Official Luthiers Forum!

I've been reading some very interesting posts in the archives, and I was wondering if any of you tune the so-called "neck mode" on your guitars? For the record (if I remember mr. Carruth's explanation correctly), the neck mode is the resonant mode where the entire guitar vibrates almost like a xylophone bar. It is my impression that tuning the neck mode is not common practice. Why is this? Is the effect too small to be worth the effort?

I checked the resonant frequency of the neck mode on two of my factory guitars, one 14-fret steel string OM and one flamenco. On the steel string, the frequency was far below the playing range of the instrument (a little over Bb 58Hz). On the flamenco, however, it was close to F# 92,5Hz (F# on the low E-string). On the flamenco, I think I can hear a small volume peak at the F#, but it is FAR weaker than the main air resonance of the box, which is at a little over G# 104Hz. I also tried tuning the E-string down to Bb on the steel string, and again, I think I heard a very small increase in volume when I hit the note, but when I tuned the string further down, approaching G 49Hz (one octave below the main air resonance), the increase was much stronger. So, to me it would appear that on its own, the main resonant frequency of the neck mode does not make a big difference... or..?

I believe I also read that if the frequencies of the neck mode and the main air mode of the box are close (but not equal) in pitch, they can "couple" and produce a single broad volume peak instead of two separate sharper peaks (someone please correct me if I'm wrong at this). How close do they need to be in order to couple effectively?

When I hold the steel string OM by the nut, put my ear to the SOUNDHOLE and bump the tailblock area with my palm, I hear only the resonance pitch of the NECK MODE. When I do the same with the flamenco, I hear the resonances of BOTH the neck mode AND the main air mode, alternating in a pulsating manner. Does this mean that they are coupling? They are about 2,5 semitones apart.

Thanks for your time!

The neck mode by itself can't radiate any appreciable amount of sound. At that frequency the wave length of sound in air is 'way too long to be produced effectively by something as small as the headstock, which is the part that's moving the most.

You are getting some coupling of the 'neck' and 'main ai' modes on your Flamenco; as you say, you can hear them both when you tap the guitar, and can hear the beat frequency as they pass energy back and forth. The main problem is that they are still too far apart of couple really effectively, so you're not getting much benefit. you could try blocking part of the soundhole to drop the 'main air' pitch, and see what that deos to the sound.

It's a bit hard to 'tune' the neck mode. About the only variables you have to work with are the neck depth and the headstock weight. If you finish after assembly you can leave the neck a bit on the deep side, and carve it to tune the mode. A tapered neck, that is deeper at the body end, than at the nut helps keep the neck mode high. CF reinforcement in a classical neck helps, too, if you keep the carbon rod low in the neck.

Swapping out tuning machines can have an effect as well. I find that it's hard to get the neck mode high enough on steel strings, especially with enclosed tuners, but the open 'vintage' types are a lot lighter. You can save the weight of two tuners with enclosed machines by changing the metal buttons for wood.

The following is sheer speculation on my part...

I have the impression that if the guitar has a resonance that is not driving the top plate, or exciting the air inside the body, then that resonance detracts from the sound of the guitar at those resonant frequencies. The resonant system excepts transfer of energy from the strings but is not in turn transferring energy to sound in the air. At some frequencies this may color the note, by shaping the frequency responce, at other frequencies it may detract seriously from the note.

I have a 5 string electric bass, 34" scale. The low B sounds floppy. I was talking to the guy who runs the bass department at Long and Macquade in Vancouver the other day, he said that a floppy sounding low B is a common problem, but indicated that a Ken Smith bass they had (34" scale) Had a great sounding low B string. A theory I have is that the problem with my bass and others that have floppy sounding low B strings is that the neck has a resonant frequency that is close to the low B and so a lot of energy goes into exciting that mode of vibration, sucking it away from the string. On an electric, I speculate, resonances will detract energy from the signal only, or color the note. After all the sound comes from the pickup that reads the string, not from the resonances in the wood.

