Unless you have had a chance to examine a double bass up close you might not be aware that acoustic basses have what is called a sound post (soundpost). Perhaps you do not know the correct setup or what it does? It is really important that you do have an understanding, as to what this rather insignificant looking stick of wood is – your bass will not potentially ‘work’ or sound at all good if it is not set-up properly.
Just to clarify the sound post is sometimes called the ‘truss rod,’ but it is the same thing. A sound post is a piece of doweling wood often made from seasoned spruce which is held in place, under the tension produced by the strings and the bridge. A peek through the f-holes on the bass and you’ll see the post vertically placed between the front and back of the bass body. It is not glued in place – it is only held by tension. This tension-hold means if you slacken off your strings too much the rod will lose its position or fall out.
What does a Sound Post do?
Vibrations caused when strings are plucked or bowed travel through the bridge of the instrument through the body and down the sound post. The sound of the deeper strings are said to resonate more in the body cavity of the bass, the higher frequency, higher notes resonate more through the sound post. This is one reason for positioning the sound post under the high string. All instruments in the violin family have sound posts. The quality of the wood used in making the sound post will in part determine the quality of sound of the bass. If you have a choice of sound posts, buy one which is made of quality wood such as spruce. The wood needs to be around 18 mm in diameter for a full-size bass and smaller for smaller sizes, it needs to be straight, with a straight wood grain. This straight grain helps transmit the sound vibrations. A poor quality sound post or one that is incorrectly placed, or not correctly situated against the bass can literally make the instrument sound terrible.
Where should a Sound Post on a Double Bass be?
Bass strings under tension exert somewhere between 60-80 lbs worth of pressure on the bass, the force varies depending on the string gauge and type. As mentioned before, it is this tension which is keeping the post in place. To ensure that the bass is supported and doesn’t buckle over time, a bass bar is fitted on the underside of the front panel on the bass. The bar runs vertically along the length of the bass distributing the tension along the top of the instrument. It also acts as a channel to help distribute the vibration and therefore sound through the bass cavity. The bridge foot that is supporting the heavier strings is aligned over the bass bar. For the higher frequency lighter strings the bridge food is aligned over the sound post in the bass body.
The set-up of the bridge directly affects the positioning of the sound post. Therefore when looking at the siting of the sound post ensure first that the bridge set-up is as it should be.
The bridge needs to be very slightly leaned towards the tail of the bass. So it is just less than 90 degrees on the vertical, but only by a few degrees. Usually, the bridge should be positioned in alignment with the notches on the ‘F’ holes. I say usually, on my own bass this is not the case, it is above the notches, this set-up is needed for my preferred bass sound. The final instrument set-up will be determined by the bass construction, the strings used, how it is played, etc.
Ideal Sound Post Position Diagram
Check out Edgar Russ on Youtube, whilst I have had lots of experience with ‘adjusting’ the bridge and sound post on my bass over the years, Edgar is a master violin maker and has some amazing videos on Youtube, he is the real expert. (measurements given above taken from one of Edgar’s videos.)
The diagram above shows the position of the bridge ‘feet’ on the outside of the bass in relation to where the sound post will sit. On the left, you will see how the left foot of the bridge is aligned over the bass bar. On the right, you will see that the sound post is positioned at equal distance to ‘A.’ So if the foot of the bridge is 4mm to the right of the bass bar then the sound post needs to be 4mm to the left. The grain on the front of the bass will run vertically so the ideal situation is for the grain on the sound post to run horizontally, in opposition.
The vertical alignment of the sound (B) on the diagram is determined by the thickness of the front body wood on your particular bass. Each bass has a different top wood thickness. If the wood is 4 mm then the gap B should be 4mm, if the wood thickness is 5mm then B should be 5mm, and so on.
This position is the ideal starting position for the sound post position if the post is moved 1mm the sound will change. After you have established the ideal starting point you can then move the sound post to adjust the sound to your liking.
If you like the sound of your bass but believe the sound post is not where it should be, do not start messing with its position, if you are happy with what you have got leave it where it is, the ideal sound post position is what sounds best for your instrument. I have experimented over the years with the set-up on my bass and it is not necessarily the same as the next bass, they are all individual. Fitting a sound post is not a simple task, you could be messing with strings, the bridge and the sound post for days. Proceed if you have to or with caution.
Prevention is Better than Cure – Avoid Knocking the Bridge or Sound Post
Prevention is better than cure and it is important not to slacken off the strings too much in the first place, and leave well alone if you are happy with the bass sound. I can’t say that enough. When you are travelling protect the bridge at all cost, to prevent it from being knocked out of position. If the bass goes into the hold of an aircraft slacken the strings only slightly, the cold of the aircraft hold will contract the wood, thereby loosening the tension naturally on the bridge.
Hire or buy a good quality flight case to use with the bass in this instance. It is for this reason and cost that most double bassists will prefer to hire or borrow a bass in-country than to go through the hassle of flying with their double bass.
It is a good idea to ensure that any ‘new’ road crew are aware of the importance of protecting the bridge on the instrument and what the consequence is to the sound if it is knocked out of position.
What to Do if The Sound Post Slips – Help!
If you do not have the confidence or desire to do this yourself look up a string instrument repair specialist and save yourself some hours worth of struggle. I could write about my own struggles with recovering and installing a sound post (successfully) but instead, this video would work much better as an explanation. You can buy a special tool called a sound post setter for double bass. If you do not live close to a Luther, it is probably a good idea to have one in your kit bag for the unfortunate time when the sound post becomes dislodged.
The sound post set-up is integral to getting a good sound from the bass. If you are happy with your bass sound do not adjust the sound post. You can spend days adjusting the bridge and sound post. If you are faced with an emergency situation you can use a combination of strings and coat hanger rods to retrieve and reset the rod, it is not a fun task especially under pressure of any kind. A classical musician who relies on almost extreme measures of accuracy will probably defer this job automatically to a Luther. I have done this myself the hard way and it did take hours. I later found out about sound post setting tool. There are some Youtube tutorials on how to do a DIY repair yourself. I won’t duplicate those here, if desperate you can do what I did, and set the post yourself and warn those around you about the potential swearing.
The video below really just shows the skill that goes into setting up the bridge and sound post but also give an almost anatomical understanding of how the bass fits together.
References & Further Reading
The Conservation, Restoration, and Repair of Stringed Instruments and Their Bows, Volume 1 by Tom Wilder