Large is better for mid-upper bass, small drivers can be less of a compromise for low bass. BUT - as everyone else already has stated, small drivers will not be able to reproduce low bass properly for full volume playback.
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Fact or Crap? There is no replacement for displacement.
- Thread starter Tom C
- Start date
I have no idea what point you had been making. In just two consecutive sentences you had been contradicting yourself. In the first sentence you implied that small drivers are better for low bass. In the second sentence you said small drivers are not suitable for low bass.
There are 2 different properties at play here - sound radiation pattern, and output capacity.
Radiation matters for all listening levels, so a small driver will always compromise sound in upper bass - low midrange, while a small driver used for bass/low bass will work well for low volume. Some like to have it LOUD occasionally, but some never listen loud and then it is really not necessary to install a large, high-capacity bass-system.
tuga
Major Contributor
Is there a direct relation between excursion and the amount of intermodulation distortion (>Xmax = >IM)?
tuga
Major Contributor
I think that I may have found the answer in a comment by Linkwitz:
"Issues in loudspeaker design - 3
O - Unreasonably high distortion in midrange drivers?
Typically distortion increases with SPL or cone displacement. For constant SPL the displacement decreases as 1/f2 with increasing frequency and it could be expected that distortion decreases correspondingly.
The dominant nonlinear parameters in a driver are the force factor Bl(x), the stiffness of suspension Kms(x), and the electrical inductance Le(x).
P - Estimate of Le(x) induced distortion at high frequencies
(...) if a low frequency signal is present together with the high frequency signal, then the large low frequency voice coil displacements can cause large amounts of intermodulation distortion products near the high frequency signal due to the large low frequency variation of Le(x)."
https://www.linkwitzlab.com/frontiers_3.htm
Another piece which may be related to the issue, this time from Purifi:
"Distortion, The Sound That Dare Not Speak Its Name
Force Factor Modulation (FFM)
The impact of position dependent force factor BL(x) and the position dependent suspension stiffness Kms(x) have been well publicised. These parameters aren’t nearly enough to predict real life distortion performance though. They only predict distortion when excursion is large, which you only get when you play bass heavy music very loud. If the BL(x) and Kms(x) duo are to be believed, all speaker drivers ought to have the same negligible distortion under normal listening conditions. This we know is not the case.
PURIFI has extended the purely statistical concept of BL(x) into a dynamic one that includes time and voice coil current. The new mathematical model correctly predicts motor force for an arbitrary combination of applied voltage signal and movement. The model reveals that all motor distortion that can’t be explained by BL(x) can be explained by something called Force Factor Modulation. FFM happens when the magnetic field created by the current in the voice coil adds itself to the magnetic field created by the permanent magnet. Ideally, the magnetic flux in the air gap is only determined by the magnet and the geometry. In practice, it varies strongly because of the current in the voice coil. The problem is exacerbated in long-stroke drivers because a larger portion of the coil is creating this unwanted magnetic field without actually producing useful drive force. This goes some way in explaining why short stroke, large diameter woofers have such a faithful following. It also tells you exactly what problem to solve to get the same sound out of a compact long stroke design.
The distortion takes the form of the signal being multiplied with a filtered version of itself, so it is predominantly second order in nature. Now, there is a common misconception that second order distortion is innocuous. This may be largely true of harmonic distortion where a second harmonic is easily masked by the fundamental, but in the case of intermodulation distortion it is patently false. Second order IMD generates difference frequencies which are below the signal frequency and don’t get masked at all. They audibly clog up the bass region in a manner which becomes extremely obvious once you remove the distortion. Also, amplitude modulation of mid frequency signals by the bass is very audible as burbling.
A second important insight was that Force Factor Modulation isn’t just linked to position dependent inductance and to reluctance force (=the attraction between the coil and the iron parts), but that all three phenomena are actually one and the same thing. This insight has driven the design of PURIFI’s new motor which is virtually free from force factor modulation.
Additionally, the new motor allows FFM and BL(x) droop to be optimized independently. It wouldn’t help much to remove FFM and then to find distortion dominated by classical BL droop, so additional refinements were made that make the static BL(x) curve extremely flat over a large excursion range. The linearity of PURIFI’s motor is significantly better than what was historically possible with shortstroke designs.
With Force Factor Modulation comes a side order of Magnetic Hysteresis Distortion. If the magnetic flux in the gap varies with current, any iron in the gap will be subject to that varying flux. Magnetic hysteresis is quite different from saturation. Saturation is a fairly benign, soft-limiting type of distortion that happens at very large signal levels. Hysteresis by contrast happens at all signal levels and lends a grainy, hazy type of distortion to almost any transducer that contains iron. In terms of measurements, hysteresis distortion tends to dominate the midrange (well above resonance and below breakup) outright. Hysteresis distortion is worst in underhung drivers because of the sheer volume of iron surrounding the voice coil.
This explains the recent arrival of so-called “iron free” drivers. Such drivers do address the problem, but at enormous expense. PURIFI’s motor combines the performance advantage of iron-free while retaining the economic and technical benefits of using iron to shape the magnetic circuit."
https://purifi-audio.com/2019/05/02/distortion-the-sound-that-dare-not-speak-its-name/
I can only conclude that the pursuit for higher fidelity, small, long-excursion midwoofers is dumb and mostly market-driven (the need to produce domestic-friendly, small, narrow-baffle 2-way speakers). But, unlike science, hi-fi, like any other business, is about solving a real problem and making a profit.
