Constant Directivity

Left: a normal driver beams its highs so only the centre seat gets them. Right: a CD horn holds the same -6 dB coverage angle at every frequency, so the whole crowd hears matched tone.

What it is

A speaker design that keeps its coverage angle the same width across all frequencies, so tone stays even as you move off-axis.

Key facts

• Speed of sound in air: 343 m/s at 20 degrees C (≈ 1 ft per millisecond).
• Wavelength formula: λ = c / f, where λ = wavelength (m), c = sound speed (343 m/s), f = frequency (Hz).
• 20 Hz wave ≈ 17.2 m long; 20 kHz wave ≈ 17.2 mm long — a 1000:1 size range.
• A driver beams (narrows) once its size approaches the wavelength it radiates — highs narrow, lows stay wide.
• Coverage angle = included angle between the two off-axis points that sit -6 dB down vs on-axis.
• CD target: hold that -6 dB coverage angle roughly constant from ~500 Hz to 16 kHz.
• Inverse-square law: each doubling of distance = -6 dB SPL in the free field.
• -3 dB = half power; -6 dB = half voltage / quarter power; +6 dB = double SPL; +10 dB ≈ twice as loud.
• Common CD horn patterns: 90x40, 90x60, 60x40, 60x60 (horizontal x vertical degrees).
• Directivity: DI = 10 x log10(Q); CD horns roll off on-axis HF ~ -3 to -6 dB/octave, so you EQ a HF shelf boost back flat.

How it works

1. A cone/dome radiates wide at low freq because the wavelength dwarfs the driver.
2. As frequency rises and wavelength shrinks toward driver size, the beam narrows (beaming).
3. A CD horn uses shaped walls (often a diffraction slot then a flared mouth) to force a fixed angle.
4. The horn mouth controls low-mid spread; the diffraction throat keeps the highs spread wide.
5. Net result: the -6 dB coverage angle stays nearly constant across the band.
6. Trade-off: on-axis HF drops, so you apply an HF shelf EQ boost to flatten the on-axis response.

Real examples

• A 90x40 CD horn covers a 90-degree-wide, 40-degree-tall room evenly — front row to back wall hear the same tone.
• Cheap unhorned tweeter: only the punter dead-centre gets crisp highs; off to the side it sounds dull and muffled.
• Line array elements use CD waveguides so every box couples and the highs do not beam into one row.
• A point-source FOH top rated 60x60 aimed across a marquee covers a tight crowd without spraying highs onto walls.
• Coaxial stage monitor with a CD waveguide gives the performer consistent tone as they move side to side.

How it helps in live sound

• Pick the horn pattern to match the room: 90x40 for wide rooms, 60x40 for deep/narrow throws.
• Aim by the -6 dB edges, not the centre — overlap edges of adjacent boxes for even cover.
• Apply the spec HF shelf boost (often +3 to +6 dB above 3-4 kHz) so on-axis tone is flat.
• Splay multi-box arrays by the rated angle (e.g. two 60-deg boxes splayed 60 deg = 120-deg cover, minimal overlap).
• Use a real polar/coverage prediction (MAPP, ArrayCalc, EASE) before flying, not guesswork.
• Walk the room with pink noise and an RTA: tone at the edges should match centre within a few dB.