DEPARTMENT OF MUSIC ACOUSTICS
WIENER KLANGSTIL (IWK)
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An Acoustical Study of Double Bass Bridge Height Adjusters


Of all parts of the violin, which one is the most important for the instrument`s tone and playability? Though it is impossible to isolate one element from the complicated resonating system of the violin as "most important", the bridge is worthy of consideration. It seems unlikely that a virtuoso violinist would cut through the legs of his instrument’s bridge in order to improve his instrument’s playing performance, yet that is exactly what 60-80 percent of American bassists do by installing bridge height adjusters on their instruments.

  1. Do bridge height adjusters affect the sound of the double bass compared to a bridge with solid feet?
  2. If so, how?
  3. Are there acoustical differences to be heard between various adjuster models?
  4. Do bridge height adjusters affect the pizzicato characteristics of the bass, and if so in what way?

These questions were answered with the help of overtone spectrum analysis, sound intensity statistics, and a listening test survey in a project of the above title.

The Bridge

The bridge is a general term for the part of an instrument upon which strings rest and through which vibrations are transmitted to the soundboard and resonating body of the instrument. Bridges are found on violins, viols, fretted instruments, and even within the pianoforte.

Fig. 1 Technical frontal and side view of bass bridge fitted with bridge height adjusters, with cutaway of bridge foot at left.

The acoustic function of the bridge is still under study. The relationship of the bridge to the soundpost and bassbar inside the instrument has an essential effect on the acoustical qualities of the instrument. Energy (vibrations) applied to the bridge through the strings is transferred through the legs and feet into the instrument. The table, bass bar and soundpost, and consequently the back and remaining body of the instrument vibrate, and these vibrations, together with the vibrations of the resonating chamber, are radiated through the air into the surrounding space.

String Height Adjustment

Stringed instruments need periodic string height adjustment, when the fingerboard is dressed, when buzzing occurs, or when the strings are or uncomfortable to play, for example. Climate plays an important role in the fluctuation of string height, and the double bass is dramatically affected from season to season, especially in North America, compared to violins, violas and violoncellos. Bassists also need to adjust to various styles quickly. Classical orchestral, solo and chamber music, and jazz and popular music all require different string height and are often played on the same instrument.

pThe traditional solution to climate-related string height fluctuation among bassists is to have two or more bridges cut appropriately for each season. It is also possible to place splints of wood, or bridge jacks, between the feet of the bridge and the table to raise height. A current revival of the bass neck adjuster reminds one of a machine found in violoncellos and basses of the 18th and 19th centuries. It is found in the neck joint and uses a key inserted in the heel of the neck to raise and lower the fingerboard relative to the strings without altering their pitch. The most common form of bridge adjustment in use, however, is the wheel-and-axle bridge height adjuster.

The Wheel-and-Axle Bridge Height Adjuster

The string height adjusters discussed in this paper are installed in each leg of the bridge. They are constructed of metal, plastic, or wood and have a shaft fixed through the center point of a flat wheel. By turning the wheel, the threaded portion of the bridge moves vertically toward or away from the table, thereby increasing or decreasing the height of the strings.

Fig. 2 Aluminum standard, brass standard, aluminum Boehm,
polyamide Boehm and maple DiLeone models (lignum vitae Kolstein not shown). Dimensions shown below.

Type name and material/code name

Weight

Wheel diameter

Axle diameter/ length

Massive Bridge/ massive bridge

-

-

-

Aluminum Standard/ AS

11.5g

28mm

6mm/ 42mm

Brass Standard / brass standard

34.6g

28mm

6mm/ 42mm

Aluminum Boehm/ aluminum Boehm

17.3g

32mm

8mm/ 45mm

Polyamide Boehm/ polyamide Boehm

9g

32mm

8mm/ 45mm

Maple DiLeone/ MD

7.8g

39mm

11mm/ 48mm

Lignum Vitae Kolstein/ LK

10.2g

35mm

11mm/ 55mm

The Tests

A list of twenty-two specific musical samples, or tasks, was played in a special echo-absorbing chamber. The recordings took place in one day, during which the same bridge was fitted with six different adjuster models. Conditions were practically identical for each recording round, allowing the rare chance to compare adjusters while controlling all other factors.

