Myths about subwoofers

# 1 Subwoofers have an RMS rating


Speakers actually have a very complex thermal compression relationship and certainly can not be quantified by just one or two numbers typically called RMS and Program or Peak. Because voice coils in traditional drivers are inherently resistors, any amount of voltage generates some amount of heat which then adversely changes the resistance and properties of the speaker. This is the principle of thermal compression: As the voice coil heats up, the resistance changes and the efficiency and performance of the driver decrease until the point of maximum thermal compression. There are some unique types of materials that have a close to zero temperature coefficient and of course there is also superconducting metals that operate at subzero temperatures with no indications of any sort of resistance. In theory, only these types of materials would have no thermal compression, but they are not employed or very practical yet. Copper and Aluminum are still the two most widely used materials for voice coils. Both copper and aluminum heat up considerably and the resistance changes as a function temperature, and there lies the problem, therefore a discrete RMS scalar value is entirely inappropriate.


Under heavy use, the TSP parameters can shift as much as 35% and in a generally un-favored direction. (higher Qts, lower sensitivity). The common ultra high RMS ratings we see of large and expensive subwoofers are at best marketing ploys to make the driver seem far more worthy than it is, or in fact they are really intended to give the customer an idea of what type of amplifier to buy. The fact is, even the highest “RMS” rated subwoofers in the world in excess of 5 digit figures will begin to compress with far less power than you would ever image, try only a few hundred watts! (no joke!). Now this doesn’t mean you still don’t need lots of power to reach the maximum potential of the driver. As a rule of thumb, the amplifier should be much more capable than what the driver needs on average. For example, quick short bursts will produce huge SPL’s and the voice coil will not have time to heat up as much, but longer term high power use will result in considerable performance regression if not failure from glues giving way due to heat or differences in the thermal expansion of materials around the glues. Under heavy use thermal compression limits begin to play a large part in SPL but most people are oblivious to this concept. It is true that woofers can be used well into their thermal compression state, and typically that is what occurs. As the power increases linearity, the SPL does not increase linearly. This is some form of compression, usually thermally related unless the woofer is beyond or close to xmax. In an ideal non-compression circumstance of either power, BL or otherwise, you can expect a 3dB increase every time the power is doubled. Rarely does this ever occur, in extremely compressed and dangerous states it can be less than 1dB!


As a woofers reaches its very 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 subwoofer. When the resistance doubles as the power doubles then absolute thermal compression has set in. In practice you can’t actually increase the power from the amplifier because most amplifiers start to produce less power as the resistance increases because almost every car, home and pro audio amplifier is a constant voltage source rather than a constant current source. So in a way this phenomenon is a self limited occurrence that accidentally works to protect the driver. However, running the driver at or near the maximum thermal compression limit will likely result in rapid failure. Ultimately, thermal compression is a very large but unavoidable shortcoming of mass controlled transducers. Likely, compliance controlled transducers, or rather subsonic transducers are not limited by their thermal properties as much, but rather their compliance or linear limits (xmax). It is believed by a few experts in the field that thermal compression plays a much greater role in linearity and distortion than we know of, but it’s rarely discussed.


# 2 More xmax means more SPL


Subwoofer drivers really can be broken down in two categories: “Mass” controlled drivers and “compliance” controlled drivers. Mass controlled drivers tend to 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 car audio subwoofers have high xmax, more weight, lower sensitivity, but more SPL in the lower frequency spectrum. Then there are of course hybrid drivers which are basically mixes of the two. Any driver in these categories can sound good or bad, but more important is being able to use the woofer where it performs the best. Using a low xmax woofer for subsonic content is probably not wise, likewise using a high xmax low sensitivity driver for sound reinforcement is not going to be very effective. In truth, there is no best driver and most drivers can overlap these zones with good results. We are not really used to the idea of a two way subwoofer, but as we demand more and more SPL and deeper bass, we may some day find that two different types of subwoofers used together are required to get the full reference SPL effect we all hunger for!


