AK157 and K15/40 specs

[guy sergeant]

I recently acquired a pair of Vitavox Bass drivers. One is the AK157 (7.5 ohm) unit and the other is badged as a K15/40. I have now had the opportunity to get the TS parameters of these drivers but have a couple of queries.

the results

AK157

Sd 0.0731
Nom Impedance 8 Ohms
DCR 6.3 Ohms
Resonance Frequency 56 Hz
Qts 0.33
Qms 6.17
Qes 0.35
Mms 49g
BL 19.8
Vas 120 Litres
Sensitivity 99 dB/W


K15/40

Sd 0.0731
Nominal Impedance 8 Ohms
DCR 6.5 Ohms
Resonance Frequency 46 Hz
Qts 0.97
Qms 6.1
Qes 1.16
Mms 57g
BL 10
Vas 157 Litres
Sensitivity 93 dB/Watt

I am a little surprised because I was expecting the K15 to be a 15 Ohm driver and this one isn't. I suppose it might have had a different diaphragm fitted. I was also surprised by the difference in sensitivity. My colleague, who measured these for me, commented that the AK157 appeared to be more suited to Bass/Mid applications whereas the K15 was better suited to low Bass material and wouldn't be so happy playing above 300 Hz. He also commented on the surrounds saying that modern drivers are better terminated than these and that such surrounds as they dry out are prone to the sort of peakiness in the impedance plots as the surrounds 'take off'. Whether the K15 is supposed to be like this I'm not sure. I can't tell whether the magnet has lost some of it's power. I also can't tell whether the diaphragm is the correct one.

Has anyone else tried measuring either of these? Any comments?

[Romy the Cat]

Guyuser, here some comments that you might try worth to consider.

1) TS parameters are very intricate thing. They are hugely important to portray how a bass driver will behave in ported, sealed enclosures but when we are talking about bass horn they become very secondary. At least the should be interpreted and considered differently. You see in bass horn there are so many different influences from horn and room that what we might attribute to Qes or even to Fs is not what it is. Beside in bass horn there is no direct relation between known to us TS and the actual sound. I know it sound like heresy but it is what it is. You may look at the TS when you place the driver into the horn and make very wide-ranging prediction but it has a little meaning. At least it is what I learned.

2) Sensitivity of 15/40 compare to anything else was ~ 99dB. I did not measure it with 1W because it alnico magnet and I did not want to screw it. I proper horn should add for you 6dB.

3) It is difficult to say what kind cone you have. The UK?s Vtx guy sells the new cones and the replacement is very simple. He says that his cones are new old stock but it is not true: they are the contemporary production. Usually the contemporary cones form any new old stock manufacture are horrible and particularly those with hard external paper suspension, however, I never used his new cones. Nevertheless, the same UK?s Vtx guy sales the contemporary production S2?s diaphragms and present them as old new stock diaphragms. The funny part that his new contemporary production S2 diaphragms are MARGINALLY BETTER then any original older diaphragms. So, it is very possible that his new cones will be very good as well.

4) Does it have to be the original Vts cone? What is wrong with the current cone in context of the given horn? Isn?t it more important question then to find who made the driver?

Rgs,
Romy the Cat

[Peter Empson]

Hi Guy,
There are some specs I stole off the web (from some forum I think) for the K15/40 here, which make for an interesting comparison.

I also measured my 157's before building my bass horns and whilst I've never been sure how accurate they are (they were measured rather crudely) they are not too far off what you've got. Here's my figures (I'm sure I've got some more info here but my old paperwork is not that well organised!).

DCR: 6.8 Ohm
Fs: 58Hz
Qms: 5.3
Qes: 0.4
Qts: 0.37

Btw my 157's were a factory refurbished pair which I think were remagnetised, painted and reconed just before I bought them.

Peter.

[guy sergeant]

These drivers are being used in The Oracle front loaded bass horn. This is a relatively small exponential bass horn with a phase plug(not dissimilar in layout to the Thunderbolt) that Vitavox made (probably) for use with the AK157. I was getting considerably more output from the AK157 than from the K15. Now I can see why. The measurements for the AK157 seem correct. They were measured on an LMS system and by adding a 50g weight to get the difference needed. The K15 was measured in exactly the same way. I believe the K15 has either a faulty magnet or the wrong cone/diaphragm. It seems just too different and insensitive to be correct.

With regard to defining and predicting a horn's performance based on the TS parameters of a given driver I'm sure that somewhere I have an AES paper by DB Keele that describes the procedure for this. Romy, do you feel that it can't be done that way?

[Romy the Cat]

I am not familiar with those papers but I have general idea what they might say. Certainly it is possible to predict the general mistakes of the bass horn upon the TS parameters. However, it is possible to predict the broad-spectrum mistakes not the successes. In the bass horns there are so many OTHER VARIABLES involved that sometimes even the obvious T/S problem are not effective, particularly when we are taking about the curved of boundary loaded horns. Even with a straight, fully-horned (all the way down), front loaded, back chamber sealed bass horn that sit in a middle of a room the ?T/S perfection? is not really a domination criteria. I feel that the T/S applications are more suitable to mass driver systems that response to acceleration. The Horns are velocity controlled and the bass horns are room controlled, cavities-controlled and so on?.

This actually funny but when I think about the best bass drivers that I even heard in context of bass-horns than I do not even knew or care about thier T/S parameters. Perhaps I am wrong and perhaps I got this feeling because I have no ways to control the T/S?. Look, in the real world, we snatch some absurd drivers here and there from the damn junk yards, drag ten home, stick then is whatever we have that fits our tiny rooms? Do we really care about the T/S application? There are thousands of contemporary drivers that have absolutely marvelous T/S characteristic and the sound like shit?. Form another point of view give me any T/S or none-T/S measurable parameter that would indicate how good a driver can discriminate the dynamic contrasts or tonal saturations? I hope you got the idea where I was coming form :-)

Rgs,
The Cat