1. More power equals output.
Often time you see this belief especially in car audio where customers ask if some particular woofer can take some particular amount of power as if that will lead to some partially high amount of SPL. The truth is that even a small constant amount of power will eventually exceed the cooling ability over a long period of time. There are probably no woofers that can even take 1000 watts RMS for over a week if not even past one day. But most woofers can take that and more over the period of a few seconds or up to a few minutes.
As a woofers reaches its power limits, unless failure occurs, there will become a point where the resistance of the voice coil is rising faster than the power going into the driver. This concept is called power compression and it implies that the more power you put into the driver, the less output you get out. For this reason it is often better to choose a driver with higher sensitivity rather than one that requires more power if all things are otherwise equal.2. More xmax equals more SPL.
Subwoofers really can be broken down in two categories: Mass controlled drivers and Compliance controlled drivers. Mass controlled tends have low xmax and high sensitivity. These tend to be punchy and very loud and mostly used in live concerts for sound reinforcement or even car SPL competitions. Compliance controlled subwoofers which tend to be the majority of home and car audio subwoofers. They have high xmax, more weight, lower sensitivity, but more SPL in the lower frequency spectrum. Any woofer in these categories can perform well or poor but it largely depends on the required bandwidth. Using a low xmax woofer for subsonic content is probably not ideal and will not only cause distortion but it won’t be efficient. Likewise using a high xmax low sensitivity driver for higher frequencies is not going to be very effective. In truth, there is no one stop solution and most drivers can overlap these areas with good results.
So yes, more xmax does mean more SPL but only for lower frequencies where power is not the limit. Generally speaking, during lower frequencies, the driver tends to run out of usable throw (beyond xmax) before high thermal compression occurs. 0-40Hz is primarily mechanical, 40-60 is in between) 60 and up is going to be thermally limited almost exclusively for most subwoofers. Surprisingly, even the largest drivers with high xmax and big voice coils can be bottomed out or run past a safe mechanical state with only a few hundred watts if the frequencies are low enough. Without a high pass (subsonic) filter, or in a low tuned system, bottoming out or breaking a driver could be a very real possibility without careful modeling and testing.3. Subwoofers are fast/slow.
Often times people make the mistake that sound quality is in fact related to the woofers quickness, but in fact the woofer’s quickness is exactly related to SPL. The faster the driver, the higher the SPL. There are two ways to change a woofer’s speed. 1. Lower the frequency of the input its reproducing or 2. increase the volume. Sounds silly, but its true. There are many other factors that go into making a subwoofer sound fast or slow (boomy or tight) but that divulges into system design. What’s important about this myth is that speed is an inappropriate concept of sound quality.4. Subwoofers can miss notes.
If your subwoofer sounds like it might be missing noted it could be a sign that its either in a box too small or tuned for a very non-linear response in a bass reflex system or ther room / car response is creating a peak or null that is affecting the output in an important frequecny range. Good subwoofer systems will play all types of music or movie material very well. A bad subwoofer system may have a null or peak in the frequency response that may benefit some material over others but in essentially this non-linear behavior in the frequency domain is not always ideal. It is true that movies have lower frequency content and perhaps more dynamic bass than music, but a good system can be used for movies or music alike.
It is also true that it tends to be more important to emphasis subsonic frequencies in the home theater environment versus the music environment where there is simply less emphasis on subsonic inaudible material. As a tradeoff, you can align a system to be more efficient above 30Hz or so. This trade off reduces the bandwidth but increases the SPL. Careful consideration should be taken to insure linear response is still maintained if that is the goal. It is very easy to have peaky bass with high BL drivers in high tuned ported systems. This is approaching the concept of basic SPL vehicles. Such systems are not very ideal for listing to music material of any kind. If you want your system louder, then it is better to add a second driver, more volume and more amplification, rather than tuning higher. It is important to understand that getting more SPL without compromise is never very cheap.5. Sensitivity doesn't matter for subwoofers.
