Qts – Total Quality Factor: The Balance Between Electrical and Mechanical Damping

Qts is one of the most critical Thiele-Small parameters when designing a speaker system. It represents the total quality factor of a driver, combining both electrical damping (Qes) and mechanical damping (Qms) into a single value. Understanding Qts is useful for determining the best type of enclosure for a driver. Getting the right Qts for a bass reflex enclosure ensures efficient output, strong transient response, and extended bass performance.

What Exactly Is Qts?

Qts is a dimensionless number that describes how well a driver controls its own resonance. It is calculated using the following formula:

Where:

• Qms = Mechanical quality factor (how well the suspension controls cone movement)
• Qes = Electrical quality factor (how well the voice coil and magnet control movement)

A lower Qts means more damping, resulting in tighter, more controlled motion. A higher Qts means less damping, allowing the driver to resonate more freely. A higher Qts driver tends to demand a larger cabinet to operate most effectively, so choosing the right driver with the right Qts is very important for almost every speaker cabinet design.

The Best Qts Range for PA Speakers

For PA speakers, especially bass reflex (ported) enclosures, the ideal Qts range is:

✅ 0.30 – 0.45 → Best for ported PA subwoofers & woofers
✅ 0.35 – 0.38 → The sweet spot, balancing efficiency, transient response, and bass output
✅ Above 0.45 → Can still work in ported enclosures, but requires a larger cabinet

A Qts below 0.3 is generally found in horn-loaded enclosures, where tight cone control and efficiency are prioritized, and the driver will work happily with a small rear chamber. There are sometimes exceptions, these are intended as guidelines only, to help make an informed choice if you’re just starting blindly at a wall of numbers.

How Qts Affects Ported Enclosures

• Qts 0.30 – 0.38 → Balanced sound with good transient response and deep bass.
• Qts 0.38 – 0.45 → More extended bass possible, but less transient snap.
• Qts above 0.45 → Requires a larger cabinet to compensate for weaker motor control.

For PA subwoofers and woofers, the ideal Qts keeps the cabinet size reasonable while ensuring powerful, clean bass.

PA Speaker Examples

Driver Type	Typical Qts Range	Best Enclosure Type
PA Subwoofer (Ported)	0.30 – 0.38	Bass Reflex (Ported)
General PA Woofer	0.35 – 0.45	Ported, some larger designs
Horn-Loaded Subwoofer	0.15 – 0.30	Horn-Loaded

🔹 Example 1: A Qts = 0.35 subwoofer is ideal for high-efficiency ported enclosures, delivering tight, punchy bass.
🔹 Example 2: A Qts = 0.42 woofer can still work in a ported cabinet, but may require a larger box to compensate.
🔹 Example 3: A Qts = 0.20 subwoofer would likely underperform in a ported box, but excels in a horn-loaded design.

Final Thoughts

For PA systems, getting the right Qts for a ported enclosure is crucial.

✅ The sweet spot for PA ported enclosures → 0.35 – 0.38 (from our experience)
✅ Avoid Qts above 0.45 unless using a very large cabinet
✅ Below 0.3 is best suited for horn-loaded designs