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How to Choose the Best Subwoofer
To choose the right subwoofer, you really have to understand how different subwoofer designs act in different situations and how they work with the rest of your system. We have always believed that the better you understand how to get the most out of your subwoofer investment, the happier you will be when you start to use your new subs. We find we need to teach most customers the fundamentals of subwoofer design and get rid of some misconceptions they may have before they really understand what they need. We don't claim to know everything, but years of experience in all situations has given us insight.
Whether you are a veteran in the business, or just getting started, this collection of articles by Rick Ramsdell will help you get the most out of your investment by helping you pick the best subwoofer for your particular application. The two biggest determining factors in choosing the best type of subwoofer you need are the subwoofer's specific application, and the type of enclosure you are going to use to reproduce the frequencies right above your subwoofer crossover point. These articles were conceived as a single article, and grew into these three. It is very important that you read all three of them before you make your subwoofer selection, because you need to have a solid understanding of everything in these articles.
Choosing the Right Crossover Point
The first step in choosing a good subwoofer is determining your ideal crossover point. Usually, the lower you can cross over, the better. A 10-inch or 12-inch full-range box will not be able to be crossed over as low as a 15-inch full range box. Below 80-100 Hz, cone excursion and power requirement in full range boxes starts to rise tremendously. The more you can keep 80-100 Hz and lower out of your "top boxes," the better they will sound at high power.
To really know the importance of a good crossover point, you need to understand an important phenomenon called source localization. Source localization is how you perceive where a sound is coming from. Ideally, you want to perceive the sound coming out of your subwoofer full-range combination as a single source, not a dual source. You do not want to hear the subwoofer functioning as a separate unit. This becomes more pronounced as the distance between the subwoofer and mid-high speaker increase. For a good starting point, calculate the distance between the two components (in feet) and then divide that distance into 1130 (the average distance sound travels in one second.) This will give you the highest frequency you can cross over at. The equation looks like this:
Maximum Crossover Frequency Equation:
Maximum crossover freq. = [distance between components) / 1130
If this Fmax equation results in a frequency higher than 100 to 120 Hz, try to cross as low as your system output will handle at the sound pressure level you feel would represent the maximum output you will need. In other words, try to cross over as low as possible keeping in mind that both components need to be capable of the same acoustic output at that crossover point. If you need to cross over higher than 120 Hz, it's best for the speakers to be as close together as physically possible.
Next, make sure that your sub and mid-high speaker can obtain the same output at the crossover point. If for example, you choose 80 Hz, make sure your mid-high speaker can keep up with the subwoofer's output at 80 Hz. Usually, the larger the mid-bass component you use, the lower it can go. This also works the other way. An 18- inch speaker may not sound as clean and tight as a 15 inch speaker if you are trying to use a higher crossover point. We at Ramsdell Pro Audio always shoot for an 80 Hz crossover point if we can get it, but a lot of our customers like slightly higher ones in the 100 to 120 Hz area.
If you need to move lots of air in the 30 - 40 Hz region you will want to consider larger units like 18 or 21 inch subs. If your low frequency needs don't go lower than the 45 Hz region, consider a good 15 inch sub. When compared, the 15 inch subwoofer is usually tighter and cleaner, while the 18 inch sub moves more air in the lowest octaves.
General Rules to Follow to Get the Most Out of Your Subwoofer Investment
1. Do it Right the First Time
We cannot stress this enough. This is probably the biggest problem we have noticed new customers encounter. Everyone wants to spend half the money and get twice the performance. When you get your new purchase out in the real world you are going to discover you need the output and reliability of the good stuff. The fact of the matter will be when your gig goes down in the middle of the show, the savings you thought you could get away with is not going to matter at all. You will probably never get hired again for that job and you're going to have to purchase the gear you should have gotten in the first place. You will painfully discover the "gig gone bad formula." It takes about 20 good shows in a row to undo a show gone bad. The reason we are stressing this issue is because we had to learn it the hard way, just like everyone else.
2. Don't Mix and Match Boxes in the Same Frequency Range
It's important that you use all the same types of sub designs together and all the same mid-high box designs together. Different cabinet designs have a tendency to fight with each other acoustically. Bass horns have a different arrival time than direct radiating, vented type boxes. 15 inch speakers have a different sound than 18 inch speakers. Horn loaded mid-high boxes throw different than non-horn loaded types. Quasi-horn loaded vented boxes have their own set of conflicts all together. Unless you are an experienced engineer it's best to stay will all the same type of box design in a given frequency range. You will have a much cleaner overall sound, prevent a mess of unnecessary headaches, and you will save amplifier power too.
