418-8lf

[airboss]

I just picked up a pair of musical instrument speakers as the thread title suggests. I have looked at various Theile/Small parameters for this speaker posted on the web and come up with something like .05 inches for Xmax. That wouldn't qualify as an LF speaker under any circumstances.

I've found problems with Altec Lansing data before. I see from the sales brochure that the maximum cone excursion for this speaker is 1/2". Did someone misplace a decimal? So what's the difference between Xmax and maximum cone excursion?

I'd like to use these in a cabinet for bass guitar.

thanks as always
Dan

[airboss]

Perhaps the 1/2" is Xmech?

[LOUDER]

There's no problem with those specs. They are correct. Voice coil and gap hiegtht are the same. Quarter inch out and in equels one half a inch. You'll like them for bass. The lf's used a slightly heavier cone which lowered the resonance by five hertz.




Louder

[joyspring]

I just picked up a pair of musical instrument speakers as the thread title suggests. I have looked at various Theile/Small parameters for this speaker posted on the web and come up with something like .05 inches for Xmax. That wouldn't qualify as an LF speaker under any circumstances.

I've found problems with Altec Lansing data before. I see from the sales brochure that the maximum cone excursion for this speaker is 1/2". Did someone misplace a decimal? So what's the difference between Xmax and maximum cone excursion?

I'd like to use these in a cabinet for bass guitar.

thanks as always
Dan

XMAX and XMECH are entirely different.

XMAX (or voice coil over/underhang) is the total linear one-way excursion of a transducer; that is, the total amount of one-way travel where the voice coil area in the magnetic gap (and therefore Bl factor - motor strength) remains the same.

xmax = abs [ (top plate thickness - voice coil length) / 2 ]

When xmax is exceeded, Bl factor drops and harmonic distortion is the result.

Vance Dickinson contends that xmax + 15% results in acceptable distortion and some manufacturers state an xmax that reflects a 30% drop in Bl rather than a true mechanical xmax. I prefer to design around mechanical xmax.

FWIW, equal top plate thickness and voice coil length always results in gross distortion at low frequency.

XMECH is the total excursion sustainable by the transducer before damage occurs... this can be the spider/voice coil junction impacting the top plate, voice coil former contacting the back plate or exiting the gap entirely, surround or spider compliance change or damage, etc.

This is the figure that Altec describes as `maximum excursion'. Large amounts of distortion result far before attaining this figure.

Instrument loudspeakers usually have small xmax and distortion is an integral part of an instrument driver's timbre; your Altec 418-8s may work for you though they may be bass-shy judging from the xmax.

BobR

[airboss]

There's no problem with those specs. They are correct. Voice coil and gap hiegtht are the same. Quarter inch out and in equels one half a inch. You'll like them for bass. The lf's used a slightly heavier cone which lowered the resonance by five hertz.




. Louder

Thanks Louder. There are several sites around the net that simply mirror bad information. I think that's the case there. .05 inches equal 1.27mm whereas .25" is more like 6.7mm. Much more like a bass speaker.

[airboss]

XMAX and XMECH are entirely different.

XMAX (or voice coil over/underhang) is the total linear one-way excursion of a transducer; that is, the total amount of one-way travel where the voice coil area in the magnetic gap (and therefore Bl factor - motor strength) remains the same.

xmax = abs [ (top plate thickness - voice coil length) / 2 ]

When xmax is exceeded, Bl factor drops and harmonic distortion is the result.

Vance Dickinson contends that xmax + 15% results in acceptable distortion and some manufacturers state an xmax that reflects a 30% drop in Bl rather than a true mechanical xmax. I prefer to design around mechanical xmax.

FWIW, equal top plate thickness and voice coil length always results in gross distortion at low frequency.

XMECH is the total excursion sustainable by the transducer before damage occurs... this can be the spider/voice coil junction impacting the top plate, voice coil former contacting the back plate or exiting the gap entirely, surround or spider compliance change or damage, etc.

This is the figure that Altec describes as `maximum excursion'. Large amounts of distortion result far before attaining this figure.

Instrument loudspeakers usually have small xmax and distortion is an integral part of an instrument driver's timbre; your Altec 418-8s may work for you though they may be bass-shy judging from the xmax.

BobR

I follow you Bob. The problem is that Altec apparently didn't disclose xmax in it's 418LF brochures. I see .05" used in some materials for that speaker published by others. As I said in the above post, that's 1.07mm. A similar driver from JBL, the K140 is published as having 5.08mm.

So take a look here: ***http://www.hostboard.com/forums/show...php?t=62653*** Post number 20. I don't trust numbers from the archives. I need to set up a jig and measure these for the whole Thiele/Small series. Unless, (hopefully) somebody has already done that.

Dan

[joyspring]

I follow you Bob. The problem is that Altec apparently didn't disclose xmax in it's 418LF brochures. I see .05" used in some materials for that speaker published by others. As I said in the above post, that's 1.07mm. A similar driver from JBL, the K140 is published as having 5.08mm.

