Perhaps the most well known is the cajon drum itself. The cajon drum is simply a rectangular wooden box. Five of the sides are thick plywood and the side which forms the playing surface is a thin piece of plywood. The back side typically has a circular hole to allow air to escape and project the sound. The typical playing position is to stand the cajon drum on its base so that the player can sit on top and hit the drumming surface with his fingers/palms/fists between his legs. I've also seen/read that players may tilt the drum backwards whilst playing. This implies to me that the bottom of the drum is left uncovered to allow air to escape like an african drum but I'm not sure whether this is common design practice given the hole in the back.
It is common to tension steel strings across the playing plate on the inside of the drum to give it a snare like effect in that position. The strings are often actual snare drum strings or occasionally steel guitar strings.
Another version is the Cajon Bongo. This is a smaller version of the drum which typically sits length ways across the lap. One long side of the drum is left open, this side faces downwards between the legs. The opposite side is the playing surface. The inside of the drum is commonly split into two compartments, one big (longer or deeper) and one small (smaller or shallower). This allows the production of two main tones from the drum, a deep one for the big compartment and a higher one for the smaller.
- Here is a practical demonstration of both drums (focusing on the bongo but demos of the big brother are available by the same author)
- Bela Fleck and The Flecktones - Over the Wall: The Cajon drum included in a band context. See around 4.26 for a percussion solo demonstrating the drum.
- cajon-drums.com (seems fairly authoritative)
The reason I mention this instrument is in relation to some design problems I've been having with my DMI. Because I decided that I would like to try and implement some vibrotactile feedback in my instrument, most of my recent (basic) designs have involved one or several loud speakers, taking inspiration from Mark Marshall's DMI The Viblotar which uses loudspeakers to localize the sound source to the instrument itself and also to provide a degree of vibrotactile feedback (with options for some clever synthesis implementations). My problem however is that most of designs are coming out as square shapes in order to accommodate the speakers!
This seems like a problem for several reasons. For a start, the designs are not particularly innovative. Most of them look like actual loudspeakers with a few sensors stuck onto them. Also a lot of my DMIs have been designed to hold and play which may not be easy as things stand due to the necessary size required for a loudspeaker enclosure, the awkward shape of a big box and also the potential weight of the instrument.
After looking into the Cajon Drum I'm beginning to think that may be a big box may not be such a bad idea, if I take some inspiration from the drum:
- The Cajon Drum is an existing instrument with an existing gesture vocabulary, there by giving first time users perhaps some idea of the kind of techniques that can be used with such a DMI. The controller would thus be classified along the lines of a instrument-like or instrument-inspired controller (perhaps dependent on the direction I went in with the software, ie imitating a drum sound or some thing a little different...)
- The Cajon Drum is a reasonably popular percussion instrument therefore there will be a good repertoire of material that could be tried on the DMI. This meets some of the ideas behind approaches to DMI design that I mentioned in a recent post
- Embracing the box design of the drum means that speakers can be reasonably easily housed in an instrument design which already exists as an expressive acoustic instrument which should address any worries regarding playing style / efficiency
- A box shape (with justification!) is perhaps actually the best design choice since it would make the woodwork and construction fairly simplistic. This is perhaps especially advantageous in this project given my time limit, the notorious difficulty of software and mapping and also outside commitments. I think its perhaps important to remember at this point that I am in no way attempting to rewrite or improve the book on DMI design. Rather I am trying to use what I have learnt from the book to produce a DMI which could be considered to have a good "feel"
- The closed box design will also allow all the electronics of the DMI to be hidden inside. It has been suggested in studies that DMIs which have visible external wiring are often seen by performers as being "experiments" and being "fragile" which perhaps restricts the level of involvement a performer could have with an instrument
- Building an instrument designed to allow the player to sit of top of it will naturally result in it being very robust which, allow with internal electronics should reduce the impression of DMIs as being fragile experiments and allow the performing to really get involved in their playing without worrying about damaging the instrument. This should also have other obvious benefits such as providing a good amount of protection to sensitive electronic components and also being reasonably resilient to bumps caused by transport etc
- One interesting thing about the "box" designs I had previously come up with is that they often encouraged the used to hold them and wrap their arms around them in order to reach the playing surface. Although granted this playing position would have been too uncomfortable it did ensure that a large proportion of the instrument body was in contact with the players body. This would be especially advantageous for the vibrotactile feedback element of the DMI. This Cajon design should still hold this advantage since it encourages the user to sit on top of it!
