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Author: Wayne Harris Click HERE to view more articles by Wayne Harris. Originally appeared in the Volume 1, Number 4 issue of Made in the USA magazine. © All rights reserved. |
Once upon a time, in a primitive install bay far, far away, some poor dude was forced by a mean, lazy installation manager to crank out woofer boxes. Day in. Day out. Morning, noon, and night, the exhausted and underpaid installer sawed and glued and nailed and screwed in his quest for the ultimate woofer box. All to no avail. Then, one day, in a Divine Revelation induced by a near-lethal (NOT!) shock from a 12 volt stiffening capacitor, the installer cried out in anguish, "There's got to be a better way! I'm tired of building these *!$#% &* boxes that don't work! Why can't I design a box that I know will work BEFORE I actually construct it?" At that point a Team America member who just happened to be in the vicinity, heard the installer's plea for help and immediately came to the rescue. "You know," the Team America member said, "I used to be a LAME-0 myself. That was before I started using TERM-PRO Loudspeaker Development software. Now look at me! I'm a Champion...One of the elite...I'm on Team America!" "Man..." the installer said, "I wish I could just place! Anyway, what's this TERM-PRO thing, and where can I get one?"
"TERM-PRO is a computer program that aides in the design of low-frequency loudspeaker enclosures." The Team America member said. "It does this by mathematically modeling various driver/enclosure combinations, which in turn allows the installer to evaluate the performance of a particular design WITHOUT actually having to build it. With TERM-PRO, the installer can model 20 different types of subwoofer enclosures including: Sealed, Ported, Isobarik Sealed, Isobarik Ported, 4th-7th order Bandpass, 4th-7th order Isobarik Bandpass, 3 chambered 4th-7th order Bandpass, and 3 chambered Isobarik 4th-7th order Bandpass. In addition, the program also does Port Design, Passive Crossover Design, Enclosure Layout and Acoustic Curve overlays." "No Way!" The installer exclaimed. "You're telling me that if I get TERM-PRO, I'll be a 'master' box builder like you?"
"Well, yes and no." The Team America member explained. "TERM-PRO is a tool. And, like most tools, a certain amount of familiarity is required to fully benefit from all of its capabilities. Fortunately, TERM-PRO is very easy to learn and use. I was designing my very own boxes within minutes of installing the program. Hey, I've got my Laptop in the car. Would you like me to show you how easy it can be to design a system using TERM-PRO?" "Do Punch amps kick ass?" The installer responded.
"Before we begin," the Team America member said, "we really need to define what we're trying to achieve. When designing a subwoofer enclosure, there are three things you should be trying to accomplish.
"It's easy to analyze low-frequency performance with TERM-PRO. First, you select the driver you would like to use from the extensive TERM-PRO driver database. Next, select the type of enclosure you would like to model. (TERM-PRO's Enclosure Preference bargraph will indicate what type of enclosure the driver would prefer if it were to have its choice.) Finally, simply design the box. TERM-PRO will calculate the required box volume and tuning frequency, and plot the predicted frequency response on the computer's monitor."
"So, how do I know if this design has good low-frequency performance?" The installer asked. "It's easy." The Team America member replied. "Look at the figure below. Notice how nice and smooth the frequency response is. Also, look in the top right hand window. TERM-PRO says the system has a cutoff frequency of 46 Hz. This is the frequency where output is down 3dB with respect to the 'flat' area of the graph. Also, notice that this particular design has only 0.034 dB of ripple. Ripple is the deviation from a flat response. You should always try to keep the system Ripple below 3 dB for best transient response." "But someone told me that I need a cutoff frequency of 10 Hz. This design only has a cutoff frequency of 46.33 Hz. What's the story?" The installer asked. "First of all," the Team America member responded, "unless you're going to be communicating with elephants or extraterrestrials, you really don't need to design a system with such a low cutoff frequency. Secondly, when you install the enclosure in a vehicle, most systems will experience a 12dB per octave rise in bass response below 80 Hz. Finally, and most importantly, most designs that have extremely low cutoff frequencies require VERY large enclosures. In addition, they also usually tend to have very poor frequency response, ripple, and power handling characteristics. This makes them impractical for most installations. As a rule of thumb, try to target a cutoff frequency in the 30-60 Hz region. Of course, if you can get a 10 Hz cutoff frequency in a 1 cubic foot box without sacrificing response, efficiency, and power handling, please feel free to do so, and then write a book about it." "Okay, okay, okay!" Exclaimed the installer. "We're done, right?" "Halfway." The Team America member responded. "We still need to verify the system's reliability."
