Box Acoustics Primer Part 1 – An Overview

Momentous changes are afoot in my life, as I am moving back to Cambridge to live as a Real Person this weekend. I also had a fantastic gig on Monday supporting the wonderful Lady Maisery at Green Note in Camden for Folk on Monday and have another gig on the 17th of this month, at Baldock and Letchworth Folk Club, supporting Jody Kruskal. So I’ve had a fair amount on my plate and somehow blogging has fallen by the wayside. I’ve started four posts, but this one I have finally finished.

This Acoustics Primer is a new series and a new venture for me; it is very much a work in progress. I am of the opinion that understanding how your instrument works helps with playing it, and that it is also very satisfying. To that end I would like to share some of the knowledge that I have gained about how the melodeon works with you, my dear hypothetical readers. Don’t be put off, I will be trying my very hardest to make this as comprehensible as I possibly can. My intention is that this will be a long series, made up of lots of short posts, not necessarily in order. The most important part of this series, as far as I am concerned, is telling you what I don’t know. A lot of rot is talked about the melodeon; I am not saying that I know the answers, but I do know what I don’t know. I think. This post is a very general overview of box acoustics, to break you in gently.

First of all, some basics of vibration, merely because there are lots of misconceptions about it. When a material is excited at a particular frequency, it will vibrate. The amount that it vibrates is known as the ‘Admittance’ (which is the inverse of the Impedance, for those interested) and this varies with frequency. There are particular frequencies where a lot of the energy passed to the material is converted into vibration and this is called ‘resonance’. The frequencies where this happens are called ‘resonant frequencies’ or ‘modes’. When a material vibrates, you can imagine it moving physically, ripples spreading out from the excitation point. When it is resonating, these ripples stand still and do not move over the surface. These are known as ‘Standing Waves’. At any other frequency, travelling waves will form on the material as it vibrates, which will interact with each other, reflect off boundaries and so on, making them difficult to visualise. It is important to distinguish between resonance and vibration. All resonance is vibration, but not all vibration is resonance!

I think that it is helpful to compartmentalise an instrument when you start to examine it. It is very easy to be overwhelmed by the complexities and subtleties of an instrument at first glance and believe it or not, a melodeon does have subtleties. I find it helpful to think about the melodeon in three phases: ‘Before’, ‘During’ and ‘After’, meaning before, during and after the sound is generated. Conveniently, there are three main mechanisms of affecting the sound, which go along with these phases.

The first is ‘Airflow’. Airflow means the path that the air takes to get to the reed, so before the sound is generated. This includes such things as flat mounted reeds as opposed to block mounted reeds, as well as more specialist things like the cassotto chambers used in high end piano accordions. But it also includes things as supposedly trivial as the holes in the palletboard. The idea of the airflow is that the direction, speed and turbulence (or lack of) of the air going to the reed can be controlled and that this can affect the sound.

The second is ‘Vibration’. This is more my field! I am referring to the vibration of solids here, such as reeds, plates, blocks and palletboards. Vibration in a melodeon occurs primarily whilst the reed is generating sound. Vibration of these solids takes energy away from the reed, which can be a good thing or a bad thing, depending on the box in question. We can also get resonance in the solid parts of a box, namely the reedblocks. This is mostly a Bad Thing.

Lastly we have ‘Acoustics’. We are back to air, rather than solids, but we’re now concerned with sound waves, oscillations in air rather than flow as such. Now of interest are surfaces, looking at how much sound they will absorb and at what frequencies. This is where casing material, internal finish and critically, grille design and grille cloth material become important. In addition, you can have resonances in air, for instance in reed block chambers, which again is probably a Bad Thing.

That is a very simplistic overview. In particular it ignores the flow component of the ‘After’ part of the analysis. The reason why it ignores it is that imposing a flow on a sound field makes everything a lot more complicated! These three phases are a simplification of what really goes on, just like everything else that I have ever been told, but it is a simplification which is helpful to get an overview of the instrument.

So the process of creating sound in the melodeon goes like this.

  1. A pressure difference within the bellows is set up by pushing or pulling. A button is pressed. This induces a flow through the reedblocks, which is affected by their shape.
  2. The reed generates the sound, with some vibration being passed to other parts of the box.
  3. The sound propagates out to the listener, some of which is absorbed by the casing or grille cloth. It is carried by the flow, so the path of the air can still make a difference to the sound.

By splitting the process into those three stages, the melodeon as a concept becomes immediately more manageable. I hope you agree. So when you look at a feature of a melodeon, try working out how it will affect airflow, vibration and acoustics and so work out what the effect on the sound might be.

The next post in this series will be on the reed, how it generates sound. I’ve already written nearly a thousand words of that post, so that one should be published quite soon. After that, I’ll be trying to work my way outwards, from the reed to the listener, probably with excursions along the way.

Nothing in this post should be taken as scientifically proven. If you have good reason to believe that I am mistaken in anything that I’ve said here then please comment and I will gladly amend.


2 thoughts on “Box Acoustics Primer Part 1 – An Overview

  1. Pingback: Box Acoustics Primer Part 2 – The Reed | Music and Melodeons

  2. Pingback: Box Acoustics Primer Part 3 – How do different reeds differ? | Music and Melodeons

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s