When it comes to DIY servo subs, the first concern that comes to mind is the stability. Servo subs at one point had bad press because too many poorly executed projects were out there. These were too often done on a trial-and-error basis, without considering the control system or theories behind them. We can assure everyone that Direct servo system is absolutely stable.

A few of the features of the Direct servo system can further demonstrate that. First, the system is stable even when the amp is in clipping. Most accelerometer-based systems are not stable when the amp clips. So the protection circuit in those systems is a necessity, rather than a value added feature. We choose not to put in any anti-clipping circuitry, because such circuitry reduces linear dynamic range. Second, there is no relay in our amps. Some servo system cannot work without relays of instability during power-on. The power on noise of our system is just like that of a regular non-servo system.

The simple answer is yes. The more accurate, more complicated answer is that it depends on the music you are listening to. On dance mixes and music with extremely high bass contents, one does not need 14 Hz extension because of what is called the "masking effect". The masking effect occurs when a high amplitude signal is present simultaneously with other smaller amplitude signals. The perceived loudness of the smaller amplitude signal is drastically reduced. In addition, human ear's sensitivity decreases in the lower frequencies. Therefore for this type of music, one may even want to set the extension setting higher with lower damping factor setting (in our A350/A370 amps). That will make the bass sound clean (by removing ambient information) and punchy. It essentially allocates more amplifier power to the signal that we hear and care about most. On the other hand, when no such signals are present (for example, in jazz, solo and other types of music) and masking effect is not strong, one can clearly hear the difference that a 14 Hz extension makes (vs say 28 Hz extension). It is not a difference that will have you jumping out of your seat say "there's the bass". Rather it is an improvement in two areas: ambience and dynamics.

Ambience: good recordings normally capture a lot of ambience information which is predominantly low frequency signals. It is generally agreed that the lower the sub plays, the deeper, taller, and wider the sound stage becomes (or one may say open and spacious).

Dynamics: during musical transitions, a lot of the non-harmonic content is produced. Most of this content is in the very low frequency band, which will be filtered out if the bass extension is not low enough.

A vented enclosure is normally larger than a sealed enclosure, therefore heavy bracing is needed. Without bracing, a vented box will sound boomy. Second, use double flared ports (that is, flared opening at both ends). They do make a difference in reducing any 'chuffing' noise that may result from port turbulence. Third, apply 1" thick polyfill to the wall. Adding too much polyfill will increase the enclosure loss and therefore make the ports less effective. A passive radiator enclosure should ideally have two radiators on opposing sides of the box so that their forces act in symmetrically opposed directions. As a result, the net forces cancel. Care should be taken to ensure that the passive radiator will not be required to use too much of its maximum excursion, as the mechanical linearity of the suspension system will be less, resulting in higher distortion. Passive radiators are often used up to their excursion limits, and this is often not considered a problem since the distortion performance of the driver itself is poor.

In a high end Direct Servo subwoofer, the distortion is very low, so great care should be taken not to compromise the accuracy by adding a poorly executed passive radiator. Audiophiles who wish to add a passive radiator should keep in mind the effects of mechanical memory. This is discussed in our technology section. In brief, Direct Servo dramatically reduces the memory effect in the mechanical suspension system, and this is a major part of the highly accurate performance with musical reproduction. When you add a passive radiator, you are adding another mechanical system with its own memory effects which Direct Servo can’t correct, because it is outside of the closed loop. Audiophiles who wish to obtain low bass extension for home theatre for high output are advised to consider a second subwoofer as an alternative to buying two passive radiators.

It is possible but certainly not preferable. The results will not be worth the effort. If you use one of the coils in a dual voice coil subwoofer, you will severely compromise the performance. Both voice coils are required for the driver to perform well, and the a regular voice coil does not perform well as a sensing coil.

The sensing coil in our driver is wound with the smallest gauge of wire. It is not only lighter, but also space efficient in the magnet gap. With a regular dual voice coil driver, half the gap space is wasted because only the half is used for producing electromagnetic force. The extra weight from the voice coil which is now being used as a sensing coil can increase the moving mass and hence increase the magnetic distortion of the motor.

Our products are ideally suited to those who feel subs just don’t cut it when it comes to music. Perhaps past experience has led you to believe this is the case. Or perhaps you have bought subwoofer and been disappointed with what it did to your music.

It is with good reason that many hifi salesmen will tell you as if it is a fact that subwoofers are for home theatre only. The reason is that they don’t have any subwoofers that are accurate enough to reproduce music. They probably haven’t heard any that can reproduce music with accuracy either.

One thing is clear – anyone who believes subwoofers are not suited to music has not heard a Rythmik Audio Direct Servo subwoofer.

Our subwoofers achieve a level of accuracy that is not possible with conventional subwoofer technology. This may sound like a bold claim, but we encourage you to examine our site. You will find that this is not sales hype, but it is in fact the only logical conclusion. Our technology section gives a background to how we achieve such a high level of accuracy.

Our subwoofers are designed primarily for audiophiles who want the most accurate musical reproduction possible. Due to their SPL capabilities, our subwoofers are also suitable for home theatre applications, which can benefit just as much from the low distortion, highly dynamic bass that they reproduce.
Not everyone needs subwoofers as good as Rythmik Audio makes. In fact, not everyone needs a home theater or music room either, or even a sound system for that matter. But if you want to have the ultimate subwoofer that can perform as well for music as it can for home theatre, you do need a Rythmik Audio subwoofer. If you buy another subwoofer expecting to achieve the same level of accuracy and performance, expect to be disappointed. You will most likely pay more and get a lot less. Would you like to know how we can make such a claim? Review our technology section and it will start to make sense.

