The Audio System

The final component of most digital pianos is the audio system—its amplifiers and speakers—which perform the same job as an acoustic piano's soundboard: making the piano's sound audible at useful volume levels. I say most digital pianos because some instruments designed specifically for stage use lack an onboard audio system, as they will always be connected to a sound-reinforcement, or public address (PA), system.

The digital pianos currently on the market offer anywhere from 12 to 360 watts (W) of output power, channeled through from two to twelve speakers. To understand why there is such a wide range of options, we need to look at how the system's power-output capability (and the type, number, and placement of speakers) relates to what we hear.

The smallest change in volume that most people can detect is 3 decibels (dB), and to achieve a 3dB increase in volume requires a doubling of the output power in watts. With these relationships in mind, let's look at some numbers.

 640.015ppp  85    1.92mp/mf
 670.03ppp  88    3.84f
 700.06pp  91    7.68f
 730.12pp  94  15.36ff
 760.24p  97  30.72ff
 790.48p100  61.44fff

Based on measurements of three of the most frequently encountered concert grand pianos—Bösendorfer model 290, Steinway & Sons model D, and Yamaha model CFIIIS—I arrived at a model dynamic range. This range extends from the softest note possible, at 64dB, to the loudest chord I could produce, at 103dB. Assigning a modest 0.015W—we're assuming a very efficient audio system—to produce the softest (64dB) note, the chart below traces the progression of amplifier power required to keep up with the increasing volume to the top of the piano's dynamic range. Different audio systems will have different starting points, depending on the size and number of speakers being powered, the efficiency of those speakers' use of power, and the notes played (bass requires more power to match the treble volume). Dynamic markings have been added to bring some musical perspective to the numbers.

If you've not seen this sort of table before, the results are startling. It's the last three or four steps of volume that really demand power from the amplifiers.

When the audio system attempts to reproduce a sound louder than it can accommodate, it goes into "clipping" and produces a distorted version of the sound. One thing to remember is that even the most powerful instruments can be driven into clipping if played loudly with the volume turned all the way up. Aside from distorting the sound, overdriving the system can damage the speakers and amplifiers. The key is to set the volume no higher than 75 to 80% of its maximum level.

If you've already peeked at the specification charts toward the end of this book, you know that only a few digital pianos produce 100-plus watts of output power per channel (left and right). Many of the models that do have that much power also separate the low-demand treble frequencies from the power-hog bass frequencies by providing each frequency range with its own amplifier and speaker(s). A very few go so far as to divide the audio system into three separate subsystems, for the bass, midrange, and highs. These designs, called "bi-amped" or "tri-amped," can make a noticeable difference in sound and power efficiency by using amplifiers and speakers optimized for specific frequency ranges rather than sending the entire frequency spectrum to a single full-range audio system.


SPRING 2010 -- page 133

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