Speakers should be (at least) 3-way and the woofer diameter should be fit for purpose.
Uncompromised, high performance, full range transduction requires a 4-way design (treble + mids + bass + sub-bass).
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Looks like you found the answer (post above).
If the driver is perfect, which we realize will never be the case, there are no significant correlation between excursion and IMD. There is the potential of frequency modulation distortion, but this effect is not large enough to be of any concern.
What is significant, though, are nonlinearities in practical drivers, which are very difficult to control when excursion increases. Suspension cm(x) Rm(x), inductance Le(x), force modulation Bl(i), force across travel Bl(x).
Small drivers can still work very well for bass, in applications where high output is never required. Which will be the case for most casual listening at home. And acoustic loading can help to reduce cone excursion and thus increase useful output, but at the cost of more complex enclosure and a little larger size.
That looks like some great info, nice find. However, I wouldn't be too quick to draw these conclusions. It just makes me want to see speaker IMD measurements at a few SPL's. @hardisj has somre really great IMD measurements of drivers on his site, including of the Purifi, but I don't know how applicable these would be to evaluating an assembled speaker.
tuga
Major Contributor
Yes, it would be interesting to see IMD measurements of the largest Purifi reproducing for example a 50Hz + 60Hz + 2kHz + 3kHz combination (or one deemed more fit-for-purpose).
I can not see the point in trying to force a small driver to reproduce low bass, and use this same driver as midrange. If a very tiny, small speaker is the requirement, why not make one that can do >120hz very well, and make a very tiny, small subwoofer - problem solved.
For such a speaker there are plenty alternatives for lf driver, and the small sub will not destroy the sound even if it distorts a little.
This solves the IMD problem, because there will not be any IMD if the driver running into problems never plays higher frequencies.
For such a speaker there are plenty alternatives for lf driver, and the small sub will not destroy the sound even if it distorts a little.
This solves the IMD problem, because there will not be any IMD if the driver running into problems never plays higher frequencies.
I understand what you meant, now but I don’t understand why would be the case. Do you mean headphones will always be compromised in upper bass for instance?
Also, can you please clarify your definition of small driver and upper bass?
Headphones is a different experience and really not comparable, for many reasons.
Small driver is a simplification, because size in itself is not what matters, it is the radiation pattern. So a 6" in a horn with large mouth can be much better than a 15" direct radiator. But while most people can relate to a 6" vs. 15", few people have experience with such horns. So it is easier to just use driver size to tell the story.
It is the radiation pattern that makes the difference, and this difference matters from somewhere down in the bass range up to the very highest frequencies. And larger speakers typically have better directivity from midrange and down in frequency.
A better pattern in the low-mid range reduces early reflections and this gives better definition, better clarity, retains timbre better, better transient reproduction.
Having listened to the same speaker, where this radiation pattern can be changed, reveals that this difference affects the sound. Better directivity sounds different from the typical small omni-radiating box speaker. Transients have more realism and impact, rendering of images improves, clarity improves. You can not see this in the frequency response, but you can see it when looking at what happens in the time domain.
The interesting aspect of this is that it is not a matter of size, it is a matter of radiation pattern. So if a small speaker can be made to produce a similar pattern as a larger speaker has, it should be possible to relicate the sound of a large speaker in a small physical size. And indeed, it was possible.
Correct me if I'm wrong, but you seem to be making the case that narrow directivity is generally better. Is this supported by the "body of work" of existing scientific studies on listener preference?
In keeping with just using driver size to tell the story, I'm going to make two assertions from a position of ignorance, having never designed a speaker. Please consider this a request to be "set straight", or in other words, why are these wrong, or if not wrong, how are they not relevant/applicable to what you are saying?
1. ka = 1 for a 15" driver at about 284Hz.
2. A 15" driver will have the same directivity as any smaller driver (ie. essentially omni) if crossed bellow 284Hz, assuming the baffle is not significantly wider than the woofer.
Just to make it clear. Are you making the comparison between a horn and direct radiator speaker?
tuga
Major Contributor
Why do you assume that what the majority prefers is in fact better?
In my view early reflections distort the transduced signal / reduce fidelity. An overlay of the listening room's acoustic footprint on top of the recorded ambience cues.
I would agree that most people seem to prefer the euphonic effects which result from wider dispersion in un- or lightly-treated rooms, both from my own observation as well as what I have read from the related literature.
Directivity drops of quite sharp, but it is still on a slope, so there will be a difference a bit further down in frequency. Say we cross at 120hz, then they will essentially be the same.
For high quality sound reproduction it is required to have controlled directivity, which means a pattern that is different from the typical small speaker, and can be described as narrow. There are aspects of sound reproduction that simply can not be achieved using speakers with too wide direcitvity.
But narrow vs wide is not sufficient to describe differences in radiation pattern, a speaker where pattern is part of the requirements specification will typically have a wider usable coverage, while attenuation at wide angles outside the listening area is larger.
There is no theoretical support for wide directivity being better, and it is easy to see how a narrow pattern improves sound quality. Once you get the chance to hear this difference, it is obvious.
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