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+---------------------------------+ | | | Loudspeaker Modeling Program | | | | (LMP) | | | +---------------------------------+ A Speaker System Modeling Program & Passive Network Design Utility For IBM and compatible computers DOS v3.xx suggested LMP was co-authored by Ralph Gonzalez and Bill Fitzpatrick. Based on an original work by Ralph Gonzalez. ---------------------------------------------------------------------------- (C)1990 Sitting Duck Software POBox 130 Veneta, OR 97487 (503)935-3982 All rights reserved THIS MANUAL This manual covers the use of LMP, provides several example designs and explains each input and menu choice in detail. The complete theory of loudspeakers, crossover networks, enclosures, "time alignment", and the 1001 other details relevant to the construction of a quality loudspeaker system from an infinite assortment of parts are far beyond the scope of this manual. The hobbyiest who wishes to explore the finer points of system design is referred to the following: - Speaker Builder; issues 1, 2 and 3 1987 - Speaker Builder; issues 1, 2, 3, 4 and 5 1988 - Speaker Builder; issue 2, 4 and 6 1989 - Loudspeaker Design Cookbook by Vance Dickason - High Performance Loudspeaker Enclosures - Bullock On Boxes - Articles from the Journal of the Audio Engineering Society All but the last item are available from Old Colony Sound Labs, PO Box 243, Peterborough, NH 03458, (603) 924-6371 ----- GETTING STARTED WITH LMP -------------------------------------------- Make a working copy of your distribution diskette and put the original in a safe place. +++++ THE FILES ON YOUR DISKETTE SD-LMP .EXE The Loudspeaker Modeling Progam run file. SD-LMP .DOC This documentation. XOVER <DIR> The sub-directory containing the models. X-OVER .BAT A batch file to run a filter program. P-FILTER .EXE The Passive Filter design program run file. FSGP .DTA A data file for P-Filter. TSGPP .DTA A data file for P-Filter. TSGPN .DTA A data file for P-Filter. +++++ MONITOR SUPPORT --------------------------------------------------------------------------- LMP supports Hercules, CGA, EGA color and VGA color monitors. If you have a Hercules monitor you must run your graphics support program before running LMP. +++++ DESCRIPTION OF PROGRAM --------------------------------------------------------------------------- LMP is a computer program designed to model 2-way loudspeaker systems. The resulting frequency and phase response curves predict the on-axis sound pressure level produced by the interaction of your choice of crossover, drivers and enclosure design. LMP assumes that the impedance curve of the drivers in the modeled system have been made reasonably flat by way of Zobel and/or impedance correcting networks. LMP also assumes that the drivers modeled have a flat frequency response from the low frequency roll-off point to the high frequency roll- off point. Quality drivers on the market today meet this criteria. LMP assumes that the user has had some experience with loudspeaker systems and crossover networks and is familiar with the terminology commonly used to describe the various parameters involved. ===== REGISTRATION ======================================================= Tens of thousands of people build loudspeaker systems as a hobby. Others do it to save a considerable amount of money by not buying the very high priced commercial speakers which often do not perform as well as a home constructed system. Home built speakers cost less because there are no dealer markups, shipping for heavy enclosures, advertising costs and profits for manufacturers. Properly designed and built, a home brew system will always sound "good" and with much attention to theory and detail can sound "superior". High quality parts for the speaker building hobbyiest are readily available from a number of sources at very reasonable prices. For example, a high quality 12" woofer can be obtain for $60 when purchased via mail. REGISTRATION of LMP for $40 provides you with the following: - A spiral bound, printed manual detailing the use of the program. The printed manual is similar to the disc documentation. - An enhancement which allows the modeling of 3 and 4 way systems. - Recommended reading list for those lacking technical knowledge. - Recommended sources for loudspeaker system parts. - User support. Register with: SITTING DUCK SOFTWARE PO BOX 130 VENETA, OR 97487 Sorry, we cannot take credit cards. Add $5 shipping for overseas. Checks must be in U.S. dollars and drawn on a U.S. bank. International Money Orders payable in U.S. dollars are acceptable. The equivalent of $45 U.S. dollars in foreign currency is acceptable. ----- AN LMP WALK-THROUGH ------------------------------------------------- We are going to design a 2 way loudspeaker system with an 8" woofer and a 1" dome tweeter crossing over at 1500Hz. Drivers that are available commercially have specifications similar to the inputs we will provide to LMP. The crossover design for this example will be done with the P-Filter program supplied. Type SD-LMP to start the program. +++++ THE MAIN MENU Starting LMP will present you with a menu screen hereafter described as the "main menu". Select the menu choice "P" to make a file which describes your printer. We suggest 150dpi, portrait mode for Laser Jet users. Select the menu choice "C" to run the batch file which runs the P-Filter program. Exit and run P-FILTER separately if you don't have DOS 3.xx. From the filter menu select choice "C" to create a new crossover