Smooth Power Response

What power response means

Power response is not another name for on-axis frequency response. The on-axis curve shows the sound travelling directly towards one measurement position. Power response is the total acoustic power radiated over all directions, plotted against frequency.

Directivity index connects the two. If the on-axis response is flat and the directivity index remains roughly constant, the power response follows the same overall spectral shape, separated by a nearly constant level difference. If directivity changes abruptly with frequency, the power response changes with it even when the on-axis graph still looks flat.

Why it matters in a room

At the listening position you hear the direct sound plus reflections from the walls, floor and ceiling. Those reflections are made from the loudspeaker’s off-axis output. If its off-axis spectrum is uneven, the room returns a differently balanced version of the direct sound.

This is why the radiation pattern matters even when the listener remains near the main axis. Moving within the room — or simply turning your head — changes the balance of sound arriving from different directions. Equalisation at one point can correct the level measured there, but it cannot change the loudspeaker’s spatial radiation pattern.

A smooth power response does not make every seat identical: room modes, boundaries and geometry still matter. It means the loudspeaker itself avoids abrupt tonal changes as radiation angle and frequency change.

Why Baffless uses dipole radiation

Controlled directivity can be created with waveguides, driver arrays or beamforming. Baffless instead uses a dipole across all four frequency bands. An ideal dipole radiates equally forwards and backwards, has deep side nulls at ±90°, and has a directivity index of approximately 4.8 dB.

The side nulls reduce the energy sent towards the lateral walls relative to the forward and rear radiation. The minimal baffle also avoids the large changes in directivity and diffraction that appear when a baffle becomes large compared with the wavelength. Each driver is used within a limited band so that the complete system stays close to dipole behaviour over as much of the spectrum as practical.

What the measurement shows

The sonogram below is a free-field horizontal measurement of the Baffless system. Frequency runs from 20 Hz to 20 kHz along the horizontal axis, angle runs from −90° to +90° vertically, and colour represents sound-pressure level. Smooth, nearly parallel contours indicate that directivity is changing gradually rather than in abrupt steps.

Pic. 1 — Horizontal sonogram, free-field measurement, 20 Hz–20 kHz and −90° to +90°, using frequency-dependent windowing at 1/6.9-octave resolution.

The dipole pattern remains well controlled through most of the range, but the measurement also shows its real limitation. The 60 × 80 mm high-frequency radiator becomes large compared with the shortest wavelengths, so the polar pattern begins to depart from an ideal figure-of-eight before 10 kHz. A smaller high-frequency dipole would extend the controlled pattern further, but suitable driver availability sets a practical limit in this implementation.

The design target is therefore not identical sound at every point in the room. It is a flat direct response, a smooth change of directivity with frequency, and no abrupt radiation discontinuities through the crossover regions. That gives the reflected sound a spectral balance that remains closer to the direct sound.

Individual polar plots and measured directivity-index values from 20 Hz to 20 kHz are shown on Dipole Loudspeakers.