BillFitzmaurice.info

Bill Fitzmaurice wrote:

modernprimitive wrote: so taking the 5.3ohm nominal for the tweeters and combining that with the 5.3ohm nominal for the drivers wired in series, should net a nominal impedance of 10.6ohm.
or did i miss something??

You don't add the impedance of the two lines, the crossover separates their loads.

ok. so what impedance will the amp see for the cabinet?? i thought i had it figured out, but now i'm kinda lost.

Ian Westwood wrote:

modernprimitive wrote:or did i miss something??

We all struggle with this at the beginning so I made a page on my site that fleshes this topic out a bit.

Hope it helps.

i can follow it all the way up to the crossover. that's where i get lost. i thought you added the impedance of the tweeter array and the driver array together to get the total impedance for the entire cabinet.

modernprimitive wrote:i can follow it all the way up to the crossover. that's where i get lost. i thought you added the impedance of the tweeter array and the driver array together to get the total impedance for the entire cabinet.

This is one bit I can't figure out myself mathematically. I'd like an answer as well!

Ian Westwood wrote:

modernprimitive wrote:i can follow it all the way up to the crossover. that's where i get lost. i thought you added the impedance of the tweeter array and the driver array together to get the total impedance for the entire cabinet.

This is one bit I can't figure out myself mathematically. I'd like an answer as well!

Remember that impedance is the sum of a real part (resistance) and an imaginary part (reactance) The elements in the crossover are primarily reactive (there is some resistance in the wire of the inductor, but at audio frequencies the parasitic resistance is quite low). The reactance of caps and inductors is dependent on frequency. The crossovers are designed so that the total impedance of the two legs in parallel with each other sum to 8 Ohms (or whatever the speakers are designed to be)

The reactance of an inductor is 2 * pi * f *L and the reactance of a capacitor is 1/(2 * pi * f * C) Since they are inverses of each other it is possible to select values of C and L such that the parallel combination stays constant.

OK, lets ask it a different way:

Q. How do you calculate the NOMINAL impedance of a speaker cab that uses a 2 way crossover such as the one in the SLA's?

Ian Westwood wrote:OK, lets ask it a different way:

Q. How do you calculate the NOMINAL impedance of a speaker cab that uses a 2 way crossover such as the one in the SLA's?

Let's see if I can do better...

In the woofer's passband, it is the impedance of the woofer, as the crossover makes the tweeter disappear, similarly well within the tweeter's passband, the nominal impedance is the impedance of the tweeter. By "disappear", at low frequencies the inductors are essentially shorts (or zero Ohms) and the capacitors are essentially open (or many many Ohms). A parallel connection of 8 Ohms and 800 Ohms is 7.92 Ohms, or about 8 Ohms.

Now, if the woofer and tweeter have vastly different nominal impedances, you can't really say the system has one nominal impedance.

If the woofer and tweeter have similar impedances, a properly designed crossover makes it so that the nominal impedance stays constant through the transition.

billkatz wrote:

Ian Westwood wrote:OK, lets ask it a different way:

Q. How do you calculate the NOMINAL impedance of a speaker cab that uses a 2 way crossover such as the one in the SLA's?

Let's see if I can do better...

In the woofer's passband, it is the impedance of the woofer, as the crossover makes the tweeter disappear, similarly well within the tweeter's passband, the nominal impedance is the impedance of the tweeter. By "disappear", at low frequencies the inductors are essentially shorts (or zero Ohms) and the capacitors are essentially open (or many many Ohms). A parallel connection of 8 Ohms and 800 Ohms is 7.92 Ohms, or about 8 Ohms.

Now, if the woofer and tweeter have vastly different nominal impedances, you can't really say the system has one nominal impedance.

If the woofer and tweeter have similar impedances, a properly designed crossover makes it so that the nominal impedance stays constant through the transition.

so in the case of the sla with the tweeter array at 5.3ohm and driver array at 9.3 ohm, then the nominal resistance should be somewhere within the range of 5.3 - 9.3 ohm??

modernprimitive wrote:so in the case of the sla with the tweeter array at 5.3ohm and driver array at 9.3 ohm, then the nominal resistance should be somewhere within the range of 5.3 - 9.3 ohm??