3. Understand High Power Subwoofer Use
All Ramsdell Audio subwoofers are designed to reach maximum driver/cone excursion at their rated power levels. We believe that you will achieve the best use of speaker output and amplifier watt requirements by using medium x-max (cone excursion), high-efficiency woofers. You will no doubt find varying opinions on this subject, but after 30 years of production and competing in product shootouts we believe that this is the best way to go. The quicker a woofer can respond to a signal, the more accuracy and control it has reproducing it. For high power subwoofer use, make sure you have a 24 db per octave high pass filter available. You will need to adjust it so that you don't allow frequencies in your signal to go below the tuning frequency of the enclosure, causing excessive cone excursion. During such cone excursion, when the voice coil begins to leave the magnetic gap, your overall system damping factor begins to deteriorate. The speaker begins to lose control of itself and the amplifier begins to lose control of the speaker. If this continues, electrical or mechanical damage to the speaker will happen and that is bad.
4. Make Use of Compression and Limiting
Even if you use multiple safety measures, make sure your amplifier comes with a soft clip limiting circuit. During use, make sure that it's not run past occasional blinking. If the limit circuit stays on to long it will turn the signal into more of a continuous level and cause voice coil over heating. This will lead to less acoustical output and eventually thermal speaker failure.
Your amplifier will also perform better running at 4 Ohms than 2 Ohms. The lower the impedance, the more demands are placed on the amp's power supply. If you're running at 4 Ohms and happen to lose an amp in the middle of a show, you can always transfer speakers to another amp. This would probably not be possible if you were already using the amp at a 2 Ohm load.
What Type of Subwoofer Design Would Be Best
There are 3 main types of enclosures used for subwoofer design. These are sealed, vented, and horn loaded designs. The rest are combinations of these three.
In professional sound applications, you don't see many sealed box subs. They are mostly used for mid-bass output in larger vented full range designs. They are more popular in home theater and car audio sound applications. Sealed boxes roll off in their low frequency limit curve at 12db per octave instead of 24 db like their vented box cousins. Cone excursion requirements are a lot higher in sealed boxes as the frequencies go lower. However, this is why they are slower to roll off below the cut off of their tuning frequency. Sealed boxes do not require as much air (cubic space inside the box) to go as low as vented boxes, but they are nowhere as efficient either.
Vented box designs are by far the most popular box for subs in professional sound applications. Properly designed, vented boxes handle much more power in the lower frequency ranges. This is because the port actually produces most of the lowest notes, reducing cone movement tremendously. If you want high efficiency in your subwoofer with modest box size this is a good compromise. You should note that your box size will grow quickly if you want your box to go lower then 40 Hz. We at Ramsdell Pro Audio achieve better all around results in the pursuit of the low note by mutually coupling smaller, punchier subs than by larger boxes tuned lower. You may notice this in the design of our subwoofer products.
Horn Loaded and Bass Horn Subwoofers:
If you are a no compromise individual, or you need to be for a particular application, then this is the subwoofer design for you. When it comes to being able to extract the maximum capabilities of a low frequency transducer, the low frequency bass horn stands alone. Horn loaded speakers can be as much as 12 db louder on a watt by watt basis then their vented box cousins! Basically, a smaller group of horn loaded subs can outperform a much larger group of vented subs and do it with less amplifier power. This smaller grouping of subs will result in more even coverage with less peaks and dips in your low frequency levels. Also, voice coils of speakers in horn loaded boxes can run much cooler for the same acoustic output levels. This 'cool coil' property of horn-loaded subs results in a lot less power compression and reduction of long term output loss. Bass horns are finding a new lease on life in the professional sound world as one of the few types of boxes that can produce the output needed to keep up with today's line array systems. As the professional sound industry rushes to re-invent the past, we encourage you to consider this unparalleled enclosure design.
Low frequency notes travel more effortlessly through air than high frequencies, because they don't get absorbed by the air as fast as high frequencies do. Because of this property, they travel farther. In technical terms, air is not an acoustically linear medium. Over the years, we've had a number of our customers tell us that bass horns "throw better" then other types of subs. We have never been able to measure this with single units placed side by side and run at the same level. However, we have measured that properly designed bass horns couple better than vented types when placed in groups of four or eight units. Also, low frequency horns need to be allowed one wave length of the lowest frequency they are going to be called on to reproduce to fully develop. In the case of a 30 - 40 Hz note, this could be 40 - 50 feet from the box. This phenomenon probably contributes to the perceived increase in "throw" of the box design. On the flip side, this property makes bass horns more difficult to work with in smaller rooms. It seems to complicate this problem even worse if more than one location in the room is used for subwoofer placement. Vented box designs usually will work better when you encounter this type of problem in an installation.
Reprinted with permission of www.RamsdellProAudio.com © 2005 Ramsdell Pro Audio. All Rights Reserved.