Optimal Load Characteristics

[FOH]

Great site....

 

 

 

I fully realize there's little new under the sun.

 

I'm guessing this has been discussed elsewhere before ad nauseam. However, I'm not sure to what extent and exactly what elements were explored. I hope someone here can enlighten me.

 

The issue I'm trying to get my head around currently, is the optimum load an amplifier output stage wants to see across it's terminals. Now, I'm not inquiring about a merely theoretical, mathematical load. I'm referring to real world amps, drivers, and reacting to LF material. Like all of us, I've read extensively, and I'm familiar with Wiggin's work/white-paper discussing inductance as a limiting factor to a rapidly changing, high drive current signal voltage swinging thru a VC.

 

So, my bias is I prefer one channel, one driver. Any opportunity to limit inductance is a solid step in the right direction. Subjectively, we're inherently poorly equipped to make such determinations, despite my protestations otherwise

 

There's a common theme associated with the AE TD motor design. Yes, is is said bass is bass. However there seems to be consistent subjective appeal to the AE TD drivers perceived tonality, transient capability, and general capability. Inductance?

 

Realistically, one can attain satisfactory, even high performance results with multiple drivers/voice coils per each amplifier channel. What I'd like to explore is if inductance is a chief limiting factor, then what work has been done exploring manipulating total inductance and the resulting transient capability? Does paralleling VC's to reduce total inductance result in a higher performance at lower frequencies? Is two VC's in series compromised in it's transient response capability? Is this phenomena exclusive to the upper octaves as some would suggest?

 

In any multi driver subwoofer load scenario/application, ie a big IB system, be best served limiting total inductance across the amp, or is this a non issue? Oftentimes a series/parallel config yields a 4ohm total impedance with satisfactory results. Where does the limit come in? How many drivers (how much inductance) can one config until a performance hit wrt transient capability arises?

 

Oftentimes I read this is of little concern at subwoofer frequencies. Is this valid? (below 120hz)

 

I'm not discussing good enough, I'm exploring state of the art.

 

Thoughts?

 

 

 

Thanks

[Ricci]

I'm still trying to determine how I feel about this myself. To some extent I usually lean towards drivers with better top end response and higher efficiency as sounding better, clearer, punchier however you want to put it. However I have not done any comparisons in a blind manner with dissimilar systems EQ'd into the same response shape. Some would say that besides inductance there is also motor force, suspension compliance, efficiency and moving mass that must be considered. Again I have never had the chance to compare these things on an equal footing without bias. Part of me believes that the audible improvements that I hear are manifested mostly in the basic frequency response differences. I do not know.

 

One thing about inductance...You may already know this but perhaps not. It is the same as motor BL in that the single parameter usually at 1khz is worthless on its own to begin with and even if some weight is put towards a single static figure you need to know the impedance to go with it at least as well. You may drop the static LE value when parallel wiring drivers but you have also cut the impedance in half so the end real world result is basically the same despite the drop.

 

As far as what amps like to see so far in my experience systems perform better with higher voltages into higher impedances rather than high current into low impedance. The higher current and lower efficiency at points of low impedance result in higher distortion, power compression and thermal buildup issues to arise, which causes the response to be less linear there. The amplifier has a much easier time driving high impedance as well.

[FOH]

I appreciate the response.

 

I too haven't performed any definitive comparos in this regard. There's are very strong argument that "bass is bass" ,... as well as the argument that frequency response is the overwhelming metric of which we form our subjective opinion. However digging deeper, I struggle with not thinking that transient capability, inductance, and the manner in which the leading edge of the transient and associated behavior is defined. There seems to be a subjective appeal to some motor designs over others. I'm uncertain if this is rooted in magnetic asymmetry flux modulation, transient capability, perhaps both,...or maybe merely an appealing FR.

 

Clearly, any attempt to minimize stroke, keeping the VC as centered as possible is commendable. Keeping operational levels low, or adding multiple drive units allows for more linear magnetic function and high acoustic output, and a "headroom cures all" approach is a sound technique. But I keep considering the very basics of electrical physics, with Lenz' Law and Wiggins work illustrating inductance governs the VC's ability to rapidly change position.

