Using a 3D Laser Doppler Vibrometer for Modal Testing of a Soprano Pan

Modal testing is a highly respected method for extracting modal parameters, for example, modal damping factors, natural frequencies, and vibration mode shapes from structures. In this example, the method is applied to a comparably recent percussion instrument known as the steelpan.

Also referred to as the Caribbean steel drum or pan, the steelpan originated in Trinidad and Tobago at the time of the Second World War. In this investigation, modal testing is performed on a soprano pan to acquire mode shapes and resonant frequencies, along with providing a spatial illustration of modal tuning in the instrument.

The response of a soprano pan excited with a periodic chirp signal from a loudspeaker was recorded by a 3-Dimensional scanning laser Doppler vibrometer.

The results show the almost harmonic relationship among tuned modes in each outer note. There is also extensive modal coupling between adjacent notes that have frequencies which are harmonically related.


With modal testing being utilized to discover the vibrational behavior of steel pans, the instrument has become the object of significant research over the past three decades.

Despite this, modal testing on steel pans has been mostly restricted to the use of time-averaged holographic interferometry and there is a range of studies which have utilized this method to study the instrument [1-13].

This technique provides high spatial resolution however, modal parameters like damping cannot be directly acquired from this technique. Modal analysis has also been significantly used to discover the dynamic behavior of various alternative musical instruments [14-19].

Alternative modal testing methods use impact excitation or sinusoidal excitation (either acoustic or mechanical). Each of the techniques commonly include an accelerometer which is fixed to the structure as a means to detect vibration response.

The structure is tapped at several points on a grid with a hammer containing a force transducer in modal testing with impact excitation. The acceleration and force data is utilized to make a transfer function where multiple algorithms can be used to extract mode shapes and additional modal parameters [20;21].

In regards to these techniques, the critical challenge is that the incorporation of the accelerometer can cause a reduction in the values of the normal mode frequencies along with the addition of damping to the structure.

If the accelerometer is located on one of its nodal lines, a mode may also be impossible to detect. While mode shapes may be correctly determined, the spatial resolution provided by these techniques is normally poor.

In this study, a loudspeaker is used in combination with a 3D scanning laser Doppler vibrometer.

Similar to time-averaged holographic interferometry, this technique also provides high spatial resolution along with providing data from which modal damping can be inferred. This method additionally enables the observation of vibration behavior without any contact with the steelpan.

The Steelpan – A Brief History

A relatively new percussion instrument called the steelpan, also referred to as the Trinidad pan or the Caribbean steel drum , originated from a process which was heavily influenced by the drumming traditions of West African slaves who were transported to Trinidad and Tobago to work on plantation estates.

Congo drums were played by earlier generations of slaves, however, these were eventually prohibited by the government, out of anxieties that the use of these instruments could incite a rebellion.

Despite this, the emergence of a new instrument came from their love for music which took the form of bamboo poles known as tamboo-bamboo. The term originates from the French word ‘tambour’ meaning drum. This was also prohibited as clashes between rival bands led to the bamboo poles being used as fighting sticks.

The people had a great passion for music and therefore began to use dustbins, car rims, car fenders, and garbage tins as instruments. However, this was quickly replaced as the increasing oil industry and the American Naval bases in Trinidad and Tobago supplied a host of used oil drums which were utilized as a raw material for musical experimentation.

The first generation of steel pans were brought to the streets of Port-of-Spain in the Carnival celebrations a few years after the end of World War II. Since then, the steelpan has developed into a family of instruments including tenor or soprano pans, double second pans, guitar, bass, and cello pans containing a musical range that covers five octaves.

A short outline on the production of the steelpan can be read in the paper on the Aubrapan which is also a segment of this conference proceeding.

A detailed history of the instrument can be seen in Blake [22]. In this investigation, a soprano pan (Figure 1) is utilized as it has the largest amount of notes and it is probable to include the majority of the vibration features found in alternative types of pans.


A soprano pan produced and tuned by pianist and tuner, Aubrey Bryan, was used in this investigation.

Adopting the Trinidad 4th’s and 5th’s note layout, the steelpan notes are categorized into three rings: the inner ring, the outer ring, and the middle ring. The lowest frequency note on the pan is the C4. The experimental setup used in this work is presented in Figure 3.

Foam wrapping was utilized to isolate the pan periphery from the frame surface and the soprano pan was mounted on an open chassis frame for 3-dimensional measurement of motion. The loudspeaker was held underneath the frame to generate excitation.

Situated between the three tripod-mounted sensor heads, the surface of the steelpan was coated with a thin dusting of chalk powder (developer spray) to decrease the reflection of light and to maximize the scattering of the laser signal back to each sensor head.

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