5.5 An extreme case: the banjo

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Having introduced the key ideas and heard some examples of synthesised plucked instruments, we are now equipped to do a serious case study. We will examine the particular case of the banjo: the target is to find out what makes the sound of a banjo distinctively different from other plucked-string instruments like the guitar. Even when strings, scale lengths, and pitches are chosen to be virtually identical, most listeners would agree that the sounds of banjo and guitar can be distinguished from just a few plucked notes.

The challenge is first to check whether the computer synthesis model captures this characteristic banjo sound, and then to use the model to do a range of ‘virtual adjustments’ to the banjo so that we can hear the effects directly. The aim, ultimately, is not simply to explore the banjo as such, but to find how far we can get with the use of models like this to understand what gives different plucked-string instruments their characteristic “voices”. The banjo represents an extreme: unless we can make good progress with this case, it would seem unlikely that the approach will be good enough to explore more subtle distinctions.

We have already seen, in sections 5.1–5.3, that there are large differences of vibrational behaviour between a banjo and an acoustical guitar: far bigger than the differences between guitars of a similar type, or between different banjos. We have begun to explore the physics lying behind these differences. More detail (a lot more detail) is given in two scientific journal papers [1,2]: one purpose of this section is to provide supporting sound demonstrations and discussion for these two papers. The two papers reveal that the key physical differences are directly associated with the use of a stretched membrane rather than a wooden plate as the “soundboard”. Membranes differ from plates in their mass, modal density and sound radiation properties. These factors, and more, play a role in understanding the measured vibration response.

This section will be quite detailed. If the technical material is off-putting, skip ahead to the sound examples!

A: Distribution of loss factors

Since the aim is to understand the characteristic sound of a played banjo, it is useful to look first at some experimental comparisons between notes on a banjo and a guitar, based on normal playing. The particular 5-string banjo used here is a Deering Eagle II, and the steel-string guitar is one made by Martin Woodhouse. This guitar, somewhat unusually for a steel-strung instrument, embodies a version of Torres-like fan bracing. Conveniently, the top strings of this banjo and guitar are extremely similar: both are plain steel strings with the same diameter, and they are under very similar tensions. This allows a rather clean comparison between plucked notes on the two instruments.

Plectrum-played notes were recorded at every semitone from the open string up to the 12th fret, on both top strings. Each of these notes was analysed using a spectrogram, as was described in section 2.4. A typical example is plotted in Fig. 1. The plot shows a set of narrow vertical bands, associated with the near-harmonic overtones of the string. Within the first 0.2 s or so after the pluck, the spectrogram shows a bright patch indicating a significant broad-band spread of radiated sound between the string overtones, extending up into the kHz region. These broad-band signals arise from transient excitation of modes of the coupled string-body system which have energy mainly in the body rather than in the string: we will see later in this section that these contribute directly to the characteristic sound of a banjo (see subsection E).

From a spectrogram like Fig. 1, it is straightforward to detect each vertical band representing a decaying mode, and to analyse the variation of level and phase with time in order to determine a best estimate of the frequency and decay rate. The results can be plotted as a “cloud” of points to reveal patterns in the distribution of loss factors with frequency. These clouds of measured points are plotted below: the guitar in Fig. 2, the banjo in Fig. 3. Two sets of plucked notes were recorded for each instrument, and processed independently to give an indication of consistency of the measurements: the two sets are plotted as red circles and stars, and a reassuring correspondence can be seen between the two over the important parts of both plots.

In each plot, the magenta line indicates the limit of applicability of the analysis method. Above this line the decay time becomes too short to be resolved by the spectrogram approach, and absence of points in this region does not mean that no such combinations of frequency and loss factor exist in the real instruments. Lines to indicate the decay time constant are plotted in green dashes: details are given in the caption of Fig. 2. This time constant $\tau$ is associated with an exponential decay of the sound proportional to $e^{-t/\tau}$, and it is related to the frequency and Q factor by

$$\tau=\frac{Q}{2 \pi f} . \tag{1}$$

(Greek letter $\tau$, pronounced “tau”.) The black points in these plots show estimates of the contribution to the loss factor arising only from energy flow from the string into the instrument body, calculated from the measured bridge admittance. This loss factor was derived and discussed in section 5.1.2, via a calculation of the reflection coefficient for waves on the string, hitting the bridge. The formula for the loss factor was given in eq. (8) of that section.

