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Qes – Electrical Quality Factor

Qes represents the electrical damping of a driver at its resonant frequency (Fs). It describes how efficiently the voice coil and magnet system control cone movement, with lower values indicating stronger motor control and higher values indicating weaker electrical damping.

How Qes Affects Speaker Performance

• Low Qes (< 0.3) → Strong motor, tight control, high efficiency. Ideal for horn-loaded and high-SPL designs. The motor force is high, sufficient to overcome resistance from air inside the cabinet..
• Medium Qes (0.3 – 0.6) → Balanced damping, suitable for bass reflex (ported) enclosures.
• High Qes (> 0.6) → When the motor force is lower, the driver depends more on its suspension (spider & surround) to return the cone to its neutral position. In a small cabinet, the trapped air acts like an additional spring, increasing resistance to cone movement. A larger cabinet provides less air resistance, allowing the cone to move more freely and extend bass response.

Qes and Its Relationship to Other T/S Parameters

Qes is directly linked to several key Thiele-Small parameters:

• Qts (Total Quality Factor) is calculated from Qes and Qms (mechanical damping)

• Efficiency (η₀) is influenced by Qes—lower Qes generally leads to higher efficiency.
• Enclosure Suitability: A high Qes driver may work better in large cabinets, while a low Qes driver is usually more efficient and can be used in compact, high-output designs.

Efficiency Bandwidth Product (EBP) is a useful guideline to determine whether a speaker driver is better suited for a sealed, ported (bass reflex), or even horn-loaded enclosure. It provides a quick way to assess how the balance between resonant frequency (Fs) and electrical damping (Qes) influences enclosure suitability.

How to Calculate EBP

Where:

• Fs = Free-air resonance (Hz)
• Qes = Electrical quality factor (unitless)

A higher EBP indicates a more efficient driver with lower electrical damping, making it better suited for ported or horn-loaded designs. A lower EBP suggests that the driver has higher electrical damping, which typically works better in sealed enclosures. Below we have listed typical applications for 18″ woofers according to their EBP. As woofers get smaller (12″) it becomes possible to have unusually high EBP which may not fit into these broad guidelines.

🔹 Sealed Enclosures (EBP < 50)

• Drivers with lower Fs and higher Qes tend to work best in sealed cabinets.
• The air inside the box acts as a restoring force, helping to control cone motion.
• Sealed boxes produce tight, accurate bass, but efficiency is lower. Usually also the bass extension is restricted

🔹 Ported Designs (EBP 50 – 100)

• Some drivers can work in both sealed and ported enclosures, depending on tuning.
• If EBP is closer to 50, it may lean towards sealed.
• If EBP is closer to 100, it will usually perform better ported.

🔹 Ported and Horn Loaded Designs (EBP 100-120)

• Some drivers can work in both ported enclosures and horn loaded designs
• High EBP usually allows for a compact bass reflex design.
• Most Horn Loaded Designs require high EBP woofers, but design adjustments can compensate for lower EBP.

🔹 Horn Loaded Designs (EBP 120+)

• Some high EBP woofer will work in bass reflex designs with VERY carefully made design adjustments
• High EBP usually means high efficiency in a horn loaded design
• Some Horn Loaded Designs with VERY high EBP woofers will be extremely efficient, but sound unnatural.

The guidelines above are intended for PA applications where maximising efficiency is the primary objective. In hi-fi applications, it is often possible to do things ‘outside of the box’ such as using a low EBP woofer in a horn, not because its efficient and loud, but because it sounds ‘nice’. This wouldn’t be appropriate in a PA application, as its very likely that you could damage the speaker when trying to operate it at high volumes.