So yes, more specified xmax does mean more SPL but only for lower frequencies. Generally speaking, during lower frequencies, the driver tends to run out of usable throw (beyond xmax) before high thermal compression states occur and mechanical failure is a greater risk. 0-40Hz is primarily mechanical, 40-60 is in between) 60 and up is going to be more thermally limited. 0-20Hz is the subsonic content and in fact there are more efficient methods of producing bass in this spectrum rather than a regular piston based transducer. 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. The difference in displacement from 40Hz to 20Hz or rather half the frequency, or one octave, is quadruple! In the simple large sealed box example, that means if your woofer displacement is 1″ peak to peak at 40Hz, you’ll bottom out just about anything in existence by the time you dip below 20Hz without protection.


Often times when people want more SPL, they really need higher sensitivity in the form of higher BL product or less moving mass, rather than more xmax because 50-60Hz is really what they are after. This is a very sensual frequency range for humans and much of the bass in music content exists in that frequency domain.


# 3 Subwoofers are fast / slow


More appropriately labeled Damping or Ringing, these concepts are really reciprocals of one another have nothing to do with speed, tightness, “boomieness” or any other misused and inappropriate term for subwoofers. Subwoofers, or rather bass drivers, all move at the same frequency when instructed to via an input single. The difference is really about the Q alignment of the system. There are many famous Q alignments which produce various frequency responses, but beyond the complex mathematics is a fundamental principal of force and acceleration and the driver will respond to a sinusoidal wave at various accelerations depending on the moving mass and force that the voice coil and motor generate on the cone. Therefore any driver can be faster or slower depending simply on the voltage! It makes little sense to call any driver faster or slower.


Damping or Ringing is really what we’re after and the amount of either is really a function of system volume along with the electro-mechanical damping factor of the driver. For example, in a sealed box system, as the volume of the cabinet becomes small, the internal pressures increase when the driver pushes in and out. This pressure is a force which, not nearly as strong as the electromotive damping force, works in the opposite direction. Contrary to intuition, higher internal pressure (which we tend to associate with tightness or stiffness) decreases damping and promotes ringing at one particular frequency (Fc in the case of a sealed box). The pressure from the air inside the box works against the driver’s natural damping factor of 1/(Qts). When the pressure becomes large relative to the motor’s damping factor, the driver will ring more and cause a peak in SPL at the given resonate frequency (Fc). This tends to be somewhere around 40-60Hz in a given sealed box, but could be outside that range under abnormal circumstances. This peak is ill desired and is accountable to the proclaimed “boomy” sounding subwoofers which tend to lack clarity, good transit response and dynamics. However some people prefer some ringing because it provides a natural boost in a very audible frequency band. Likewise, in a larger box, the Q will decrease and the ringing and SPL around that frequency will too, but the low end will open up and you’ll have more deep bass. This tends to sound better and more controlled.


On the flip side, over dampened drivers tend to have poor low frequency response and require equalization to boost the low frequencies. They tend to work better in vented boxes where their larger motor force factor (BL^2/Re) is put to good use with a resonator which then makes the low end much more efficient with its increased displacement. Likewise, drivers with high Qts will work better in sealed boxes and should be exempt from being used in a ported system without careful consideration. When high Q drivers are used in a vented system they will ring at the tuning frequency of the box (Fb in this case) and the “boomy” problem is considerably worse.


# 4 Ported boxes don’t sound as good as sealed


In most cases this is strictly a result of linear response vs non-linear response and it could go both ways. 4th order systems or “vented” boxes tend to be far more particular to volume, port size and length and the driver TPS’s rather than sealed systems. Misalignments are therefore amplified and greatly affect the frequency response. Often times in car audio, ported boxes are not tuned low enough, or the volume is too large and there is a large peak in the frequency response from literately too much sensitivity or SPL at a very narrow frequency band. The other issue is if the driver does not have enough BL or has too high of a Qts and becomes under damped at resonance. This again leads to drastic peaks at the resonating frequency; however in this case, the driver will be peaky there regardless of content and it will sound ultimately less dynamic and very bottom heavy. However, a well designed vented box may have considerably lower distortion and higher dynamics than a sealed box because of the added SPL gained from the port without increasing the active driver displacement requirements. Sealed systems evoke the most non-linear driver behavior to reach any given SPL, so in fact, they could be the worst sounding system if your SPL demands are considerable. It is important to model a ported design or ask the manufacture for a recommendation. It is also critical to include a high pass filter on the active driver in a ported box for protection.