 

Fig. 3 Test player, instrument and equipment inside anechoic chamber. Note the sound-absorbing wall and floor of the chamber. The instrument is stabilized in its position while the player sits comfortably. The musical samples, or tasks, are seen at upper right. The amplifier for the accelerometer, digital thermometer and metronome (white background, l-r) and pitch meter (on stool facing player) are shown. Video camera, microphones and barometer not shown.

 

After the sound files were recorded, processed and stored into the memory banks of the IWK server, the sound analysis could begin.

FFT Spectrums

Three bowed tasks (Contra E, Large B, and D 1) were analyzed using FFT (Fast Fourier Transfer) diagrams created in the sound analysis program S_Tools. Spectrums represent the timbre resulting from the overtones present in the selected sample. The frequency increases from left to right and the intensity of those frequencies from bottom to top. The graphs below compare six of the seven tested variables as they appear in the high range of the double bass (D 1).

D 1 (ca. 293 Hz)

 

 

 

 

Massive Bridge

 
 

 

Aluminum Standard

 

 

 

 

Brass Standard

Aluminum Boehm

Maple DiLeone

Lignum Vitae Kolstein
 

Fig. 4 Comparison of FFT spectrums: massive bridge, aluminum standard, brass standard, aluminum Boehm, maple DiLeone, and lignum vitae Kolstein

This was the highest tone on the list of tasks. The Massive Bridge curve was consistent in both and characterized by four peaks between 250 Hz (mean) and 4.8 kHz. The "thin" sound of the Aluminum Standard model can be seen in that the fundamental peak at 250 Hz (293Hz, to be exact) is weaker, and the higher frequencies also fall off significantly. The Aluminum Boehm model shows a stronger fundamental and richer overtones above 2 kHz, including a sharp peak at around 2700 Hz, which support the observation that this model sounds "brighter" than the massive bridge but "rounder" than the standard model. The two wood models show a very similar curve to the Massive Bridge, especially in the case of the maple model. Lignum Vitae’s lack of strong peaks above 4 kHz probably shows why it sounds "slightly more dampened" than the solid bridge.

Comparing control task FFTs (separate recordings of the same variable) and listening to the sound files corresponding to the curves during my work showed a somewhat large margin of variation in graphic depictions of double bass tones, and I found that diagrams such as those shown above may be misleading if not interpreted with caution and in combination with other data. In spite of some inconsistencies, the diagrams that were created with S_Tools clearly show some general tonal tendencies of bridge height adjusters. See Conclusions below for tonal characteristics of adjuster types.

RMS Pizzicato Decay Time

The characteristics of plucked notes are of special interest to bassists. Not only the foundation of jazz and popular bass style, pizzicato is also a specialty of the bass in orchestral ensembles because of its resonance and sustain. Background information for this study indicated that bridge height adjusters have an effect an pizzicato notes, which led to an examination of how adjusters affect dynamics and sustain.

Two studies were made, both using S_Tools RMS analysis and Microsoft Excel tables. The first was to define the sustain of the various adjusters by the amount of time (t->, in seconds) necessary for a tone’s amplitude to fall from maximum loudness (max) to 40 dB below maximum (max-40 dB= 100 times less intense). This results in a single number which can be represented in a bar graph.

Pizzicato Test 1

Fig.5 The above tables show inconsistency in the sustain time of pizzicato notes among bridge height adjuster variables, including the massive bridge.

Pizzicato Test 2

The second test plotted the maximum dB level, dB level after 100ms, and every 200ms until 1100ms. This results in a curve of seven points which are roughly equivalent to the RMS curve of the pizzicato note when plotted on a linear graph. This information was processed with Microsoft Excel, and two examples are shown below.

Again, variables were inconsistent, and it is difficult to generalize about their sustain characteristics. It is interesting, however, to compare the massive bridge with the aluminum adjusters and the wood adjusters. The only real tendencies seem to be that the notes A, G and g harmonic vibrate louder and longer with bridge height adjusters within this 1.1 second period.

