Several folks have inquired concerning the construction of a sealed 1801. I constructed a pair of these and believe they still rest in Dennis Murphy's living room. I had the opportunity to listen to them in the Spring of 2003. I will address the pertinent issues herein. This discussion has the potential of getting very lengthy.
The first issue is the rate of lower frequency spl roll off. ALL sealed cabients roll off their sound pressure at a slower rate. The rate is typically about 12db/octave, but this will vary slightly due to the size of the cabinet. Slightly bigger cabinets will have a slower/shallower roll off. Smaller cabinets will have a slightly steeper roll off. Generally, the roll off remains about 12db/octave. This contrasts with the 24db/octave roll off of a ported enclosure. The ported enclosure rolls off much quicker.
The second issue is the dampening effect of the cabinet. This issue is somewhat complicated and will likely get lengthy. The language here may get somewhat confusing, and I will attempt to be clear. There are two effects impacting the driver cone - spring and dampening. These effect the cone exactly like an automobile suspension. The spring on a car is mechanical and there are springs on a loudspeaker driver too. These are both mechanical and electrical. The dampening on a car is provided by shock absorbers, and the loudspeaker driver's suspension.
The spring on the loudspeaker driver is rooted in its suspension AND the cabinet. The cabinet spring effet will vary depending on the size and type of cabinet. Bigger sealed cabinets have less spring. Smaller sealed cabinets have more spring. The size of the cabinet changes the amount of spring applied to the driver.
The dampening of the loudspeaker driver remains wholly rooted in its suspension. Hence, changing the cabinet size does not change the "shock absorbers" in the driver. The shock abosorbers are eternally constant.
Herein the language gets messsy. The terms "underdamped" and "overdamped" enter discussion. I'll get on my soapbox as I progress further. I am not really sure why these terms are used. I think the terms should be "oversprung", and "undersprung" since this is the variable that changes in the loudspeaker system. The spring effect changes. Oversprung corresponds to underdamped. Undersprung corresponds to overdamped. Unfortunatly, I didn't create the loudspeaker industry culture and I don't have the luxury of changing the lexicon. Nonetheless this lexicon exists and requires further dissertation.
Up to this point I have primariy addressed these issues individually. I will now address the cabinet & driver SYSTEM. The driver IN the cabinet comprises the loudspeaker suspension SYSTEM. I will do so in bullet format. Hopefully the logical progression will be clear.
1. The driver's dampening never changes.
2. The cabinet size changes the spring effect impacting the driver.
3. The RATIO of CABINET spring versus DRIVER dampening determines the SYSTEM "dampening"
4. Smaller cabinet SYSTEMS are "underdamped". This is because there is more spring and the same driver dampening.
5. Bigger cabinet SYSTEMS are "overdamped". This is because there is less spring and the same driver dampening.
Hopefully this makes sense. Now I'll toss another wrench in the discussion - the "Q" factor. The Q factor for sealed enlosures numerically defines the system dampening. A standard sealed cabinet has a Q of .7 (.707 to be exact). Low Q sealed cabinets (i.e. .5 - .6) are overdamped. High Q sealed cabinets (.8 - .9) are considered underdamped.
All ported enclosures impress a Q of 1 or greater on the system. On the same Q scale the ported enclosures are severely underdamped.
Given these remarks about dampening it should be clear that sealed cabinets are clearly superior with regard to dampening. Indeed the sealed cabinet is superior.
Now to the third issue. The onset of lower frequency roll off in the sealed 1801 is 50hz. This contrasts with 40hz for the ported 1801. The lack of bass between 50hz and 40hz is VERY audible IMO. I would never build an 1801 monitor in a sealed version for better sound quality and/or bass performance. Despite all the issues discussed above, the ported cabinet has more bass, and better sounding bass IMO. The ported 1801 has some degree of authority in the bass region. The sealed 1801 doesn't. I believe the simple difference is the lack of bass in the sealed cabinet from 40 to 50hz.
The fourth issue is subwoofer integration. This issue is relatively insiginificant due to room modes swamping the integration of the sub/monitor crossover. Most subwoofer plate amps, such as the Apex Sr., have a 12db roll off. Some have a 24 db roll off. Since the sealed 1801 has a 12db roll off, it should be paired with a subwoofer plate amp also having a 12db roll off. The ported 1801 has a 24db roll off and should be paired with a subwoofer plate amp having a 24db roll off. Again, I believe that being overly concerned with this issue is not appropriate due to room modes. They are NASTY down low because the wavelength is VERY long. Wall coupling, pile-ups and cancellations will overwhelm a theoretically correct subwoofer crossover.
The Fifth issue is the crossover. Stated simply, no changes are necessary. The affected bass region is below 50hz and subsequently below the contouring effect of any 2-way filter circuit.
The sixth issue is the cabinet recipe. They follow the same general construciton method as the standard 1801s, but with different dimensions. There are two vertical braces that should be drilled like swiss cheese. The sealed cabinet is slightly smaller. Internally it measures 7 1/2" wide, 10" deep, and 15 1/2" tall. Externally it measures 9" wide 12 1/4" deep and 17" tall. I recommend stuffing it with nice fluffy fiberglass.
I am sure other issues will arise. I will address them as needed. Please ask questions!