The plots tell an interesting story. It is simplest to explain the guitar case first, Fig. 2. The majority of the red points mark out two slightly fuzzy lines, one with loss factors of the order of $10^{-2}$ and the other of the order of $10^{-3}$. These indicate “body modes” and “string modes” respectively, as mentioned above in connection with Fig. 1. The body modes often show as clusters of many points, because in principle these modes are excited by the transient nature of every plucked note, regardless of the played pitch, so that many estimates of these modes are obtained from the chromatic scale. Strictly, each body mode is not exactly the same for every played note, because it is perturbed by coupling to the string. However, except for special cases where a string overtone falls very close to an unperturbed body mode, the shift is small. Probably the line of points for these body modes would continue approximately horizontal beyond the magenta line but for the limitations of the analysis technique.

The “string modes” consist of an approximately harmonic series based on the fundamental of each successive note, so that the plotted points are spread out along the frequency axis. The body modes have Q-factors around 100 or lower, while the string modes have Q-factors of a few thousand and their decay times determine the duration of each played note. It is very striking in Fig. 2 that the line of the string modes is fairly featureless, and mostly lies significantly above the black points. Rather unexpectedly, the decay rate of string modes in the guitar is dominated by the damping of the string itself, and loss into the body of the instrument is usually only a small perturbation. There are exceptions where the guitar body has a strong resonance, but remember that that this particular plot is confined to the frequency range relevant to the top string, and the strongest body resonances of the guitar lie lower in frequency (see section 5.3).

The plot for the banjo, Fig. 3, is strikingly different. The black points lie considerably higher over much of the frequency range, as a direct result of the higher input admittance of the banjo. There is still a trace of two lines of red points showing string modes and body modes, but whenever the black curve crosses above the position where the line of string modes occurred for the guitar, it carries the actual loss factors up with it. Energy dissipation arising from different physical mechanisms is additive, so in theory the total loss factor cannot be lower than the black curve.

In the main this expectation is borne out by the data. There are a few red points lying below the black curve, especially in the frequency range around 1 kHz, but these are probably associated with an aspect of the physics not taken into account in this simple description: each string mode can occur in two different polarisations. The description here, and the basis for the calculation of the black points, considers only the string polarisation perpendicular to the banjo membrane. Vibration in the plane parallel to the membrane is likely to couple much less strongly, and thus exhibit lower loss factors. The real plucks during the test procedure will have involved a mixture of both polarisations. Probably a few peaks associated with the second polarisation have been caught by the analysis. This would be expected even if the second polarisation is associated with initially quieter sound. It has a slower decay rate, so after a while it will dominate and be picked up by the computer analysis.

The contrast between Figs. 2 and 3 has several consequences for the behaviour of notes played on the banjo. For frequencies up to about 1 kHz, the string modes often have significantly higher damping than for the same string attached to a guitar. The decay time will be faster, and at some frequencies it will be so fast that the distinction between string modes and body modes is lost: this is flagged in the plot by the black points reaching levels comparable with the line of body modes. Any note with a fundamental or low overtone lying in one of these frequency ranges might be perceived by a player as “falling flat”: it will not ring on as much as usual. Secondly, over most of the frequency range plotted here the string modes follow the black points quite closely. This means that most of the energy put into the string is lost by being transferred to the body, whereas in a guitar most of it is dissipated by other loss mechanisms.

The result is that a note played on the banjo with the same player gesture as a note on the guitar is likely to sound louder, and decay faster. No player is likely to quarrel with that description. This makes it seem likely that at least part of the essence of “banjoness” might be captured by a model based on the effects revealed in these plots. However, in musical acoustics it is often found that perceptual characteristics do not correlate in a straightforward fashion with features that seem obvious in physical measurements. To address this issue, it is necessary to listen to sounds from the synthesis model and find out if they do in fact strike listeners as convincingly banjo-like.

B: Models and datum cases

In the following subsections, a collection of synthesised banjo sounds will be presented. The first step is to check whether the synthesis models can succeed in “sounding like a banjo”. After that, by varying parameters within the models we can explore the predicted perceptual effect of some variations, and see whether these match the expectations of banjo players and makers. The same short musical extract is used for all examples: the first few measures of a banjo arrangement of the tune “The Arkansas traveler”.

