Acoustic Center
Canonical Definition
The acoustic center is an effective origin used to describe the phase or arrival behavior of a radiating source for a stated frequency range, axis, loading condition, and measurement method. It is not always a single fixed physical point.
Its useful location depends on what frequency band, axis, filter state, mounting geometry, and measurement question are being discussed.
Why Physical Landmarks Are Only Starting Points
| Driver type | Common tape landmark | Why it may differ from effective origin |
|---|---|---|
| Cone driver | Dust cap, cone plane, or grille | Motor, suspension, cone breakup, enclosure, and filter response affect phase |
| Dome tweeter | Dome apex or faceplate | Dome geometry, rear chamber, waveguide, and high-pass response affect timing |
| Compression driver and horn | Horn mouth | The effective source can lie behind the mouth because energy travels through the throat and horn |
| Subwoofer system | Cone or enclosure face | Low-pass filters, enclosure alignment, cabin loading, and limited bandwidth broaden the response |
Frequency Dependence
A driver does not necessarily behave like one rigid point source across its whole operating range. Radiation pattern, cone or diaphragm modes, horn loading, baffle interaction, and filters can change the effective phase origin.
Therefore, an offset inferred at 500 Hz should not automatically be applied as a universal center at 5 kHz or 50 Hz.
How Acoustic Center Appears In Measurements
Impulse response
Relative arrival can suggest an effective offset when measurements share a timing reference. The first or largest peak is not a universal center marker across driver bandwidths.
Phase slope
A consistent relative phase slope through an overlap band can be converted into an effective time or distance offset after bulk delay is accounted for.
Crossover summation
Adjusting delay until the intended normal-polarity sum and reverse-polarity null appear can reveal the useful effective alignment for that crossover band and observation point.
Acoustic Center Versus Acoustic Path Length
Path length is the distance sound travels to the microphone or listener. Acoustic center is the effective origin used in the phase or timing model. The useful delay correction includes both:
Effective relative delay
= propagation-path difference
+ driver / enclosure / filter timing difference
+ any channel processing-latency differenceCar-Audio Examples
- A dash tweeter may be physically closer than a door midbass but have additional acoustic or filter delay.
- A horn mouth may look close while its effective origin lies farther back.
- A subwoofer in the cargo area may measure later than tape distance predicts because of low-pass and enclosure behavior.
- A seat or console reflection may make one apparent arrival marker unstable with microphone position.
Measurement Workflow
- Define the microphone position and frequency band of interest.
- Use the same timing reference for every driver.
- Measure drivers individually with their intended crossover filters.
- Estimate physical path difference.
- Compare impulse and relative phase through the overlap band.
- Adjust delay and polarity while measuring normal and reversed sums.
- Record the effective offset as band-, axis-, and setup-specific.
Common Mistakes
- Calling the voice coil, dust cap, or horn mouth the acoustic center without qualification.
- Applying one inferred center across every frequency.
- Using measurements with different timing references.
- Confusing polarity or filter-shape problems with physical offset.
- Ignoring how the center changes with microphone angle or position.