 

Back to real world applications; is there any hit in performance that you can think of with a set of multiple drivers, series-paralleled across an amps terminals into say 4 ohms, and the same set of drivers, with their own amp channel/same impedance/same power per driver? Any real performance benefit at all to an approach utilizing a dedicated amp channel,.... per driver? I've always preferred the simplicity and in my head preferred the dedicated wiring per driver/amp channel,... and I'm the first to admit it's a purely subjective preference. For one practical reason, I can quickly determine which driver is potentially problematic,....but for the purposes here, I'm inquiring about performance.

 

 

Additional thoughts?

 

 

Thank you

[Kyle]

FOH, you need to also keep in mind that inductance is a function of frequency. It really shows up in the impedance because you can see the resistance climb as we reach higher and higher frequencies. We (data-bass) don't really have a perfect way of measuring our drivers, we're still debating which system to use and which parameters to put fourth. The inductance is one of the most inaccurate in our opinion in terms of the predicting the final frequency response. But in so far as subwoofers and especially low frequencies, the inductance is not something that dominates or really even has any meaningful effect on the performance or sound quality there. When we start to climb, then it starts to matter, and it matter big time. The ratio we provide on the driver page Le/Re is mH/Resistance which becomes milliseconds. This is the time duration of which about 95% of the inductance is discharged after that period of time. So for longer strokes (lower frequency) this time becomes smaller and smaller relative to the period, hence less and less important.

 

Now the inductance value also changes with frequency because we have this whole moving coil issue But still even a changing value is somewhat unimportant if its already a non factor and we get the biggest changes at the lowest frequencies so we tent to luck out in that department.

 

Dealing with higher mid bass, we want to reduce inductance obviously. Speakers tend to sound more efficient and more effortless. Inductance does reduce power across the amp which is why EQ is often used to combat it. Now inductors by themselves are not bad and are used all the time in signal processing, but the main issue with the speaker motor is its a terrible inductor that can add its own distortion signature. There are a few speakers that attempts to tackle the problem from the opposite approach and simply rather than trying to make the inductance lower, they make it more linear by using a powered iron core commonly found in inductive. This core is a poor electrical conductor, but a reasonably good magnetic conductor (about 80% of what low carbon steel can do). Inductance will be high, but at least it won;t add distortion...Now in practice this will still reduce power draw from the amp, but in theory you can just apply a filter and boost the voltage to the driver to draw more power.

 

As far as amps, i always just get the biggest thing I can afford... perhaps not practical from a cost standpoint, but power is cheap these days, drivers are not getting cheaper.

[Bossobass Dave]

IMO, inductance roll off is irrelevant in that you can shape the signal to change that. Inductance is relevant in that it causes harmonic distortion where it's audible and changes the sonic sig.

 

Of course, the number one problem in all of home subwooferdom is playback level. There is far too much emphasis put on how "hard" the sub "hits", "punches", "slams", "pounds", etc. Seriously, WTF does any of that mean anyway? It means how loud can you push the hardware before it pushes back.

 

Sort of like asking asking a guy how fast his car can take a corner as they're towing it out of the canyon.

 

If the system is properly sized and powered for the job, harmonic distortion is all but irrelevant and if the signal shaping flexibility is designed to accommodate the desired X-over point, inductance is all but irrelevant.

 

I believe there is one area that hasn't been properly explored regarding a measurement methodology and a metric.

 

Latent release of stored energy through the surround/cone/dust cap.

 

Years ago I asked gurus like Wiggins what effect the cone/surround/dust cap design/material had on sonic signature and the immediate and unanimous reply was "none".

 

Rubber surround vs foam vs accordion. Coated non-pressed paper cone vs aluminum. Large, stiff dust cap vs no dust cap (a la LMS U). I'm not talking about resonance or break up, just transmission of latent stored energy. This phenomenon affects the sound, one way or another, in every alignment.

 

Is it audible? Does it affect sonic signature? Is it measurable? Is it different from sealed to PR to horn to ported to BP?

 

I have always realized that this is a grope in the dark theory, but so has the entire evolution of subwoofer assessment process been, IMO.

 

Siegfried first turned me on to the LRSE (Latent Release of Stored Energy) phenomenon. It is the single reason he moved on from sealed to dipole. It's the only reason I always stuff the box, because any dissipation of stored energy is a good thing. Siegfried even discussed sintered metal filter products to absorb the stored energy, putting the idea out there and leaving it for others to pursue.

 

There is no question in my mind that frequency response (what the mic tells you you're hearing primarily) is the dominant factor in subjective results and it amazes me that anyone would think otherwise. But, having taken part in many, many discussions of intermod distortion, harmonic distortion, group delay, compression, frequency response and roll off order, etc., I tend to dismiss all of those phenomena based on actual listening and the factual data that's available.