Any synthesis model requires information about the strings, and the position of the plucking point: information relating to the actual strings with which the Deering banjo is fitted is given in Table 1. The default plucking position is 120 mm from the bridge.

The sound files are organised in groups, each of which illustrates a particular type of variation. Within each group of sounds, the scale factor used to generate the sound files is kept constant, so that any variations in loudness are preserved. However, between groups the scale factor may be different. The files fall into two main classes: those based on the use of measured admittance, and those based on what we will call the “square banjo” model: it will be described shortly. For each class, a datum case has been chosen as the baseline for comparisons. For measured admittance cases, the datum is based on the admittance for the Deering banjo measured near the first string position. This is the banjo admittance that has been seen repeatedly in sections 5.1–5.3. The same admittance is applied to all five strings. Here is the resulting sound: it is encouragingly banjo-like.

As will be seen and heard in the next few subsections, a number of interesting questions can be explored using measured admittances. However, there are other questions that cannot be addressed that way, at least without making a lot of banjos with variations in their physical details (such as the diameter of the banjo head). For that purpose, a model is needed that is accurate enough that it still sounds realistically banjo-like, but which allows physical parameters to be changed. This is where the “square banjo” model comes in. It takes advantage of some pre-existing mathematical results for the vibration and sound radiation efficiency of rectangular membranes (there are no corresponding analytical results for sound radiation by a circular membrane). Details of this model are given in the next link.

SEE MORE DETAIL

The bridge admittance for this datum case is compared with measured admittances in Fig. 4. Measured admittances are shown for two cases: with and without the resonator back fitted to the banjo. The case with the resonator back (solid red line) is the datum case, used for Sound B.1. The blue curve shows the datum case for the square banjo model: but this model does not include any allowance for the effect of the resonator, so it is more logical to compare it with the dotted red curve, measured on the real banjo without the resonator. As expected from the discussion in section 4.2.2, the main effect of fitting the resonator is to turn the lowest strong peak in the dotted red curve (around 300 Hz) into a pair or peaks in the solid red curve, lying on either side of the original peak frequency. Apart from this, the two red curves look very similar over the entire frequency range.

The blue curve follows the trends of these two red curves fairly well. At low frequency, it looks recognisably similar to the dotted red curve, except that the first peak frequency is a little higher in frequency. This is an expected deviation: a circular membrane has a lower fundamental frequency than a membrane of any other shape with the same area, mass and tension. The formant centred around 800 Hz is followed quite convincingly by the square banjo model. The most obvious deviation between the blue and red curves comes at frequencies above 3 kHz, because the real banjo has a “bridge hill” which is missing from the square banjo model. This was discussed in section 5.3, and sound examples to illustrate its influence will be given in subsection H below.

The sound of the datum “square banjo” model appears below. While not identical to the sound based on the measured admittance, it seems sufficiently similar that we can hope the model will capture the perceptual effect of parameter variations well enough to be useful.

For both types of synthesis model, there are some additional parameter choices relating to internal “housekeeping” issues of various kinds: filtering the output signal to be more like radiated sound, details of damping models, and so on. These are somewhat secondary to our main agenda, but for completeness some discussion and sound examples relating to all these choices is given in the next link.

SEE MORE DETAIL

The most obvious thing to explore using synthesis based directly on measured admittance is simply to use bridge admittances from different instruments, or with different setup details on the same instrument, and synthesise the sound assuming the same string behaviour. The first set of sounds relate to admittance measured at different points on the banjo bridge. Sound C.1 is simply an alternative measurement of the datum admittance, on a different occasion. Sound C.2 uses the admittance at the bridge centre, at the position of the third string.

This admittance was plotted in section 5.3, reproduced here as Fig. 5. The comparison of the red and black curves in this plot makes it hardly surprising that a very different sound is produced. Sound C.3 uses a different admittance for each of the five strings, and so is closer to the real instrument than any of the cases using the same admittance for all five strings.

The next group of sounds gives a similar comparison, but using a set of admittances measured with the resonator back removed from the banjo. Sound C.4 corresponds to the measurement position of the datum case, plotted as the dotted red line in Fig. 4. The sound is somewhat different from the datum case: compare with either Sound C.1 or Sound B.1. Part of this difference is presumably the direct result of the change in the low-frequency modes, but it also has an element of undesirable “zinginess” about it, to which we will return in subsection I. Sounds C.5 and C.6 correspond to Sounds C.2 and C.3 respectively, using admittance at the bridge centre, and different admittances for the five strings. Sound C.5 has even more of the “zinginess” effect.

