Tech Talk
Loudspeaker Linearity
Mark Glazer | Senior Principal Engineer
To reproduce a wide bandwidth of frequencies, loudspeaker systems require multiple size drivers. For waveform fidelity, each loudspeaker driver is required to move ‘pistonically’. This means, ideally when the voice coil moves back and forth, so would the diaphragm, without flexing or bending to all the movable parts.
In practice, this is not the case throughout each drivers operating range, as a loudspeaker diaphragm is not infinitely stiff. In addition, the diaphragm needs to be damped to attenuate the effects of cone ‘break up’ at higher operating frequencies. These two properties need to be balanced for best performance.
Scanning the Vibrations of Diaphragms:
The Klippel Scanning laser vibrometer system performs a non-contact measurement of the geometry and mechanical vibration of the loudspeaker diaphragm, visualizing the vibrational behavior, scanning hundreds of points mapped around the dimensions of the diaphragm throughout the required frequency range.
Figure1: The PerformaBe 5 inch midrange Laser Scanner set-up.
Revel Performa 5.25 inch Midrange Comparison Using the Scanning Lazer Vibrometer:
The scanning vibrometer was utilized to further improve the performance of the Revel Performa, 5.25 inch midrange for the PerformaBe line.
The scan frequency range is 100 Hz to 10 kHz of a period over 12 hours
The Performa3, 5.25 inch midrange consists of an aluminum cone and a rubber surround.
The PerformaBe, 5.25 inch midrange consists of Deep Ceramic Compositive (DCC) cone and a rubber surround engineered specifically for this driver.
Figure 2 is the Perfoma3, 5.25 inch midrange.
Figure 3 is the PerformaBe, 5.25 inch midrange.
At lower operating frequencies both drivers operate pistonically.
Figure 2: 300 Hz, both diaphragms, positive direction
Figure 3: 300 Hz, both diaphragms, negative direction
From 650-800 Hz the Perfoma3, 5.25 inch Midrange cone moves in the positive direction, (Blue) but the surround moves mostly in the negative direction (Red)
Figure 4: Performa3 midrange at 800 Hz.
From 650-800 Hz the Performa Be cone and surround move in the positive direction (Blue), improving midrange clarity. This is primarily due to the optimized surround design in the PerformaBe diaphragm.
Figure 5: Performa Be midrange at 800 Hz.
At 3.2 kHz the Performa3 illustrates a circumferential resonance, in which the cone-surround edge resonance causes bending motions in the cone.
Figure 6: The Performa3 midrange at 3.2 kHz.
Figure 7: The PerformaBe midrange at 3.2 kHz
Conclusion.
Advancements in loudspeaker analysis tools such as the Scanning Lazer Vibrometer, is how modern scientific tools helped us advance the superb Performa3 midrange into the exceptional PerformaBe midrange.