Low frequencies travel further… image source
Choosing Recording Distance
Let’s take a simple real life situation where a bass player plays an acoustic bass in front of an audience. And now suppose you want to record the sound of that bass. At what distance would you place the microphone or pickups? The bass player in this example is so close to the instrument that he /she will hear each little scratchy friction sound of the fingers on the strings, while the audience 15 meters away will probably not hear these little sounds (BTW: I can imagine a lot of bass players do not even know what their own bass sounds like from the audience perspective).
So when it comes to recording and you can choose between a microphone 15 metres away and one up close, which recording represents the ‘real’ acoustic bass sound? Well, they both represent the real bass and none of them do. Meaning for each position this is what apparently, the bass (that is real) sounds like, there isn’t one single ‘real’ bass sound… or… is there…?
The point is, when you play an acoustic bass and you want to record and/or amplify the bass, you introduce the problem of choice; do I record the close-by sound, the far away sound..? How to solve this?
Just for thought experiments sake, what if you could amplify the sound of the acoustic bass to the exact same volume that the acoustic bass has naturally. What would then happen to ‘hearing each little scratchy friction sound of the fingers on the strings’ as you move further away from the speaker? Wouldn’t the difference between the sound close to the speaker (‘the sound the player hears’) and the sound the audience 15 metres away hears not be just the same as with an acoustic bass played acoustically? Well yes, I guess the difference in the ability to hear the high scratchy finger sounds would be more or less the same for a speaker as for the instrument itself.
Towards an answer…
So to answer the question, where to place the microphone for recording? Well, of course it depends on what you want. For instance, do you want to record the bass or do you want to record the performance…? What I want, is to place the pickups or microphone to get a sound that is as close as possible to a real acoustic bass. To achieve the effect of ‘real’, of ‘it is just like the instrument is here in the room with me’ I think the best choice would be to record at the source, which is at the instrument. Because that way when you walk through the room away from or towards the speaker, the diference in ‘hearing each little scratchy friction sound of the fingers on the strings’ as you move through the room, would feel the most ‘natural’; this method would therefore be my choice for getting as close as possible to a real acoustic bass sound ‘experience’.
So, I’ve narrowed it down to ‘record at the source, at the instrument’. But where on the instrument do you place the pickups or microphone and why? Hmmm.. the answer to that is a lot more complicated than this first part was…
Projection within the violin family
The body of the bass, especially the top plate, will vibrate, resonate in certain patterns. These patterns are called Chladni patterns. If you don’t know what these are, I advice you to first read my article on Chladni patterns. These patterns occur at the resonance frequencies that are tuned in by the luthier to give the bass its character. What actually happens when such a pattern emerges, is that the plate gets divided into a number of areas.
Here I have a short video of some patterns that form in a violin plate. A violin has more or less the same shape and therefore the same patterns appear as in its big brother, the acoustic upright bass:
What actually happens here, is that while one area moves outward, the other moves inward; the areas vibrate in opposite phase. The lines where the tealeaves collect, are the lines where the plate does not move inward or outward, it is a nodal line and it marks the borders of the areas that vibrate.
Now this complicates the ‘ideal’ pickup placement on the bass al lot!
The closer you get…
As can be seen in the video, a violin shaped plate (an acoustic upright bass has about the same shape) will have areas moving inward or outward. As you stand away from this violin plate, say 10 metres, you will hear more or less the same volume for a particular pattern when you step 1 metre to your left or right. But the closer you get to the plate, the more you will hear just one particular spot of the topplate. So when you want to place a pickup on the plate, it really matters a lot if the pickup is placed on a nodal line of a pattern – where a particular resonance is barely recorded because at the nodal line there is minimal vibration, or at an antinode – where the plate moves inward and outward at maximum amplitude. When the pickup is placed at a nodal line, the particular resonance that is in the instrument will hardly be present in the recording, on the other hand, when the pickup is placed on an antinode, the particular resonance might be ‘too present’.
The ‘Net’ result
So, as the image with the + and – signs that indicates the areas moving inwards/outwards shows; the plate is divided into areas that vibrate in opposite phase. But -as you might think… + plus – equals… zero? Yes. The plate pushes air forwards at one area while it pulls air backwards at another, the net result would be zero. And this is true, theoretically. If you were in a special laboratory setting of an echo-free room and you would place a microphone on exactly the right spot where the areas cancel each other, you would not (hardly) record sound. In practice this does not happen, you will always hear something, but this doesn’t mean the effect does not exist…
The ‘pattern’ at which the whole area of the plate moves inward or outward (called the ‘ring mode’) is by far the loudest pattern of an acoustic bass (and violin). If a pattern divides the plate into 3 areas where 2 move in opposite phase of the other, the net result can also be quite high. But for patterns where just as much area moves inward as outward, the net result is a lot lower than for the ‘ring mode’.
