Understanding loudspeaker measurements

No single graph completely describes a loudspeaker. Good evaluation combines several measurements.

On-axis frequency response

The on-axis response shows what a microphone receives directly in front of the speaker. It is important, but listeners also hear reflected sound.

A smooth on-axis response is most useful when accompanied by well-controlled off-axis behavior.

Directivity

Directivity describes how output changes with angle.

A small radiator is nearly omnidirectional when its dimensions are much smaller than the wavelength. As the radiator becomes large relative to the wavelength, its output normally narrows.

That is why a large woofer may become directional before a small tweeter does. A poorly chosen crossover frequency can therefore create an abrupt change in radiation pattern.

Polar plots and contour maps

Polar plots show output at different angles for one or more frequencies. Directivity contour maps show angle and frequency together.

Look for:

• Smooth narrowing with frequency
• Consistent left-right behavior
• No sudden off-axis gaps around the crossover
• Controlled rather than irregular radiation

Diaphragm breakup

At low frequencies, a cone may behave approximately like a rigid piston. At higher frequencies, bending waves and resonances appear.

Breakup can cause:

• Response peaks and dips
• Ringing
• Narrow or irregular radiation

Materials with high stiffness-to-mass ratios may move breakup higher in frequency, while materials with greater internal damping may reduce the severity of resonances.

Harmonic distortion

Common sources include:

• Suspension compliance changing with excursion
• BL changing as the coil moves through the gap
• Voice-coil inductance changing with position and current
• Cone and surround resonances
• Voice-coil heating
• Port turbulence

Distortion should be measured at stated sound-pressure levels. A percentage without level and distance is incomplete.

Power compression

As the voice coil heats, its resistance rises. For a constant amplifier voltage, this reduces current and output. The loudspeaker therefore produces less additional sound than an ideal model predicts.

Transient behavior

A high-Q resonance stores energy and takes time to decay. This can produce ringing or “overhang.”

Transient behavior is related to frequency and phase response.