Call it 5.3 up to the crossover point and 9.3 after. Nominal is just some arbitary figure though, as the impedance is changing all the time with respect to frequency. If you are looking at amp sizing, just pick the 5.3 (woofer) impedance, as most of your power is in that region.

thank you for your input.

i understand that impedance is not static, but nominal impedance gives me something to go on. and from what i understand, the nominal is closer to the least amount of resistance in the circuit. so if nominal is 8ohm, then in reality it may be somewhere between 6ohm and 16ohm during operation. that way the general consumer is able to select the appropriate speakers for their receiver/amp, without causing damage. since an amp can handle loads well above it's intended impedance, but rarely can handle impedance lower than intended.

the reason i was asking is that a lot of home receivers that i've looked into do not drop down to 4 ohm loads. they are often 6ohm to 16ohm. i understand that 5.3ohm is close to 6 ohm, so there should not be an issue. just that as impedance drops, current rises and driving more current through an amp than it's intended to handle is normally a death sentence. i wanted to make sure that my receiver would be fine with an sla as a center channel.

just trying to understand how all of this works.

Some of the reading I've done suggests that nominal impedance is, first and foremost, the goal or target of the impedance of the driver when it is at the design stage, as distinct from what the impedance turns out to be once manufactured.
Perhaps that's a great way to look at it.

Since the "standards" in driver impedance are 4, 8 and 16 ohms, speaker/driver manufacturers need to build devices that are roughly in line with what the amplifier manufacturers are building in terms of load handling capability, as you have pretty much said.

It's not uncommon for a speaker/driver to have a nominal impedance of 8 ohms, but average impedance might be between 5 and 6 ohms.

The TS parameter Re gives you a measurment of the coil for DC resistance in ohms, but it's only an impedance plot of any given driver that will tell you what the lowest impedance of a driver is and at what frequency (or frequencies) that occurs.
You may well find a whole band of frequencies well below nominal impedance, and it's not uncommon for these bands to be also well below the rated load capability of brand X amplifier. The answer there is to perhaps limit the amount of cabs (to one cab) on any given channel to limit the increase in amperage requirements to mutiple cabs at frequencies the amplifier supposedly can't handle, or buy amplifiers that handle lower load limits.

I guess in relation to what you are actually discussing though, it is quite normal to calculate parallel, series, series-parallel wiring etc, as well as crossover/filter points and their calculations, being based on nominal impedance. If you don't have an impedance plot (because one isn't available, is to hard to find, or simply doesn't exist), there's nothing else to go on.

If you go with that premise, then you only need to wire your multiple drivers so that you arrive at a rough 8 ohm load, and tweeters/HF drivers to also arrive at a rough 8 ohm load, because your component values for the filters/crossovers (ie caps resistors and coils) are based on an 8 ohm load calculation (or their component values would need to be different).
As a consequence, you can assume the entire cab is nominally an 8 ohm cab.

Hope I got that right...

thank you.

yeah, from what i've read, the resistance is dynamic based on frequency and current across a given wire of a given material. i've read a lot about resistance and reactance, but a lot about reactance is just "blah, blah, blah" to me. i've tried to understand the formulas used to figure out the values for capacitors and inductors, so i could better understand how the whole system works, but it's just over my head. so i'll leave that kind of thing to people who understand it, and i'll just hope they're willing to help me when i have questions. but if you have questions about coanda effect, vena contracta, or bernoulli's principle, then i think i've got ya covered.

Hello... hello... hello...

where did everybody go??

Was it something I said?? Do I smell funny??

Nope, not at all. Sometimes folks just don't respond; only read.

Tom Smit wrote:Nope, not at all. Sometimes folks just don't respond; only read.

+1 71 replies: 1583 views

Nothing you said but you do smell.