# 5 Subwoofers care what they play


Your subwoofer driver does not have a conscience, and it does not perform better with one type of music over another. It’s just a driver. 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 essentially this non-linear behavior is not ideal. It is true that movies have lower frequency content and perhaps more dynamic bass than music, especially with the recent compressed CD’s of the last 10 or so years, but a good system can be used for movies and music alike if it is indeed a “good” system.


It is also true that it tends to be more important to emphasize 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. It is very easy to have peaky bass with low Q drivers in high tuned ported systems. This is approaching the concept of basic SPL vehicles which use low Q, highly sensitive drivers tuned very high for very narrow but ferociously peaky response. 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!


# 6 Sealed box can take more power than ported


There is some truth to this, and some myth, but as far as the thermal limits of the driver are concerned, it can’t take more power one way or another. However, in a sealed box the driver will require more power to reach the same SPL as the frequency range lowers. A ported system is simply more efficient so it wont need as much power to reach the same SPL. Based on the mechanical limits of a driver, different frequencies can take different power loads. At higher frequencies, driver can be pushed hard and won’t necessarily be in a mechanical-risk state. However the driver tends to be in a higher thermal compression state and could be thermally at risk. This is true for both ported and sealed boxes. However, for lower frequencies, the sealed box also acts as a filter in a way because the internal air pressure prevents the driver from over excursion. In a sealed box, the compliance of the suspension system almost always forgoes that of the air spring system unless the box is very large. In a vented box, there is no pressure to protect the driver and furthermore, when the system unloads below resonance, the active driver’s excursion increases exponentially and a high pass (subsonic) filter is critical to prevent mechanical failure.


# 7 Sensitivity does not matter for subwoofers


Sensitivity is indeed 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 or more accurately BL product and moving mass, but not by maximum driver displacement. Higher sensitivity means more SPL and ultimately better performance especially for upper bass punch or kick such as a “kick drum” which resonates at 63Hz. In fact, all good SPL competition drivers need to have high sensitivity not xmax!


There are several standards for sensitivity. SPL at 2.83 volts or SPL at one watt. The SPL at one watt is the more accurate number as 2.83 volts could correlate to more than 1 watt which would not be relatively appropriate to go by. Also sensitivity is a function of, in part, the driver’s cone area which is never quite explicit and could be exaggerated slightly. Ultimately as engineers, we do strive for high sensitivity because not all bass resides in subsonic domain and many good sounding subwoofers are in fact good because they have great sensitivity and not necessarily high xmax


# 8 Smaller drivers sound better than bigger drivers


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 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 but not BL product. 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 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 dampened drivers, it’s just they require larger boxes and less internal pressure to prevent ringing. Ultimately there becomes a point where a driver really should be used in an infinite baffle where its actual Qts and Fs becomes the system Qtc and Fc. As enclosure volume decreases, Qtc increases and it will take a driver with a low Qts to make for an average Q system. So in conclusion, the only reason to use a smaller bass driver is for space, weight and potentially power considerations, but likewise, it is inappropriate to try and fit a larger driver into a space smaller than it is ideal for.


# 9 I can compare two drivers using the same box


What you will find is primarily how different TSP’s work in different boxes. And the differences usually observed are of course differences in TPS’s with a given system, rather than performance. The best way to compare two drivers is to make two different systems based on the driver itself and ensure that the frequency responses are linear to the range you desire, and then compare those two systems in terms of dynamic headroom, SPL and distortion. Simply saying one system is “louder” or “deeper” in the same box is inappropriate. In one case it could be a something as simple as an under dampened driver ringing a lot more than an over dampened one at resonance causing a larger peak in low frequencies throughout. It does not mean it’s louder or deeper or better outright, it is simply non-linear, and all bets are off. Proper enclosure deigns and/or EQ should be used for any system.


