Ultra Low Harmonic Distortion (HD)

If a driver exhibits low HD it may be a suitable candidate for further evaluation. A driver with high HD will cause audible coloration or alteration of the music and is not a suitable component choice if the goal is a pure and transparent audio system.

PURIFI's quest to eliminate sources of nonlinearities and reduce HD to the levels comparable to those of good amplifiers, frequently 60dB or more below the signal level, and magnitudes better than competing high-end transducers. 

Look for HD vs. frequency plots in logarithmic (y-axis) scale. Plots in linear scale may appear to have low HD, but this is an optical illusion that makes the curve appear to approach zero.  

While HD is an important metric, it is not a 'seal of approval'. Different distortion mechanisms may cancel out and produce seemingly low HD in single-tone frequency sweeps, but performance can completely deteriorate in multi-tone situations. 

Ultra Low Intermodulation Distortion (IMD)

Intermodulation occurs when, e.g., a mid-frequency tone is modulated by a low-frequency tone. Practically speaking, IMD can cause a bass guitar note or drum kick to warble and distort a more delicate midrange instrument or voice. IMD is a direct contributor to inaccurate sound staging and garbled music reproduction.   

The PURIFI PureDrive and PURIFI NeutralSurround reduce IMD, often by a factor of more than 10x compared to competing equivalent-sized high-end drivers.

A simple  approach to analyzing IMD is to perform two-tone FFT measurements. IMD will reveal itself as sidebands around the fundamentals. The lower the sidebands, the better. PURIFI's drivers show exceptionally clean FFT's over a wide range of frequencies and amplitudes, indicating ultra-pure sonic response across the spectrum. 

Constant Force Factor vs. Voice Coil Position, Bl(x)

Force Factor (Bl) tends to change with position. A drooping Bl curve will cause mid-frequency signals to be amplitude modulated by low-frequency tones resulting in significant amounts of IMD and a warbling sound or muddy midrange.

The PURIFI PureDrive motor reduces both position- and current dependent Bl a factor of 10 or more compared to existing high-end drives. The result is a crystal-clear midrange, even at high SPLs.

Look for Force Factor vs. Position plots, which should ideally be standard content in any transducer datasheet. PURIFI's drivers show very flat and symmetric curves over a wide excursion range, compared to competitors' often parabolic and asymmetric slopes.

Constant Inductance vs. Voice Coil Position, Lgen(x)

The inductance of the voice coil tends to vary with position, which modulates the current and produces amplitude modulation type IMD.

The PURIFI PureDrive reduces this effect by more that 10x over conventional high-end drivers, resulting in lower IMD and purer audio.

The electrical behavior can be visualized in Inductance vs. Position plots, preferably repeated at multiple frequencies. The ideal behavior is a set of ruler-flat lines, as achieved by PURIFI drivers, whereas most competing products will display sets of heavily slanted or asymmetric curves, indicating large amounts of distortion.​

Ultra Low Current Distortion vs. Current Amplitude

On a deeper technical level, the current distortion vs. current amplitude is important because it reveals Magnetic Hysteresis Distortion (MRD), which is an audible artifact caused by the millions of magnetic domains in iron having individual memory in the form of hysteresis. The resulting distortion is both unpleasant and dominant in the upper operating frequency band.

PURIFI’s PureDrive motor design reduces MRD distortion to unprecedented low levels and is easily recognized sonically by the grain-free, deep black background to the music.

When evaluating current distortion, look specifically for extremely low 3rd order harmonics in Current Distortion vs. Current Amplitude plots. Unmanaged MRD will surely manifest itself in the form of high 3rd order harmonics, although other issues may also exhibit this behavior.

Smooth Frequency Response w/ Excellent Dispersion

Frequency response (FR) and dispersion are common parameters for comparison in audio systems.  

PURIFI utilizes a combination of proprietary IPs, including cone geometry optimization, to achieve maximum response flatness. The cone geometry, surround shape (PURIFI NeutralSurround), and the edge stiffening ring (PURIFI ConeRing) are designed to mitigate and manage edge breakups, resulting in a more gradual and soft behavior. Additionally, the high frequency roll-off, where the drivers become directional, is well-controlled and has minimized peaks.

USHINDI paper/fiber drivers use a proprietary pulp mix designed to achieve specific material properties considered optimal for superior sound quality. The aluminum-cone versions are shape-optimized for a high break-up frequency easily integrate into cross-over networks.

Problem: Surround Radiation Distortion (SRD)

The phenomenon behind SRD is easily conceptualized by pushing on the cone of a conventional "half-roll-surround" driver. When the cone is pushed inwards the effective radiating surface area of cone+surround (Sd) increase, and conversely, when pulled outwards the radiating surface area decrease. This position-dependent effective area deviation can often be as much as 20% of a drivers radiating surface area and is therefor a direct contributor to distortion.

The industry standard method of dealing with Sd modulation is to completely ignore the problem and rely on the generic notion that 'bass distortion is inaudible'. This is certainly not the case. Sd modulation is fundamentally a signal-dependent change in overall sensitivity that is sonically most obvious as amplitude modulation of midrange content. All in all a MUST-fix problem if the goal is excellent sound quality.

Solution: PURIFI NeutralSurround

PURIFI has addressed the SRD issue with its proprietary NeutralSurround technology. This technology uses a special surround geometry that is optimized mathematically, keeping Sd modulation to a minimum, effectively eliminating SRD and significantly reducing IMD effecting the full range of the driver.

Furthermore, the geometry has a number of additional benefits all working collectively to enable unrivalled performance and low distortion from PURIFI's extended-stroke drivers:

    1. Practically eliminates Sd(x) Modulation
    2. Controls the critical Cone/Surround edge breakup for improved FR, HD and IMD
    3. Resilient against Back-Pressure; enables use in very small cabinets

Problem: Force Factor Modulation (FFM)

FFM is a complex beast with negative implications on transducer performance reaching way beyond the established "position dependent force factor", Bl(x), understanding of the root cause of distortion. One of its elements is the modulation that happens when the magnetic field in the voice coils adds itself to the field created by the permanent magnet. Another component which cannot be ignored is the undesirable attraction force caused by voice coil currents and nearby iron parts. This force is quadratic in nature, and to make matters worse, always act in the same direction independent of signal polarity. The position- and frequency dependent inductance of the motor is a telltale and an important metric that should be widespread within the transducer industry as performance indicator. Conventional wisdom, e.g., use of shorting rings, is far from enough to combat FFM. 

Solution: PURIFI PureDrive

To address FFM, PURIFI uses a patented combination of topology and numerical geometry optimization to concurrently and independently correct both Bl(x) and FFM. The result is a motor system with exceptional linearity, allowing small cone/long-stroke drivers to outperform much larger transducers.

The larger linear cone displacement lowers harmonic and intermodulation distortion and enables speaker designers to build powerful compact systems with loudness and clarity beyond what has traditionally only been possible with very large drivers.