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MEASUREMENTS

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 Bases 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 Carbide Base. This improvement would be compared to the base case of the loudspeaker enclosure sitting on steel floor spikes on a concrete floor.

Test Loudspeaker

To perform vibration tests we first constructed a test loudspeaker enclosure. We created our own enclosure to minimize the unknown variables which could influence the measurements. 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 woofers were wired in parallel to a Class AB amplifier. 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.).

Measurements

This experiment was limited to measuring vibration dissipation which is different than vibration isolation. To measure vibration isolation, the vibration source and the location where the measurements are taken are typically on opposing sides of the isolation device under test. The lower the transmission of vibration energy through the device to the other side, the greater the isolation. It is possible for a device to achieve a high level of vibration isolation yet have a low level of vibration dissipation. Such an underdamped isolator will do little to remove vibration energy from the system. Oscillations are allowed to persist long after the excitation force.

 

In our vibration dissipation experiment the vibration source and the the location of measurements were both located on the same side of the isolation device. The measurements were taken on exterior panels of the loudspeaker enclosure. The vibration source was a pair of woofers mounted in the same 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. Again the same measurement was then taken with the enclosure sitting on Carbide Bases.

 

To measure vibrations we utilized a Measurement Specialties ACH-01 piezoelectric accelerometer sensor. The sensor was attached to the enclosure using double sided tape. An amplifier with an integrated analog signal processor was used to amplify the analog output of the ACH-01 sensor. The amplifier was calibrated for the sensitivity of this particular ACH-01 sensor allowing for absolute acceleration measurements. In turn the sensor amplifier fed its analog output into a Tascam US-366 USB interface which was used to record the signal digitally on a PC. A log swept sine signal from 35 Hz to 200 Hz was fed into the Class AB amplifier which powered the woofers with a 3.8V driving voltage .

 

Waterfall graphs were generated with a 500 ms window and a 100 ms rise time over a 400 ms duration at a 4.72 ms slice interval resolution. A waterfall graph was used to show the decay of the vibration amplitude over time. The y-axis represents dB below full scale of the recorded signal relative to the maximum peak level before clipping. The y-axis was limited to a minimum of -60 dBFS to avoid noise floor artifacts.

 

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

Bottom Panel

On Floor Spikes
On Carbide Bases

Upper Side Panel

On Floor Spikes
On Carbide Bases

Results

Measurements confirmed that low frequency resonances within the panels of our test loudspeaker enclosure were subdued when the loudspeaker was placed on Carbide Bases instead of floor spikes. This damping effect occurred not just locally near contact with the Carbide Bases but also at a location near the opposite end of the enclosure. Both the amplitude and decay of most of the resonances present in both panels was reduced when the loudspeaker was on Carbide Bases. The exception was the resonance at 150 Hz in which there was a decrease in amplitude and an initial decrease in decay, followed by a small increase in decay 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 Society51(4), 234-243.

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