Can we discuss intonation/compensation?

I'm on the tail end of my very first build from a kit and I have some question on intonation/compensation. Some of you may have seen my thread: Build Thread: StewMac Tenor Ukulele Kit

There has been some awesome UU members giving me tips and advice along the way, and now I've encountered the hurdle of locating the bridge. Here's what I'm unsure about:

The stewmac product page lists the scale length as 17-3/32" (17.09375"), however, the instructions state to place the bridge with the center of the saddle at exactly 17-3/32". Furthermore, when I use the fret calculator on the stewmac site and enter 17.09375 as the scale length with 19th frets on an ukulele, it tells me to place the bridge at 17.19460".

So I'm real confused.
I'm a gigging musician and hope to use this instrument at gigs. I have some ukes I've purchased with real bad intonation, so I want to get this right. Can someone give me advice on how to make sense of this?

Thanks so much in advance!

There may be a confusion between 'scale length' and 'string length'. To get the scale length, double the distance between the face of the nut and the 12th fret. Run that figure in the Stewmac calculator and see if the actual frets line up with the figures you get. I'd hazard a guess that the scale length is 17 inches, and then 3/32 would be a decent compensation, resulting in a string length of 17 3/32. I am guessing a bit since I haven't seen this kit or read the instructions. But knowing you from your thread you will persevere and make an awesome uke with perfect intonation.

One more thing. What action do you prefer at the 12th? If you're a hard strummer with 3.5 mm (note the shift of units) you might need more compensation than I do, I'm at wimpy 2 mm and play softly. No matter what I say about being hard ass.

Measure the fretboard from the face of the nut to the center of the 12th fret, double that measurement and add .125". Make that measurement the center of the saddle slot.

Sven, thank you so much for your continued help, you have no idea how grateful I am.

I actually already tried to plug in the 17" scale length into the stewmac calculator only because my measurement on the uke showed the 12th fret dead center on 8.5" from the nut edge of the fretboard. The stewmac calculator says that the bridge should be placed at 17.1003" +/-0.030...which sits right around 17-3/32 (17.09375)" if you subtract the tolerance shift. The engineer in me really would like to understand how the compensation distance is calculated, but from quick research, it's not so straight forward (i.e. there are too many variables to consider). With that being said, it makes me question the accuracy of kit instructions.

For action, I'm a bit weird because prefer it actually to be right at around 2.25mm - 2.5mm (usually measure to 0.09"). I say I'm weird because I'm a heavy stummer and fire picker! Haha! Just by natural development over the past 15 years, when I add intensity to my strum, I slightly change my angle of attack to accommodate it. But, when I start ripping out the strings during solos, I end up hearing slight buzzing and that signals me to let up a bit.

I'm still trying to think this through, for some reason, I'm really not comfortable just placing the bridge where someone recommends it. Again, the engineer in me is requiring a justification or proof of how to calculate the location. Sometimes its a blessing, but some view it as a curse! Haha!

The calculation is an estimation based on some average for string stiffness and maybe mass. FWIW, the reason you add to the scale length is because the stiffness of the string essentially keeps it from vibrating for a certain distance from the nut and saddle so the vibrating length of string is shorter than the measured distance from nut to saddle. Stiffer strings have a longer segment that doesn't vibrate and therefore they need more compensation.

The calculator should get you close. One thing I've found is that different brands and gauges of strings can make a relatively large difference in compensation. The difference between D'Adarrio Pro Arte's and Worth's can be greater than the width of most saddles. If you want to experiment with different strings, I'd suggest using a wide saddle to make sure you have room to compensate.

Some people will glue an un-slotted bridge on in the calculated position, string up the instrument with a floating saddle, move it around until the intonation is good, mark the spot, then rout for the saddle. I probably wouldn't recommend that to an inexperienced builder but it's a good way to get it dialed in for a specific set of strings.

Hope this helps.

Kent, thank you so much for this information. I posted this in my build thread, but I'll be doing something similar to your suggestion of locating after finding the correct intonation. One thing that makes this easy is I only use Aquila's, it's my preference. I'll dial it in with Aquila's and hopefully have it right forever.

An excerpt from my build thread:

I've decided to locate the bridge using a more effective method. I'll be attempting tip from Dan Erlewine and making a similar jig like this:

This jig allows you to string up the instrument and move the bridge forward or back to find the correct intonation. Considering this is how I adjust saddles of an electric guitar to set intonation correctly, this is how I plan to go about this.

Again, this will be my first attempt at locating a bridge for an acoustic instrument, so if anyone has any advice or suggestions, I'd be so grateful!

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Joe, I don't know if you have heard of this website but it is a treasure trove of information.http://www.ukuleles.com/HouseKeeping/sitemap.html
He talks about compensation there and since you are an engineer you might like his book "Left Brain Lutherie". I have it, read it several times, still scratching my head.

It is possible that the scale (of the fingerboard) is 17" and they have added 3/32 in the plans for compensation without mentioning it.????
3/32" is closeish to correct.

That was my thought as well. But please make your jig and let us know how it turns out! I'd bet a pint of Adnams that the C string will need some more.

Measure from the the nut end of the fret board to the centre of the 12th fret slot. If it's spot on to 8.5 inches then the scale length is 17 inches. So they have given you the compensated length to use to the centre of the saddle slot.

Beau Hannam Ukuleles said:

It is possible that the scale (of the fingerboard) is 17" and they have added 3/32 in the plans for compensation without mentioning it.????
3/32" is closeish to correct.

Click to expand...

That's what I was thinking too. 17" is a very standard scale for tenor ukes and the fret calculator at Stew Mac says 0.1" compensation. You have +/- 1/32" on the saddle so that makes sense
I don't measure from the nut to the saddle anymore though. (burned by a bad ruler) I butt a ruler or straight stick against the nut and make a mark at the middle of the 12th fret. Then I add the compensation with calipers. Then move the end to the middle of the 12th fret and use the compensated mark to show where the saddle will be.

Wow, what timing, this article was posted on the AGF a couple of days ago:
Nigel Forster on Intonation