© Tolvan Data 2004-2006
The Edge program is a baffle diffraction simulator. Typically it is intended to use for estimation of the "baffle step" that occurs when a loudspeaker is mounted in a baffle of finite size. It is common that the baffle step is approximated with a pole-zero pair to approximate the 6 dB raise towards higher frequencies. The Edge uses the geometrical theory of diffraction (GTD) to estimate this transition better.
The Edge is and will probably remain a pure baffle diffraction simulator. The results from the Edge should of course be added to other properties of a loudspeaker system, such as those described by Thiele-Small and others and also the effects of crossover filters etc, such as can be done in the Basta! loudspeaker simulator (http://www.tolvan.com/basta). The Edge can simulate a simple baffle step compensation, or an open baffle bass drop compensation, but that is probably as far as the development will go. The edge can also export the response data in text format, for further use in other programs.
The Edge has a home page at http://www.tolvan.com/edge
Suggestions and opinions both on the program and this documentation are happily accepted by firstname.lastname@example.org
With the baffle designer, the dimensions of the baffle are entered into the Edge, the loudspeakers are placed on the baffle and the microphone is placed in front of the speaker system. The baffle shape is determined by a number of corners, and they can either be dragged with the mouse to for the shape of the baffle, or they can be moved to form a baffle with a width and height as defined in the "size" group box.
Figure 1. A baffle of size 300 x 500 mm, with the loudspeaker placed slightly to the right. The Edge models this particular system by means of one point source at the speaker centre (blue), and 36 sources (green) of opposite sign at the baffle edge (red). The microphone is in the far field (100 m away), so the positioning of the "mic" symbol has little effect in this case.
Up to 50 drivers can be placed on the baffle, and each of these is modelled by a set of point sources and their mirrors. The "speaker source density" spin box determines the number of point sources used per driver. Increasing this number is essentially a way of modelling the speaker directivity. It does not, however include cone break-ups, since all of the sources are of equal magnitude and phase. Each of these point sources has an amplitude of 2, corresponding to the sources and their mirrors. For each of these sources, a number of edge sources are placed along the edge of the baffle. The sum of these sources has an amplitude of -1. The edge sources are distributed along the edge of the baffle such that each edge source represents the same angle, as seen from the driver source(s). As can be seen in Figure 1, this method automatically results in a higher edge source density when the driver is placed near the baffle edge and that each source can have the same amplitude. In the case of the open baffle the amplitude of the edge sources is doubled, to model the diffracted radiation from the back of the driver. The number of sources used in the simulation can be large, and this may slow down the calculation of the response. Some experimenting with the number of edge sources and speaker source density is recommended. In general, a higher number of sources are needed for a more accurate simulation of higher frequencies. The response is normalized so that the sum of all driver sources corresponds to 0dB if they had been located at the same distance to the mic.
The driver shape option can be used to simulate drivers of circular, elliptical, square and rectangular shaped membranes. In order for the shape option to have any effect the "Speaker source density" has to be 2 or larger. The Edge spreads out a number of point sources over the area covered by the membrane. The rectangular shape can also be used to approximate an array of equally spaced loudspeakers. This might be convenient during the design process, since the whole array easily can be moved on the baffle as if it was one speaker.
To simulate the frequency response of the system, all of the point sources are added taking the distance to the microphone into consideration. The distance will affect both the phase and magnitude of each contribution to the summed signal. The resulting frequency response is shown in the response window.
Figure 2. The response of the system in figure 1 (red). This graph shows the response as a function of the frequency. Above the graph, the wavelength is also shown. The blue curve shows the effect of compensation, which is not used in this case.
Note: To change the axis scaling, simply drag it with the mouse. There are three zones on each axis, which enable either stretching or moving of the scale.
The help menu in this window can open the home page of the Edge. Here you can find this document and also download the latest version of the Edge.
The Edge can apply two types of compensation to the response curve. The first is used for boxed systems, and the other is used for open baffle systems. There are other circuit configurations than the suggested ones that give identical responses.
Figure 3. Compensation of the diffraction effects. The Edge can simulate the effect of two commonly used types of diffraction compensations.
For boxed systems, it is common to compensate for the baffle step by means of a pole-zero pair. In the Compensation designer such a pole-zero pair can be entered as the "corner" frequencies f1 and f2 in the response. By entering a resistor value, R1, some help is provided to design the compensations circuit. Either of circuit 1 or circuit 2 can be used, circuit 1 is probably best fit for passive filters, and circuit 2 is best fit for line level filters. The Edge suggests values for L and R2 for circuit 1 and C and R2 for circuit 2.
The Edge can suggest values for the f1 and f2 frequencies. It does so by numerically finding the frequency where the far field baffle response has increased by sqrt(1.25/2) or approximately 2 dB. The Edge takes this frequency as f1 and twice this frequency as f2.
For the open baffle ("dipole speaker") there is a tilt of the frequency response of 6 dB/octave. This can be compensated for by an active line-level bass boost. The Edge can simulate the effect of adding such a network. Like the baffle step compensation, this compensation is added by entering two "corner" frequencies, f1 and f2. The lower frequency f1 will be the lower frequency limit of the system (neglecting the response of the driver), and f2 should be set to the start of the bass drop of the baffle design. Again, some design help is provided if the resistance R1 is given, the Edge suggests value for C and R2.
The edge can also suggest a value for f2. It does so by numerically finding the far field response for 0.1 Hz, and extrapolating by a slope of +6 dB/octave up to the frequency where this slope reaches +6dB.
