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Problems and Solutions...
For decades the inherent limitations of the direct radiator loudspeaker have been tolerated but never fully accepted. Over the last half century a huge amount of effort has been put into correcting the deficiencies in bass reproduction from loudspeakers but little advances have been made to make it economical and practical for the masses.
Typically, bass reproduction is achieved with a relatively large direct radiating moving coil loudspeaker mounted in a suitable sealed or vented enclosure and optionally fitted with some external EQ applied to extend or flatten the response - the bare minimum needed to make a system ! It is probably the most economic solution available for the amount of money invested. However it still has many shortcomings when it comes to accurate reproduction of the original source material. Bloated, muddy and slow bass are usually the adjectives that are used to describe distorted sound of one form or another. In the end the owner being dissatisfied with the reproduction of music then seeks out something better at yet more expense. Technologically speaking when it's all said and done there is really nothing to distinguish one design from another apart from appearances, marketing hype and one-upmanship in the SPL wars.
The moving coil loudspeaker has a number of inherent limitations. In particular it exhibits both frequency and amplitude limitations, which manifest themselves as linear and non-linear distortions respectively. Linear or frequency response limitations can easily be corrected with equalization. However, the increased drive requirements of speakers mounted in small enclosures means that the amplitude limitations can become rather dominant so much so as to completely change the timbre of the sound.
It is not uncommon for a woofer to exhibit 10-50% of distortion in the low frequency range. This is unacceptable unless you are willing to tolerate gross coloration of the original source material. High definition audio reproduction from digital sources requires low coloration and low distortion from all of the components in the chain. Quite simply, existing open loop (no negative feedback) low frequency loudspeaker systems are inadequate for the job. Clearly a new technology that is both effective and economical at reducing these distortions is needed to fix this problem that has been ignored for decades.
Loudspeaker Distortion is the forbidden topic in the Hifi industry
Have you noticed how talk of loudspeaker distortion is completely frowned upon or discouraged when it comes to loudspeakers but is totally accepted when it comes to amplifiers and other source equipment. In fact manufacturers of speakers go to great lengths to omit this information in their specfications. Why ? Considering that loudspeaker distortion is orders of magnitude greater than the worst amplifiers it is puzzling why this aspect of speaker design is glossed over. The simple answer is that loudspeaker designers cannot do much about it so it's easier from a marketing perspective to completely ignore it and hope that the consumer doesn't look to closely into it either. And since nearly all loudspeakers share the same weaknesses then it becomes an unwritten rule to not talk about it or make excuses as to why it is not important.
Would you buy an amplifier that performed as badly as this ?
If someone tried to sell you an amplifier that reproduced a 20Hz sinewave as poorly as depicted in the waveform to the right what would you think ? Either the person is mad or the equipment is faulty or poorly designed ! And yet this is what was produced when a typical subwoofer was fed with a 20 Hz sinewave signal from a high quality amplifier that for all intents and purposes is distortionless ! The top yellow waveform represents the acoustic output of the woofer and the bottom blue waveform is a measure of the current in the voice coil. Both are distorted which is not what you want when trying to accurately reproduce music as the artist intended.
If the speaker does this to a simple sinewave, just imagine what it would be doing to a complex music passage which is why amplifier designers have expended huge amounts of effort in erradicating distortion of all kinds ! However the sad fact of reality is that nearly all speaker systems and subwoofers measure this badly whilst the vendors quietly ignore the problem and look for other sales gimmicks to sway your purchase decisions. That old chestnut about not being able to hear bass distortion is usually wheeled out as well even though our hearing is more sensitive to the higher frequency distortion artifacts !
Even vendors of expensive speakers built out of exotic materials are not immune to this. Whilst focussing on the cabinet, wiring and crossovers they have forgotten that the driver is still the Archilles heel and suffers from the same motor induced distortion problems as a lesser speaker system.
At first glance you’d be forgiven for thinking that the sub-woofer in the picture to the right is just another 18 inch high powered driver mounted in a sealed box. But don’t be fooled, appearance is only skin deep with our sub-woofers. Beneath the protective dustcover is an ultra-sensitive, highly linear industrial grade accelerometer which is designed to provide instantaneous feedback of the driver’s diaphragm motion to an advanced digital servo controller. This digital servo controller monitors cone motion at nearly 50,000 times per second and provides correction when the diaphragm departs from its intended motion thus making sure that the diaphragm follows the input signal as accurately as possible. The result is low distortion, exceptional transient response and extended bandwidth which is all unachievable without servo control.
As you can see from oscilloscope snapshot to the left, with servo control the input signal (top yellow waveform) and measured acoustic output signal (bottom blue waveform) are virtually indistinguishable from each other. To the listener this translates into tight, articulate, clean and transparent bass reproduction of the original source material which after all is what music reproduction is all about ! No other non-servo sub-woofer can even approach this level of accuracy and currently available servo speaker technology falls far behind by at least an order of magnitude in performance objectives.
Negative feedback. Nothing new under the sun...
Traditionally you had to pay tens of thousands to get decent bass performance from a speaker system and even at those prices you were still only getting mediocre bass performance from an expensive driver mounted in a sealed or vented enclosure. It is not uncommon to see speakers that sell for hundreds of thousands of dollars with one or more bass drivers running open loop (no feedback). As a result you seldom see distortion performance from the manufactures simply because it is usually mediocre and embarrassing to say the least!
On the other hand it is standard procedure to publish distortion performance of an amplifier or source unit such as a CD player ! Some might argue that in some cases distortion colors the sound and makes for a more impressive sounding engagement but is that what the original artists wanted when they were mixing their work in a studio or recording at a live performance ? Were they really relying on the users reproduction equipment to add coloration to their original performances in order to complete their works ? Somehow I don’t think so !
You are probably not aware that you are using an amplifier or preamplifier that employs negative feedback and in most cases uses a large amount of feedback to reduce distortion and noise as well as extend bandwidth. Even in the digital domain your hi-end ADC or DAC most likely uses one or more feedback loops to improve conversion accuracy. The same advantages of employing negative feedback in amplifiers and in the digital domain is also used in our sub woofers for exactly the same design objectives. It seems counterproductive that of all the techniques used to reduced distortion in the play back chain are essentially mitigated by a non-servo speaker system which adds even more distortion back again !
Last Updated November 2, 2014