How cello sound is made

Before looking at tension, string materials etc.. you really need to understand the core mechanics of how your cello actually makes sound.  These concepts will form part of any topic on instrument acoustics.

Sound is created by the body of your instrument, vibrating back and forth, just like a loudspeaker.  The string's role is to transmit the energy needed to create those body vibrations.

This energy moves through the string (and onwards through the bridge, sound post etc..) in the form of waves.

These waves can vary in shape, speed and pattern depending on the string and how it is played.  And it is these differences which make one string sound different to another.

The materials used to make the string, the quantity used, and how they are constructed together, all have an impact on these waves.  So when considering differences between strings, always try and relate this back to waves and how they might change.

What is this sound we hear?

When we play a single note what we actually hear is a combination of different frequencies.  For instance, when the A is tuned to 220 Hz your string is also vibrating at 440 Hz, 660 Hz, 880 Hz, etc. The vibrations at 220 Hz are the strongest, which contributes to the reasons why our ear perceives this frequency as the actual pitch of the sound, but the addition of other frequencies contributes to the overall acoustic richness.  So when we say "I prefer the sound of this string" we actually mean "I prefer the combination and balance of frequencies".  

Your string vibrates at other frequencies too, ones that do not divide evenly by the fundamental. Together, these frequencies can be put into two groups:  harmonic and inharmonic.  Changing the balance and composition between these two groups of frequencies affects the timbre of sound we perceive.

Of course we don't talk about sound in terms of frequencies, their distribution, and pattern.  We use names like "bright", "warm", and "deep".  At Rostanvo, we looked into whether we could show empiricallythat one string/or instrument had these qualities (and others) more/or lessthan another. Luckily, some excellent research has already been done on this subject at Cambridge University in 2010 but concluded that listeners were unable to agree on the terms when given a choice of sounds.

What we hear is hugely subjective and ultimately down to personal preference. A "warm" sound you perceive might seem muffled and muted to others for instance. So always be weary of such descriptions as you might hear the sound differently.