Sd (Effective Diaphragm Area) is the active surface area of a speaker cone that moves air to produce sound. It’s usually measured in square meters (m²), but sometimes also specified in square centimeters (cm²). Sd is most often used for calculating other TS parameters, and its fairly common for all woofers with a certain diameter to have virtually the same Sd, this is because it can be calculated directly from the speakers diameter:
Where:
Sd = Effective diaphragm area (m²)
D = Effective cone diameter (meters)
π (pi) = 3.1416
Note: The effective diameter usually excludes the surround—only the part of the cone that actively moves air is considered – this can be hard to determine in some cases as some of the surround does move with the cone. For precise Sd, advanced methods are required to accurately determine the active surface area.
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.
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
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.
