Some of the main reasons for a
phase plug are to:
a) Reduce path length differences between driver diaphragm and
horn throat
b) Cancel resonant modes in the
front (throat) cavity (only the important first few)
c) Reduce front cavity volume
so that [Hf] may be extended (second resonance)
d) Assist in determining
(controlling) dispersion pattern
The formulas below will help
with assessing some of the above.
[fp] = [c]/(2*[pi]*[h]*[T] -
high pass cutoff frequency for phase plug (Hz)
and
[W] =
4*([pi]^2)*([f]^2)*(X^2)*[p0]*[c]*[Sd]*[T] – acoustic power
at [St] (Watts)
where,
[c] = 345 m/s – sound velocity
[h] - diaphragm to phase plug clearance
[T] = [Sd]/[St] - compression ratio
[f] - reference frequency ([f] > [fc]) (Hz.)
[X] - diaphragm excursion
[p0] = 1.18 kg/m^3 - air density
And if you are wondering where
you take your diaphragm reference point from, if you’ve done
it right then [h] will be a constant and so can be taken from
any point along the inside edge. If a constant has not been
achieved then reading will be the mean diaphragm to inside
edge clearance.
The
Power equation is acoustic watts
d)
can include assisting in dispersion pattern by allowing the
use of near parallel and hence tight dispersion expanding horn
walls that would normally dictate a large throat section. The
phase plug would be used to occupy volume in the throat and
reduce its area down to that dictated by the mouth and length
for the chosen fo.
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