Wednesday, May 28, 2014

Bass Guitar Scale Length, String Gauge, and Tuning

Back in March I wrote about wanting to shift from guitar to bass. Since then I went ahead and made the jump, picking up a beautiful, used Yamaha bass at a ridiculously low price given the materials and workmanship. (For more on why I chose a used instrument, read my Eco Guitar Ponderings post from back in 2010). The choice of a used instrument was easy compared to the back-and-forth thinking that I did about whether to get a standard 34" scale bass or to opt instead for either a medium scale (32") or short scale (30") bass to give my aching wrists a break.

I've written a few times before on this blog about guitar scale length and its relation to pickup placement and fret placement, tuning, and intonation. It's a topic that I'm pretty comfortable with in terms of the underlying physics and how they make the instrument sound and play. I've also gone through a couple of months in the past when I was unable to play much because of shoulder and wrist pain and had considered switching to a travel guitar to take some stress off of my wrists. So it's not like I am the sort of person who is going to choose the longer scale bass simply because I was convinced that the longer scale is inherently better or more "authentic" than a short scale bass. Indeed, the first bass I looked at was a short scale Hagstrom 8-string to match my Hagstrom Swede. And had I done that, I'm sure I would have loved it. But I didn't. When the time came to commit, I chose a 34" scale RBX instead.

Why? The answer to the scale question bears some examination and, like the previous entries about scale length, involved looking up a lot of charts and comparing the numbers. (Thank goodness that D'Addario does an excellent job of giving their players as much information about string gauge, scale length and tension as they do). I'll try to keep the numbers to a minimum and make the explanations as short and clear as I am able.

Scale Length Basics

As noted before:

...a guitar's scale length is the length of the string from where it leaves the nut towards the fingerboard to the place where the string crosses the bridge saddle. This distance is (more or less) the same as the "speaking length" of the string -- the part that vibrates when the string is plucked. The strings speaking length and gauge form the basis for all the math that goes into acoustics that I'm mostly going to ignore here because we don't need all that math to get a basic understanding. Just wrap your head around this one thing:

If two guitars have different scale lengths but are using the same gauge of string, the one with the longer scale length will need more tension on the string to reach the same pitch as the one with the shorter scale length.

Put another way, comparing the same string on two guitars:

Longer string - same gauge - same tension = lower pitch

Same length string - heavier gauge (thicker string) - same tension = lower pitch

Same length string - same gauge - lower tension = lower pitch

Therefore: longer string - heavier gauge - same tension = even lower pitch

As you can see the pitch is determined by all three things (length, gauge, tension) and not by any one of the three alone.

If we assume, therefore, that any competently built bass will allow us to alter string tension to accommodate the tuning requirements of the other two factors, and we assume that we are discussing conventionally fretted basses of short/medium/long scale, then we can control our tuning through a combination of string gauge and speaking length. In practical terms this means that in order to maintain a consistent level of string tension across a range of tunings, the shorter scale instrument will need to use heavier gauge strings than will a longer scale bass. In other words, a short scale bass strung with a standard set of 105s and tuned to EADG will have less tension on those strings than will a long scale bass with strings of the same gauge.

Now what if you want to tune lower? Assuming we want to keep a similar tension on the strings to keep the playability, neck adjustment, and overall sound quality as close as possible to the standard tuned version, we'd need to use strings of a heavier gauge. Comparing tension charts for D'Addario Nickel Rounds as a control standard since they are widely available and come in a fairly representative range of choices.

Changing Pitch - Gauge and Scale

Since a 34" scale is the most widely used scale, we'll start there. A .105 gauge Nickel Round tuned to E has 40.3 lbs of tension on it. Tuning it down to D drops the tension to 32 (around the same tension as a 30" scale bass tuned to E) and C drops it to 25.4 lbs. In order to keep the tension on the string consistent as the pitch drops, one would have to gauge up to .125 (40.3) for D and .135 (40.4) for C. On the other hand, if one doesn't mind, or prefers the string tension of the short scale bass, then the .125 can be tuned down to C (32.0) without the string becoming too floppy. We could tune even lower using these principles, but the general idea remains the same.