Conventional thinking puts it this way – accuracy for music, maximum SPL at 20 Hz for home theatre. The problem with this is that movies contain music, and often contain bass from familiar sounds, so we know what we are hearing isn't accurate.

We think differently. Both music and high end home theater demand accuracy. The primary difference between a subwoofer optimized for each relates not to accuracy but the nature of what is being reproduced. With home theatre, relatively high SPL capability at 20 Hz is important. Due to the masking effect, deeper bass will often not be noticeable, unless at very high in amplitude as a special effect. Of course, there is music with high amplitude bass that will mask very low bass. Some movies will also benefit from bass below 20 Hz.

Our recommendation for a subwoofer for music are our sealed Direct Servo subwoofers. If your interest is also home theater as well, one or a number of our sealed subwoofers will still be an ideal choice. If you are a home theater enthusiast on a modest budget and don’t listen much to music, you might consider one of our vented subwoofers. We provide advice on choosing a suitable subwoofer in our product selection guide.

The important consideration here is that there are two numbers we can arrive at as an answer to this question. One is maximum SPL which is the most that you can measure without damage to the subwoofer. This is relatively simple to determine for a given frequency. The other number is the maximum SPL which can be reproduced with low distortion, definition and accuracy. In practice this is the more significant limitation of a subwoofer. Most subwoofers lack accurate bass at any level. Typically even subwoofers normally considered high end are not able to reproduce what we would consider accurate bass at 10 dB less than their absolute maximum SPL.

While our subwoofers are not designed to impress with high SPL, the SPL they can produce without high distortion is higher than competitive subwoofers.

Maximum SPL at low frequencies (without considering distortion) is determined by the amount of air that the subwoofer can displace. In a sealed subwoofer, this is the only factor, but in vented subwoofers, the output from the vent must also be considered. In theory, a vent or passive radiator can contribute 6 dB in the bottom octave, but this gain can be reduced by port compression in vents and by the increase in mechanical stiffness of the passive radiator at high excursion.

The amount of air displaced can be limited by a number of factors that are difficult to estimate accurately. In practice, a maximum estimated SPL capability is of little use since it is not related to the distortion involved.

The following points should be considered:
Higher excursion drivers will not necessarily have higher SPL
Drivers designed for a very high excursion have significant compromises which result in an increase in distortion
Higher amplifier power does not always mean more output
While a vented subwoofer can have similar output at 20 Hz to two sealed subwoofers, the sealed subwoofers will have higher output above 40 Hz

If high output is important to you, in most cases the best way to achieve it is with a number of well-designed subwoofers. Rather than buying one very expensive subwoofer with ultra high excursion and a massive power amplifier, we advocate multiple subwoofers with a sensible and intelligent design. This removes the compromises used in the ultra high excursion ultra powerful subwoofer which we consider to be more useful for making noise than for reproducing music. Our subwoofers are superior in the areas that count the most – accuracy and value for money. In many cases they are superior in SPL and cheaper as well. This is only possible because we use superior technology.

Many people categorically put any spurious signal component that is not in the original signal as distortion. Noise, when beginning to correlate to the signal, becomes distortion; Distortion, after being totally randomized in relation to the original signal, becomes noise.

In the real world, it is never black and white. Distortion can have various degrees of correlation to the original signal. And that is the key to explaining why some distortions are more pleasant to the ear because they have strong correlation with the signal (for example, distortion generated by tube amplifiers). This also explains why some distortions are less measurable, and often leaves us scratching our heads why the sound is not as good as our distortion measurements lead us to expect. In our opinion, the most subtle distortions are often those that are not quite measurable (such as memory effects). Very often the effects of such distortion is to obscure the life-like dynamics of the music and/or limit the resolution of the system. Now going back the difference between noise and distortion: the best example of this is digital dithering. Digital quantization error (or distortion) is one of the nastiest sounds that can be introduced into the recording/playback chain. The objective of digital dithering is to randomize this quantization error so that it becomes noise. The result is very dramatic. One of the Chesky Record test CDs has 2 demo tracks that demonstrate the effect of dithering. The track without dithering is just unbearable. The track with dithering, in comparison, is much more listenable despite a high amount of background hiss noise (as a result of dithering).

This is a common misconception. If you have access to simulation programs, you can easily see why so-called "optimal box configurations" are in fact engineering compromises.

First, let us look at the vented box. Set the box size to 100 liters and tune it to 20 Hz. You can plug in any reasonable values for T/S parameters and play with them. You will then see that maximum output at 20 Hz depends only on two parameters: Re (voice coil resistance) and BL (actually, it depends on 3 parameters: Re, BL, and enclosure size, which is fixed). All other T/S parameters will change the response curve, but not the maximum output at 20 Hz. So the job of driver designers is to find a set of T/S parameters that will level off the response above 20 Hz so that we get a flat response. But in the process, the efficiency at the mid-bass band (40 Hz-100 Hz) is sacrificed. The mid bass from professional drivers is generally very good. This is mainly because of their high efficiency. The goal of our Direct Servo driver is to borrow that idea so that the mid-bass efficiency is as high as possible. In terms of frequency response, we rely on the servo technology to give us a flat response. So that we have best of both worlds: good mid bass sound and great bass extension. A similar argument applies for sealed box designs. Overall speaking, there are 3 parameters that we care most: Re, BL, and enclosure size.