Loudspeaker Norton//Thevenin electrical equivalents and LTspice models - Page 1
Forum section: electronics / projects

author: reinerterig
author_rank: Contributor
timestamp: May 19, 2018, 01:56:43 am
content: Hey Guys,I am trying to understand how I can model loudspeakers in LTspice with their electrical equivalents.I found these spice models by Duncan Munro.http://www.duncanamps.com/spicels.html.SUBCKT VINT30 A BR_RL A $N_0002 7.5R_RD1 $N_0002 $N_0003 12.5L_LD1 $N_0003 $N_0002 20mHC_CD1 $N_0002 $N_0003 250uR_RD2 $N_0003 $N_0004 0.5C_CD2 $N_0003 $N_0004 50uL_LD2 $N_0004 $N_0003 100uHL_LD3 B $N_0004 100uHR_RD3 B $N_0004 20.ENDSI know I can paste this .SUBCKT statement in LSspice and it would do all the dirty work for me but I specifically want to build to the whole model from Spice components. Below I will Attach a Speaker simulation I found on another forum from someone building an active load box for a guitar amplifier, I guess what they are aiming to do is simulate the parasitics of a loudspeaker and how its impedance changes with frequency -the same as this .SUBCKT model would try and emulate as well-.Can I substitute the values in this .SUBCKT model into my active load circuit?if so where does everything go?What measurements can I do on my own loudspeakers to create an electrical equivalent model of my own?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 19, 2018, 05:58:36 am
content: Heh, here's one I solved in Excel actually. Apparently it was a homework example.Netlist format is pretty easy to work with, once you know what it means. The format is:Code:<Type><name> <net1> <net2> [net3 [... netN]] <value> <Type> is a single letter: R = resistor, L = inductor, C = capacitor, etc. These components only have two pins, so there's only <net1> and <net2>; other components have more pins (net3, ..., netN). So, resistor "_RL" is connected between nets "A" and "$N_0002", and has value 7.5 (ohms). (Units are ignored in SPICE; unit multipliers are understood as usual, except that SPICE is case-insensitive so mega is "MEG", while "M" is milli. You can just as well use floating point, and I'd say both are used about equally. That is, "1.25e-6" vs. "1.25u".)So, in capturing a schematic from a netlist, you want to place all the components, name them accordingly, and set their values. Then, start connecting pins that are in common nets,and name the nets. Watch carefully what you're doing, and don't mind if you're making a terrible mess, just make sure it's consistent! Clean it up later, making sure that you don't swap any components or nets in the process.As for measuring, start with an anechoic chamber (or as free space as you can get -- no reverb!), and simply measure the terminal impedance (e.g., apply a constant current and measure the voltage drop, or connect a resistor in series with the speaker, to make an impedance divider, and measure that). Fit the curve with a rational polynomial -- each factor (zero/pole) in the result corresponds to components in the network.Tim

author: reinerterig
author_rank: Contributor
timestamp: May 19, 2018, 07:24:04 am
content: Hi Tim, Thank you for the help!QuoteThat did the trick!It looks like these "reactive loads" are just a series of filters (R2,L1,C1), (R3,L2,C2) and (R4,L3)that sums over Rload(R1). Does this mean that this is just an emulation of the frequency response of a vintage 30, or is it an accurate electrical equivalent?QuoteIs the effect of reflections on the speaker that extream in effecting its impedance? If so can I just put it in an Iso-cab? (which would basically just be me settling on the type of inaccurate I am ok with). and then you lost me with "rational polynomial".

