Passive loudspeakers – why do they still exist?

A walk around last week’s135th Audio Engineering Society Convention at the Javits Center New York reminded me again of how passive (non-self-amplified) speakers seem like antiques, despite their continued dominance in high end home audio. Studio monitor speakers are pretty much all self-amplified “active” designs.

At a basic level, amplified sound consists of electrical amplification, to make the electrical signal “bigger,” and speaker transducers, to turn that signal into sound waves. The first step is purely electrical; the second is electrical and mechanical. There is no law that says these must be together or separate, so why does it matter?

The strategy of keeping amplification separate from the speaker requires that any amp be compatible with any speaker, demanding adherence to norms of electrical impedance and other characteristics, power level, and speaker sensitivity. These norms limit the designer, because above all, the system must retain its compatibility, or it won’t be commercially viable.

A consolidated amplifier/speaker system – particularly one employing digital signal processing control of equalization and other parameters – opens up tremendous opportunities for performance increases. Counter-intuitively such systems are in a sense more difficult for the engineer, because all these possibilities mean the project is less bounded and therefore larger. However, the potential benefits are enormous, as can be seen in small active consumer models like the Sonos Play:1 or the Boston Acoustics MC100Blue.

I think for different reasons engineers and consumers both help perpetuate passive speakers. Engineers (especially inexperienced ones) who are under pressure to complete designs in limited time appreciate more bounded projects. Consumers – particularly in the high end arena – like the bragging rights of saying they have an amplifier that can drive any speaker, or have fancy high end speaker that challenges all but the most capable amps. It’s similar to a consumer’s desire for a high performance sports car that is in reality very challenging to drive well. Consumers also may not buy the speakers and amplifier at the same time, so they don’t want to be limited or forced into an interdependent decision.

Professional audio monitors, use in mix, production, and broadcast studios, are commonly self-amplified. This originated out of convenience and reliability requirements – a person who uses gear to make a living prefers systems with fewer connections, components, and other opportunities for trouble. And some key brands like Genelec have set the trend that many other companies have followed. The fact is that these self-powered speakers offer higher performance than their amplifier plus passive speaker counterparts. And in the monitoring environment, where high sound levels are expected, the self-powered systems are also self-protected and less likely to blow up.

An interesting exception to the trend are large scale professional systems – commercial live sound and cinema – that are elements of a system that is custom designed and tuned in situ. Such systems are planned and installed as integrated units, often with additional components providing custom equalization and other adjustments.

My belief is that long term passive consumer speakers will disappear, or at least be relegated to fringe hangers-on who use them with their turntables. Sales numbers bear this out, with traditional components declining in the face of docks, soundbars, etc. But this trend doesn’t have to be all “low end” – high end product designs would do well to make the leap to amplified systems. Such systems, combined with automatic room tuning (like Audyssey) and room acoustic treatment (absorption and diffusion), provide the greatest opportunity for amazing performance.

© Copyright 2013 Stephen Shenefield

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