The next two sounds relate to a study carried out in reference [2]. In the process of refining models of the banjo behaviour, some measurements were made with the bridge replaced by a solid circular bridge, placed either at the centre of the head membrane (Sound C.7) or offset to a position similar to that of the regular bridge (Sound C.8). Figure 6 shows a test in progress with this bridge. Only a single string is carried by this “bridge”, and the tests were done with the resonator back removed.

The corresponding admittances are plotted in Fig. 7, in comparison with the result with the normal bridge (also measured without the resonator), the same as the dotted line in Fig. 4. It is perhaps not very surprising to discover that these admittances produce distinctive sounds, but reassuring that both sound (to my ears, at least) banjo-like. As would have been expected, the “bridge hill” around 3 kHz has disappeared with the rigid bridge.

Notice one feature of Fig. 7, which will be significant when we come to subsection G. At the lowest frequencies, both cases with the circular bridge show significantly higher admittance than the curve for the regular bridge, and the formant has shifted to somewhat lower frequency (peaking around 500 Hz rather than 700 Hz). As was explained in section 5.3, this low-frequency behaviour is influenced by a stiffness acting at the bridge, arising from several effects all associated with the strings and their break angle over the bridge. Care was taken to keep that angle the same with the circular bridge. The difference, as can be seen in Fig. 6, is that only one string was carried by the circular bridge, instead of five. This gives a large reduction in the stiffness, raising the low-frequency admittance and lowering the formant frequency; despite the fact that the mass of the circular bridge is in fact lower than that of the normal bridge, which would tend to raise the formant frequency.

The final set of sounds makes use of bridge admittances from entirely different stringed instruments: two different guitars, and a violin. Sound C.9 is the Woodhouse steel-string guitar used to generate the results in Fig. 2, while Sound C.10 uses the admittance of a flamenco guitar by the same maker. Both guitar syntheses give sounds that are much quieter than the banjo cases, and the strings ring on for a lot longer as a result of the lower bridge admittance. These findings are entirely in keeping with the results plotted in Figs. 2 and 3. The start of each note tends to have an audible “thump”: this slightly unnatural effect is probably a consequence of listening to what is essentially the body motion, not the radiated sound. All the modal responses to the pluck are coherent in the body motion, so they add up constructively at the initial instant after the pluck release.

Finally, Sound C.11 gives an indication of what a violin might sound like with a banjo neck and strings fitted to it. In some ways the sound is intermediate between that of the banjo and the guitars. This is consistent with the pattern of the bridge admittances: the admittance of the violin is significantly higher than the guitars in the low kHz range, as a result of the “bridge hill”. The result is a sound which strikes some listeners as reminiscent of a harpsichord.

D: Strings and plucking

The next set of sounds are all based on the datum measured banjo admittance, and they explore variations in strings and playing details.

One easy comparison is to make otherwise identical simulations using different gauges or materials for the strings. The original banjo has plain steel strings for all except the 4th string, which is wrapped. Other materials commonly used for musical instrument strings are nylon, fluorocarbon and natural gut. These materials have all been well characterised in an earlier study [3,4]. The only choice to be made is a set of string gauges for each material. From a scientific standpoint, there is a very straightforward approach to that issue. Since each string has a known length and tuned frequency, the transverse wave speed is fixed. So a natural choice is to keep both the tension and the mass per unit length the same in every material. As a consequence, the characteristic impedance will also be the same. To achieve this, the string diameter $d$ must be adjusted in inverse proportion to the square root of the density ratio of the two materials. The resulting set of gauges for the four materials is shown in Table 2.

Sounds D.1, D.2 and D.3 respectively relate to nylon, fluorocarbon and gut strings, and they should all be compared to the datum case Sound B.1 (reproduced below for convenience). All the polymeric strings have higher intrinsic damping than the steel strings, and this comes out in a difference of brightness in the sounds. Even so, the brightness may be over-estimated in these sound examples because we have not taken account of the fact that these polymeric materials all have far lower Young’s modulus than steel, which would affect the added stiffness effect: see the discussion around Sounds G.8–G.13.