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 19, 2018, 11:37:03 am
content: I believe there's a standard speaker model/load, maybe IEC or similar. Long ago I wanted to physically build one, but the inductor values needed to model the stored energy of a moderately large woofer far exceeded what I wanted to spend. Entire books have been written on how to extract models from measured data. Some things, crystals for example, have simple and well established formulas that work well. Loudspeakers are a bit tougher. A bit of into near the very end of this-http://sound.whsites.net/z-effects.htmAlso,http://www.epanorama.net/documents/audio/speaker_impedance.htmlandhttp://audiojudgement.com/speaker-equivalent-circuit/It seems reasonably direct to get to a model if you have all the usual T&S parameters available or can measure them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 19, 2018, 01:31:07 pm
content: Quote from: reinerterig on May 19, 2018, 07:24:04 amYes. I mean, it's as accurate as the model is."Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?"A simulation is only as good as the models put into it. Doubt everything, and test as much as you can -- both physical components, to see how they really behave, and the simulation models to verify if they're actually doing the things they're supposed to be doing!Mind, not the frequency response -- that usually means the transfer function from electrical input (usually voltage, but power would be more theoretically correct) to acoustic output (which, again, has to be quantified without reverberations, else you get peaks and valleys in the response that aren't characteristic of the speaker).A very flat speaker ought to have a fairly flat electrical impedance, but the converse need not be true -- simple example, a piezo speaker with a resistor attenuator in front of it, is just as sharply resonant as any piezo, but exhibits a flat impedance thanks to the resistors. It also has terrible efficiency, because of the resistors. You can always make a transducer worse, but you have very little wiggle room on a truly good transducer (wideband, flat, high efficiency -- like a horn, one of the best-performing types).QuoteIt is -- the lowest resonance (parameters F_s and Q_ts) is entirely due to the cone's mass working against the suspension's springiness. A very tight speaker might have a low Q_ts, but in that case, it's a matter of resistance (whether electrical resistance, say in the coil, or damping in the suspension, or useful acoustic radiation), and the electrical and acoustic forces are coupled, to some extent or another, regardless.You might not think quartz crystals do very much -- the piezoelectric expansion rate is in the parts per million. Yet, crystals are a powerful and wonderfully effective component! In this case, the equivalent resonance manifests as an equivalent circuit of -- yep, more RLC components! Even though the motion is very small, the coupling (at least, right at resonance) is quite good.Rational polynomials: remember them from high school algebra? Turns out they have a use!It's the form of math which represents the frequency response of a system. The impedance of an inductor goes as L*freq, and a capacitor, 1/(C*freq), so when you have combinations of L and C (with R thrown in, which is a constant), you get various (whole numbered) powers of freq added together, i.e., a polynomial. It's a ratio of polynomials, one divided by another, so, "rational". If you've not touched on circuit analysis or algebra yet (or it's been much too long), that probably still doesn't help much, but that should at least give you an idea where to start.Tim

author: Audioguru
author_rank: Super Contributor
timestamp: May 19, 2018, 06:18:30 pm
content: Why simulate a loudspeaker? All half-decent speakers have a detailed datasheet showing all its Th-Sm spec's and its frequency response in a recommended enclosure design.A woofer has upper mid frequency cone-breakup that causes a "shriek" sound that is not wanted for hi-fi but might be normal for a guitar speaker.

author: Zero999
author_rank: Super Contributor
timestamp: May 19, 2018, 08:35:09 pm
content: I suppose it's less noisy simulating the amplifier design in SPICE.The speaker enclosure will make a difference to the impedance. Any measurements should me made whilst the speaker is in the proper enclosure, otherwise they won't be accurate.

author: reinerterig
author_rank: Contributor
timestamp: May 19, 2018, 10:02:54 pm
content: Quote from: Conrad Hoffman on May 19, 2018, 11:37:03 amThanks, Conrad. It seems that "Thiele / Small" are the keywords here. I found a similar article last nighthttp://sound.whsites.net/tsp.htmalthough I have only skimmed through all of these articles, I will be sure to give them a thorough read thru once I am a bit more awake.Quote from: T3sl4co1l on May 19, 2018, 01:31:07 pmNo, I don'tIt has been a couple of years since high school, and I've only been dabbling in electronics for a couple of months now. But I'm sure I'll figure it out.Quote from: T3sl4co1l on May 19, 2018, 01:31:07 pmI get it, the air pushing back on the cone adds to the Mechanical damping which would be represented by a resistance. I see there is also a capacitance associated with the moving mass but won't that capacitance increase as the cone extrudes and decrease as it comes back?either way this makes me agree with Hero999Quote from: Hero999 on May 19, 2018, 08:35:09 pmAll Measurements should then, in fact, be made in the proper enclosure.The end goal here is to see how accurately I can model the Acoustic properties of my guitar cab using a combination of the electrical model and impulse responses.So the room and enclosure should absolutely be taken into account for the most accurate result (for my application).

author: b_force
author_rank: Super Contributor
timestamp: May 20, 2018, 10:42:24 am
content: I think you missed a few thinks about loudspeakers.You are also kind of reinventing the wheel here.Basically you can divide the response of a loudspeaker into two parts.1 - piston area, were a loudspeakers behaves like a mass-spring-damper system (which can be translated to equivalent electrical part)2 - The part were the frequency response and things like the cabinet, directivity etc etc are dominant.The first part is important to determine what cabinet size you need.Also it tells you how much excursion you can expect and how much power you need for a certain SPL level.You can calculate and simulate this part pretty accurately with just the thiele-small parameters.Although I would high recommend measuring these yourself, since there is quiet some tolerance.The second part you can simulate to some extend, but at the end you do need to measure it with a microphone.There are some software programs who can help you with this.I should start with BASTA! and WinISD (take the older alpha version).For measuring loudspeakers you can use ARTA (recommended) or REW.At this moment I am actually working on a software program to combine all these things together.

author: Audioguru
author_rank: Super Contributor
timestamp: May 20, 2018, 04:33:39 pm
content: The speaker manufacturer has done all the work for you and shows their cabinet design for a flat frequency response that is shown. If you do not want a flat frequency response but you want a one note boom box or a bongo drum sound then do the calculations yourself.