#10 cone material affects the sound


For low frequencies, the cone on a driver makes no difference in the sound whatsoever. The only 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 1000 to 2000Hz: Well beyond a bass driver’s usable limits.


Various cone materials are used for various purposes. Some cones, such as composite core with fiberglass or carbon fiber skins are extremely light and very stiff, especially when pressed with epoxy. Other cones such as aluminum provide excellent thermal cooling to decrease voice coil operating temperatures when the heat is conducted though the (if possible) conductive former. The cones job is to push air, not break, and ideally not be too heavy (easier said that done). But they don’t change the tone, pitch or timbre of a subwoofer system whatsoever. Anyone who tells you otherwise is probably hearing differences in the motor distortion, likely related to BL, compliance or other non-linear distortions not relating to the cone.


#11 bigger magnet means more magnetic force


The motor is essentially the steel and magnets on the bottom of the driver. Its job to create a magnetic circuit that has an air gap where flux lines cross in one direction so that a coil can rest in this field and carry current which then produces a force up and down and moves the piston to create SPL. The force that this motor creates is dependent on the amount of power or rather current inside the conductor F = B*L*I. So we need a more intuitive understanding of how a motor affects a driver’s performance without considering how much current it receives. This is the simple concept of “force factor”. Larger motors will ideally have higher force factors, but this number not only affected by the motor, its affected by the voice coil size, length, distance to the motor (gap) and conductive martial used too. The end result is in fact the BL squared divided by Re (resistance of the vc). This is literally Newtons squared per watt and is called the force factor. The higher the number, the more efficient the motor voice coil combination is and the more performance you get out of the motor.


BL, one of the many TS parameters you are probably somewhat familiar with. It is literally the magnetic field “B” crossed with the conductor length “L.” L does not in fact depend on the number of turns on the voice coil, but rather the actual cross section area of the coil itself which is inside the gap. While force factor is entirely important for any high performance driver, one should also consider the moving mass. A 600 horse power engine in a semi truck is pretty typical, but in a sports car it’s certainly something to gloat about. Together, the force factor, moving mass and the piston area account for sensitivity. This number is very important even for subwoofers, especially for frequencies above ~60Hz.


#12 Double bass kick, only good sounding drivers can do it


We have all heard that only good “SQ” drivers can do double bass kick because they have good transient response or something to that extent. This is really nothing more than linear frequency response and lack of ring. If high Q subwoofers are in small boxes or if low Q subwoofers are in large ported boxes, the frequency response of the system will likely be greatly non-linear. This non-linear response compromises relative SPL and can drown out certain sounds and frequencies. Room acoustics can also do the same thing. The same subwoofer may sound completely different in another room simply because there could be poor coupling and non-linear frequency response as a result of standing waves and peaks in the response curve. A peak at 80Hz may make for a rather anemic 60Hz response, and while 60Hz appears to be the problem, it’s actually from the nonlinear response else where! The bottom line is “double bass kicks” are usually not a function of the driver or driver’s performance but rather the system design, linear frequency and room equalization.


Often times people associate double bass as something to do with speed and only good drivers are fast. Believe it or not, even the largest and heaviest drivers, have no problem producing low frequencies, even 300Hz is a relatively slow long wavelength with a slow impulse time. Subwoofers are in fact MUCH faster than you would expect. Bottom line is, the lack of double bass, within the working limits of a driver, is not a problem with the driver so much as it is probably a problem with the system design, room and/or EQ settings.


#13 Transient response is better with sealed boxes


The fact is “transient response” is truly misleading and probably entirely unimportant at least for low frequency response. What people hear is really a function of the linear frequency response and distortion. It is often accepted that transient is a function of timing, but our ability to hear differences of a few milliseconds of low frequencies is quite negligible which is why the low frequency group delay of a 4th order system is quite unimportant next to the sensitivity advantages provided. Transient does not exclusively depend on sealed or ported designs, high Q, low Q, in fact, even drivers with high inductance don’t outright suffer from “transient response” insofar as we can physically distinguish certain sporadic behaviors because within their working range, they may be very efficient and dynamic. The fact is, what makes bass indeed bass, are long wavelengths that take considerable time to pass our ears. The perception of transient is really a function of perceived sound quality and there is really not appropriate example for good “transient response”. We as humans hear two things, distortion and SPL, and in the end that’s really want matters. What does improve “transient” response or perceived quality is usually more headroom, more drivers (usually larger boxes depending on the Qts of the driver), better efficiency and ultra low distortion within the prescribed limits of the system or drivers within the system. Sealed systems in fact don’t offer better transient response no more than ported even with their lower group delay tendencies, at least to human ears!