It should be explained that some banjos do indeed use strings of these other materials, but usually in a context of seeking an “old-time” sound, often with a natural skin head with somewhat lower tension than a typical Mylar head. In that context, the actual choice of gauges would typically be lighter than the values given here, and these strings would often be combined with a lighter bridge (to be discussed in subsection G) to increase the brightness of the sound.

A different variation in string choice would be to keep steel strings, but choose heavier or lighter gauges than the datum case. Examples of the synthesised result of such a change are given below. The realistic range of gauges is fairly limited, so the cases explored here range from 20% thinner (Sound D.4) to 20% thicker (Sound D.8) than the values given in Table 1. The main difference in sound is in the decay rates: lighter gauges ring on a little longer than the datum case, while heavier gauges give a fast-decaying, more “plunky” sound. Of course, using a lighter or a heavier string gauge may also affect the feel of the instrument in the hands of the player, but that question would take us into different territory and we ignore it for now.

Another thing that is easy to explore is the influence of two variables under the player’s control: the plucking point and the nature of the plectrum or fingertip. We have already heard a synthesised example of moving the plucking point on a nylon-strung guitar, in section 5.4. Sounds D.9 and D.10 give two similar examples for the banjo. The familiar change of sound associated with this shift is captured well by these sounds.

The effective width of the plectrum or fingertip can also be varied: this models the range of sounds produced by changing from a hard plectrum, to a fingernail, to the flesh of a fingertip. This effect is achieved by a rather crude filter in the model, but the familiar change of sound is quite well captured by Sounds D.11, D.12 and D.13.

E. Alternative synthesis methods

There are two variations in synthesis method that reveal useful information about what contributes to “characteristic banjo sound”. So far, all examples have considered a single polarisation of string motion. But by measuring the $2 \times 2$ matrix of admittance at the bridge, the frequency domain approach can be extended to include the second polarisation [5]. Allowing for both polarisations brings a new variable into play: the initial angle $\theta$ of the pluck (remember, “theta”), defined so that $\theta = 0$ corresponds to a pluck normal to the banjo head, while $\theta = 90^\circ$ is the opposite extreme, plucking parallel to the head. The output variable is still the motion at the bridge perpendicular to the membrane, since this is the component of head motion mainly responsible for the radiation of sound.

Sound E.1 uses only a single polarisation, as in the previous sounds, but it uses the bridge admittance taken from this matrix set, measured some weeks after the admittance used so far. Comparing this sound with the datum (Sound B.1) gives a direct check on the effect of variability, either arising from experimental details not perfectly repeated, or from physical changes in the banjo head over time, for example as a result of changes in temperature or from creep of the tensioned Mylar head.

The remaining three sounds are all calculated using a two-polarisation model, with three different values of $\theta$: Sound E.2 has $\theta = 0^\circ$, Sound E.3 has $\theta = 45^\circ$, and Sound E.4 has $\theta = 90^\circ$. It is hard to know what is the typical angle $\theta$ for normal banjo plucks, but $45^\circ$ seems a good guess. To my own ears, Sounds E.1, E.2 and E.3 sound rather similar, suggesting that the precise pluck angle may not make all that much difference. Sound E.4, though, is clearly different. This case is quieter, as would be expected from weaker coupling of string motion to head motion, but it also sounds rather unrealistic. There is a sense of something ringing on in the background. This ringing seems to be an artefact of the synthesis process when damping is too low. We will come back to this issue in subsection I.

All the synthesised sounds so far have been produced by the frequency domain approach. We now compare the results with some obtained by a modal approach, based on approximating the measured admittance by a modal summation, then using that to calculate the couple modes of string plus body. Sound E.5 gives the modal version of the datum Sound B.1 (given again here for convenience of comparison). The sounds are extremely similar. This confirms that the modal decomposition is sufficiently accurate for the present purpose, and also provides an internal check on the synthesis coding since these two methods are independent.

The modal approach provides an opportunity to do something interesting, which is not possible by the frequency domain method. Each coupled body/string mode can be classified as either a string mode or a body mode, by computing how the energy in the mode is partitioned between string and body. Having separated the modes into these two groups, it is easy to do the synthesis separately for each group. This gives a “string only” sound (Sound E.6) and a “body only” sound (Sound E.7). Adding these together gives Sound E.5 that we have already heard. On a casual listening, the string-only sound is quite similar to the full synthesis with all modes. However, the difference is clearly audible (provided your audio playback quality is good enough), but it is hard to put into words.

