Single 21" or dual 18"
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abcdmku
diyAudio Member

Join Date: Mar 2012
Location: Chicago
Single 21" or dual 18"

I had a question about my 21" if the larger box is better. My answer soon became 'dual 18s > single 21'. I figured I'd post it here to see your input.

I just copied and pasted my reply:

The larger box does preform well, but be honest for the price of one MCM 21", you could get 2 MCM 18"s or this other 18".

Lets do a little math:
The Sd of the 21" = 0.1706 m^2
The Sd of the both 18's = 0.1232 m^2
Xmax of the 21" = 7mm
Xmax of the 55-2954 18" = 4mm
Xmax of the 55-2964 18" = 6mm

We'll double the Xmax to show the max air the driver can move linearly:
1x 21": (0.1706 (m^2)) * 14 mm = 2.39 liters
2x 55-2954 18"s: 2 * ((0.1232 (m^2)) * 8 mm) = 1.97 liters
2x 55-2964 18"s: 2 * ((0.1232 (m^2)) * 12 mm) = 2.96 liters

If we go by air movement the 55-2964 (Other 18") 'wins'.
Now the best way to find out is to put them next to each other:

Sadly I have the dual 18's taken apart right now. The 18's in the picture are the cheaper MCM 18's. I did give them a quick test out there, the dual 18's were louder, but they are in half space.

Here's the SPL curves of them:

Single 21" (55-2985):

Dual 18" (55-2954):

Dual 18" (55-2964):

Quote:
 Originally Posted by abcdmku I've had the question the question, "Is having two 18's better than one 21?" in my head for almost 2 years. I've been leaning to one side and then to the other, never able to make up my mind. Well I fixed that 18", and now answered the question. Even though the 21" will move 2.39 liters, and the dual 18's will only move 1.97 liters, the dual 18's are still louder.

Last edited by abcdmku; 5th April 2013 at 08:23 PM.

 5th April 2013, 05:06 PM #2 anwaypasible   Banned   Join Date: Aug 2007 well i know it is stupid to say one speaker is better simply because of it's size, since sound quality differences ever occur. but the reason why i decided to post & say something is to point out the 4 inch voice coil on the 18 as the same size coil on the 21 inch more voice coil surface area can be helpful if it is utilized. **edit** it also says the magnet is the same size? well it says 100oz for both, but it doesn't say how strong the magnetic field is .. it says the larger speaker has 500 more watts RMS .. and i figure that amounts to inefficiency of the magnet in relation to the same voice coil size and magnet weight? . Last edited by anwaypasible; 5th April 2013 at 05:09 PM.
 5th April 2013, 05:06 PM #3 abcdmku   diyAudio Member   Join Date: Mar 2012 Location: Chicago Now that I posted this, I'm going to try and fix the dual 18's so I can test this. One of the 18's were blown when my friend had them in his car, he gave them clipped signals . The subs are his anyways, and hes ordering a new cone. I just have them to install the new cone. It will be difficult to get the blown one working, but its possible:
abcdmku
diyAudio Member

Join Date: Mar 2012
Location: Chicago
Quote:
 Originally Posted by anwaypasible well i know it is stupid to say one speaker is better simply because of it's size, since sound quality differences ever occur. but the reason why i decided to post & say something is to point out the 4 inch voice coil on the 18 as the same size coil on the 21 inch more voice coil surface area can be helpful if it is utilized. **edit** it also says the magnet is the same size? well it says 100oz for both, but it doesn't say how strong the magnetic field is .. it says the larger speaker has 500 more watts RMS .. and i figure that amounts to inefficiency of the magnet in relation to the same voice coil size and magnet weight? .

Actually the magnate on the cheaper 18" and the 21" are both 125 oz, MCM has an error. As for power rating that 800W has to be peak, Its about 300-400w RMS. I gave it 600w and it started burning.

According to the specs the calculated BL would be:
55-2954 18": 35 Tm...?
55-2965 18": 20 Tm
55-2985 21": 24 Tm

