How cello sound is made. It's not what you think..

This is the first in a series of articles aimed to comprehensively explain how cellos make sound in an easy to understand way, what’s important in creating this sound, and what isn’t.  Hopefully it will of use to you.  If for any reason something isn’t clear, leave a comment below and I will do my best to answer.  If I don’t know the answer, I’ll find out for you too!  Lastly if there is a specific topic you want explain, send us a message via our contact form or Facebook and we’ll add it to the list of topics to cover in the future.


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Our world is a complicated place.  There’s just too much to learn with so little time.  And it’s no different with the cello.  A complicated instrument to make (well) and difficult to play too.  So we skip learning about all the techy stuff, and instead try to squeeze in a little more cello practice or focus on other things we have to do. 


A shame really because even just knowing a little will help you make better choices when it comes to buying an instrument, strings and accessories.  What’s more it’s actually quite straightforward and interesting too!


This article will cover some basic concepts. Be sure to read the other articles in the series for a more in depth analysis of some of the subjects raised.


Going backwards, let’s start with:

Understanding what sound is

What the cello does to create sound

How the cello does this

What instrument factors influence the quality of sound.


What Is Sound?


Sound (also known as “acoustic waves” to the scientific community) is actually tiny changes in air pressure, travelling through the air.  Our ears pick up these air pressure waves, which then get converted into electrical signals and then sent onto our brain.


In terms of frequencies what we generally hear ranges between 40 vibrations a second up about 10,000 vibrations a second, although our hearing range is slightly greater than that.  Just to put things in perspective the cello A is around 220 vibrations a second, or 220 hz.


The way your ear actually “hears” sound is a fascinating subject in itself, expertly explained in the video below.  




Sound travels through your ear canal and then causes your ear drum to vibrate.   This in turn vibrates fluid in the cochlea tube behind your ear drum, a bit like a micro tidal wave travelling down the tube.  High frequencies make the start of the cochlea to vibrate, while low frequencies make the other end vibrate. 


Inside there are row upon rows of tiny hair cells that move due to the waves of the fluid inside tell the brain which part of the cochlea is vibrating.  Each hair cell corresponds to a frequency and can determine how loud it is.  These hair movements are absolutely tiny and can measure only the width of an atom!  It’s quite remarkable.


Your cello, the portable loudspeaker. 

So we understand what sound is, but how does your cello do this?  Well it does this by vibrating back and forth, just like a loudspeaker.  Pushing outwards and inwards to create these air pressure waves travelling through the air.  A common misconception is to think the string is what is making the sound.  But with so little surface area, the string has a negligible contribution.  The air around it is left nearly unaffected.



The string's job is to capture the energy coming from your bow movements (or fingers, if pizzicato) and transmit this energy in the form of waves needed to create those body vibrations.  Strings are of immense importance because it is here, and here alone where these waves are formed and defined before moving onwards through the bridge, the sound post etc..


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.


So what influences these waves patterns and why?


The materials used to make the string, the quantity used, and how they are constructed together, all have an impact on these waves. 



Let’s take the example of a whip and let’s compare it to a steel rod.  The whip bends freely.  Try the same whipping action on the rod and it flexes only slightly.  Both will carry a wave but the shape and speed will be very different to one another.  The whip will have a large wave and would travel slow enough for us to make out its shape and motion.  The rod would result in a smaller shaped wave but one that would travel quickly.


So what the above example shows us is that the materials used and the physical properties they have can result in dramatically different shaped waves and thus sound, even if the energy/action applied is the same.


And strings on cellos, or any instrument for that matter, are no different.  So when considering differences between strings, always try and relate this back to waves and how they might change.  Some materials are naturally more bendy or stiff.  And the way they have been constructed will also influence how flexible or rigid they are.


Bow movement and string waves


One thing we haven’t discussed is the relationship between the bow and string and how that distinctive bowed instrument sound is created.  What happens is that your bow continually latches onto the string (with the help of sticky rosin on the bow hair), and then slips, before catching the string again only to slip back again etc..  This happens many, many times over.  The motion was discovered by the German physicist Hermann von Helmholtz 155 years ago.  For a full explanation of this movement read this excellent article written by Professor Jim Woodhouse and Paul Galluzzo.


Given that the bow hair and string need to stick to one another, albeit briefly during the Helmholtz motion, it goes without saying the that the rosin used will determine the level of stickiness and thus how the string motion behaves.  We’ll look to cover this in more detail in a future article.



How do we define 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 (technically described as the “fundamental”) are the strongest, which is the main reason why our ear perceives this frequency as the actual pitch of the sound. 

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.




So the addition of other frequencies contributes to the overall acoustic richness and help form the character of the sound we hear.  Put another way, when we say "I prefer the sound of this string" we actually mean "I prefer the combination and balance of frequencies".  Some strings will have greater levels of inharmonicity, while others might give off more harmonics and resonate with adjacent strings to a higher degree.



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 empirically that one string/or instrument had these qualities (and others) more/or less than another.

The good news is that some excellent research has already been done on this subject again written by Jim Woodhouse in 2010.  The bad news it 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 wary of such descriptions as you might hear the sound differently to someone else.


So this concludes a quick(ish) overview of how the cello makes its sound, how we perceive it and what the role of strings are in all this.  The key take away is that it is the string’s degree of flexibility which is key in shaping the balance of harmonic and inharmonic frequencies that the cello produces. 

But isn’t tension important? 

The answer is no and I’ll explain why in the next article..


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