# Speaker questions...technical



## greco (Jul 15, 2007)

I am trying to find the answers to some of the mysterious aspects (for me) of speakers :

What is the difference in the *construction* of a 4 ohm vs an 8 ohm vs a 16 ohm impedance speaker?

What is the difference in the *construction* of a 30 watt vs a 100 watt power rated speaker?

Can a speaker contribute significantly to sustain? 
If so, what are the factors in the construction of the speaker that contribute to the sustain?

What role does doping play in the speaker?
What influence does doping have on the tone/output of the speaker?

I am also interested in the ribbing, cone angle and other aspects about the cone..but I will leave these questions for later.

Many thanks

Cheers

Dave


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## Swervin55 (Oct 30, 2009)

All great questions which I also await the answers to. Also, the differences between ceramic and alnico magnets and what differences they impart on sound?


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## kw_guitarguy (Apr 29, 2008)

This may help with #1 and #4

How is a speaker's resistance/impedance created during construction? - CARSOUND.COM Forum

~Andrew


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## Hamstrung (Sep 21, 2007)

May not answer your questions directly but helps in visualizing the process...

[video=youtube;VN0tmyyC0ak]http://www.youtube.com/watch?v=VN0tmyyC0ak[/video]


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## keeperofthegood (Apr 30, 2008)

Dave you like to "experiment"? You can work some of this out with some logical experiments. If you get even just two cylinder magnets, something easy enough to work with say 1/2" diameter by 1" high and some thin wall acrylic tube (or pill jar) that will glide and not bind on the magnets you can set yourself up a/some fun Sunday Afternoon experiment(s).

Pronine Electronics Design - Multilayer Air Core Inductor Calculator

For the "sake of argument" (and use #45 as it is the smallest on the calculator) if you have an air core coil (in metric, its easier) that it 12.9mm in inside diameter (.1mm for the winding form, .1mm air space between form and magnet) and 12.7 in length then you have an inductor that is 0.51mH (this computes to 242 turns of wire)

Ok, doing some fancy work with Inductive Reactance Calculator you will see that this coil, #45 wire air core wound on a 12.9 diameter space will have an inductive resistance of 4 ohms at 1250 Hz. 

XL = 2piFL
= 2 x 3.141592653 x 1250 Hz x .51 mH
= 4 ohms (to 2 significant digits)
= 4.0 ohms 


So, there is your "base line" from which you start. Use a 1volt source and measure the degree of levitation of the coil(s) while on the magnet(s). You can get more and more funky with this as you add to it and no reason why you couldn't  Using 1 volt is good too because it does maths out better. 242 turns of #45 wire is about 107 ohms, about 9mA and as it is 1 volt, it would also be about 9 mW in power. So no big risk of fires or over heating things.

The DC values are going to not change. The AC values will. Putting an AC signal across that coil that is 1250Hz it will act as though it is 4 ohms. 1 volt, 4 ohms is 0.25A (1/4 amps and 1/4 watts). Putting anything inside the coil will change its inductive value. From a little to a lot. Changing the value you will raise or lower the current, so a 1 volt pp AC 1250 wont be 0.25 amps when the coil is on the magnet but the 1 volt will be still 1 volt. You can work out what the resistance has become and from that back work what the inductance has become. 

For working out the cone's you can do that easy enough with some sheets of yellow manilla paper or heavy parcel paper. It is simple cones, you set the diameter and height and cut them out and tape them up. I think you will find that as cone angle increases you gain directional focus and higher frequency reproduction but lose out on power transmitted.

The outer edging rigidity damps the cone and if I recall right increases bass response but diminishes treble.


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## starjag (Jan 30, 2008)

... and all along I thought that the cap itself had something to do with the sound, but it's only there to prevent dust from coming in. I guess that's why they call it dust cap kqoct So does the radius/diameter of the hole that is covered by the cap matter?


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## keeperofthegood (Apr 30, 2008)

I thought it had to do with material. Felt caps are sonically transparent, but the various plastic caps affect high frequency cancellations (they are an upside down small diameter cone) and the caps that are forward facing mini-cones themselves generate the higher frequencies.

Now, that could all be sales brochure malarkey too kkjq


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## keeperofthegood (Apr 30, 2008)

OH and....

I have always been owned by cats. Had my first cat purchase me back in kindergarten. What can I say, I'm just loved by cats.

Speakers with the doping that is sticky also LOVES pet hair and can become a mess of it  Have had a couple like that over the years. The foam edged ones, that foam breaks down with the NOx and SOx in the air and by the time 30 years has rolled by can be quite delicate or even powdery. 

I believe Weber sells a kit to repair/replace the outer edge of the speaker cone though.


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## greco (Jul 15, 2007)

Many thanks for all of the information and links. 
I have a lot of reading/watching and learning ahead of me.

Cheers

Dave


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## mhammer (Nov 30, 2007)

greco said:


> I am trying to find the answers to some of the mysterious aspects (for me) of speakers :
> 
> What is the difference in the *construction* of a 4 ohm vs an 8 ohm vs a 16 ohm impedance speaker?
> 
> ...


the difference between a 4, 8, and 16 ohm speaker is purely in the amount of wire used to produce the voice coil. The voice coil is simply a cylinder with wire wrapped around it.

Your guitar pickup is also a wire coil, but wrapped around a magnet. When something moving near it disturbs the magnetic field that produces an output. In the case of speakers, the same things are involved, but in a different way. There, we have a wire coil, and we also have a magnet, but the coil is allowed to be disturbed itself, in response to current fed _*into*_ it.