Corresponding spectrograms are shown in Figs. 9 and 10. The comparison of these spectrograms reveals that it is hardly surprising if the body modes make some audible difference to the sound. The body modes are strongly excited, and although they generally have much faster decay times than the string modes, typical banjo music like the passage used here has notes that come thick and fast, and the body modes ring on for long enough to bridge the gap to the next played note. They surely make a significant contribution to the “punctuation” at the start of each note, and it seems likely that this effect forms a significant part of characteristic banjo sound. Note that this body mode contribution to the sound gives a possible mechanism for recognising a particular instrument, to some extent independent of what music is played. The mix of body modes is similar for every note, and constitutes a kind of acoustical fingerprint of the instrument.

F. Tension and size of head membrane

For the remaining sound examples we turn to the square banjo model, in order to have access to parametric variations. The first set of sounds illustrate the effect of scaling the plan dimensions of the rectangular membrane by various factors, from a half (Sound F.1) to double (Sound F.5) the nominal size. This seems quite a drastic change (the area changes from four times the real banjo down to a quarter of that area), but the effects on sound are surprisingly small.

To see a possible explanation, Fig. 11 shows three of the admittances associated with this set. It is clear that all the individual resonances are shifted when the size changes, as expected.But the overall envelope of the admittance, governed by the formant discussed in section 5.3, does not change: it is centred at around 700 Hz in this case. As explained in the earlier discussion, the formant frequency is mainly determined by the mass of the bridge, and by a combination of several sources of stiffness associated with the presence of the strings on the instrument. None of these factors is significantly influenced by changing the size of the head, so the formant does not move.

Slightly different behaviour is seen in the next case, when the effect of changing the head tension is investigated. Values range from a half (Sound F.6) to double (Sound F.10) the nominal tension. Figure 12 shows a selection of the resulting admittances. In the judgement of the authors, the sounds differ rather more than in the case of changing the size of the head. The formant frequency is affected by the change of tension, as expected and as the plot confirms. This set of sounds, compared with the previous set, suggests that changes in the formant might have larger perceptual significance than changes in the individual resonances within a fixed formant structure. We will explore this idea further with other parametric variations in the next subsection.

The effects of head size and head tension are only incorporated in this study through their influence on the bridge admittance. On a real banjo these changes would also affect radiation from the instrument, and coupling to the internal air cavity. Both effects were discussed in references [1,2], but the model we are using here is not sophisticated enough to capture the perceptual consequences.

G. Bridge mass and added stiffness

The next set of sound examples concern effects that are concentrated around the bridge of the banjo: the mass of the bridge, and the additional stiffness from effects of the strings and break angle. Adjusting the bridge mass is something commonly done by banjo players. A rather extreme range is explored in the sound examples below, from an unfeasibly small value of 0.1 g (Sound G.1) up to a very heavy 10 g bridge which would be considered by a player to be a mute (Sound G.7). The sound changes are very significant, and broadly in line with expectations.

Figure 13 shows examples of the associated admittances. The formant is shifted drastically, as would be expected with these large changes of mass. The choice of a bridge mass of 1.5 g for the datum case was motivated by examining these admittances: it gives a formant frequency and bandwidth which is a fairly close match to the measured admittance. By contrast, using the actual bridge mass of 2.2 g shifts the formant significantly too low in frequency.

Figure 14, and the next set of sound examples, illustrate the effect of changing the additional stiffness. Values vary from essentially zero (Sound G.8) to double the value used in the datum case (Sound G.13). Not surprisingly, increasing the stiffness shifts the formant to higher frequency. The effect is clearly audible.

One way to change this additional stiffness has already been seen in Fig. 7: a bridge carrying only one string rather than the full set of 5 clearly resulted in a similar change to what is seen in Fig. 14. More relevant to normal banjo experience is another parameter change that would lead to an increase in this stiffening effect: it would arise from an increase in the break angle at the bridge; and the effect would disappear with a zero break angle. A prediction would thus be that increasing the break angle should make the sound brighter. Such a change is in accordance with banjo lore.

