More on DD Cabinet Construction
We wrote this in response to the recent criticism of DD cabinets, but Mark S pretty much has it covered! Nonetheless, we hate to see writing go to waste - so here it is...
A Word on Digital Drive Cabinet Construction
Submitted by
Bruce Hall
Velodyne Acoustics, Inc.
There has been some recent criticism regarding the lack of internal bracing for Digital Drive cabinets, and it has even been suggested that we’ve cut corners on the design of the cabinet. To summarize our position: we have studied the cabinet design for DD, and its predecessor, HGS, extensively. Everything you see inside the cabinet is by design, and there were no corners cut to save money.
In a sealed high-pressure subwoofer cabinet there is high risk of extraneous noises coming from the joints of the enclosure. It is common to inadequately seal and/or reinforce the joints so that rattles, minute air leaks, and other resultant noises occur. In cabinets such as these, internal bracing helps to minimize the strain on the joints so as to minimize these noises. For DD, we liberally caulk and reinforce the cabinet joints, as well as use special tongue and groove routing cuts to ensure that the cabinet joints fit tightly together and never leak or rattle. Since the cabinets are made this way, no internal bracing is needed.
There is a popular misperception that with a high-area piece of MDF, such as the wall of a DD cabinet, the lack of a brace will somehow deteriorate the sound quality of the unit. When we observe the actual flex of the MDF at high pressure, we see that its actual travel area is very small (this is because it’s resonance is very high – well out of the operating range of the woofer). The resultant SPL introduced into the room by the MDF is likewise small, and we have found it to be insignificant. So, we found that internal bracing designed to address this issue had no measurable effect on the sound quality of a DD sub.
There is a penalty for internal bracing, however, and that is reduced internal cabinet volume. A sealed woofer’s efficiency is limited by several factors, and one big one is how much air the woofer needs to compress for each cycle. Said another way, the less internal cabinet volume that exists, the harder the woofer has to work to properly reproduce the input signal because it is expending energy compressing the air inside the cabinet. The Audioholics reviewer noted that, “The DD-18 sports what is just about the smallest cabinet I’ve ever seen an 18” driver bolted into.” This is by design to maximize WAF.
With DD’s high gain digital servo, all the amp power is directed at the faithful reproduction of the input signal, and internal bracing would make the amp and driver work harder to overcome the reduced air volume in the cabinet. So again, we found that internal bracing actually made the woofer less efficient and robbed it of performance.
Others have testified that they cannot hear any sound degradation of DD subs, even close up. That is because the flexing of the MDF walls has not proven to be a factor in woofer sound quality, and we have thus not paid the penalty of reduced internal cabinet volume to address it.
Incidentally, the actual MDF thickness for DD cabinets is 1” all around, not .75” as mentioned in the review. This is our bad – we were offered a fact check on the review and we missed it. This thicker wall means even less of the effects described above. Also, we should note that while the exterior of the DD cabinet is veneered, the inside is also veneered. This is to prevent deterioration of the MDF over time.
This is not to say that the DD-18 cabinet does not vibrate, nor that it has the world’s most pleasing “thud” when you knock on the side of the cabinet. It is trivial to design for those niceties, but we’ve found they do not improve the performance of the woofer. And performance is what DD is all about.
I hope this helps clarify our position on this issue, and again I apologize for any redundancies with Mark's post.
Bruce
