Thiele-Small Parameters Explained
Decode the specifications that define subwoofer performance. Learn what Fs, Qts, Vas, Xmax, and other T/S parameters mean for your build.
What You'll Learn
- ✓Understand each major T/S parameter
- ✓Use parameters to predict subwoofer behavior
- ✓Match subwoofers to appropriate enclosure types
- ✓Compare subwoofers objectively using specifications
Prerequisites
In This Article
What Are Thiele-Small Parameters?
Thiele-Small (T/S) parameters are a set of electromechanical measurements that define a speaker's low-frequency behavior. Named after A.N. Thiele and Richard Small, these parameters allow us to:
- Predict how a subwoofer will perform in different enclosures
- Design enclosures mathematically (rather than guessing)
- Compare subwoofers objectively
- Model bass response before building
Without T/S parameters, enclosure design would be pure trial and error.
Resonance Parameters: Fs, Qes, Qms, Qts
Fs (Free-Air Resonance)
The frequency at which the subwoofer naturally "wants" to resonate when not in an enclosure.
- Measured in: Hz
- Typical range: 25-50 Hz for subwoofers
- Lower Fs = better potential for deep bass extension
- Higher Fs = may struggle with very low frequencies
Qes (Electrical Q)
Describes the electrical damping of the speaker (from the voice coil motor).
- Lower Qes = stronger motor, more electrical damping
- Higher Qes = weaker motor control
Qms (Mechanical Q)
Describes mechanical losses (suspension, spider, surround).
- Lower Qms = more mechanical damping
- Higher Qms = less mechanical loss
Qts (Total Q)
The combination of Qes and Qms. This is the most important Q value.
Qts = (Qes × Qms) / (Qes + Qms)
| Qts Range | Best Enclosure Type |
|---|---|
| 0.3-0.4 | Sealed |
| 0.4-0.5 | Flexible (sealed or ported) |
| 0.5-0.7 | Ported |
| 0.7+ | Infinite baffle or free air |
EBP (Efficiency Bandwidth Product)
A derived parameter that helps quickly determine enclosure suitability:
EBP = Fs / Qes
| EBP Value | Recommended Enclosure |
|---|---|
| < 50 | Sealed (acoustic suspension) |
| 50-100 | Flexible (either works) |
| > 100 | Ported (bass reflex) |
EBP is a quick heuristic—always verify with modeling software, but it provides useful initial guidance.
Volume Parameters: Vas, Vd
Vas (Equivalent Compliance Volume)
The volume of air that has the same "springiness" as the subwoofer's suspension.
- Measured in: Liters or cubic feet
- Larger Vas = requires larger enclosure for optimal performance
- Smaller Vas = can work in compact enclosures
Think of Vas as indicating how "stiff" or "loose" the suspension is.
Vd (Volume Displacement)
The volume of air the subwoofer can physically move.
Vd = Sd × Xmax
Where: - Sd = effective cone area - Xmax = maximum linear excursion
Higher Vd = more potential output. This is one of the best indicators of a subwoofer's SPL capability.
Excursion Parameters: Xmax, Xmech
Xmax (Maximum Linear Excursion)
How far the voice coil can travel while staying in the magnetic gap—one-way, measured in millimeters.
- Higher Xmax = more potential output
- SPL subwoofers often have 25-40mm+ Xmax
- SQ subwoofers may have 10-15mm Xmax
Important: Xmax is measured differently by manufacturers. Some spec "peak-to-peak" (double the one-way value). Always verify how it's measured.
Xmech (Mechanical Excursion)
The absolute physical limit before damage—where the voice coil hits the backplate or the suspension tears.
Xmech > Xmax. Operating near Xmech causes distortion and potential damage.
Why This Matters
A 12" subwoofer with 30mm Xmax will out-bass a 15" with 8mm Xmax in most scenarios. Xmax and cone area together determine output capability.
Sensitivity and Efficiency
Sensitivity (SPL @ 1W/1m)
How loud the subwoofer is with 1 watt of power at 1 meter distance.
- Measured in: dB
- Typical range: 82-90 dB for subwoofers
- Higher sensitivity = louder with less power
Efficiency (η₀ / Eta-0)
The percentage of electrical power converted to acoustic power.