 

LRSE would also, of course, include the enclosure design and construction. I've found that to be quite easy to measure, but have no idea how to quantify it. The drivers construction, OTOH, is a moving target and much more difficult to measure... at least I think it is at this point.

 

Just rambling about a subject that has been banging around in my head like a ping pong ball for a decade. That usually means that there's something there. Just thought I'd throw it out there because the recent inductance jargon has left me pretty unfulfilled.

[deepthoughts]

I have always realized that this is a grope in the dark theory, but so has the entire evolution of subwoofer assessment process been, IMO.

 

Siegfried first turned me on to the LRSE (Latent Release of Stored Energy) phenomenon. It is the single reason he moved on from sealed to dipole. It's the only reason I always stuff the box, because any dissipation of stored energy is a good thing. Siegfried even discussed sintered metal filter products to absorb the stored energy, putting the idea out there and leaving it for others to pursue.

 

There is no question in my mind that frequency response (what the mic tells you you're hearing primarily) is the dominant factor in subjective results and it amazes me that anyone would think otherwise. But, having taken part in many, many discussions of intermod distortion, harmonic distortion, group delay, compression, frequency response and roll off order, etc., I tend to dismiss all of those phenomena based on actual listening and the factual data that's available.

 

LRSE would also, of course, include the enclosure design and construction. I've found that to be quite easy to measure, but have no idea how to quantify it. The drivers construction, OTOH, is a moving target and much more difficult to measure... at least I think it is at this point.

 

Just rambling about a subject that has been banging around in my head like a ping pong ball for a decade. That usually means that there's something there. Just thought I'd throw it out there because the recent inductance jargon has left me pretty unfulfilled.

 

It only matters above the frequency where you have a dimension long enough to allow for a reflection off an internal surface and back to the cone. For most subwoofers this occurs well above 100Hz and typically above 200Hz. It is a very real issue for main speakers, and the drive for designs like the Nautilus. It can be a real pain to deal with in the midbass-midrange and can make useful components sound like a huge clock radio if not treated at all in offensive bandwidths. How much effect this has in a subwoofer is not as clear a matter.

[Bossobass Dave]

It only matters above the frequency where you have a dimension long enough to allow for a reflection off an internal surface and back to the cone. For most subwoofers this occurs well above 100Hz and typically above 200Hz. It is a very real issue for main speakers, and the drive for designs like the Nautilus. It can be a real pain to deal with in the midbass-midrange and can make useful components sound like a huge clock radio if not treated at all in offensive bandwidths. How much effect this has in a subwoofer is not as clear a matter.

 

I think we're maybe talking about 2 different phenomena. In any case, stored energy doesn't stay in the box, otherwise the box would eventually explode. The length of the sound wave is irrelevant.

 

 

 

When a speaker driver is mounted in a box it radiates as much energy into the space in front of the cone as it does into the much smaller space behind the cone. What happens to the air borne energy inside? At long wavelengths it is common practice to store it in resonant structures to extend the steady-state low frequency response of the speaker. In general, the energy leads to very high sound pressures inside the box. A small amount of the energy is lost as heat in the stuffing material, some in the process of flexing the cabinet walls. Much of it reappears outside the box, because the thin cone presents a weak sound barrier. Just how much is difficult to measure, but it is a contributor to the frequency response.

[deepthoughts]

I think we're maybe talking about 2 different phenomena. In any case, stored energy doesn't stay in the box, otherwise the box would eventually explode. The length of the sound wave is irrelevant.

 

Since were talking about energy in the form of air pressure modulation (SPL), wavelength or period always matters. In this particular case the entire subwoofer range qualifies as "long wavelengths" in the quote you referenced. Sound can only be transmitted by air displacement, so this truly does come back to how stiff the cone remains in operation. Since the wavelengths are long, the pressure is the same across the entire area of the cone (especially below ~100Hz).

 

Any of the energy loss or bleed you are referring to would have to come from un-desired motion or flexing of the cone. To think of it another way, the energy goes into the air spring of the box which is in parallel with (combines with) the spring behavior of the driver's suspsension. A case of zero internal energy is that of a theoretically ideal infinite baffle. Of course things change significantly once operating at a frequency much greater than 1/4 to 1/2 wavelength of any box dimension.