Rms – Mechanical Resistance of Suspension

Rms, or mechanical resistance, describes how much damping the speaker’s suspension provides to control cone movement. Think of it like shock absorbers in a car—too much resistance, and the suspension is stiff and unyielding; too little, and it becomes too loose, leading to uncontrolled movement.

Rms is directly linked to Cms and Fs, as can be seen in the formula below:

What Does Rms Actually Do?

Rms affects how quickly the cone stops moving after being displaced. A higher Rms means more damping, which helps prevent unwanted resonances but can reduce efficiency. A lower Rms means less damping, allowing for more movement but potentially leading to excessive ringing or overshoot.

How Rms Affects Speaker Performance

• Higher Rms (More Damping) →
  • The cone stops moving quickly after a signal ends
  • Prevents excessive ringing and improves transient response
  • Often found in PA and midrange drivers, where control is crucial
  • Can reduce efficiency because more energy is absorbed
• Lower Rms (Less Damping) →
  • The cone moves more freely, leading to longer decay times
  • More efficient at converting electrical energy into sound
  • Often found in subwoofers, where extended low-frequency response is desirable
  • May require careful tuning to avoid unwanted resonances

How Rms Relates to Qms

Rms directly affects Qms, the mechanical quality factor of a driver.

• A low Rms results in a high Qms, meaning the driver has lower mechanical losses and rings for longer.
• A high Rms leads to a low Qms, meaning mechanical energy is dissipated more quickly, reducing ringing.

For example:

• A PA midrange driver may have Rms = 5 kg/s and Qms = 2-3 for precise, controlled response.
• A subwoofer may have Rms = 1.5 kg/s and Qms = 7-10 to allow for more free movement and extended bass.

Rms and Speaker Efficiency

Higher Rms means more energy is absorbed as heat in the suspension, making the driver less efficient. That’s why high-efficiency speakers (like horn-loaded designs) often have low Rms, reducing mechanical losses.

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Real-World Example of Rms in Different Drivers

Driver Type	Typical Rms (kg/s)	Effect on Performance
PA Midrange	4 – 6	Tight control, minimal ringing
Hi-Fi Woofer	2 – 4	Balanced damping for clarity and bass extension
Subwoofer	1 – 2	More excursion, deeper bass, less damping

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Final Thoughts

Rms might not be the most commonly discussed Thiele-Small parameter, as typically most people focus on Cms or Vas. These are ways of quantifiying more or less the same thing, just in slightly different ways.