As noted in the parentheses above, the other way to approach this question of pitch is from the direction of scale length using strings of identical gauge. From this perspective, as noted above, a bass with a 34" scale strung with a .105 gauge Nickel Round tuned to E has 40.3 lbs of tension on it. In comparison, a bass with a 32" scale strung with a .105 gauge Nickel Round tuned to E has only 35.7 lbs. of tension on it and a a bass with a 30" scale strung with a .105 gauge Nickel Round tuned to E has 34.0 lbs of tension on it. Tuning the same 32" and 30" basses to D reduces the string tension to 28.3 and 27.0 lbs. respectively, and to drop the tuning to C the tension on each string would need to be reduced to 22.5 lbs. on the medium scale bass and 21.4 lbs on the short scale bass. That's almost a 20 pound difference in tension from a long scale base tuned to E and a short scale bass tuned to C and that difference in tension is going to translate into a very different sound and feel. Evaluating the Possibilities

What all this demonstrates is that there is a wide range of possibilities to be considered before choosing. That does not, however, mean that all these possibilities are equal across all the examples. To illustrate the differences we need to think in terms of specific goals we have for our bass guitars and the ways in which we can achieve them with different instruments. The single most limiting and expensive choice involved in this conversation is the choice of the bass scale. Changing string gauge is relatively inexpensive in comparison, requiring only the purchase of new strings and perhaps a bit of attention to or replacement of the nut to accommodate the change in string diameter. Given this, scale is an important consideration.

Before we settle into the topic of scale, however, we need to consider that scale only measures the maximum speaking length of the string. Fretting a string creates a new speaking length, effectively shortening the scale of the instrument for as long as the string is fretted. What this means for our comparison of short, medium and long scale instruments is that a 34" long scale bass can be made to feel and function very much like a medium or short scale bass by placing a capo at the first (32.1") or second (30.3") fret respectively and a medium scale bass can function much like a short scale bass with the addition of a capo at the first fret (30.2"). Both these compromises do affect the overall playing experience a bit: it reduces the number of playable frets by one or two and it changes which fret acts as the octave fret, making the position markers on the neck misleading and problematic. (I suppose the latter problem might be relieved somewhat by installing vinyl inlay stickers on your instrument by way of compromise...). Otherwise, though, this accomplishes many of the same things that make people search for a short scale bass. It brings the (relative) first fret up to 3.75" closer and reduces the stretch required to span the lower notes in the lower and open positions. The only thing it does not do is reduce the overall length and weight of the instrument, but even compared within scales, some instruments are lighter and shorter than others depending on bridge placement, body size and headstock shape and configuration.

Looking the other direction, aiming at a shorter scale and relying on changing string gauge to allow for lower tunings, we begin to see greater limitations for the short scale instrument. The largest gauge offered in a short or medium scale Nickle Round string set is the .105, which means that in order to gauge up, you would need to cut your strings down (and risk ruining the string, according to many pro bass techs), or go in search of alternative strings. This often translates into much more limited availability, longer out-of-stock waits, and thus the necessity of ordering extras to have on hand for emergencies. It's not a problem that cannot be overcome, but it is less convenient than the alternative.

The same laws of supply and demand, (or is that "economy of scale"?), that make larger gauges more available for the long scale bassist who wants to tune down by gauging up work to make it possible to use both "cheats" at the same time. Gauging up and using a capo means being able to use the thicker strings to tune down and then capo. A 34" bass with a .135 can be tuned to C and capoed at the second fret to function like a 30" bass that is tuned down to D with the same string tension as a 34" instrument tuned to E.

Which is why I ended up choosing a long scale base in the long run. It gives me the widest range of possible strings while also leaving open the possibility of making it play like a shorter scale instrument should that need arise.