- Typical range: 0.1% - 1.0%
- Yes, speakers are very inefficient
- Higher efficiency = better for low-power systems
The Tradeoff
High-excursion SPL subwoofers often have lower sensitivity because their heavy motors and long voice coils require more power to move. This is normal—they're designed for high-power applications.
Power Handling: RMS vs Peak
RMS Power Handling
The continuous power the subwoofer can handle indefinitely without damage.
This is the spec that matters. Match your amplifier's RMS output to the subwoofer's RMS rating.
Peak Power
Marketing nonsense in most cases. Theoretical maximum for brief transients.
Ignore peak power ratings. A "3000W peak" subwoofer with 750W RMS should be paired with a ~750W amplifier, not a 3000W amp.
Thermal vs Mechanical Limits
- Thermal: Voice coil overheating
- Mechanical: Exceeding Xmax/Xmech
SPL subwoofers often have mechanical limits (Xmax) before thermal limits. SQ subwoofers often reach thermal limits first.
Using T/S Parameters in Practice
For Enclosure Design
- Note Fs, Qts, and Vas
- Use modeling software (WinISD, BassBox, etc.)
- Input parameters to simulate different enclosure volumes and tuning frequencies
- Choose the design that meets your goals (flat response, max SPL, etc.)
For Comparing Subwoofers
| If You Want | Look For |
|---|---|
| Deep bass | Low Fs, Moderate Qts |
| High SPL | High Xmax, Large Vd, Low Fs |
| Small enclosure | Low Vas, Higher Qts |
| SQ focus | Moderate Fs, Lower Qts (0.3-0.4) |
Real-World Example
Sundown X-15 v3: - Fs: 31.5 Hz - Qts: 0.54 - Vas: 88.7L - Xmax: 38mm
This subwoofer wants a larger ported enclosure (high Vas), can dig deep (low Fs), and has massive output potential (huge Xmax).
Frequently Asked Questions
Why do manufacturers measure Xmax differently?
There's no universal standard. Some measure one-way (voice coil overhang), some measure peak-to-peak, some include the 10% distortion point. Always check how the specific brand measures Xmax when comparing.
Can I use a high-Qts subwoofer in a sealed box?
You can, but it may produce a peaky, boomy response rather than flat output. High-Qts subwoofers (0.5+) generally perform better in ported enclosures or infinite baffle applications.
What if the manufacturer doesn't publish T/S parameters?
This is a red flag. Reputable manufacturers publish T/S parameters. Missing specs often indicate cheap OEM drivers where performance is inconsistent. You can measure parameters yourself with test equipment, but consider choosing a transparent brand.
Do subwoofers really need to 'break in' before they sound good?
The 'break-in' phenomenon is real but overstated. New speaker suspensions (spider and surround) do become slightly more compliant with use, which can shift T/S parameters slightly—Fs may drop 5-10%, Vas may increase. However, this happens mostly within the first few hours of use, not over weeks. It's not dramatic enough to design your enclosure for 'broken in' specs. Build for the published parameters.
Sources & References
- ●Thiele/Small Parameters - Wikipedia
Comprehensive overview of T/S parameter theory and applications
- ●A.N. Thiele: "Loudspeakers in Vented Boxes" (1961) - PDF
Original AES paper establishing the mathematical framework for vented enclosure analysis
- ●Richard H. Small: "Closed-Box Loudspeaker Systems" (1972) - Semantic Scholar
Foundational AES paper defining parameters for sealed enclosure modeling
- ●Richard H. Small: "Vented-Box Loudspeaker Systems" (1973) - PDF
Extended T/S framework for ported enclosure design and prediction
- ●Measuring Thiele-Small Parameters - SB Acoustics PDF
Technical guide on parameter measurement methodology
- ●T/S Parameters Guide - SoundImports
Practical guide to understanding and applying T/S parameters
- ●Understanding Loudspeaker Data - Eminence
Manufacturer explanation of how to interpret T/S specifications
- ●How the Q Factor Impacts Audio Projects - MISCO
Technical explanation of Qts, Qes, and Qms and their effects
- ●EBP Calculator - Efficiency Bandwidth Product Guide
Practical guide for using EBP to determine optimal enclosure type
- ●Thiele Small Parameters - Room EQ Wizard
Software documentation for T/S parameter measurement
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