called “nominal impedance” by most loudspeaker manufacturers.

A mismatch in impedance between an amplifier and loudspeaker can result in poor performance, in the form of improper frequency response and excessive distortion. Serious damage to one or both components is also a very real possibility.

Most professional loudspeakers are rated at either 8 ohms or 4 ohms, although a number of power amplifiers are specified to drive loads ranging from 16 ohms to 2 ohms.

You’ll notice that the lower the impedance, the greater the amplifier power rating. More power is required to overcome the increased resistance. For example, a loudspeaker delivering 110 watts at 8 ohms will also likely be specified to deliver about 150 watts at 4 ohms. Consider our prior discussion of power handling, and you quickly see why its important to note a loudspeaker’s impedance specification in the matching process.
It’s usually best to avoid 2-ohm operation, which can unnecessarily stress an amplifier, and also can require much larger (heavier gauge) cable to effectively and safely transport the load.

And speaking of cable, why go to all of the trouble of properly matching loudspeakers and amplifiers if the link between them is subpar? We recommend using heavy gauge (#14-#10), professionally manufactured cable in all applications. A good rule of thumb is the heavier the cable, the more efficient it will be as a conduit between the devices.

To learn more about the issue of power handling, EAW offers an application note entitled “Understanding Loudspeaker Power Handling and Selecting the Proper Power Amplifier.” To obtain a free copy, contact us at 800/992-5013.