Listening Test

A closed system programmed onto laptop was used for flexible listening tests and exact data processing. Soundcomparison is a program written especially for such tests which presents a written dialogue to a test participant on the monitor screen, plays tone samples from its database through headphones, asks for a decision and records the results. The test parameters were defined, then sound files of three types of tones were selected and stored in the program’s databank.

Sound file Selection and Processing

Three types of tones were chosen to be compared: a low pizzicato (contra E, ca.41 Hz), a low bowed tone (contra E, ca.41 Hz), and a medium-range bowed tone (B, ca. 123 Hz). These three tones were extracted from the recordings of all adjuster variables and processed by equalizing volume and eliminating initiation and decay. A second set of separate massive bridge tones was also chosen as a control.

Testing Method

Using SoundComparison, candidates were asked to give some personal statistics and introduced to the test. The processed sound excerpts were played in pairs in a specific order which remained the same for each separate test. One series of massive bridge tones was used as a comparison and always present in the pairs. The control pairs played the identical file twice; all other pairs were either a different recording of the same adjuster variable (massive bridge) or different recording and variable. Test participants were asked to judge examples as "the same" ("gleich") or "not the same" ("nicht gleich"). 24 pairs (3 tones x 8 variables) were played twice during each test, resulting in 48 judgements per test and 960 judgements in all. Twenty people participated in the test, the majority of whom are music students.

The bar on the left shows that the identical files were judged by almost all participants (99.16%) to sound the same. The next bar shows results of the massive bridge comparison between different recordings of the same variable (63.1%). Though there is a large difference between the identical control files in the first bar and the massive bridge comparison in the second column, the closest bridge height adjusters, the maple DiLeone and brass standard models (17.43%), were judged the same significantly less often. Adjusters that were judged the fewest times to sound the same, such as the aluminum Boehm (4.15%), aluminum standard (8.3%) or lignum vitae Kolstein (9.13%) models, sound the least like a massive bridge with these tones.

Bowed tones were found different more often than pizzicato among all adjusters, while the massive bridge comparison remained more or less proportional to the control. Especially noticeable was the arco tone, which was judged the same only 5 times out a possible 240 (2.1%) among all bridge height adjusters compared to the massive bridge, whereas the massive bridge control was the same 30 out of 40 times (75%). The brass standard model came closest to the massive bridge on the arco low E with 27.5% "the same", compared with 47.5% of the control.

Conclusions of the Listening Test

Bridge height adjusters generally make a substantial audible difference in sound compared to a massive bridge. There is tonal variance among models of bridge height adjusters depending on the frequency of the note played. These differences are more audible with bowed tones than with pizzicato.

Of the tones tested, maple DiLeone sounded most like the bridge without adjusters on the low pizzicato, brass standard sounded least different on the low bowed note, and all models sounded significantly different on the mid-range note. The aluminum Boehm model ranked furthest from the massive bridge overall.

Conclusions

Preliminary research showed that there is no previous literature on the acoustical characteristics of double bass bridge height adjusters.

Local and international surveys showed current tendencies in adjuster use. Between 60-80% of North American bassists use them, while they are practically absent from the European music scene. Wood adjusters are preferred by bassists for tonal and aesthetic reasons, but aluminum models are more commonly used.

A test was prepared with a massive bridge and six types of bridge height adjusters. Digital analysis shows that bridge height adjusters make a significant difference in pizzicato decay time, but vary irregularly throughout the range of the double bass. FFT graphs and listening to sound examples defined the sound characteristics among the tested variables as shown below in The Sound Character of the Various Adjusters.

Listening tests indicate that all types of bridge height adjusters cause an audible difference in sound compared to a bridge with no adjusters, and that individual models and materials have unique tonal characteristics.

The Sound Character of the Various Adjusters

In summary, the above research has led to the following generalizations about bridge height adjusters’ sound characteristics:

A RealAudio version of various bridge adjusters is found below. Short samples of different adjuster models and materials give clues to their sound characteristics.

1) A bridge with no adjusters

2) A comparison of maple, none, and maple again

3) A comparison of aluminum, maple, and aluminum again

4) A comparison of none, aluminum, brass, plastic, and maple

For the RealAudio-files you need kostenloser download des realplayer-basic!

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