Sensitivity is very important for subwoofers. Not all frequencies are limited by xmax. In fact most of the bass frequencies for music are really limited by sensitivity rather than driver displacement. Higher sensitivity means more SPL and ultimately better performance especially for upper bass output. There are several standards for sensitivity. SPL at 2.83 volts or SPL at one watt. The SPL at one watt is the more appropriate number for a fixed amount of energy while 2.83 volts is the small single frequency response standard. Also sensitivity is a function of, in part, the driver’s cone area. Its also a common mistake to assume that for example a 12" driver has a 12" cone. In truth, they are more often less than 10". You need to read the manufactures' sd specification cone area comparisons.6. Smaller woofers sound better than bigger woofers.
One of the biggest myths about woofers is that 8’s and 10’s are “tighter” and “cleaner” than 15’s or 18’s. Nothing is really further from the truth. What tends to happen is that the smaller drivers have lower Q’s because manufactures tend to put large cones on smaller motors to increase SPL and sensitivity. Well unless the motor can compensate for the extra mass it has to push, then the Qts will not be the same as the smaller drivers the and ultimately the driver may not be suited for the same kinds of alignments and could ring too much and compromise the perceived sound quality. Having said that, high Qts drivers are not any less “tight” or “musical” than well damping drivers, it’s just they require larger boxes and less internal pressure to prevent ringing. All things equal, a well designed 18” woofer will sound louder with lower distortion than a well designed 12” or 10” woofer. Bigger is better, but its almost more expensive.7. Cone material effects the sound.
For low frequencies, the cone on a subwoofer makes no difference in the sound whatsoever, end of story.
The only remote possible affect it could have is in the case of a metal cone or very stiff composite cone that resonates at a high frequencies and buzzes. However this frequency would be up around 500 to 2000Hz and unless you play the subwoofer at those frequencies, you’ll never hear it. . The cones sole job to push air, not break and ideally not be too heavy. But they don’t change the tone, pitch or sound of a subwoofer whatsoever.8. Big woofers require big amps.
Often times larger drivers require less amplification, that’s sort of the idea. The concept that bigger woofers need more power is not always true and plays into a major misconception common in car audio. What you should consider is the efficiency of the subwoofer. Efficiency will literally tell you how much acoustic output you will get given a discrete amount of power. If the driver is bigger, has a larger motor and has a higher sensitivity, there is no mystery about it, you are going to get more SPL with the same amplifier provided the impedance is similar. Sensitivity is most easily achieved by a weight reduction usually from the cone surround and voice coil. Sensitivity is often a tradeoff for xmax.
However there are many larger drivers that don’t have ultra high sensitivity. A good pro audio subwoofer may have 6 to 10dB higher sensitivity over an average high excursion car audio subwoofer making them very capable with quite a bit less power, at least for their frequency range which is usually above 40Hz. Likewise, SPL drivers ironically enough don’t need much power! Let me repeat. True SPL drivers ironically enough don’t need much power! That’s because they are used in the higher frequency range and generally have great sensitivity numbers. They need this in order to get the excursion and ultimately SPL. High sensitivity and lots of power means lots of SPL provided the driver is still reasonably linear and does not break. It’s important you know the TSP’s of the driver you buy, otherwise it could be the wrong driver for you!9. Frequency response is efficiency response.
This is a common assumption, especially when doing simulations. The most obvious example can be done theoretically. Take any woofer and double the BL product and compare it to the original in a simulated sealed box design. The driver with double the BL has a 6dB advantage in sensitivity but it will role off quicker making it appear to make less bass. This is not completely true, but rather shows how system design often requires going beyond the box and woofer to achieve the SPL goals you need. If the driver has very high BL product it will have very high dampening and impedance. The high impedance reduces the current from the amplifier under the same voltage (gain) and while the driver is extremely efficient, it also pulls less power from our voltage source. The driver with lower BL product will pull more power from the amplifier and displace more air but with lower dampening.
To correct this low Q problem we must place EQ into the system to shape the response and boost the amplifier voltage where appropriate to pull the same amount of power as before. If the driver has headroom (xmax) Then it will move more air than its lower BL counterpart with the same amount of power even for extremely low frequencies. The lesson here is don’t assume the box + driver raw frequency response is the final say in your performance as port or additional filters can also play equally important roles in our final response. The real limits are power and displacement.