"The biggest factor affecting reliability is power handling. When designing a subwoofer enclosure, there are two types of power handling you should be concerned with: electrical power handling and mechanical power handling."
"Electrical power handling simply refers to the maximum amount of electrical power (in Watts) that can be applied to a driver's voice coil without the voice coil burning up. In most instances, this is the power rating (Pe) given to the driver by the manufacturer. You can never exceed the electrical power rating of a driver or you will burn up the driver's voice coil. It is important to note that the type of enclosure selected for the design has absolutely no effect on the electrical power handling of the system."
"Mechanical power handling, on the other hand, is affected by both the driver and the type of enclosure it is installed in. The Calculated value represents the maximum amount of electrical power (in Watts) that can be applied to the driver's voice coil without causing the driver to exceed its Xmax rating. (Xmax is a rating given to the driver that describes the maximum distance the driver's cone can move 1-way in a linear fashion.) You can never exceed the calculated mechanical power handling of a driver/enclosure combination or you will physically damage the driver."
"I get it!" The installer exclaimed. The Team America member replied. "Look at the figure below. This is a plot of the system's power handling capability. If you look in the top left window, you'll see that the driver we're using has an electrical power rating (Pe) of 600 Watts RMS. Looking at the graph, we can see that the system has full power handling down to 30 Hz. (Remember, the system's power handling can never exceed the driver's electrical power rating Pe.) Below 30 Hz, however, the system exhibits mechanical power handling limitations at 10 Hz, the system is only capable of handling 19 Watts without mechanical damage occurring."
"Why?" The installer asked. "Several reasons." The Team America member responded. "Look at the next figure. This is a plot of the driver's cone excursion versus frequency for 600 Watts of applied power. Note that as frequency goes down, cone excursion increases. At the tuning frequency of the box (38.61 Hz), there is a reduction in cone excursion. This is typical of ported enclosures. But, look how cone excursion rises rapidly below the tuning frequency of the enclosure. This is also typical of ported enclosures. At around 30 Hz, the cone excursion of the driver exceeds the rated Xmax of the driver. If you'll refer back to the second figure, you'll see that this is the very same place where mechanical power handling limitations occur."
"So, this is a bogus design...right?" The installed asked. "Wrong!" Replied the Team America member. "First, there's practically no music below 30Hz. Secondly, this design assumes a 'worst case' scenario where we are applying maximum electrical power to the system. If we reduce the applied power, cone excursion will also be reduced." "So, what would I do in this situation?" The installer asked. "That depends..." said the Team America member. "You need to look at the customer the box is for. If all the guy listens to is 'Big Bertha Butt Bass' discs, I'd say you better try another design. If he listens to normal music, however, you're probably not going to have any problems. Remember, the key to a successful design is to evaluate your customer's desires and expectations, and make them come true." "Thanks, dude!" The installer said gratefully. "I'll order my copy of TERM-PRO today!"
If you would like to contribute to the 'Wayne Harris Retirement Fund', order your copy of TERM-PRO today! Better yet, order two. If you would like more information on TERM-PRO, please call (800) 366-2349 or (480) 517-3079.
If you currently use TERM-PRO, please call your TEAM AMERICA representative (Suzanne Angulo in the East and Lou Anne Loesche in the West) at (800) 366-2349 for your FREE TERM-PRO patch for your Team America jacket.