Acoustic properties · Composite indices
Acoustic conversion index ACE
What changes when radiation ratio is combined with loss?
ACE combines a light-and-stiff material index with damping. The term “efficiency” is used in literature, but this index is not a measured percentage of energy converted to sound.
Try the interactive demoStart with something familiar
A combined shopping score can reward two desirable features, but two very different products may tie. ACE similarly combines R with damping: one number is convenient, yet can hide how its ingredients differ.
R = 12
Q = 100 · ACE ≈ 1200
R = 10
Q = 120 · ACE ≈ 1200
Notice what changes
R already combines modulus and density. Q adds separately measured information about loss under specified conditions. Combining them does not make any missing input known.
Understand why
An increase in one factor can compensate for a decrease in the other. Two equal ACE values therefore need not mean equal mass, stiffness or damping.
Connect it to wood and making
Use a combined index to organize a comparison, not to bypass the maker’s goal. Whether extra R or extra Q is useful depends on the part and the structural task.
Put names to the ideas
Define the convention
Here ACE = R/η, approximately RQ for weak damping. Since η is dimensionless, ACE has the same units as R: m⁴/(kg·s).
Damping must be available
If Q or η is missing, ACE is unknown. Filling Q with zero or a species-average value makes the output look measured when it is not.
Inspect the ingredients
The same ACE may come from high R with greater loss or lower R with less loss. Keep the underlying measurements so that a single combined number does not hide the trade-off.
ACE ≈ RQ says to multiply R by Q in the weak-damping approximation. Missing Q means missing ACE. Despite its historical “efficiency” name, this result carries R’s units.
A worked example
R = 12 and Q = 100 give ACE ≈ 1200; R = 10 and Q = 120 give the same value. The materials are not mechanically identical despite the equal combined index.
Explore it yourself
Try changing loss while holding density and modulus fixed. R stays constant while ACE changes.
- c
- 5345 m/s
- E′/ρ
- 28.57 10⁶ m²/s²
- R
- 12.73 m⁴/(kg·s)
- Z
- 2.24 MRayl
- Q ≈ 1/η
- 100.0
- ACE
- 1273 m⁴/(kg·s)
E′ + ρ → c → R, Z η → Q → ACE