[Paul Spencer]
While I'm not an authority on horn design, I present what I'm learning
while in the process of attempting to design an effective and compact
bass horn.
1. Compile a list of drivers which appear suitable
It appears that its possible to design a narrow bandwidth horn for most
drivers, however, in general, drivers with an xmax of 12mm, a light moving
mass (mms), a fairly low Qts and high BL are preferred.
Look for TS parameters that are required by Horn Response
2. Enter parameters, then use Hypex Designer as a starting point
In the Tools menu, select "Hypex Designer" and enter in the bandwidth
over which you want the horn to be optimised.
In my case I choose 40 - 80 Hz
3. Check compression ratio
This is the ratio of SD:throat area. If the throat is half SD, then you have
a compression ratio of 2, which appears to be a commonly recommended maximum.
Often you will get a higher ratio from the Hypex designer, so you may wish
to increase throat area.
Then you will notice that the response will change
4. Downsizing
This is where you start to compromise. If you plan to make a stack of horns,
you might reduce mouth size by half and stack two together. Avoid shortening
the horn length too much, as this will mean you will lose bottom end and there
will be a dramatic loss of performance.
5. Tweaking
You may not have a very flat response. If at the bottom of the response there
is a peak followed by a trough, you can tweak this with the rear chamber volume
(VRC) and throat area (S1). Experiment with this and you will find you can
often smooth the bottom end fairly easily.
Try changing one thing at a time, then "calculate."
Press F4 while looking at the charts for SPL and displacement and you will see
the previous version.
Change the horn length (HYP) and you will notice that this changes the corner
frequency above which it rolls off.
Change VRC and you will notice it affects excursion below fc (cut off frequency),
as well as the steepness of the rolloff slope.
6. Check excursion and power
Double click on "ES" where you enter input power in Volts RMS. It will calculate
volts when you enter in impedance and power. For a max SPL check, enter in the
nominal power rating of the driver then "calculate." Most likely, you will be
well below xmax above fc, but exceed it below fc where excursion increases rapidly.
If you use a high pass filter with an fc @ F3, the excursion will decrease approx
30% at that point. If you use a 2nd order high pass, then half an octave below fc
the excursion will be reduced by at least half. After such a high pass filter is
used, the excursion limited power will increase so that the power handling will be
limited by excursion at fc. The actual power handling will be typically 140% of
the excursion limited power hanlding at the F3 point without the filter.
Example: Suppose you use a driver with an xmax of 10mm in a horn with an F3
at 40 Hz, crossing to a subwoofer below this point with a 2nd order high pass filter.
Driver reaches xmax @ 40 Hz with 100w without the crossover. When the crossover is
used, the power handling will be 140w.
Above this point, the power handling will be thermally limited. Horn loading
increases the impedance and less current goes through the voice coil. The driver
will fail due to overheating rather than overexcursion in this range.
