Sonic port dimensions, too large?

[acoustic360]

I am planning on building a cabinet for a JBL E140, possibly using the plans for a Sunn Sonic 1X15, but have issues regarding the port dimensions, as they seem too large for proper loading, and air cushion for the speaker. JBL recommends a smaller port area, and depth, for this speaker; and I fear that something as large as the Sonic port will not give enough air cushion to prevent excessive cone excursion, and damage. Although this cabinet is reported to be the best design, even “dead on”, by a JBL engineer; I am reluctant to build with this design, contrary to the JBL recommendations, without some sort of reinforcement from those who have experienced the performance first hand.
     Does anyone here have any experience using this, or the 200S cabinet, that can verify it’s effectiveness, and ability to avoid speaker damage? It seems to me that such a large port is operating at nearly free-air conditions, and I am wondering if it would not be safer to just use a ported enclosure, with the recommended specs that JBL provides. I hate to make a mistake that would not only damage the speaker, but make the cabinet itself unusable. I have heard so many positive reports about the Sunn design, that I feel it is the way to go, unless my fears of inadequate acoustic dampening are legitimate. Can anyone provide any insight into this situation, or explain why the Sunn cabinet can get away with allowing so much air to be sent freely out the port?

[JoeArthur]

I am planning on building a cabinet for a JBL E140, possibly using the plans for a Sunn Sonic 1X15, but have issues regarding the port dimensions, as they seem too large for proper loading, and air cushion for the speaker. JBL recommends a smaller port area, and depth, for this speaker; and I fear that something as large as the Sonic port will not give enough air cushion to prevent excessive cone excursion, and damage. Although this cabinet is reported to be the best design, even “dead on”, by a JBL engineer; I am reluctant to build with this design, contrary to the JBL recommendations, without some sort of reinforcement from those who have experienced the performance first hand.
     Does anyone here have any experience using this, or the 200S cabinet, that can verify it’s effectiveness, and ability to avoid speaker damage? It seems to me that such a large port is operating at nearly free-air conditions, and I am wondering if it would not be safer to just use a ported enclosure, with the recommended specs that JBL provides. I hate to make a mistake that would not only damage the speaker, but make the cabinet itself unusable. I have heard so many positive reports about the Sunn design, that I feel it is the way to go, unless my fears of inadequate acoustic dampening are legitimate. Can anyone provide any insight into this situation, or explain why the Sunn cabinet can get away with allowing so much air to be sent freely out the port?

You seem to be an engineer.

Based on your acoustic360 handle, I suggest you direct your question to Harvey Gerst.  He not only designed the original "D" series of JBL musical instrument speakers, but he also worked for Acoustic Control Corporation afterwards and was prohibited from using the very speakers he designed.

The difference you seem to be seeing is easy to explain.  The recommendations are based on "reproduction" of sound.  Musicians are not concerned with a reproduction of sound - but the initial production of it.  

In other words, proper protection of the driver based upon the driver's various parameters (e.g. Vas, Xmax) is not of any real concern.

And Harvey will confirm this.

Hope this helps.

[Isaac]

Another factor is that the optimum port has two characteristics. One is the tuning frequency. This is dependent on both the port area and its depth. The larger the area, the higher the tuning frequency. The longer the port, the lower the tuning frequency. So, a larger port that is also longer may well be the same frequency as the smaller and shorter one.

The other factor is directly dependent on area. This is the maximum velocity of the air through the port. If it is too high, it will become audible as "port noise", or "chuffing". In this department, the larger the better, assuming the tuning frequency is correct.

A third area is the resistance to air flow of the port. Most agree that less is better, but this may not be at all significant. Many disagree, but, again, larger is better.

So the JBL spec may be fine, or it may be the smallest practical solution, and larger may well be better. But, as long as the tuning frequency is correct, larger will never be worse.

[acoustic360]

Assuming the Sunn design is superior, since it seems to be regarded as "the ultimate"; my real question is not which port size is better, but rather, is this large port liable at all to allow excessive cone excursion, and subsequent damage to the speaker itself? Isn't a bit of air cushion benefitial in limiting cone excursion, and increasing power handling? I suppose this problem would have manifested itself in the Sunn amps themselves, had it been problematic; but then again, they were being driven by 60 watts or so, and I will be using a Crown DC300A, with more than enough power to blow the E140.
     A port measuring 24"X 5", and 3 1/2" deep before the start of the angled board, (or half that size, for the 1 X 15) just seems excessive to me, but if the design worked well for Sunn, as it seems it did, who am I to question success? Are my concerns here unfounded, and can I assume that this design will out-perform one that uses the JBL recommendations? Opinions and experience with this design will be appreciated.

[Isaac]

I think your concerns are unfounded. At the port resonance frequency, the mass of air in the port is vibrating in phase with the cone motion, such that the cone motion is nearly zero. At that frequency, it provides more 'cushion' than any sealed cabinet. Above and below that frequency, the oscillations become progressively out of phase, and cone excursion increases. At higher frequencies, this is not a concern because the driver is operating in the mass-controlled region. Below the resonance frequency, excessive excursion is possible, and drivers have been damaged by this. However, the Sunn cabinet is tuned fairly low (my guess is 50-55 Hz), and I haven't heard of this being a significant failure mode. It could be, I suppose, if you're using a larger amp, as you are, and even more so if you're using a bass with a low B or lower. Overall, though, I think you'll be fine, especially if you listen to the sound of the speaker, and turn down if the drivers sound distressed. Also, keep in mind that none of this has anything to do with the size of the port. It could just as easily happen with the smaller, JBL-recommended port as with the larger Sunn port.

Of course, I could be wrong...

[Soundmasterg]

I made myself a 1x15 cabinet using the E140, and made the port size exactly half the height, but the same depth as the vintage Sunn 2x15. It sounds very much like the larger 2x15, with the exception that it is quieter and doesn't throw the sound out quite as well overall. The cabinet has also been frequency tested and specs out pretty close to the original Sunn 2x15 cabinet. To make the overall frequency response more flat up in the higher ranges, I would suggest to install some smaller tweeters of some sort. I didn't in mine, and while it sounds and performs fine with a Sunn 200S and 2000S, it would benefit from some higher frequency emphasis I think.

I wouldn't worry at all about cone excursion unless you are driving the speaker with too much power.

Greg

[Isaac]

Well, he is using a Crown DC300A!

[Soundmasterg]

Yes...that is a possible problem! I would assume he wasn't planning on turnin the thing up to where it would overload the speaker.

Greg