BTW my bass is a fretless, with a movable bridge. I slid it back, new scale length 35". Sounds a little less floppy. Could this be because the new tighter tension on the neck is changing the resonant frequencies of the neck?

Any comments, insight much appreciated

Stefan

Al, I'm curious. Couldn't you change the neck frequency with some design changes like using a stiffer piece of wood for the neck (say maple instead of mahogany)? Or could you use a laminate neck made up of 3 or more sections to stiffen the neck as opposed to a 1 piece neck?

Stefan brings up a good point......maybe an acoustic would loose volume or sustain if the neck mode resonance falls within the range of notes produced by the guitar.

_________________
Formerly known as Adaboy.......

Alan, thanks for commenting! I tried covering part of the soundhole like you said, and I did notice an increase in the coupling effect as the pitches came closer to one another. Very interesting!

If the nodes of the neck resonant mode are typically found near the nut and somewhere across the lower bout, doesn't that mean that the antinode (which I thought was the part that moved the most) is in the middle, near the neck block, and not the headstock? In any case, if the goal is to raise the pitch of the neck mode, is it such that reducing mass and increasing stiffness is most effective if it is done as near the antinode as possible? If so, in addition to beefing up the neck near the heel, do you think it would be an idea to extend the CF rods into the headstock, and maybe stiffen up the upper bout as well? Thanks again, and sorry for all the questions.

Stefan, by increasing the scale length, the strings will have higher tension at the same pitch. Maybe that is part of the reason why the B-string sounds less floppy? Or maybe, when you changed the scale length, the bridge was moved closer to one of the nodes (stationary points in the resonant mode) of the neck mode, so that less of the string's energy is used to drive the neck mode? This should give you less acoustic volume and more sustain.

Darryl, from what I understood, lamination alone does not make the neck stiffer, only more stable. Unless of course, the lamina is stiffer than the rest of the neck material... Thanks again for the comments, folks!

The 'neck mode' can couple with the top in such a way as to pump air through the soundhole, and that's what makes it effective in changing the sound. Most guitars have a top that is arched upward to some degree. If you push inward on the top from both ends it will "puff out' a little, and suck some air in through the soundhole. Pulling on the ends flattens the top a bit, and pushes air out. When the guitar is vibrating in it's lowest bar mode the top is being pulled and pushed in this way.

Normally this doesn't move much air by itself. However, if it's close in pitch to the 'main air' resonance then it becomes much more effective. After all, that's what a resonance is; the frequency where you get the most output for a given input of power. And, of course, the air also pushes on the neck via the same mechanism. Again, off resonance, it's not very effective, but near the resonant pitch it makes more of a difference.

This sort of bar mode has three active areas, separated by two node lines. As the headstock and tailblock go 'up' the neck block goes 'down'. The innactive node lines are near the nut and across the wide part of the lower bout. If you think about it for a while, you'll see that this has to be so to conserve momentum. So, yes, the neck block area is moving a lot, but the headstock and tailblock can be moving more.

I think Stefan is right, at least about solidbodies. If you hold up a Strat, say, you'll find that you can get those bar modes moving really well if you hold near the bridge, and tap on the tail end or the headstock. The bridge is near the nodes for the lower two or three bar modes (which are the ones that are within the pitch range of the string fundamentals), and this makes it hard to drive those modes with forces exerted at the bridge. It's like trying to move a see-saw by pushing at the pivot. In trying to keep the bass as short as possible they move the bridge 'way down to the end, which is going to be an antinode for every bar mode. No wonder they end up with dead notes!

On an acoustic the neck moves too, of course, but nowhere near as much as the top, so the top is where the action is. I've looked at the lower three bar modes of a number of acoustic guitars, and found no particular relationship between the frequencies of them and the tone. There's so much going on with the top, back, and air, that most of those neck modes are swamped: part of a complex mix that's hard to sort out.