#14 It’s a bigger driver, then I need a bigger amp


Often times larger drivers require less amplification, that’s sort of the idea. The concept of bigger woofers need more power is not always true and plays right into the ever progressing misconception of 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 an amount of power (assuming linear limits of course). 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 and the amplifier can produce high voltage at impedance peaks when the driver naturally draws very little current for a narrow range. If a driver is more efficient and has a larger voice coil, well you just got your cake and you can now eat it. Not only will it be louder, but it will have less thermal compression and ultimately more sound provided all else is equal (but such is not usually not the case). It’s often difficult to make voice coils larger and increase sensitivity too. This usually requires very large motors and expense. Sensitivity is most easily achieved by weight reduction usually from the cone surround and voice coil. Sensitivity is often a trade off of xmax and thermal compression limits.


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. That advantage makes 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 either! Let me repeat. True SPL drivers ironically enough don’t need much power! That’s because they are used in the higher frequency domain not limited by displacement and generally have great sensitivity numbers. They need this in order to get the excursion and ultimately SPL they need to win contests. High sensitivity and lots of power means lots of SPL provided the driver is still reasonably linear and does not physically break of course. Note: Strictly for SPL contest, drivers are normally burped at Fc (system resonance) which is the point of maximum current draw and minimum active driver displacement which is why excessive power must be used. Do not confuse that requirement with the much lower power requirements for sound reproduction outside that single SPL frequency. It’s important you know the TSP’s of the driver you buy, otherwise it could be the wrong driver for you! Who buys a car without knowing the horsepower? Just because a driver big and the manufacture claims pie in the sky RMS numbers doesn’t mean a thing!


#15 Neodymium will lose its strength with heat


Of course it will, and so will ceramic motors too, but the fact is, under even extreme operating conditions, it’s not likely the motor will ever reach these temperatures. There is just too much steel to absorb the heat from the voice coil in almost any practical case. In practice, gradual demagnetization due to use simply does not occur. We have been making high power neodymium based drivers for many years now and we have never once measured a discernible number from heat.


While Neodymium is nearly 10 times as strong as a similar sized ceramic magnets, it can cost up to 50 times too which is almost exclusively why it is not used often. Also, traditional overhung motors, which account for more than 95% of all car audio designs, can get everything they need out of a ceramic magnet assembly and stronger neodymium would be perhaps unnecessary. If we could use neo more, we would, but because it’s a patented martial, it’s just not economically practical for most designs. Furthermore, in order the magnetize neodymium, A magnetizer with over twice as much power and energy needs to be used. Many manufactures lack the capabilities of even magnetizing neodymium, so it becomes impractical to not only use it, but to manufacture.


#16 Its all about maximum displacement


A DIY’er favorite statistic, displacement / dollars. If you’re considering any bass above 40Hz then throw it out the door right now. Often times people assume that simply because one or more drivers have more maximum displacement over another type of woofer, than they will ultimately be the better performer(s). In many cases this is true, but it’s not true in general. Displacement alone does not guarantee SPL. In fact, SPL depends on not only displacement, but frequency range, sensitivity, box size, and BL product too. This is simply a matter of converting energy into acoustic sound pressure level and different devices work more efficiently than others for different frequency ranges. For subwoofers, it is generally accepted that BL product is the dominate factor that accounts for much of the performance or rather system efficiency, especially in a bass reflex or more complex system where there is a lot of air mass to displace. But keep in mind, depending on the type of system, size, frequency range, power and thermal limits, there may be even more critical and dependent variables that determine the overall performance of a system. None the less, high displacement is usually a good indicator that the subwoofer can excel in deep bass SPL. Of course there are other factors to consider depending on the system of system.



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