Acoustic Suspension Issues.

From the Madisound BBS, a few years ago. 

I have a somewhat different take on your question about acoustic suspension speakers than others that have replied so far, FWIW. I believe that most sophisticated designers are finally aware of the situations where acoustic suspension speakers would make the best choice, and where a different approach might be preferrable. Efficiency seems to have dropped off the average customer’s radar screen. Power is now so cheap, and the ultimate difference in max SPL between AS and vented is small enough, so that I don’t think this is a major issue these days. I can’t back that up with hard data, but it is common industry wisdom. Also, it is not as simple as using any woofer that happens to work in a sealed box. If the mechanical suspension is contributing most of the spring force, it isn’t AS. It’s more like an infinite baffle. To get all the characteristics of a proper AS design, the box compliance must dominate. How important this is depends on how good your mechanical suspension is, and how linear your motor is. (I’m not debating the merits, just clarifying the terminology.)

The problem is that manufacturing a proper acoustic suspension woofer is a real pain. Few driver companies are anxious to put them in their standard line. In turn, this means that a designer/brand wishing to make an acoustic suspension speaker has to devote extra effort towards engineering a full-custom driver, and paying the premium that this entails.

To summarize a complex issue, the high compliance of acoustic suspension woofers make them ill-suited to automated production. It’s difficult to handle very soft parts, keep surrounds in shape as you glue them, maintain positions exactly as the glue dries, etc. Also, on a production line, its not easy to rapidly test an Fs below 20 Hz. Data acquisition time and ambient vibrations are both the culprits here.

(After I chose to design the 1259 woofer for the NHT 3.3 as an acoustic suspension, it took a great deal of arm-twisting to get any of our suppliers to even quote on a driver with an Fs below 25 Hz. The lack of AS woofer availability was one reason I decided to make the 1259 available to DIY.)

There’s one more factor that I personally think inclines speaker designers away from AS. Let me try to explain:

With an AS design, there is only one, and only one, box that will lead to the target response. There is no way to tweak the response of the system to "tune" things after the fact. (Besides active EQ, and a very small effect from stuffing.) If the production driver does not match the prototype, or if the designer wants to fuss with the results at the last minute… too bad. This means that AS design is less forgiving of error or uncertainty. On the other hand, many designers like to tweak the bass response with port tuning, long after the drivers have been placed on order. Given a typical woofer production lead time of several months, this can be significant.

Sorry if this was too much information….


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    • Ken Kantor  On November 13, 2016 at 6:06 pm

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