|
|
|
|
||||
|
|
|
|
||||
DIY Project # 12 - True Audio's WinSpeakerz Design Software and Introduction to Loudspeaker Design October, 1999 Ralph Calabria
Introduction Aren't
computers great? As we close the
20th century, it's hard to imagine going through a typical
day without one. Sending
and receiving e-mail from all over the world, searching the Internet,
sending FAX messages, not to mention the thousands of other software
programs available (O.K. I'll mention a few) like word processing,
scheduling and project organization, and the slew of 3D games on the
market. Suffice it to
say, the computer is not just a glorified typewriter anymore!
Many things that were done using tedious, repetitive calculations (on a hand-held calculator) can now be glorified using Excel spreadsheets. The same holds true for a variety of other tasks that took much longer before the computer emerged to its current status. Take loudspeaker design, for example. Hand calculations using the formulas originating from Thiele and Small took quite a bit of time to crank out and plot on a graph (also by hand, point, by point, by point) to get a response curve. Now there are several CAD (Computer Aided Design) programs on the market that use these formulas to calculate AND plot a frequency response curve. One such piece of software is True Audio's WinSpeakerz: The Speaker Design Toolbox, by John L. Murphy. And what better way to augment loudspeaker design software than with a book that teaches you about the basics of loudspeaker design. As coincidence would have it, Mr. Murphy has also written a book entitled 'Introduction to Loudspeaker Design', a tutorial of sorts, with an intention to supplement the information included in the WinSpeakerz user's manual. WinSpeakerz:
The Speaker Design Toolbox Like all loudspeaker design software, WinSpeakerz uses formulas to calculate a response curve for a given driver based on the Thiele/Small (T/S) parameters of the driver. So, in order for WinSpeakerz to do its thing, you'll need to have the T/S parameters handy to load into the program. (There is also a very large driver database that is included with the software.) The program's System Editor supplies the user with space to completely file your driver system info. In addition to the T/S parameters, useful information such as the physical and mounting specifics, driver materials of construction, and picking an enclosure response for the system (including baffle diffraction loss and automotive applications) can be stored as part of the system 'package'. O.K.,
so now you've put all the T/S parameters into the program;
now what? Well, the rest
is pretty intuitive, and the user's manual guides you through all
the options you have to choose from.
Let's take a look at some of the more useful ones. WinSpeakerz
can calculate box volumes for the following systems: 2nd
and 3rd order closed box, 4th order vented box,
and 4th, 5th, and 6th symmetric
bandpass alignments. Besides
plotting frequency response, WinSpeakerz can also plot Phase, Group
Delay Impedance, and Excursion responses.
To
best illustrate what this program can do, let's use the example of
Dayton's Titanic 12' subwoofer driver.
This driver looks great on paper, with excellent power handling
and a decent Xmax (maximum
excursion of the voice coil front to back). Let's plug it
into WinSpeakerz and see what happens!
Figure 1A is a snapshot of the screen once the driver is loaded
into the program. You'll
notice that on the x-axis is the frequency (Hz) and on the y-axis is
dB. At the bottom
left is displayed driver and system parameters, and on the bottom
right is the alignment we've selected (in this case, a 2nd
order closed box). FIGURE 1A: WinSpeakerz default screen with Titanic driver loaded
To
get various alignments for this driver, WinSpeakerz takes some of the
guess work out of . . . well . . . guessing.
By keeping the box volume set at 0 and asking the program to
calculate the frequency response, WinSpeakerz will supply you with 10
different alignments, while storing them in memory.
By selecting or de-selecting each plot using the store buttons
on the top tool bar, one can view each alignment individually and
determine which alignment is best.
Figure 1B shows you what to expect when asking WinSpeakerz for
suggestions FIGURE 1B: Array of responses suggested by WinSpeakerz
You'll
notice that some of the suggested alignments can be eliminated because
of a sharp peak in the frequency response, while others may be
eliminated because bass extension is weak.
The main point here is, YOU can pick what alignment that works
best for your design, fully aware of the tradeoffs involved before
making the speaker enclosure. OK,
back to our subwoofer design. After
doing some weeding, we come up with a couple of alignments that look
good in terms of frequency response vs. ripple peaks.
Figure 2 summarizes the two best alignments WinSpeakerz came up
with. FIGURE
2: Best calculated
responses from array
By
changing the system Q, the box volume changes dramatically at the
expense of a small ripple at 70 Hz.
You, as the designer, choose the tradeoffs.
As it turns out, by adding acoustical stuffing to system #1,
the small peak may be tamed to a great extent, if not eliminated
completely. However, man
does not live by frequency response alone.
There are several other things to consider to ensure your
design is successful. Excursion Response How
loud will the design play, and how much power can the system handle
before it crashes and burns? These
are good questions to ask yourself (particularly the latter).
Well, WinSpeakerz can help you answer them.
First, let's explain excursion.