model. Enter 2 for the number of drivers. Type "8 inch woofer" for the 1st driver description. Type N for NO polarity reversal. Type "1 inch tweeter" for the 2nd driver description. Type N for NO polarity reversal. Type 3 for crossover order. Type 1500 for crossover frequency. Type 8 for each of the impedance values. YOU ARE NOW BACK AT THE MAIN MENU OF P-FILTER. Select menu choice "S" to save this crossover design. The design is saved in the directory XFILES under the name PASSIVE.TMP Select menu choice "V" to see the hook-up diagram and parts selection for the network you have designed. Print this diagaram if you wish and return to the menu. Select menu choice "R" to view the curve. Press <ESC>. Select menu choice "E" to exit the crossover design progam. YOU ARE NOW BACK AT THE MAIN MENU OF LMP. Select menu choice "L" to load the network you have designed. From the provided menu, highlight PASSIVE.TMP and press enter. You are again at the main menu and your crossover design has been loaded. +-------------------------------------------------------------------------+ | Although your "model" now contains information about the crossover net- | | work, it does not yet have any information about the drivers you are | | going to be using. You will provide that information now. | +-------------------------------------------------------------------------+ Select menu choice "M" to modify the model. Note that information from the crossover design is already shown on this screen. Press <ENTER> to accept the 2 driver configuration. YOU WILL NOW ENTER INFORMATION FOR THE WOOFER WHICH IS DRIVER #1. Assume that the woofer is in a sealed box, the resulting Low frequency corner is 40Hz and the Low frequency damping ratio is .7. Input that information. Assume that the woofer high frequency roll-off causes it to be 3db down at 3000Hz, the damping ratio is .7 and the roll-off is order 4 (24db/o). Input that information. Press <ENTER> to accept the Polarity Inversion default of "N". According to the manufacturers data sheet, the sensitivity of the woofer is 90db and it was measured on a EIC standard baffle (53" x 65") at 1 meter with 2.82 volts input. Our enclosure is going to have an average baffle size (Height x Width/2) of 16". The on axis output will begin to DECREASE as the wavelength of the sound gets longer and begins to equal the average baffle size. This frequency is provided by the formula: F = 13500/BaffleSizeInInches. and is called the Step Frequency. For our example enclosure, the Step Frequency will be 843Hz. Since our small cabinet will be mounted on stands and placed well into the room, the room boundaries provide no extension to the baffle. In this case the step will cause a loss of 6db. This loss will just begin to show up a 843Hz. The shape of the curve which describes this "diffraction loss", shown in Figure 1, is for 843Hz with a 6db step. sorry, curve not available in text file We need to allow for this loss within the program and will do so by REDUCING the inputted value for Sensitivity by 6db. So, rather than accepting the 90db default sensitivity value provided by the network design procedure, reduce the value to 84db. Accept the 0 default for Depth Displacement, input 843 for the Step Frequency, 6 for the Step Height and accept the crossover default choice of 8. +-------------------------------------------------------------------------+ | The screen you now see is for entering the crossover component values. | +-------------------------------------------------------------------------+ Press <ENTER> four times to accept the default values of your crossover design. YOU WILL NOW ENTER INFORMATION FOR THE TWEETER WHICH IS DRIVER #2. Assume that the tweeter has a Low frequency corner of 600Hz, a Low frequency damping ratio of .7, a High frequency corner of 20000Hz, a High frequency damping ratio of .7, and a High frequency roll-off order of 4 and no polarity inversion. Because the tweeter is mounted on the same baffle as the woofer, we need to duplicate the Step information and adjust the sensitivity value as we did with the woofer. Actually, we don't need to do this with a 3rd order network since 843Hz is far enough from 3000Hz that the step will make little difference. But, it won't hurt. Accept the displacement of 0, input 84 for the sensitivity, 843 for the step frequency, 6 for the step height and accept the default crossover choice. Accept the defaults for the crossover component values. YOU ARE NOW BACK AT THE MAIN MENU. Select "R" to view the response and phase curves. Make a print-out of this curve if desired. Figure 2 sorry, curve not available in text file Response of model before modification. We are assuming that Zobels or impedance correcting circuits are going to be used where needed and that the tweeter will be set back on a stepped baffle board so there is no offset. This, then, is a fairly accurate model. The resulting design is not particularly good for 1990 standards though it is similar to some designs of the 70's which were known as "West Coast" speakers and had a forward quality. Let's see if we can apply a couple of quick fixes. Since reversing the phase of one of the drivers does not help this example (try it and return the phase to normal) we'll have to try something else. One possible approach is to reduce the amount of mid-range output and forget about the 90db effeciency that we were expecting. Let's try increasing the first inductor (k3) in the Low Pass network to 2.2mh to restrict the amount of mid-band energy getting to the woofer. Let's also reduce