H. The effect of the 3 kHz bridge hill

The final change to be illustrated in this discussion is the effect of the “bridge hill” occurring at about 3 kHz in the datum admittance. As was explained in section 5.3, this feature is associated with dynamic response of the banjo bridge, in conjunction with the underlying membrane. This feature is not included in the square banjo model. It is of interest to hear the effect of adding the feature back in.

The approach that has been used is to take weighted mixtures of the two datum admittances, one measured and one from the square banjo model, using a scaled version of the error function $\mathrm{erf}(\omega)$ to give a smooth transition over a bandwidth of 200 Hz. The different sound examples below correspond to different choices of the transition frequency. Sound H.1, with a transition at 400 Hz, is more or less the original measured admittance. Sounds H.1 and H.3 have transitions at 1.5 kHz and 2 kHz, both low enough that the bridge hill feature is still included in the combined admittance. Sounds H.4 and H.5, with transitions at 5.5 kHz and 9 kHz, do not result in the 3 kHz hill feature being included. To my ears, the biggest change in sound quality comes between Sound H.3 and Sound H.4, when the bridge hill is first removed.

Selected examples of these admittances are plotted in Fig. 15}. The plot is a little hard to interpret at first glance, because large sections of these curves are identical. For example, the appearance of a blue curve turning into a red curve around 5 kHz is misleading: The red curve is obscured by an identical black curve below that frequency, whereas above that frequency the red curve obscures the blue curve until 9 kHz, where it in turn is obscured by the black curve. Comparing the black and blue curves in this figure gives an illustration of the level of agreement in the formant frequency and bandwidth resulting from the chosen bridge mass and stiffness. This good level of agreement is presumably the reason that Sounds H.1, H.2 and H.3 sound rather similar: swapping between the two datum admittances in frequencies up to about 2 kHz makes relatively little difference.

These examples are now repeated, this time using the square banjo admittance without the phase compensation fudge that was described in section 5.5.2. The final sounds in this set illustrate the “zinginess” problem mentioned earlier, and discussed in the next subsection.

I. “Zinginess” and the damping question

Several of the sound examples in this section have exhibited a phenomenon that results in synthesised sounds that strike many listeners as unrealistic. The effect was first noticed in the context of synthesis using the simplified “square banjo” model. Initial efforts to use this model, after adjusting details to give a reasonable-looking match to the measured bridge admittance, suffered from an undesirable “zingy” sound. The source of this sound was traced to the fact that the real part of the admittance, containing the information about energy absorption from the string, was significantly lower than the measured values in the frequency range above about 3 kHz. This resulted in coupled string-body modes in that frequency range with damping that was too low, perceived as the zingy sound. Sounds X.15–X.19 and the associated discussion in section 5.5.2 illustrate the phenomenon and the somewhat unsatisfactory “fudge” procedure that was used to control it.

It is not surprising that synthesised banjo sounds might be sensitive to details of damping. Human perception of transient sounds is often sensitive to decay rates, and hence to damping. The results shown in Figs. 2 and 3 suggest that decay rates may give a very important cue for identifying an instrument as a banjo. In a wider context, modal decay rates or Q factors provide the main cue for identifying the material of an object as “metallic” or “wooden” based on the sounds it makes [6]: we heard examples in Chapter 3, associated with various synthesised percussion instruments.

Part of the problem with the original square banjo model, therefore, could stem from inadequacy of the damping model used. Radiation damping is captured quite well, but structural damping is difficult to model and predict. This issue is by no means unique to the banjo. In most contexts of vibration prediction and measurement, theoretical models can be expected to give an accurate representation of effects of mass and stiffness, but not of damping. Sophisticated commercial software packages frequently offer only very crude models for damping. This is not evidence of laziness among software engineers, but of a shortfall in understanding of the underlying physics of damping. There is no universal theory of vibration damping on a par with the general linear theory of undamped vibration, based on the Lagrangian approach and leading to concepts like the mass and stiffness matrices (see section 2.2.5).

However, a poor damping model cannot be the whole story. Unrealistic sounds associated with the effects of low damping are not confined to cases based on theoretical models: some of the synthesised sounds based on measured admittance exhibit related effects. The most striking example is Sound E.4, resulting from a two-polarisation synthesis of a pluck parallel to the banjo membrane. Other cases are Sounds C.4 and C.5. These do not sound identical to the square banjo case, but they are all characterised by a sense of “something ringing on too long” in an unrealistic way. The admittances involved in all these examples share the characteristic that the real part gets very low in mid-kHz range: the first example involves the admittance on the bridge top parallel to the membrane, while the other two involve admittance at the bridge centre.

