XO - Excel Crossover Design Tools
Parallel XO Series XO Target Model
The Tweeter modeling tool allows you to refine HF data before you input it into a crossover design tool. Typically, you would build a target model, extend the driver response over the desired frequency spectrum, build impedance models, adjust the phase wrap due to microphone measurement distance, and possibly provide impedance compensation elements using series or shunt R-L-C values. There are three distinct modules in this spreadsheet. The target model allows you to create magnitude and phase data for use in either target or response calculations. The filter module allows you to select inputs from either target data or measured input data and perform response shaping or impedance compensation functions. The filter sections use a pair of Series / Shunt R-L-C elements. There is also a driver impedance generator model which I have called Q-Builder. It lets you derive the T-S values graphically, or you can input the parameters if they are available. All sections allow the output to be saved to file.
Since this is the only XO spreadsheet using T-S
values, those calculations which require T-S parameters (such as a Zobel or
resonant circuits) are performed here.
If you implement these circuits as part of your driver model, then the
impedance and response values that you save in output files will assume that
your circuit now contains such components and that they are physically located
between the crossover and the drivers. The order in which you build these
models is important. For example, you
may want an attenuator (series + shunt resistor) circuit after your crossover
and a Zobel (parallel R-C) circuit connected after the attenuator. You can build these elements by first adding
the Zobel and saving the output to a file.
Then input that same file back into the Tweeter spreadsheet, add an
attenuator circuit, and again save the file.
It is the user’s responsibility to book-keep any components added and
the order in which they have been implemented.
As you know, cookbook formulas for a Zobel or resonant trap may not
always be ideal and you may want to optimize them. In that case, you can wait to add impedance
correction components in the crossover module, using the values calculated here
as a rough starting point.
Select
Desired Bandwidth
As in all
of the XO design spreadsheets, you must initialize the frequency spectrum. Set the number of data points to use, and the
lowest and highest frequency values. All
input and output will be based on the frequency ranges that you set. These values are located on the “RespData”
sheet. The imported frequencies in your
data files will be interpolated to the new values used by this spreadsheet, so
you MUST set up the desired spectrum before you begin your inputs.
Input/Output Driver Response / Impedance Data
The
input/output fields are in the RespData" section of the design tool.
When you import data, the imported frequencies will be interpolated to
new values based on the #Points, FMin, and FMax, so you MUST set up the desired
spectrum before you begin your inputs.
Build
a Target Response Model
The target function is the acoustic design
criteria for your system. In order to
build a target model, you must set the (acoustic) HP and LP corner frequencies,
the designed SPL, and roll-off shapes.
You can use the predefined shapes (Butterworth, Linkwitz-Riley, Bessel,
etc.), or model your own shape by inputting the required coefficient
values.
Like all Excel spreadsheets, you can also copy
and paste values into the crossover design tool. This becomes helpful whenever
your measured data is missing portions of the desired frequency spectrum. In this case, you can “add tails” to the
response by matching the shape of your response with a target functions model,
and then copy the missing values into the input columns. This works well if you are just filling in
out of band data, such as the tweeter frequency response at 20 – 500 Hz. If your data files don’t have phase
information, it would be best to derive the missing values using a Hilbert
transform calculation program. (You
could still do a quick “what-if” study using estimated phase values.) Remember that in Excel, you "Paste
Special" and select "values" to paste into the crossover
spreadsheet. Otherwise you will be copying formulas, not values.
Model
Driver Impedance
The impedance models are based on the Re, Le,
Fs, Qes, and Qms values of the tweeter. By varying Re, Qes and Qms, you
establish the shape of the driver's resonance peak and base resistance.
By varying Le, you shape the high frequency impedance. When you use
the Q Builder function, you can derive the Qes and Qms values, given Rmax
(maximum resistance at resonance) and F1 (the frequency at which the impedance
is .707*Rmax). While I have included
some sample components, they are just to get you started and you must get your
own data. To set Le, you can model the
inductance based on manufacturer's specs, measured impedance, or by using the
impedance graphs published by other hobbyists.
Note that after you finish your model, it must be stored in one of the
data rows on the Driver Sheet using the white shaded cells. You must then select the Driver option number
using the Spin Buttons located on either the Driver or Options Sheets.
OPTIONS
Series / Shunt Impedance Compensation elements - Miscellaneous filter elements that help shape response defects or provide impedance compensation. Those picks which have * or # in the description calculate values based in selectable input. If you enable any other combination of R, L, and C elements, you must input the values in the appropriate fields. Note that while these components affect system response, they are modeled as impedance elements to allow the use of “standardized filter shapes”. Note that as impedance elements, the crossover calculation values shift whenever you implement impedance correction.
Offset calculation - Horizontal separation distance for the acoustic centers of the woofer and tweeter.
Tweeter Phase Inversion Option - Checkbox to alter the HF phase response by 180 degrees. The simplest method to determine if phase inversion is required is to look at the combined amplitude response both with and without phase inversion.
Filter Source Input – You have the option of routing either the target response or the input response data through the filter section as a viewing tool or in order to piece together response segments in a partial input file. (Remember that any impedance or response file that is saved while the filter values are enabled will include the filter as part of the system.) You must manually to book-keep the values which you have implemented. Print the Options page as a record!
Note: Any selectable input field (white shaded cells) is normally zeroed out. All input must be numeric. If the spreadsheet misbehaves, it is typically because no value has been entered or a space has been used instead of a zero.
“No human thing is of serious
importance.”
Plato
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