the first capacitor (k3) in the High Pass network to 7.0mfd to restrict the amount of mid-band energy getting to the tweeter. Now, run the curve again. Figure 3 sorry, curve not available in text file Response of model after modification. Not too bad. The room boundary effects are going to help lift the low end so this might be a listenable speaker after all. Note that the fix applied to our example does not represent the only fix possible nor does it necessarily represent the best fix. RETURN TO THE MAIN MENU. +++++ SAVING YOUR DESIGN Select "S" from the menu. Assign a DOS legal name and short description of the model. This model is now saved and can be loaded in again during a subsequent session with LMP. YOU ARE NOW BACK AT THE MAIN MENU AND HAVE COMPLETED THE WALK THROUGH. ----- MAIN MENU CHOICES --------------------------------------------------- The following are brief descriptions of the various choices available from the main menu. <L>oad model Loads a model file from the sub-directory XFILES. Loading a model overwrites the model currently in memory. You are presented with a loading menu containing a list of models available. <S>ave model Saves the model currently in memory. If it is a new model you are given the opportunity to assign a file name and brief description. The file name must be DOS legal; 8 legal characters, a period and a 3 character extension. The model is saved in the subdirectory XFILES. <M>odify model If you have no model loaded, pressing "M" will begin a new model. If a model is currently in memory, pressing "M" allows you to modify it. <Q>uit Exits to DOS, giving you the opportunity to save the model currently in memory if it has not been previously saved. <P>rinter setup Lets the program know about your printer. This information is written to a file and loaded the next time you run the progam. <R>esponse Displays the frequency and phase response curves of the model currently in memory. <D>elete file Deletes the disc file corresponding to the model currently in memory and then removes the model from memory. <C>rossover Temporarily suspends execution of LMP, exits to DOS and runs the batch file named XOVER.BAT. If XOVER.BAT does not exist, you will be returned to LMP with your model intact. The first line of the XOVER.BAT file must contain the name of the crossover program you wish to run. The next line must say EXIT. We supply XOVER.BAT which is written to run P-FILTER. You may substitute your favorite crossover design program by modifying the batch file. Only P-FILTER will save a design to a file readable by LMP. <B>ox Temporarily suspend execution of LMP, exits to DOS and runs the batch file named BOX.BAT. BOX.BAT may contain the name of your favorite box design program. We do not supply a BOX.BAT file because we do not supply a box design program. <F>ree Temporarily suspend execution of LMP, exits to DOS and runs the batch file named FREE.BAT. FREE.BAT may contain the name of any program you might wish to run from within LMP. +---------------------------------------------------------------------+ | In order to exit LMP and run the above batch files, DOS must be | | available. Because leaving LMP loads an extra copy of COMMAND.COM | | and retains LMP in memory, the ability to run other programs from | | LMP will be dependent upon the available memory. You should have | | no trouble running most of the box and crossover programs which are | | available. Note that the procedure for leaving and returning to a | | program with the original program data intact is called "SHELLing". | | Shelling requires DOS 3.xx and if you have an earlier earlier | | version of DOS DO NOT use the Crossover, Box and Free menu choices. | | +---------------------------------------------------------------------+ <O>ptions <T> Allows you to toggle between Q and damping ratio. Use the form you are comfortable with. <R> Sets the resolution for the response display. <F> Sets the frequency range the curves will cover. <!> Clear Clears the model currently in memory so you can begin anew with a clean slate. ----- ENTERING INFORMATION FOR THE MODEL ---------------------------------- +++++ THE DRIVER DATA FIELD The following provides details for each item that is entered in the driver model field. +++++ LOW FREQUENCY CORNER The resonant frequency of the driver. Use an approximate value for a woofer if the value is unknown or the system is other than an acoustic suspension. Mid-range drivers are almost always sealed systems and tweeter are always sealed so use the manufacturers resonant frequency specification. +++++ LOW FREQUENCY DR/Q Some prefer to work with Damping Ratio, others prefer to work with Q. The Options selection from the main menu allows you to choose. Speaker manufacturers always specify Q so it may be more convenient to follow suit. You can convert with the formula: DampingRatio = 1/2Q. If the Q specification is not given but a response curve is provided, compare the response curve to the figure below and pick the shape with the closest match. Some conversions are provided for your convenience. sorry, curve not available in text file Figure 4 Low Frequency Damping, Second Order, 1KHz +++++ HIGH FREQUENCY CORNER, DAMPING RATIO and ROLL-OFF ORDER Refer to FIGURES 5 and 6. The two sets of curves represent various Damping Ratios for a 1KHz High frequency corner. Compare the curves to the high frequency roll-off of your driver as shown on the manu- facturers data sheet. Select a curve that most closely approximates the high frequency roll-off of your driver. You can estimate