I will here describe the process of using the Edge for simulating a baffle design. We start out with a single driver is placed on the middle of a 300 x 300 mm baffle. The driver is modelled by 44 point sources, and 36 x 44 edge sources, a total of 1628 sources. The response shows a peak at 1 kHz of almost + 9 dB before flattening out at + 6 dB towards higher frequencies.
Attempting to compensate this baffle design with a pole-zero pair essentially fails, due to the large peak at 1 kHz. The peak remains, but the higher frequencies are successfully brought down to 0 dB.
The Edge suggests two different compensation circuits, circuit 1 is suited for passive realization in the crossover, in this case the driver should be connected in series with L and R2 in parallel. Selecting a driver impedance of 6 ohms gives that R2 = 6 ohms, L = 2.653 mH. Note that the impedance of a real driver is not resistive, so there might be some deviances here. Circuit 2 is more suited for line level connection Selecting R1 to 10000 ohms gives that R2 and C should be 10000 ohms and 44,21nF respectively.
Moving the driver to a more asymmetric position greatly reduces the peak height, now the peak height is only about 1.5 dB after compensation. Experimenting with the baffle shape and driver placement can possibly reduce the peak even more. Finally, it should be remembered that other parts of the loudspeaker system may actually be compensated by a leftover peak, so the absolutely flat diffraction response is not always the goal.
The microphone position is given by the "mic distance" and the positioning of the "mic" symbol in the baffle designer. The mic distance is given in meters, and the default distance is 100 m, which is about the same as "far field". The mic distance is the perpendicular distance (z coordinate) between the baffle plane and the microphone. The x and y coordinates are given by the placement of the mic symbol.
It should be noted that if the distance is not very much larger than the baffle size, the baffle step will not be a full 6 dB. For example, a square 250-by-250 mm baffle will only exhibit a ~5 dB step at a microphone distance of 0.25 m. The reason for this is the longer pathway from the edge to the microphone. The longer distance combined with the distance law will reduce the sound pressure produced by the edge sources relatively more at close range. This is true also in real life and is particularly visible when making extreme close field measurements, where the baffle step is hardly visible at all.
The bottom line here is that the microphone distance should be set to a normal listening distance (usually ~4 m) when designing a system, but it should be set to the actual microphone distance (usually ~1 m) when comparing to measurements if the comparison should be valid. The default microphone distance is set to 100 m just to make sure that the microphone is in the far field.
The baffle designs can be saved to files. The files are of the windows standard ini type. This means that the values stored in them can be modified with a text editor, such as notepad. This may be useful e.g. if you prefer to enter numbers for the dimensions instead of dragging symbols in the baffle designer.
Start the edge from start|run with the command line switch "/n" (without quotes) to start with a fresh baffle design. This might be useful if the present design has become irreparably complex J, or if any of the windows have ended up outside the screen etc.
-First version based on the experimental program diffract.exe
Added in version 1.1:
-In addition to circular drivers, it can now also simulate elliptical, square and rectangular membrane shapes.
-Multiple response curves can be shown for comparison. The piece of ice on the toolbar freezes the current curve.
-Redrawing is done without recalculation if possible. This speeds things up a bit.
-Redrawing can be shut off. For systems with many sources, the automatic redrawing was quite annoying.
Added in version 1.2:
-Display of phase.
-Rectangular shape distributes sources at approximately equal x- and y-wise pitch.
-Notification when a source is off the baffle.
-A "New" button.
-Placement of edge sources for strange baffle shapes improved (bug fix)
-Sign of phase at export fixed (bug fix)
-Frozen phase curves are now OK when changing the mic distance (bug fix).
-Mic distance text box is now updated when opening a system (bug fix).
-Background of frequency response graph is greyed if sources are placed off the baffle.
-Suggestions of compensation frequencies
-Export of compensated, uncompensated and compensation curve.
- Hint text changed (+3dB -> +2dB) for "Suggest f1 & f2" button.
- Correction of the way the strength of the edge sources was calculated. Previously, the amplitude reduction of the edge sources by the distance law included the driver-to-edge distance. This was wrong, since this part has already been taken care of when dividing the edge in sectors seen from the driver sources. A slight difference from this change can be seen when the microphone distance is near or shorter than the dimensions of the baffle. The effect of the change is a lowered level at low frequencies in the new version.
-The minimum number of corners, edge sources and speaker source density is now 1. This makes the editing of these values slightly more well-behaved, even though there still are issues (which are related to the implementation of the spinedit control in Delphi).
-Automatic check for new versions. The Edge now automatically checks http://www.tolvan.com/edge for a new version if the computer is connected to the internet. This check is run when The Edge is launched, but no more than once a day. You can disable this function from the help menu.
-Fixed a minor bug regarding redrawing after opening a file.
-Added information about why The Edge accesses the internet (to check for new versions as introduced in ver 1.6) on first startup.
-Added a “donate” item in the help menu for users who want to contribute to the development of The Edge.
-Added a “Basta! loudspeker simulator home page!” item to the help menu. This change was added 2006-02-09.
-Added copy of the response image
-Strange baffle shapes (like "L" shape or "T" shape), where several edges can be "seen" by the source for a given direction, only the first edge is modelled.
-Some computer systems experience BSOD (blue screens) when The Edge is run. See http://www.tolvan.com/blue_screen_Q_A.htm. I am very interested in having reports from those of you who experience this. I am particularly interested if the test program on the page above (http://www.econos.de/fpu) also causes your system to crash.
© Tolvan Data 2004 - 2006