Excursion is a term used to describe the movement of the driver
cone back and forth. The
maximum allowable movement of the cone before distortion occurs is the
Xmax. This
translates to a distance the voice coil can move within the magnet
assembly without popping out. This
relationship is usually given in millimeters for the distance traveled
in one direction, say 10mm. If
the Xmax is exceeded, you will usually hear distortion and
occasionally a popping sound coming from the speaker.
Should this happen, turn down the volume, as this may cause
damage to the speaker. The
excursion of the speaker system is related to the box design, and the
power fed to the speaker. In
Figure 3, you'll see that the driver and system parameters window
gives you the option to select multiple drivers and/or the input power
used. By plugging in a
power number, then calculating the frequency response, excursion data
is also generated. The
more power used, the higher the excursion response becomes.
For system #1, WinSpeakerz tells us that the maximum power
handling is 170 watts, after which the excursion limit of the driver
is exceeded. FIGURE 3: Excursion plot vs. power and dB output
Along
with the excursion data, comes system SPL, or how loud the system will
play at a given power input. WinSpeakerz
can toggle between a dB plot and a dB SPL plot.
The dB SPL plot displays how loudly the system will play at a
given frequency and power input.
Figure 3 shows that system #1 will play at 112.5 dB through 52
Hz, then drop off sharply as frequency is decreased (note the
excursion plot is in red). This system has the capability of playing
at 104.5 dB at 20 Hz. Not
too shabby! As a side
note, system #2 can only handle 110 watts before reaching the
excursion limit of the driver, and will play at 110.5 dB at 52 Hz. Group Delay Response The
time delay experienced by the audio signal between the speaker's
electrical input terminals and the acoustic output is known as the
group delay. What does
this mean? Well, if the
group delay is large enough, it will be audible.
The audible threshold for group delay is about 30 milliseconds,
although lower is desirable. WinSpeakerz
calculates group delay to let you know if your design is within the
desired range See Figure 4 for the Titanic system group delay (blue
plot). As you can see,
our example has a maximum group delay at 23 Hz of 9.2 milliseconds. Phase Response The
difference in phase between the input of the speaker and the
speaker's output is known as the phase response of the system.
The phase response of a system should produce a smooth curve
with not much in the way of large humps.
This would indicate that the group delay is large, approaching
the audible limit. Again
for our example, the response is smooth, as would be expected by the
system's low group delay (Figure 4, purple line). Impedance Response Calculating impedance for your system is important to ensure the impedance does not drop too low. Knowing the minimum impedance of a system will guarantee you won't run into amplifier problems at the impedance minima. For a closed box system, one sees a sharp increase in impedance at the resonance frequency of the box (in the example of the Titanic, about 75 ohms at 31.5 Hz). The impedance minima is about 5 ohms for system #1 (see Figure 4; yellow line). WinSpeakerz is also equipped with an Impedance Compensator calculation, which may be use in the crossover of a system to smooth out the impedance curve. FIGURE 4: Group delay, Phase, and Impedance Responses
Crossover Design WinSpeakerz
has the capability to calculate a variety of crossover networks
including 1st, 2nd, and 3rd order
Butterworth, 1st order Series, and 2nd and 4th
order Linkwitz-Reilly. As
mentioned before, there is an impedance compensator as well as a
tweeter attenuation calculation and a resonance compensator.
As in all loudspeaker designs, these crossover calculators are
a very good starting point. To
fine tune a crossover to get the response just right may take some
tweaking. This tweaking
usually requires equipment that actually measures the frequency
response of the speaker system. Various Other Goodies Aside
from the above mentioned functions, WinSpeakerz can model diffraction
loss, as well as auto-cabin response for those who are looking to get
into the hi-end car stereo business.
For vented (ported) systems, WinSpeakerz has a vent calculator
that summarizes the size, length, and number of vents needed for a
given system. There is
also a box calculator for rectangular, trapezoidal, and bandpass
enclosures, which gives you the option of 'freezing' a dimension
(length, width, or height), and WinSpeakerz calculates the remaining
dimensions. Also provided
in the box calculator are displacement compensation for driver,
bracing, and other displacements, and acoustical fill compensation
which calculates V(b) increase due to filling. I've
touched on the major strong points the program can do.
There are many more that allow you to play with data once
compiled, save systems for future work, and input your own drivers
into the already 1080 driver database that comes with the software. Summary In short, WinSpeakerz is a great piece of speaker modeling software that can be used by both the novice and experienced designer. The combination of its pull-down menus and user's manual make this package a breeze to use; very easy, very intuitive, very nice (for all you Macintosh users out there, True Audio also offers an all-new version of ). There's a lot of speaker modeling software out there to choose from. If you're in the market to design and build your own speakers, take a look at this one. It's got all the features you need to get you started. And unless you've got enough dough to plunk down on state of the art LEAP and LMS design system software, this package is a bargain at $129 US. - Ralph Calabria - * Copyright 1999 Secrets of Home
Theater & High Fidelity |
|
|
|
|
|
|
|
True
Audio Home Page | Catalog | Tech
Topics | Audio Links | Book
Store Your comments are welcome at webmaster@trueaudio.com |