for curves that fall in between those shown. Example: A manufacturers curve most closely approximates the curve corresponding to DR=1.0/Q=.5 on the Second Order graph. The curve is 6db down at 1KHz. On the manufacturers curve, the 6db down frequency is 2300Hz. Then: High Frequency corner = 2300Hz sorry, curve not available in text file Figure 5 High Frequency Damping, Second Order, 1KHz sorry, curve not available in text file Figure 6 High Frequency Damping, Fourth Order, 1KHz +++++ SENSITIVITY The sensitivity of the driver. For a system, the sensitivity of each driver should be determined by the same method. The standard is 2.82 volts input and measured at 1 meter; this is equivalent to 2 volts in and measured at .5 meter. +++++ POLARITY Input "N" if the - lead from the amplifier is connected to the - lead of the driver. Input "Y" if the - lead from the amplifier is connected to the + lead of the driver. This is based on the assumption that a positive signal on the + lead causes the cone to move foreward, which is almost always the case. +++++ DEPTH DISPLACEMENT Each driver will have a different "distance to the listener" when mounted on a flat baffle. The output of drivers further from the listener will be delayed in time and thereby phase shifted. The acoustical center of each of the drivers in a system would be the ideal value to input here. This information will not generally be available so we must do with an approximate physical measure- ment. Measure the height of the cone or dome of each driver and use one half of that value. Enter 0 for the driver having the forewardmost cone. Enter negative values in inches for the remainder of the drivers based on how far behind 0 they will be when mounted on the baffle. For example: We have a dome tweeter whose AVERAGE dome height will be .25" forward of the flat baffle on which it is mounted. We have a woofer whose AVERAGE cone depth will be 1" rearward of the flat baffle on which it is mounted. Effectively the woofer will be 1.25" behind the tweeter when mounted on the baffle. The proper inputs for this example would then be: 0" depth displacement for the tweeter. -1.25" depth displacement for the woofer. If these drivers were mounted on a stepped baffle, where the woofers mounting surface was 1.25" forward of that for the tweeters, you would then input 0 for each of the depth displacement values. Specifying the proper depth displacement does not "time align" the drivers. +++++ RESPONSE STEP FREQUENCY and HEIGHT In the 2 way example model for the LMP walk-through, we used an average value of the height and width for the baffle size to determine the response step frequency. In reality, the baffle does not have a single size, but many. For a driver which is mounted asymmetrically on a baffle; measuring from one baffle edge, across the center of the driver, to the other baffle edge every 22.5 degrees on a circle will yield 4 DIFFERENT distances and 4 different step frequencies, each contributing 1.5db to the step. Fortunately, the step curve produced by the averaging method will be within 1 or 2db of a multi-step curve if the height to width ratio of the baffle is not unusually large. If sound absorbing foam is applied to the surface of the baffle, the step frequency will increase to a degree related to the absorbtion characteristics of the foam. This should be considered as somewhat unpredictable. Speakers designed to be wall mounted will have NO step frequency or step height as the wall acts as an extension of the baffle. +++++ CROSSOVER CHOICES Select the number corresponding to the crossover choice that you want to use for the driver # that you are inputting information for. For a woofer this will usually be a low pass; the mid-range, a band pass; the tweeter, a high pass. You may often opt for no network, allowing only the drivers natural roll-off to provide filtering. ASYMMETRICAL BAND PASS NETWORK SECTIONS MAY BE MODELED. If you want a 3rd order high pass cascaded with a 4th order low pass, select the 4th order band pass and "open" the second parallel inductor by inputting a value of 1000 mh. +++++ ENTERING INFORMATION IN THE CROSSOVER FIELD Enter values for each element in the crossover when prompted. All inputs are in standard units; microfarads, millihenries and ohms. To accept the displayed default value, just press <ENTER>. Parallel INDUCTORS may be OPENED by inputting 1000 mh Series INDUCTORS may be SHORTED by inputting .001 mh Parallel CAPACITORS may be OPENED by inputting .001 mfd Series CAPACITORS may be SHORTED by inputting 1000 mfd Note that "asymmetricalizing" bandpass filter designs imported from P-FILTER will cause the bandpass gain to be non-constant across the bandwidth of the filter. ----- BI/TRI AMPLIFICATION ----------------------------------------------- Some believe that bi or tri-amplification solves all the problems that are inherent in systems with passive crossovers. This is not true. While multi-amplification does eliminate the need to consider the drivers impe- dance curve and apply Zobels or other correction networks, the systems drivers still have their own roll-off characteristics, depth displacement and step frequency considerations. Multi-amplifier system designs can be modeled with LMP as easily as passive network systems. Each network choice in the P-Filter program and any asymmetrical design you wish to create can be duplicated with active circuits. There are a number of active filter design programs available for the PC from Old Colony Sound Lab, even one from Sitting Duck called A-Filter.