Understanding Key Loudspeaker Parameters(7):Equivalent Moving Mass (Mo/Mms)-The Role of Inertia in Speaker Response

Published by IWISTAO

In loudspeaker engineering, Equivalent Moving Mass — often expressed as Mms or Mo — is one of the most influential Thiele–Small parameters. It represents the total mass that the speaker’s motor must move and control to generate sound. This includes the diaphragm, voice coil, suspension components, and even the mass of air that moves with the cone.

Mms plays a critical role in determining bass extension, sensitivity, transient response, and enclosure behavior. Understanding this parameter is essential for designing or selecting high-performance loudspeakers and subwoofers.

 

1. What Is Equivalent Moving Mass (Mo / Mms)?

Mms is the total moving mass of the speaker’s mechanical system, including:

• Cone (diaphragm)
• Dust cap
• Voice coil former and winding
• Half of the surround and spider mass
• Air load (the air that moves with the cone)

Mms = Mmd + Mair

Mmd is the diaphragm assembly mass, and Mair is the added acoustic mass of the air in front of the diaphragm. This combined mass determines how much force the motor must produce to accelerate the cone.

2. Typical Mms Values

Driver Size	Typical Mms	Notes
1–2″ tweeter	0.1–0.5 g	Extremely lightweight
3″ full-range	1–3 g	Fast transient response
6.5″ mid-woofer	8–20 g	Common Hi-Fi woofer
10″ woofer	25–45 g	Good low-frequency capability
12″ subwoofer	40–80 g	Deep bass, heavy cone
15–18″ pro sub	70–300 g	Extreme SPL capability

3. How Mms Influences Loudspeaker Performance

a. Resonance Frequency (fo)

Mms is a major factor in determining the speaker’s resonance frequency:

fo = 1 / (2π × √(K / Mms))

• Higher Mms → lower fo → deeper bass
• Lower Mms → higher fo → stronger mid/high response

b. Bass Extension

A heavier moving mass allows deeper low-frequency reproduction, making Mms crucial for subwoofers and large woofers.

c. Sensitivity (Efficiency)

Higher mass requires more force to move:

Sensitivity ∝ (Bl)² / (Re × Mms)

• High Mms → lower sensitivity
• Low Mms → higher sensitivity

d. Transient Response

• Low Mms → fast, detailed, dynamic
• High Mms → smooth, heavy, slower response

e. Enclosure Interaction

Mms affects:

• Bass-reflex tuning
• Sealed box resonance
• Required enclosure size
• Maximum output before distortion

A driver with very large Mms may need strong motor force (high Bl) to maintain control.

4. How Mms Is Measured

Method 1 — Added Mass Technique

1. Measure the driver’s resonance (fo) without added mass.
2. Add a known weight to the diaphragm.
3. Measure the new resonance frequency.
4. Calculate Mms from the frequency shift.

Method 2 — Derived from Cms and fo

Mms = 1 / ((2π fo)² × Cms)

Measurement tools like DATS, CLIO, and ARTA compute Mms automatically.

5. Real-World Examples

Driver Model	Size	Mms	Description
Full-range A	3″	2.1 g	Fast, open midrange
Woofer B	6.5″	15 g	Balanced Hi-Fi woofer
Woofer C	10″	35 g	Strong low-frequency output
Subwoofer D	12″	78 g	Deep bass, large diaphragm
Pro Sub E	18″	235 g	High SPL, professional use

6. Choosing the Right Mms

Choose low Mms when you want:

• High sensitivity
• Fast transient response
• Clear midrange
• Full-range driver behavior

Choose high Mms when you want:

• Deep bass extension
• High air displacement
• Subwoofer-grade output
• Strong low-end authority

The key is balancing Mms with Bl, Cms, Sd, and Xmax to achieve the desired performance.

Conclusion

Equivalent Moving Mass (Mo / Mms) is a foundational parameter in loudspeaker design. It influences resonance behavior, bass extension, sensitivity, transient response, and enclosure alignment. Understanding Mms helps engineers and enthusiasts design loudspeakers that deliver the desired combination of power, clarity, and control — whether it's a fast full-range driver or a deep-reaching subwoofer.