The coupling of the resonance frequency of the cabinet structure and the output frequency of the pickup is the hidden power that shapes the purity of the fundamental frequency of the tone. As the resonance carrier of the string vibration, the cabinet itself has a specific resonance frequency due to its wooden structure and internal space design, while the pickup is responsible for converting the string vibration into an electrical signal and outputting a complex frequency combination including the fundamental frequency and overtones. When these two frequencies are in a harmonious state, the cabinet resonance will strengthen the fundamental frequency energy output by the pickup, making the tone sound solid and clear, and the overtones are orderly distributed around the fundamental frequency, spreading naturally like ripples. This coupling does not bring about a simple volume enhancement, but a consolidation of the core position of the fundamental frequency, allowing the tone to maintain a clear pitch contour under various playing forces.
Once the frequency coupling between the two deviates, the purity of the fundamental frequency of the tone will be disturbed. If the cabinet resonance frequency is lower than the fundamental frequency output by the pickup, the inherent vibration of the cabinet will slow down the response speed of the fundamental frequency, like a fuzzy veil over the clear fundamental frequency. At this time, the high-frequency overtones output by the pickup will be perceived before the fundamental frequency, and the fundamental frequency gradually emerges in the slow resonance of the cabinet, causing the head part to appear sharp, while the main part is unclear, the dominant position of the fundamental frequency is weakened, and the overall timbre loses a stable anchor point. In this case, even playing a simple single tone will make people feel that the pitch is not focused enough, as if multiple frequencies are pulling each other.
When the cabinet resonance frequency is higher than the fundamental frequency output by the pickup, another distortion will quietly occur. The high-frequency resonance of the cabinet will be superimposed with the high-frequency overtones output by the pickup to form unnecessary resonance peaks. These abrupt peaks will cover the presence of the fundamental frequency, making the timbre sound harsh and floating. For example, when playing low-frequency notes, the fundamental frequency that should be stable will be cut into pieces by the high-frequency resonance of the cabinet, and the high-frequency components in the overtones will be over-amplified, causing the fundamental frequency to be submerged in the messy high-frequency noise, losing the original thickness and clarity. This coupling imbalance makes the fundamental frequency become the background of the overtones, destroying the foundation of the timbre.
The material difference of the electric guitar equipment box structure will aggravate the effect of this coupling on the purity of the fundamental frequency. The solid wood box has a relatively stable resonant frequency and slow decay due to its dense wood structure. The coupling with the pickup output frequency tends to strengthen the continuity of the fundamental frequency. Even if there is a slight deviation, the excessive interference of overtones can be alleviated through the natural damping of the wood. The resonant frequency of the composite board box is more susceptible to environmental influences. Slight changes in temperature or humidity will cause the resonance point to drift, and the coupling with the pickup output frequency will become extremely unstable. This instability will cause the fundamental frequency purity to be good and bad from time to time. The same song played at different times may present completely different timbre textures, and the fundamental frequency may be clear and fuzzy at times.
The installation position of the electric guitar equipment pickup also plays a key role in the coupling process. The pickups near the bridge have richer mid- and high-frequency components in their output frequency. If they are paired with a cabinet with a high resonance frequency, the coupling between the two is likely to excite too many high-frequency overtones, forming a "siege" on the fundamental frequency; while the pickups near the neck have heavier low-frequency components in their output. When combined with a cabinet with obvious low-frequency resonance, the fundamental frequency may fall into the quagmire of low-frequency resonance. Only when the pickup position and the cabinet resonance characteristics are mutually adapted can the frequency coupling between the two serve the prominence of the fundamental frequency rather than creating chaos. This adaptation is not fixed, but needs to be adjusted according to the playing style. For example, when pursuing a heavy tone, the strong coupling between the high-frequency resonance cabinet and the bridge pickup should be avoided.
Changes in playing techniques will indirectly change the pickup output frequency through string vibration, thereby affecting the coupling state with the cabinet resonance. When the strings are lightly plucked, the fundamental frequency energy output by the pickup is weak, and the overtone components are relatively prominent. At this time, if the cabinet resonance frequency is close to the overtone frequency, it is easy for the overtone to dominate, and the fundamental frequency appears pale and powerless. When the strings are plucked again, the powerful fundamental frequency output may trigger a strong resonance of the cabinet. If the frequencies of the two match, the fundamental frequency will be even more powerful and show full power; if they do not match, the fundamental frequency and the cabinet resonance will cancel each other out, making the tone appear hollow, as if a piece of the fundamental frequency has been dug out. This coupling change caused by the playing force makes the purity of the fundamental frequency show dynamic fluctuations.
The sound-absorbing material configuration inside the electric guitar equipment cabinet is an invisible barrier to adjust the frequency coupling and protect the purity of the fundamental frequency. The right amount of sound-absorbing material can absorb the excess resonance energy inside the cabinet, especially those stray frequencies that may interfere with the fundamental frequency, making the resonance frequency of the cabinet more concentrated and more stable. When the sound-absorbing material is insufficient, multiple resonances will form inside the cabinet, and the coupling with the pickup output frequency will become complicated. The fundamental frequency will be torn by various irrelevant resonance frequencies and lose its original purity. However, excessive filling of sound-absorbing materials will inhibit the normal resonance of the cabinet, resulting in the lack of cabinet support for the fundamental frequency output by the pickup, which appears thin and dry. Although the overtone interference is reduced, the vitality of the fundamental frequency itself is also weakened. This balance is directly related to whether the frequency coupling can serve the purity of the fundamental frequency.