And of course, the more the pattern divides the plate, the more area of the plate is taken up by nodal lines at which the plate vibration has a minimal amplitude. So modes at which the plate is divided into more areas have less volume anyway.
When it comes to pickup placement, this means that you would want a mix of the different volumes of all (plate) modes in the instrument, where that mix represents an average as if you were standing at a distance. But… the pickup is not placed a distance, at least, in the opening of the article I narrowed it down to placement at the instrument itself. Because placing a microphone at a distance would again introduce other issues.
Pickup placement under the bridgefeet?
The bridge carries the vibrational energy from the strings to the body. The feet of the bridge rest on the body right at the center, between the F-holes. What might be immediately clear to you when seeing the picture with the bridge position drawn in, is that the violin plate in the video and at these pictures, is not tuned. The bridgefeet rest on nodal lines of several patterns, which is the worst place for driving the plate into motion. The only mode that will be driven into motion by the bridge quite efficiently, is the ‘happy face’ mode.
But then again. Finding the perfect spots for the pickups to get an ideal mix of all modes is virtually impossible, because the nodal line of one pattern may coincide with the maximum amplitude of another mode. So the answer to the question ‘where to place the pickups on an acoustic upright?’ I give you might be disappointing to you; I do not think there is an answer to this puzzle. That said, don’t despair; most bassists find a solution that is satisfying to them.
You might think, well, what if I give each pattern its own pickup? Still, you would have the big ‘ring mode’ where the whole plate area moves inwards and outwards influence the recording of the individual patterns. So again, I’m sorry, although I think that under normal conditions placing the pickups under te bridge would provide the best solution to record an acoustic upright bass, because that is so to say ‘the most important point on the instrument’, I have no answer for an optimal recording of the bass. For an acoustic upright bass that is. For an EUB however… Well… 😉
What about microphones placed on the instrument?
Well, I have a few comments on that:
By placing a microphone on the instrument you have essentially the same problem as illustrated above. The microphone is so close to the topplate, that you only record a small section of the topplate. So for instance, if the microphone is placed between an f hole and the bridge, then the belly of the instrument, where the maximum amplitude of the instrument is most likely to occur*, will not be represented at the volume it really has (* compare a string, the most likely place for the maximum amplitude to occur is at the middle).
The position where the microphone is attached to the instrument also vibrates. The consequence of this, is that the microphone gives a relative value from the vibrating microphone to the vibrating air it is recording. This may cause phase problems that distort the character of your sound. And beware; the microphone itself -being a vibrating mass- can also have resonances.
What about the f-holes, isn’t this where the sound comes from? hmmm… No, not really, I wouldn’t say the sound of the acoustic bass ‘comes from the f-holes’. If you were to close them up with tape you would hardly notice any difference in volume – if any. That is because it is the large surface of the body, it is the vibrating wood that pumps a column of air into motion; inside and outside the bass. In fact, the air pressure wave coming out of the f holes might even be out of phase with the topplate, which would mean it substracts from the overall volume.
That said, there is a factor that might be interesting for microphone placement… The bodyvolume of the bass, the air inside the bass, can resonate also. Like when singing in the bathroom, or blowing over a bottle; these are air cavity resonances. The air inside the bass’ body acts like a spring and the pressure fluctuations of the air cavity resonances can be picked up with a microphone at the f holes.
Without getting into theory any deeper (because I guess I am already stretching it quite a lot here), these air cavity resonances have ‘modes’ also. This means the air in the body also organizes into patterns, which have nodes and antinodes. At the f holes, you wil find high amplitude spots of some of these air resonances, especially the low ones; so placing a microphone near an f hole, can give your overall sound more bottom, a fuller sound.
If you only use pickups that are clamped under the bridge don’t worry, you do not ‘miss’ these air cavity resonances, because the resonating air is pushing and pulling ‘the walls’ of the body into motion, so these resonances are already merged with the vibrations /deformations in the topplate.
The ideal situation would be to catch all the resonances, all the patterns that could be present in the instrument, at the volume as they really are. And for that a microphone at a distance would be a good idea. So if you can record your acoustic upright bass in good controlled studio situations, I would definitely choose that option. For less optimal situations (environmental sounds, stage) I prefer the modern piezofilm pickups that are clamped between bridgefeet and body.
All in all this is to illustrate the quite impossible task of recording the acoustic upright bass. It comes down to finding the right mix of resonances in the instrument, while you know this is impossible. And If you’ve found the right mix, cross your fingers and hope the sound engineer who sees an acoustic upright and immediately thinks FEEDBACK!!! doesn’t top of the resonance frequencies that give the instrument its character.