I think the above statements make a compelling argument for using the Cajon drum as inspiration. I think at this point and after all this research, that finalizing the fundamental design choice in this manner is the next step to take and it would be good to advance in this way. There are however, some important points that I would have to consider if I took this direction:
- Gesture capture system. The main method that I had in mind was to use a piezo microphone mounted on the inside surface of the playing plate to monitor the vibrations of the plate and the rest of the instrument. Some thing to consider about this approach is that it would involve a certain degree of signal processing which could become fairly complex. For example I would have to distinguish between the vibrations caused by the loudspeaker feedback system and the vibrations caused by the actual player. There are possibly some interesting things to be done here such as distinguishing between amplitudes to trigger different effects and possibly distinguishing playing location and technique. On a related note here's an example of a commercially available Cajon Drum which includes a "DST pickup microphone" which I believe is a contact mic of some kind. The mic is mounted on the inside and outputs through a 3/4 inch jack plug with volume control.
- If the same gesture vocabulary as the original instrument is used then the DMI is going to give primary acoustic feedback from the players hands hitting the wood whilst also giving the secondary acoustic feedback from the synthesizer. The question is do I want this primary feedback and if not, how shall I remove it? In the DMI the speaker will stand in for the air hole so this should provide some damping. I might also consider packing the inside with soft material, much like stuffing a bass drum with pillows.
- What kind of software approach should I take. I'm thinking more of what are the sounds that I would like this instrument to make. They needn't be percussive. Interestingly, I think that if I used percussive sounds exclusively then this DMI would perhaps be classed as instrument-like since it is designed to look, be played and sound like a cajon. If i used some other type of sound though it would perhaps be instrument-inspired since it is based on the concept of a cajon drum but is designed for a different purpose.
- One element that could inform the sound choices I make is the choice of transducer technologies I include with the DMI. I think placement of transducers here will be an especially important decision given the fairly unique playing position of actually sitting atop the instrument and bending down over it to play. Looking at some videos the arms tend to be fairly stretched at any given moment and their left-right movement is limited by the position of the legs. Two Cajon designs could possibly help with this:
The Cajon Bongo since the instrument is placed on the players lap and the player typically interacts with the top surface on which could be placed the transducers for easy access. With this design the controller would not be too dissimilar to Mark Marshall's Viblotar
The angled surface Cajon since the front plate playing surface is angled to slope downwards away from the player in order to allow slightly easier access to the various tones available from the edge of the plate to its center. In the same way it would allow easier access to any transducers which were placed on the plate. On a side note I'm not fairly convinced that typically the Cajon Drum does not have an open bottom and the reason for tilting the drum backwards during play is again to allow easier access to the full playing surface.
Here are some Cajon materials which are perhaps slightly more authoritative and better references than those given at the top of the post. I'll keep all Cajon stuff within this post for now until I take an active decision to follow this line of design. Could do with some even more authoritative works and perhaps some scientific studies. So far after a little google searching I can find no examples (aside from simple amplification) of digital or analogue modified Cajons.
- Frequency and Dynamics Analysis of Bass Tone of Cajon Box Drum - Peter Kicak - This article is fairly bad from what I've read especially since the English is rather poor, but the first section does give some background on the Cajon which I presume is reasonably reliable.
- Studies for Cajon - Martin Rottger - just a book preview entry found on google scholar but again there is a history section and also some information regarding damping the Cajon and about the tilted playing style
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