PMC TB2+ analysis, measurements and foam replacement - Part III

erikjan · Thursday at 5:55 PM

Simulations of the PMC TB2+ with AJHorn (based on measured driver data)
Simulating the TB2+ is not trivial.

It is a transmission line.
The tuning of the line i s off, with line quarter wave resonance around 75Hz and driver resonance around 60Hz.
The line damping is heavier than usual.
The line exit is damped with a foam plug.
The line exit is not at the end of the line (strictly speaking).
The line exit (port) is at the back of the speaker, with a distance of about 40cm between the port center and the baffle plane.
The port cross section is actually 20cm² smaller that the line cross section (108cm² vs. 128cm²).

Personally I have used AJHorn, software from a German developer, for the simulation of transmission lines and other "horn" like speakers. For a description of the parameters in the screenshots you should consult the AJHorn 7 manual.

There are basically two ways to simulate the TB2.

As a simple straight line transmission line, ignoring the fact that the line exit is not placed at the end of the line.
As a straight line, with a closed mouth, and an added port. this is actually a more precise representation of reality, because the port has the correct cross section (108cm²), and the damping of the port can be set separately from the line damping.

I chose the second option. The screenshots show the parameters used. There is no crossover used in these simulations.

Parameter	Represents	Remarks
AT	Cross section of the start of the line
L	Length of the line
H&B	height and width of the end of the line (mouth cross section)
ß1 / ß2 / ßET	Damping for first, respectively second half of the line, and the port.	Parameter value ranges from 1-1000 and simulates medium density foam damping.
AET	Port cross section
LET	Port lenght
xD1 / xET	distance of driver and port to the start of the line.
dLET	Distance between port center and baffle plane
Position	Acoustic conditions	Free=Free field, the condition used for these simulations.
Input		Simulations use a standard input signal of 2,83V

AJHorn sim speaker A - include damping line + port - config parameters.png

AJHorn sim speaker B - include damping line + port - config parameters.png

AJHorn simulation parameters Speaker A = left, Speaker B = right

Sketch drawing and additional parameters Speaker A = left, Speaker B = right

The calculated volume of the "horn" matches the real volume closely. The raw volume of the speaker cabinet is 15 liters, and will be slightly higher in reality due to the effect of the damping material. The dividers account for about 0,8 liter, both drivers around 0,5 liter.