I’m new on the forum but I’ve being working
hard to understand this fascinating audio work.
I’m talking about Horn because the decision was
already made for my PA but I’m asking for help regarding what drive should I choose
my cabinet.
The target is: Lower frequency as possible with
higher SPL considering the model available on the list at the final of the
post.
Taking into account three very knew and well
successful Low frequency horn designs - JBL 4818 (W-HORN) è It uses JBL 2240 drive
- Cerwin Vega B36 (Earthquake) à It uses Eminence Sigma Pro 18A2
- Electro Voice T18 è It uses Electro Voice DL18MT
Doing reverse engineering comparing all those
drivers the main Thiele/Small parameters they have in common are:
High VAS (> 440 L) and also high CMS once
those two parameters are direct correlated.
Vas represents the volume of air
that when compressed to one cubic meter exerts the same force as the compliance
(Cms) of the suspension in a particular speaker.
Why high VAS is desirable? What is the negative
effect of using low VAS on Low-frequency Horn?
Low QTS (< 2,9)
A unitless measurement,
characterizing the combined electric and mechanical damping of the driver. In
electronics, Q is the inverse of the damping ratio.
The value of Qts is
proportional to the energy stored, divided by the energy dissipated, and is
defined at resonance (Fs).
No doubt about QTS
High BL
This is a measurement of the motor
strength of a speaker. Think of this as how good a weightlifter the transducer
is. A high BL figure indicates a very strong transducer that moves the cone
with authority!
No doubt about BL
Low Xmax (< 6mm)
Short for Maximum Linear Excursion.
Speaker output becomes non-linear when the voice coil begins to leave the
magnetic gap. Although suspensions can create non-linearity in output, the
point at which the number of turns in the gap (see BL) begins to decrease is
when distortion starts to increase
Low Vd (< 0,725 L)
This parameter is the Peak Diaphragm
Displacement Volume — in other words the volume of air the cone will move. It
is calculated by multiplying Xmax (Voice Coil Overhang of the driver) by Sd
(Surface area of the cone).
Here I see the first controversial parameter
for Horn once usually high Xmax is desired for low frequencies cabinets and
also high Vd, but looks like for Horn those variables are desired low to reduce
throat pressure and as consequence reduce the distortion. Could someone confirm or explain?
My cabinet is a close of Electro Voice T18 but is
not possible to buy the original T18’s drive DL18MT and change the design is
not an option, so I’m looking for local driver to install on this cabinet and
see what is the best tradeoff it can reach regarding low frequencies and SPL.
I’ve being also trying to study deeply the
theory behind the Horn design but without time and experience with this product
have been very hard to proper understand the things.
Some references I tried:
Low-frequency horn design using Thiele/Small
driver parameters (1977) è By D.B. Keele Jr
On the Specification of Moving-Coil Driver for
Low-Frequency Hon-Loaded Loudspeakers (1979) è By W. Marshall Leach Jr
I also tried to simulate the T18 cabinet using Hornresp
but the result is a little different from the original curve and maybe due to
some simplifications, losses or wrong input data.
The throat was cute in sections like indicated
below.
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