Speaker Reasonant Frequency - Different Impedance

[Littlewyan]

I just noticed on Eminence's website that the reasonant frequency of their speakers changes depending on the impedance. The 16Ohm Speakers have a lower reasonant frequency compared to the 8Ohm Speakers of the same model. However on Celestion's site they show the same frequency for both. Is it normal for them to be different? Or is it just Eminence changing it on purpose for whatever reason?

[pdf64]

Those specs are a misnomer, as real engineering specs require a tolerance to be associated; rather they may be better seen as nominal / typical values.
Eminence publish full TS characteristics, but the validity of the numbers may be questionable, as guitar speakers are generally non-linear by design, so the numbers may differ depending on eg the power level of the excitation signal.
It seems reasonable that the slightly differing voice coil weights of the different impedances will affect various characteristics to some degree.
But my guess is that the differences will be massively swamped by those resulting from differing test conditions, eg environmental conditions, signal level, and normal production variance.
So it may be more beneficial to focus on other aspects.

[xtian]

Having just reviewed the Jensen specs, I noticed the same thing. They show different impedance curves for 8 vs 16 ohm speakers.

[JMFahey]

Well, I wind my own voice coils, so deal with it all the time.

There is some truth to that, but wire weight is only part of the full picture.

For the same speaker, same specs, comparing two same diameter coils (that's a given) and winding length , the 4 ohms one will use somewhat thicker wire than the 8 ohms one, so will be slightly heavier .
How much? ..... think some 20% heavier.
Same rough ratio applies between 8 and 16 ohms.

But, putting it into numbers, the coils will weigh around 2 or 2.5 grams, while a typical 12"guitar cone weighs around 12 grams (American sound) to 15 grams (British sound) to 18-25 grams (PA/Jazz speakers), you also have around 0.5 to 1 gram adhesive plus most of the spider (the far end does not move), also tinsel wires , eyelets and solder, and the dustcap.
Considering all, a 20% variation in a 2.5 gram part does not change the game by much.

See it by yourself: some Thiele Small parameters include Mms , Moving Mass, which is the grand total of what I've been naming above, in a clasic Jensen C12N it's around 25 grams, now you know why

Sadly Eminence has been dumbing down its datasheets and does not post that any more.

Celestion? .... they were never very open publishing speaker Tech data.

So of course a small change in weight will change resonant frequency somewhat, but it's not an *electrical* problem, just a weight/mechanical one.

Almost forgot: on *some* brands, for 16 ohms speakers they would need to go to a very thin, harder to wind wire, so often they cheat a little, us a not-so-thin wire, and wind it a little longer than otherwise needed (you'll see it reflected in a higher X-max spec), so *that* longer coil will be somewhat heavier than expected.

Speaker design is a tight compromise so sometimes tough decisions have to be made.

[pdf64]

Wow Juan, that's very insightful!

Following on from that, I'd be grateful if you could advise on this; for a given voice coil, above bass resonance, is its freq/impedance response (roughly) the same no matter what the magnet / cone / doping of the speaker it's used with?

[JMFahey]

The rise in impedance, usually starting between 400Hz and 1kHz, depends only on VC inductance, and can be killed by sheathing the polepiece with copper foil (very popular in 60's and 70's dual cone PA speakers, specially Goodmans , and RCF in the 80's), gluing a thick aluminum disk to the polepiece top (JBL) and/or gluing an aluminum or copper ring to the inside of the top plate (again JBL) .
[img:750:575]http://www.troelsgravesen.dk/9710_files ... epiece.jpg[/img]

[img:750:559]http://www.troelsgravesen.dk/9710_files ... epiece.jpg[/img]

[img]http1283]http://www.speakerdesign.net/scan_speak ... te_big.jpg[/img]

Lots of people, even speaker repairmen ignore this, because it's "invisible" and rarely mentioned in datasheets and of coyrse you have to fully disassemble the speaker to see it.

Part of the secret of some speakers being so bright and punchy.

There's some dual cone RCF speakers 12L something or 15L something which hold their own side by side with speaker + horn combinations, go figure.