The 0.05" xmax is probably correct, especially if Altec chose to maximise sensitivity by closely matching voice coil length and magnetic gap depth. This is fine for an electric guitar loudspeaker since the lowest fundamental achievable by a guitar is above 80 hz, the linear excursion requirement is quite modest, and driver breakup is a feature.

Despite the `LF' designation - judging from the range featured in AL-1839 `Altec Musical Instrument Loudspeakers' - I believe that the 418 (`full-range') was intended for electric guitar and the 421 (`bass') for electric bass similar to the recommended application for the D/K/E130 and D/K/E140 JBLs.

So take a look here: ***http://www.hostboard.com/forums/show...php?t=62653*** Post number 20. I don't trust numbers from the archives. I need to set up a jig and measure these for the whole Thiele/Small series. Unless, (hopefully) somebody has already done that.
Dan

Judging from my product literature published by Oklahoma compared to the Anaheim operation, I'd agree.

While driver Q and compliance are easily calculated using impedance measurements and mass loading, xmax cannot without dissecting the driver and physically measuring the gap and voice coil heights.

Thankfully xmax is not necessary for calculating enclosure tuning -- it's only required for large signal performance.

You then have two options to test for maximum SPL (actually three):

1. Trust the 0.05" xmax you found online. This is probably accurate for an MI loudspeaker.

2. Take harmonic distortion measurements at various frequencies and SPL.

3. Build an enclosure and try it out.

BobR

[airboss]

The 0.05" xmax is probably correct, especially if Altec chose to maximise sensitivity by closely matching voice coil length and magnetic gap depth. This is fine for an electric guitar loudspeaker since the lowest fundamental achievable by a guitar is above 80 hz, the linear excursion requirement is quite modest, and driver breakup is a feature.

Despite the `LF' designation - judging from the range featured in AL-1839 `Altec Musical Instrument Loudspeakers' - I believe that the 418 (`full-range') was intended for electric guitar and the 421 (`bass') for electric bass similar to the recommended application for the D/K/E130 and D/K/E140 JBLs.



Judging from my product literature published by Oklahoma compared to the Anaheim operation, I'd agree.

While driver Q and compliance are easily calculated using impedance measurements and mass loading, xmax cannot without dissecting the driver and physically measuring the gap and voice coil heights.

Thankfully xmax is not necessary for calculating enclosure tuning -- it's only required for large signal performance.

You then have two options to test for maximum SPL (actually three):

1. Trust the 0.05" xmax you found online. This is probably accurate for an MI loudspeaker.

2. Take harmonic distortion measurements at various frequencies and SPL.

3. Build an enclosure and try it out.

BobR

Thanks BobR. I sure wish I understood horn loading. I don't at all. Direct radiator ducted ports are simple by comparison. Jeff Markwart (sp?) wrote this back in 2003: "The parallel non-flared sides of the 816 horn are 16.125 inches apart, which will produce a fundamental resonance at 419Hz. Jeff" He was referring of course to the phenomenal 816A PA cabinet. Bill Fitzmaurice (henceforth BFM) has a design that ports the horn enclosure into the back of the horn flair with the parallel sides being more like 40" apart. One would then tune the port using the well known impedance peaks procedure. This would be time consuming but may be able to maximize what looks like pretty wimpy bass spl performance with this driver. Using this chart ***http://www.phy.mtu.edu/~suits/notefreqs.html*** I would like to get to 41.2hz with equalization If I use the cabinet for tenor bass (a likely scenario) all I need to reach is 55hz. BFM insists on using a high xmax driver for his alignments. He's right of course. If I understood horn design it would be an interesting way to maximize the performance of these drivers. Thanks for your help Bob.
Dan

[Old Guy]

You can't compare the xMax specs Altec released to those of other manufacturers directly. the reason? Altec measured it's xMax figures at a very low distortion level. therefore they are artificially low compared to other manufacturers specs. To a manufacturer who disregards distortion, xMax is a much bigger figure.

[joyspring]

You can't compare the xMax specs Altec released to those of other manufacturers directly. the reason? Altec measured it's xMax figures at a very low distortion level. therefore they are artificially low compared to other manufacturers specs. To a manufacturer who disregards distortion, xMax is a much bigger figure.

Altec always had the advantage of underhung topology distortion-wise (and disadvantage LF SPL-wise) - at least in the 515 and 604 - so what was its testing methodology for determining xmax? Specifically, what is the distortion limit? How much harmonic content at what frequency?

While Thiele never specified, Small suggested voice coil overhang (`Closed-Box Loudspeaker Systems; Part I: Analysis', JAES, Dec. 1972). JBL's Gander stated that overhang didn't consider fringe flux for overhung topology (`Moving-Coil Loudspeaker Topology as an Indicator of Linear Excursion Capability', JAES, Jan 1981).

I note that most manufacturers have and still use Small's recommendation of mechanical xmax (or at least provide magnetic gap depth and voice coil length) unless otherwise specified (i.e. - Precision Devices UK and B&C, which both quote xmax at % Bl).

BobR