The "wattage" of a speaker is an index of how much current the wire in the voice coil can pass before going pffft, like a fuse. Note that since the voice coil is a cylinder inside a slender space, moving back and forth at high speed can create friction-related heat build-up. The current passing through the coil can create heat too, but the remainder of the design assists in dissipating heat to keep that coil as cool as it can be. That's why you will sometimes see speakers with fins around the magnet structure, and also why you will see speakers sometimes described as "ferrofluid cooled". The idea is to dump heat away from the coil.

The gap between the coil and the magnet it surrounds plays a role in the efficiency of the speaker. If the space is wide, you need to pass more current through the coil for it to move around the magnet. A smaller/narrower gap will make the speaker more efficient. Moving back to pickups, if you wind the coil directly around the magnet polepieces, Fender style, you get a pretty efficient pickup. If you wind the coil on a plastic bobbin and then position that around the polepieces, such that there is a space between coil and polepiece, it's a less efficient pickup. So, same thing in reverse. Unfortunately, while a narrow magnet gap makes for a more efficient conversion of current into coil movement, it also makes for a riskier construction.

That a speaker can handle lots more current passing through its voice coil does not necessarily imply that it will be louder. The loudness will be a function of how much air it moves, and how much it moves that air. So if the voice coil travelsback and forth an inch, it is moving the air more forcefully than if it only travels a half inch at most. A 12" cone will move more air than a 6" cone. AFAIK the two properties are additive (i.e., bigger cone with more travel = mucho loudness).

The farther the coil travels back and forth, the greater a challenge it is to keep that voil coil flawlessly aligned so that it doesn't go off-axis, rub up against the magnet, overheat and go pfffft. So speakers vary in how "compliant" they are; i.e., how freely they permit the cone/coil to move back and forth. A stiff paper cone and paper surround, properly aligned places less onus on the spider around the cone, near the coil, to keep things perfectly aligned. If the surround and spider are extremely well-designed, and the magnet gap not too tight, you can extract remarkable efficiency out of the speaker (maximum loudness for minimum wattage).

The surround, spider, and cone struture (including doping) will kind of set the "acceleration speed" of the speaker. One of the reasons why you can find 300W 10" subwoofers for $15 is because the cones are heavy to impair the acceleration of the coil. You generate much less heat rubbing the coil against the magnet 500 times a second than 5000 times a second. Those subwoofers will typically have big magnets, heavy voice coils with thick wire, and large magnet gaps. The coil wire will be able to handle hig current, the big gap will keep the coil away from danger.

It's also fair to say that there is a certain degree of "shock" applied to the speaker cone as it jerks back and forth. the design of the surround, and often the rest of the cone, is partly intended to reduce the artifacts created by that "shock", and absorb some of it.


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## keeperofthegood (Apr 30, 2008)

Ferro Fluid is fun  The "long and short" is "really super tiny iron filings in oil". When the nano-scale particles are suspended in oil it can transfer heat very efficiently and it can permanently lubricate bearing surfaces and it can form air tight seals.

[video=youtube;XUz1ZI-w6LQ]http://www.youtube.com/watch?v=XUz1ZI-w6LQ&amp;feature=related[/video]


It's also cool as hell to watch as art


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## J-75 (Jul 29, 2010)

I don't know if this has been mentioned, but a speaker is a motor: current into a coil in the presence of a magnetic field = motion (in this case, linear).
For every _level_ of current delivered to the coil, there exists a specific _displacement _(position) of the cone - like the pointer in an meter.
Like a (rotary) motor, a speaker doubles as a generator. If the cone is pushed, an EMF (voltage) is induced in the coil.

When the speaker is connected in a circuit, the entire output stage is tuned to a specific efficiency: the cone displacement faithfully tracks the coil current at some resonant frequency (based on electrical & physical factors). This resonance is noted by a peak in volume, and the output stage is efficient, and running 'cool'.
Outside the resonance band, the speaker is not working in its 'comfort level', that is, its moving mass, suspension, cone area, etc. are not contributing to what it is supposed to be doing, i.e. faithfully tracking the coil current in a _proportionate_ manner.

The greater the signal frequency is from resonance, the more 'dysfunctional' the speaker becomes. This physically is represented by lagging and/or overshooting its correct coil/cone position with respect to proportionate response to coil current transients (signal).
Whenever the moving coil is not in positional alignment (tracking) its instantaneous current, it is generating a "_back-EMF_" in opposition to the signal, which leads to thermal build-up and, in extreme cases, mechanical failure of the speaker. This is analogous to a car being driven beyond its intended speed/handling envelope.

Another analogy: In a rotational AC motor, running at its synchronous speed, it runs fairly cool. Put a load on the shaft, and knock it off its synchronous speed (resonance) and it starts building up heat (_from "eddy-current' losses_).

Output sections have a specific _damping factor_ which plays a part in how well the system provides its working bandwidth.
It's a trade-off: the more power it's designed for, the more mass & stiffness in the moving components = the lower & tighter the resonance.

Often referred to: "harmonic richness" is a characteristic which is interpreted in two schools:

'Bell-like clarity, clean' - which represents a system operating well within its performance envelope, tuned to broaden, or flatten its resonance.

'Distortion, break-up, metal' - full of artificial overtones and harmonics, symptomatic of a system well beyond the bounds of its resonance, and, in the _natural_ case, on the verge of a thermal failure of some kind, but a lot of this is synthetic these days - engineered by pedals, effects loops, tube selection, etc. so as to be more operationally stable - sometimes.


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