Kawai’s Digitals: Part 2 – Sound Technology

PART II: SOUND TECHNOLOGY

Playing a piano can be a rewarding experience. The slightest change of pressure applied to a key not only affects the volume of the note, it changes the tonal character. Going from the smooth, demure pianissimo tones to the thunderous, percussive fortissimo tones, the piano has one of the most complex sound palates of any instrument. The physics involved with the making of a piano’s sound is complicated. It’s widely accepted that the sound of a piano is the hardest to replicate in the digital realm. This is where Kawai has made the most progress of any digital piano maker. They may not offer 400+ sounds, but they do recreate the most important one better than anyone else – the intricate sound of an acoustic piano. If you are looking for a digital piano for the purposes of playing it like a piano, Kawai is the clear choice.

A Bit Of Physics

A piano’s sound is arguably the most difficult to replicate digitally for a few reasons. The first is that more than one sound is produced when a piano note is played. These additional sounds are called harmonics. Figure 1 shows the fundamental sound plus 3 harmonic overtones (more than a dozen harmonics can be produced by a single piano note). These harmonics ‘stack’ on top of the fundamental, creating a complex waveform. The waveform at the top of figure 1 illustrates the resulting shape created by stacking the waveforms below it. Notice that it does not repeat a pattern. By contrast, instruments like violins and flutes produce simple, repetitive waveforms making them far easier to reproduce digitally (as shown in Figure 2).

Secondly, the piano’s sound is non-linear, meaning that the shape of its waveform radically changes when played loudly or quietly. It is almost as though the piano were many instruments, not all playing at once, but all playing together. There are other factors such as whether the sustain and soft pedals are engaged, the position of the top lid, the speaking length of the string, the scale design, types of wood used in the piano, etc. All of these influence a piano’s sound.

Sampling

Adding to the complicated task of digitally reproducing a piano’s sound is the process of sampling the piano. Because the piano makes a non-linear sound, taking multiple samples of each note offers a more complete profile of a piano’s sound. It is not a simple case of pointing a microphone at a piano and pressing record. There are many factors involved in the sampling process.

• choosing which piano(s) to sample
• mono or stereo sampling
• the number and types of microphones to use
• microphone placement
• number of samples to take
  • at various volumes (ppp – m – fff)
  • with and without the sustain pedal
  • staccato and legato
  • with or without room ambiance

There are 88 keys on a piano. If each note is sampled 5 different ways in stereo, that’s 880 sampled sounds. That’s a lot of data for a CPU to stitch together in real time to recreate the sophisticated sound of a piano. Many digitals only use one sample per note and rely on algorithms to recreate the tonal variations, but these algorithms fail to account for much of a piano’s nuanced sound, which is why they are dissatisfying to play. Kawai starts with numerous samples on their entry-level digitals and goes beyond 12 on their flagship models (they use proprietary technology, so getting more detailed information is near impossible). Below are overviews of Kawai’s sound technologies, starting with Harmonic Imaging and continuing through to their latest SK-EX Rendering Piano Sound Engine.