One structurally and sonically very important feature of violins, violas and cellos which is hidden on the inside of each instruments box is the bassbar. A bar, traditionally of the same material as the front, is glued to the inside of the belly, placed somewhat parallel to the centre joint but outwards crossing the bass foot of the bridge. It counteracts the downward pressure from the strings through the bridge and is an important acoustic support opposite the soundpost. Material choice, placement, fit and its shape are critical to achieve the best tonal outcome of an instrument.
Historical bassbars are rarely found in their original place, we are lucky that some have been carefully removed and being kept in museums or with their instrument. They look rather different from a typical modern bar, a lot shorter and lower sometimes made of slab cut spruce.
I will show a few photos of how I install a bass bar in one of my violins.
This is a finished Stradivari model violin front, the next step is to fit a bassbar. I use light spruce (with a density of about 0,38) perfectly split and with medium grain, my preferred thickness is 5,5mm.
I start the process by finding my desired position, depending how much I want to stiffen up the front I can alter the inclination. Traditionally I determine the length of the bar by marking 4cm from each end of the violin plate. I than divide the upper and lower bout width by 7 and use the first mark away from the centre joint for a rough guide. Most important placement guide is the bridgefoot, I want the foot to outreach the bar by 1 to 1,5mm.
Once I have marked the position I use a chisel to roughly fit the bar to the shape of the front.
This is a frame I like to use- the belly plate is kept in a flat position while fitting and glueing the bar. I want to fit the bar to the belly and not the belly to the bar which would happen without the frame as the front is more flexible than the bar. Without useing a frame there is a risk of deforming the long arch when the bar is glued in.
Here you can see the bar in the fitting process, the profile is roughly cut down.
Once perfect fit is achieved I use hide glue as reversible glue and quite a few clamps to spread the forces as evenly as possible.
After a drying time of 8 hours plus I take off the clamps and shape the bars profile. Flexing the plate with my hands during the shaping process tells me how stiff the structure still is and where to take off more wood. Here it is nearly finished, the ends still need to be cut down in a slope.