Guitar Bridge Retrofit
Our objective is to produce an acoustic guitar bridge that has the best possible intonation that the guitar can deliver. If you’re having sharp intonation or you want to add more saddle compensation this is a great way to do it. One of the best things about playing guitar is when is feels and sounds nice. A client sent us his seagull guitar to retrofit, he heard about our intonation improvements and wanted to try it on his instrument. Jay hadn’t performed this task before but being an engineer, he thought it a good task to take to help spread the bridges to the world. A jig was made to hold the guitar in plate while a new channel could be routed. A scary thing to do once you put someone’s favorite guitar in the surgeon’s chair. Fortunately, everything came out a success and the intonation was improved on this guitar. The improvements were measured before and after the bridge was put on so we got a good view of the changes the bridge and saddle can make.
What follows is first a breakdown of the improvements that were made and an analysis of them with graphs. The next piece is a photo diary of making the bridge retrofit jig.
For our purposes we will measure the difference between the actual note and the desired note and plot the differences on a graph. In these graphs the note played is plotted on the horizontal axis and the difference between the played and desired noted is plotted on the vertical axis. The vertical axis is measure in cents (100 cents to the half step). To aid in the analysis a linear least square fit is applied to each graph. The graph below shows the results of the E6 string of the Seagull guitar before the split saddle bridge was installed. We can see that the notes trend sharp as the string is played up the fret board.
For comparison purposes the results for the E6 string after the split saddle bridge was installed are graphed below.
We can now see that as the string is played up the fretboard the linear fit is constant. We also must note that there is still significant variation from the ideal. This is for the most part a consequence of the physics of the vibrating system we call a guitar.
The following graph shows all of the results for the Seagull guitar before the split saddle bridge was installed. We can we that in total the guitar tends to play sharp as it is played up the fretboard.
The following graph shows the results for all of the strings after the split saddle bridge was installed.
We can see that we have adjusted the intonation so that the trend lines are nearly constant.
The following graph shows an expanded view of the final results without the trend lines. We can look at each individual note on a string and compare its intonation to the rest of the notes on that string, or we can compare a note to the equivalent note played on a different string. The ideal would be for all of the individual notes to lay right on top of each other. This can be approached by installing a split saddle nut. Nonetheless, the results are pretty good.
A note and comment on the feature in the graph around the D#. We can see that the intonation suddenly goes from somewhat flat to somewhat sharp. This is a repeatable and consistent feature of all guitars. This guitar has an air resonance at the D#. When a note is played near but below this resonance it will play flat and if the note is played above the resonance the string will play sharp. This is a feature of the physics of resonant systems and is one of the phenomena that makes a guitar sound like a guitar. If you tried to make this go away you would end up with a 2X4 with strings.
Here is our photo journal of how we changed the acoustic guitar bridge.
Here is the seagull fresh onto the bench. The strings must be taken off.
The saddle must be taken out. It appears there was a shim underneath to lift up the saddle in the bridge a little bit.
First the saddle plates are made on the cnc
Then the saddles are routed out.
Here is one of the saddles put together
The plan in making this acoustic guitar bridge retrofit jig is to make a plate which will hold a router so that a channel can be made in the right place. The top plate makes the shape needed available
Here it is tested out on a piece of scrap wood. And it works.
Here is what it will look like
The mounts to hold the guitar are installed. The jig is complete, it’s ready to use.
Foam is placed on the guitar to protect the surface of the guitar.
Placed on the guitar. The side mounts are adjustable and squeeze the outside of the guitar providing a sturdy surface to route on.
We’ve got to hold the guitar down, so elastic bands are used because they won’t scratch the finish.
The pegs are drilled out and filled in.
The router in action
Here it is after the first pass. It’s always a good idea to go slow.
The saddle plates fit like a charm
The new pin holes are drilled
It is complete
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