6 Rattle Noise Suppressor

The rattle noise suppressor is a special purpose frequency sensitive compressor that reduces rattle noise. When loudspeakers are driven hard near the resonance frequency of the speaker, the mechanical movement of the diaphragm causes air turbulence through the firing ports and results in high-frequency rattle noise. The threshold of the compressor depends on the signal level at which the rattle noise is produced. If rattle noise is produced at low signal levels near the resonance frequency then the threshold has to be lowered and vice versa. Only the audio signal near the resonance frequency needs to be suppressed, hence the compressor is applied around the frequency region near resonance. The compressor is typically used as a limiter to keep the signal level close to the threshold limit. The side-chain detector also analyzes energy levels in both the resonance region and the high-frequency region. The detector analyzes the energy levels to determine the severity of rattle noise and activates the compressor only if the high-frequency energy does not exceed the resonance energy by a certain level. The resonance region, the high-frequency region, and the level are all programmable.

Figure 6-1 Rattle Noise Characteristics

Figure 6-2 Rattle Noise Suppressor Controls