 5th April 2013, 07:05 PM #5 anwaypasible   Banned   Join Date: Aug 2007 sound quality aside.. i think the difference in cone size does a lot with the localization of the sound. for example, i've got some 5.25 inch midranges and i had some 6.5 speakers, so i decided to hook one up and hear how it sounded. well the larger cone size certainly made it's presence known .. but i don't think i'm being totally fair because the 5.25 is rated for 75 watts rms and the 6.5 speakers are stock door speakers from a vehicle and i assume they are 30 watts rms when i first hooked them up they weren't much louder in decibel or amplitude, but the radiated output matched the increase in cone size. it made me think the 6.5 inches for a midrange that close to the listening position is simply too much, even if the woofer is 12 inches and while midranges might not be as clearly viewed as subwoofers.. looking at the situation of the subwoofer isn't all too difficult. the cone outputs, the room's boundarys interact with the output.. unless you are using them outside, because that is one place where i can't imagine smaller size would be helpful. when outside, it helps to use a bigger cone with the hope of pushing on some air.. then grabbing that air and pulling it backwards, only to shove some more air into the pocket of air grabbed as if to create a slapping of air in front of the cone that is audible .. because doing that can help the amplitude of the small details normally perceived as clarity that would otherwise be lost in the air as it travels far away from the cone. but that is why there is a difference between PA speakers and regular speakers. you might say PA speakers aim their focus to cast out a soundwave as far as it will go, but in reality it doesn't serve a stable purpose because nobody knows the size of the room where the equipment is setup.. and the differences in size can prove to be really big. which brings it back to grabbing of the air in front of the cone to make as much use of it as it can before it creeps away. even if you use the sub in the house facing a wall, there are going to be different sizes in the rings expanding from the wall no matter if the same test tone is playing. it is the size of those rings that travel throughout the room that can help answer why one size is more appropriate than the other.
 5th April 2013, 07:14 PM #6 anwaypasible   Banned   Join Date: Aug 2007 one example for subwoofer size that i know works.. a 6.5 inch subwoofer that is very rich and muddy sounding.. it can bring the feeling of 'too loud' faster than my two 12 inch woofers. but listening to the soundwave rise and fall, i can still tell the cone is only 6.5 inches yes the small sub sounds comparable to a 500 watt halogen bulb on the floor compared to two 50 watt bulbs on each side of the room but with the light and shadows in place, you can literally see the localization differences. **edit** oh, and when the two 12's add up to more lower extension than the small subwoofer.. it is hardly a comparison that i think the two 50 watt bulbs with their better localization wins hands down.
 5th April 2013, 08:04 PM #7 abcdmku   diyAudio Member   Join Date: Mar 2012 Location: Chicago anwaypasible, I agree with what you're about the sound localization, it does matter what you are going for. For home theater you want to know where the sound is coming from, hence 2.1, 5.1, 7.1, ect. For concerts you dont need that clarity of sound, like you said, you just need the sound to be coming form the front. As for the the bass extension, typically the large the cone/Xmax the lower you can go for any speaker.
 5th April 2013, 08:13 PM #8 abcdmku   diyAudio Member   Join Date: Mar 2012 Location: Chicago I've had the question the question, "Is having two 18's better than one 21?" in my head for almost 2 years. I've been leaning to one side and then to the other, never able to make up my mind. Well I fixed that 18", and now answered the question. Even though the 21" will move 2.39 liters, and the dual 18's will only move 1.97 liters, the dual 18's are still louder. Last edited by abcdmku; 5th April 2013 at 08:22 PM.
j.michael droke
diyAudio Member

Join Date: Jan 2009
spl

Quote:
 Originally Posted by abcdmku I've had the question the question, "Is having two 18's better than one 21?" in my head for almost 2 years. I've been leaning to one side and then to the other, never able to make up my mind. Well I fixed that 18", and now answered the question. Even though the 21" will move 2.39 liters, and the dual 18's will only move 1.97 liters, the dual 18's are still louder.
Hi there a: Looking at your posted SPL curves:
21" (2985) at lowest note on a piano 27hz 88db
2x18(2954) " 27hz 85db
2x18(2964) " 27hz 88db

21" (2985) 55hz 99db
2x18(2954) 55hz 101db
2x18(2964) 55hz 102.5db

21" (2985) 50hz to 100hz 102.5+-0.5db
2x18(2954) 50hz to 100hz 97+-1.5db
2x18(2964) 50hz to 100hz 103+-0.5db

The posted curves bear out your listning experience 2x18" (2954) has a substantial bass peak, but lower overall response across the band width (BW), while the 2x18" (2964) and the 21" (2985) have similar response with a slight edge to 2x18 across the BW and a higher peak at 55hz. Now, how did you generate the curves? Did you feed the speakers at the same level throughout testing? Agreed, you should place the speakers identically on the surface for testing. The cabinets do not appear to be of the same type, such that they maybe accounting for the difference in SPL and BW variations. Good work on the enclosures and working on testing to determine best or your application. ...regards, Michael

 6th April 2013, 07:27 AM #10 bentoronto   diyAudio Member   Join Date: Jan 2008 Location: Toronto and Delray Beach, FL 21" and prolly 18" have too large an expanse of unsupported cone to be quality drivers. Likewise for the amount of motor compared to the amount of cone surface. My 2-cents Ben __________________ HiFi aspirations since 1957. Currently working on motional feedback again... the final frontier in audio Last edited by bentoronto; 6th April 2013 at 07:30 AM.

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