All Sirena instruments are built around these Bruce Johnson designed truss rods that we build here in the shop. The story goes that sometime in the 90’s, Bruce got hired on by SWR to develop The Workingman’s Bass with a dream team of engineers and designers. The bass was really cool, they built a few prototypes, but SWR just couldn’t bring the bass to production affordably.

Part of Bruce’s job was to develop this double-action compression truss rod. The rod is made of stainless steel, while the head and anchor assembly are made of aluminum and brass. The allen adjuster screw is made of stainless steel. The installation process involves embedding the truss rod into the neck with epoxy, creating one solid piece. These truss rods are made to be really strong and reliable.

What is the advantage of this truss rod and how does it affect the sound of the bass? From my experience, the necks I’ve built using these rods have felt very stiff and are very sensitive to playing dynamics. Additionally, the basses ring with tonal clarity.

When I asked Bruce about his truss rod design, this is what he told me:

A well designed truss rod can make a real, noticeable difference in the overall sound, through the amp, of an electric instrument.The physics of why are about changing the way the neck bends under load. A single-rod (“compression” type) truss rod, set back deep into the neck, adds a stiff tension strap down the back of the neck. It’s as if you had added a steel rod and a turnbuckle down the back of the neck from the headstock to the heel. Pulling the strings up to tension tries to bend the neck forward, and the truss rod goes into tension. The steel is stiff, so it doesn’t stretch much. The bending load forces the wood in the forward part of the neck (like the fingerboard) to compress. So, as the string is plucked and cycles back and forth, the small flexing of the neck is happening with the back of the neck stiff, and the wood in the front of the neck cycling in compression.

In comparison, a double-rod truss rod doesn’t act like a tension strap at all. It’s basically free-floating inside the neck, running approximately down the centerline. When you tighten it, it pushes straight back at the two ends and forward in the center. That’s how it counteracts the bending from the string load, and adjusts the relief. But it doesn’t work at all like a tension strap. It isn’t capable of restricting the stretching of the wood on the back of the neck. Now, in cross section, there’s more wood in the front of the neck than in the back. So, when the neck flexes under load, almost all of the deflection is in the stretching of the wood in the back of the neck.

That’s the big difference:
In a neck with a single-rod truss rod, the deflection is happening by compressing the wood in the front of the neck.
In a neck with a double-rod truss rod, the deflection is happening by stretching the wood at the back of the neck.

Here’s how it affects the sound: When wood is stretched or compressed, it also provides damping. That is, it acts like a car’s shock absorber, dragging and providing resistance as it moves. But, one of the interesting characteristics of wood is that there’s much more damping effect in compression than there is in tension. It’s like 4 to 1. The effect of this damping is that it resists the cyclic flexing and causes background frequency spikes on the string. That’s the warmth and richness that fills out the sound of some instruments.

Normally, there’s a big trade-off in how stiff the neck is built. A neck that’s built really stiff, with hard wood, laminations, graphite, etc., will give the bass plenty of clarity and sustain. It flexes very little as the strings cycle, so it isn’t pulling energy out of the strings, or adding in background frequencies. Build the neck soft and more flexible, and it flexes more as the string cycles. Less sustain, and a warmer, mushier tone. Particularly the bottom end. Usually, you have to choose: Clarity and a cold sound, or warmth and mush.

My work with the single-rod truss rods is to cheat that trade-off. The tension strap down the back of the neck provides the clarity and sustain that you’d get from a really stiff neck, while allowing the front of the neck to cycle in compression, bringing out the warmth and richness that you’d expect from a much softer neck.

That’s what it’s all about. Careful design of the truss rod and the neck’s internal structure allows for a better combination in how the neck reacts to the movement of the string. Clarity in the bottom end, combined with warmth in the midrange.

– Bruce Johnson