Question:
A while ago, I wondered what caused a Constant Directivity (CD) horn to exhibit a roll-off different from any other.

I made the assumption the phenomenon existed after seeing numerous processing products that include “CD horn compensation.”

I was told that no such thing existed, and that the roll-off exhibited was due to the break point associated with the diaphragm mass of the driver.

Could you please elaborate on what the truth is here? And what is this break point due to diaphragm mass? Does this exist?

Answer:
CD horns slot-load the driver at the horizontal break point of the horn. You can see it, if you look at the front of the horn into the bell. For most CD horns, this will be a vertical slot where the two sides of the horn bell meet.

This slot creates a diffractional effect that causes all the driver’s energy to radiate evenly between the horn walls, thus creating “constant directivity” from the lowest frequency. The horn loads the driver in the horizontal plane up to the frequency where the slot is wider than the wavelength.

This low-frequency loading point is evident in the vertical beam-width plot of the horn’s specifications. However, a driver’s total acoustic power output declines with increasing frequency, therefore the overall output level must decline as the frequency increases above the mid-band range where the driver is most efficient.

CD horn EQ boost is employed to counteract this phenomenon, producing a flatter response to the upper limit of the driver’s response.

Radial horns narrow in beam-width as frequency increases. Thus the lesser energy of the driver at higher frequencies still produces a flatter response on axis and needs no boost EQ.

CD horns were invented to counteract this narrowing of beam-width at high frequencies that the then-common radial horn exhibited. In order to maintain the flat on-axis response that everyone was used to, HF EQ boost was necessary.

The Diaphragm Break-Point phenomenon is more commonly known as the first break-up mode. The first break-up mode occurs when the wavelengths through the material of the diaphragm, not through air, become smaller than the diameter of the diaphragm.

This is not a roll-off of frequency response, but a deep notch in the higher frequencies usually followed by resonant peaks and more notches even higher in frequency. This notch can be 20dB deep, if your measurement system’s sample points are at the right frequencies.

A typical aluminum four-inch diaphragm will have this notch at a lower frequency than an aluminum three-inch diaphragm. Likewise, a titanium or beryllium diaphragm will have a higher first break-up mode than a similar diameter aluminum diaphragm.

This is because the speed of sound through those materials is faster than it is through aluminum. Paper cone speakers also have the same characteristics occurring at much lower frequencies. Break-up modes and resonances of 15-inch drivers show up, if the LPF of the crossover is removed.

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