There are two possible interpretations of this problem. First, it is possible that there is something not quite accurate in the admittance measurements. The obvious candidate would be the process of compensation of phase to allow for the measurement delay, as described in section 5.5.2. However, that process was no different for these admittances than for others which resulted in synthesised sounds that do not exhibit this problem.

The second possibility is that this is a real effect, but one that is not obvious from normal banjo playing. It should be recalled that what is computed by the synthesis algorithm is not the radiated sound, but the motion of the body at the bridge. Different modes of the banjo head have very different levels of radiation damping (discussion and measurements can be found in refs. [1,2]). Modes with low radiation damping, and thus high Q values, presumably do not feature very strongly in the sound received at a distance from the banjo, but they may be strongly present in the body motion. In principle this effect could be allowed for in synthesis using the theoretical model, but it would rely on using the modal approach to synthesis. Synthesis direct from measured admittance can only be done in the frequency domain, so the option to weight the modes differently according to their radiation efficiency is not available.

This idea would suggest that a real banjo might sound rather harsh if some kind of body pickup was used to allow amplified sound. It might also sound harsh in a recording using a close microphone, which would pick up near-field sound from modes with low radiation efficiency. Both predictions are consistent with anecdotal evidence about amplifying and recording banjos. So perhaps the banjo, with very low intrinsic damping in both strings and body, really is on the edge of making unpleasant sounds like those heard in the synthesised files.

J. Summary

This extended case study has given examples of how physically-based sound synthesis can be used to shed light on questions of interest to makers and players of instruments, as well as to scientists. The banjo has been used because it represents an extreme case in more than one sense. The sound of the banjo is extreme among familiar plucked-string instruments, for reasons that have been illustrated here. The strings of a banjo are coupled more strongly to the body of the instrument, leading to loud sound that decays rapidly, and to a significant component of “body sound” accompanying each plucked note (recall Sounds E.5–E.7).

But the banjo is also extreme in the sense that more of the key variables can be adjusted by the player. The choice of strings is always available to the player of any instrument, but in the banjo the head tension and the mass, break angle and other design features of the bridge can also be changed. Synthesised sound examples associated with all these changes have been given here. In the main, the impression given by these sounds agrees quite well with the generally accepted view among banjo players of the tonal effects of changes to head tension and bridge details.

Postscript: experiments with different bridges

All the results we have been describing in this section were concerned with measurements and modelling of a single banjo, and a single bridge. After the study described in this section had been completed, we thought it would be interesting to see if the predictions of the modelling would carry over to bridges of different types: banjo bridges are available in a wide variety of shapes, weights and timbers. Many different bridges were fitted to the banjo used here, and the admittances measured at the 1st and 3rd string positions. Those admittances were used to make another set of synthesised sound files, to illustrate at least some aspects of the predicted differences of sound. To see and hear these results, go to this side link.

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[1] Jim Woodhouse, David Politzer and Hossein Mansour. “Acoustics of the banjo: measurements and sound synthesis”, Acta Acustica 5, 15 (2021). The article is available here: https://doi.org/10.1051/aacus/2021009

[2] Jim Woodhouse, David Politzer and Hossein Mansour. “Acoustics of the banjo: theoretical and numerical modelling”, Acta Acustica 5, 16 (2021). The article is available here: https://doi.org/10.1051/aacus/2021008

[3] N. Lynch-Aird and J. Woodhouse “Comparison of mechanical properties of natural gut and synthetic polymer harp strings”. Materials 11, 2160, (2018)

[4] J. Woodhouse and N. Lynch-Aird “Choosing strings for plucked musical instruments”. Acta Acustica united with Acustica 105, 516-529, (2019)

[5] J. Woodhouse “On the synthesis of guitar plucks”.  Acta Acustica united with Acustica 90, 928–944 (2004).

[6] S. McAdams, A. Chaigne and V. Roussarie “The psychomechanics of simulated sound sources: Material properties of impacted bars”. Journal of the Acoustical Society of America 115, 1306–1320 (2004).