Vibration Dissipation Using Carbide Base Footers

It is known that a loudspeaker enclosure contributes significantly to the total radiated sound at its lower resonance frequencies[1]. Even though the surface velocity of the panels of a loudspeaker is small, the panels radiate with an efficiency many times greater than that of the drivers. This is due to the large radiating area of the panels relative to the radiating area of the drivers. Sound radiating from the enclosure panels can impart audible distortion and should be mitigated. Damping the enclosure panels is one effective way to reduce the amplitude of resonances[2].

The goal of this experiment was to determine if placing Carbide Base footers under a loudspeaker could reduce low frequency resonances within panels of the loudspeaker enclosure. The reduction in panel resonances would help quantify the improvement in vibration dissipation provided by the footers. This improvement would be compared to the base case of a loudspeaker enclosure sitting on steel floor spikes on a concrete floor.

Test Loudspeaker

To perform vibration tests, we first constructed a test loudspeaker enclosure. The enclosure was machined out of High Density Polyethylene (HDPE) sheets with 25 mm (1 in) thick panels used on the exterior and 50 mm (2 in) thick panels utilized for the internal bracing. Two Accuton AS250-6-552 250 mm (10 in) woofers were mounted on opposing sides of the enclosure. The enclosure was sealed with an internal volume of 129 liters yielding a Qtc of approximately 0.64. No stuffing was present inside the enclosure. The total mass of the enclosure with the woofers mounted was 83 kg (183 lbs.).

Vibration Dissipation Measurements

In our vibration dissipation experiment, the measurements were taken on exterior panels of the loudspeaker enclosure. The first set of measurements were taken on the bottom center of the enclosure. The second set of measurements were taken on upper portion of the left side panel at a height 76 cm (30 in) above the bottom of the enclosure. Measurements were first taken with the enclosure sitting on steel floor spikes directly contacting a concrete floor. The same measurement was then taken again with the enclosure sitting on Carbide Base footers.

To measure vibrations we utilized a Measurement Specialties ACH-01 piezoelectric accelerometer sensor. The sensor was attached to the enclosure using double sided tape. A log swept sine signal from 35 Hz to 200 Hz was played through the woofers and the panel vibrations were measured. Waterfall graphs were generated to show the decay of the vibration amplitude over time.

The blue waterfalls represent measurements with the enclosure on Carbide Base footers and the red waterfalls represent with the enclosure on steel floor spikes directly contacting the concrete floor.

Bottom Panel

On Floor Spikes

On Carbide Base Footers

Upper Side Panel

On Floor Spikes

On Carbide Base Footers

Conclusion

Measurements confirmed that low frequency resonances within the panels of our test loudspeaker enclosure were subdued when the loudspeaker was placed on Carbide Base footers instead of floor spikes. This damping effect occurred not just locally near contact with the footers but also at a location near the opposite end of the enclosure. The amplitude and decay time of most of the resonances present in both panels was reduced when the loudspeaker was on the Carbide Base footers. One notable exception was the resonance around 150 Hz in which there was a decrease in amplitude and an initially faster decay, followed by a small increase in decay time below -40 dBFS. In the lowest frequency region where enclosure resonances are most audible, the vibration amplitude was reduced in some instances by over 80%.

References

[1] Bastyr, K. J., & Capone, D. E. (2003). On the acoustic radiation from a loudspeaker’s cabinet. AES: Journal of the Audio Engineering Society, 51(4), 234-243.

[2] Juha Backman, Effect of panel damping on loudspeaker enclosure vibration, 1996, Nokia Mobile Phones, Finland.