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Polk Audio MM521

5.25" 2-way Mobile Monitor Series Coaxial Speakers (Marine Certified)

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General Features

  • 5-1/4" 2-way MM Series Marine Certified Coaxial Speakers
  • Power Handling:
    • Peak: 400 watts per pair / 200 watts each
    • RMS: 180 watts per pair / 90 watts each
  • Woven glass composite cone woofer
  • Butyl rubber surround for a smoother response
  • 1" Silk/Polymer composite swivel tweeter
  • Klippel optimized components for distortion-free sound
  • 2.7 ohms impedance
  • 25mm Large diameter voice coil increase reliability
  • Carbon composite basket
  • Neodymium magnet structure for more power
  • Top-mount depth: 2-1/2"
  • Authorized Internet Dealer
  • 90 Day Sonic Electronix Warranty

Priced Per Pair

Important Note: Polk Audio specifies the nominal impedance of these speakers to be 2.7 ohms. The product box states the impedance to be 4 ohms and the speaker magnet shows 2 ohms, this has been confirmed by Polk Audio to be an incorrect specification. The product manual is correct in stating the nominal impedance of the MM series of speakers to be 2.7 ohms.


Second Depth
The second depth is used only when the box is slanted or angled. This value is the top measurement and it is measured in inches.
The front-to-rear depth of the item, in inches. If the item is angled or slanted, this value is the bottom measurement.
The height of the item, measured from top to bottom, in inches.
The width of the item from left to right (side-to-side) measured in inches.
Color of handle.

Warranty and Return Policy

Parts Warranty Duration
90 days
Labor Warranty Duration
90 days
Warranty Provider
Sonic Electronix

Summary of Customer Ratings & Reviews

Rating Distribution

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Average Attribution Rating

Browse 8 questions and 25 answers
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Why did you choose this?
Sonic Electronix Store
Luv Polk Audio
terry w on Oct 31, 2016
Polk makes great products, I've used them in the past with great results
Kelly H on Oct 13, 2015
Recommended by someone who installed the same items.
Michael P on Oct 29, 2015
excellent speakers
Darrell S on Aug 24, 2015
are these compatible with 4ohm speakers (rf 282 8in.)or any other 4ohm speakers?
A shopper on Oct 15, 2014
Best Answer: Hello,

Unfortunately , you may be asking the wrong question. The impedance of Speaker (x) will have absolutely no bearing on its compatibility with Speaker (y) . Instead, your concern must be whether the Amplifier that you wish to hook up Speakers (x) & (y) to will accept this kind of setup. Often being the case, it will NOT. If THIS is your concern, contact the manufacture of your amp for details.
Reply · Report · Customer SStaff on Oct 15, 2014
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I'm running a Pioneer de-80 PRS and want to run in network mode. I'm currently running 4 Polk MM521's in my jeep + a sub and I'm wondering if I can ditch the polk passive crossovers and run two separate NVX MVPA4 4ch amps amping the mid woofer and the tweeters on these 2-ways separately? I really don't have room to easily install the component MM5251. This would allow me to control all crossover points for Mid, Hi, and Sub directly through my head unit. Little help would be appreciated.
Longdrive on Aug 21, 2015
Polk Audio MM521
Polk Audio MM521
Pioneer DEH-80PRS
Pioneer DEH-80PRS
Best Answer: Yes it can be done active this way. You'll have to run 3 amps if your going this route. One for highs, another for mids and one for the sub stage. A couple RCA splitters and you're in business. You have many options with this deck, its up to your imagination and problem solving abilities. This HU can actively run 4 networks if you power the front tweeters off the deck. You don't really need the rear tweets. Just move the front ones from the mid and put them up on the dash. It will clean up the sound and you wont have to drive them as hard. Hope this is helpful.
Reply (1) · Report · Kent H on Aug 22, 2015
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is 12ga. speaker wire overkill for these speakers with 400watt amp? looking for best sound I can get
A shopper on Nov 7, 2014
Best Answer: good quality 16 or 14 ga. cable is good. at short distances, should be ok. haven't tried 12 ga. but watch your gain settings and volume if over 100 rms per channel.
Reply · Report · John F on Nov 7, 2014
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WHatis the difference in 2.7 an 4.0 ohm ???I
A shopper on Oct 31, 2014
Best Answer: Hello,

There are a few ways to answer this, so please do a bit of homework if this doesn't answer your interpretation. The difference in impedance simply suggests the impedance of the given speakers, which will then relate to how much power it is able to take from your amp. In theory, the higher impedance is 'better', but the margin of difference is SO SLIM that getting into that is a waste of time. Generally speaking, neither is "better" than the other; Like subwoofers, it comes down to which configuration works best for Your project.
Reply · Report · Customer SStaff on Nov 2, 2014
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what does MM mobile monitor mean,why extra boxes for speakers?what do they do?
A shopper on Oct 8, 2014
Best Answer: The MM stands for Mobile Monitor, Polk's upper line of car speakers. As for the "extra boxes," they are outboard crossovers, their function is to send the midrange to the mid and the high frequencies to the tweeter. Hope this helps, they sound great in the fairing of my Victory Cross Country.
Reply · Report · Brian B on Oct 8, 2014
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what is the best mini amp to use with the polk audio mm521 speakers?
A shopper on Apr 22, 2016
Best Answer: I would imagine there are too many variables involved to answer that question. I mounted my MM 521 in a motorcycle fairing and used one of Rockford Fosgates Amps designed for motorcycles and have been very pleased with it. I think it'll depend on your application.
Reply · Report · Eric G on Apr 22, 2016
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I was looking at your Q&A on youtube and was wondering if you could do one with a baseline on how to set the eq on a stereo? I know all stereos,cars, woofers, etc are different but thought it would be very helpful to a lot of people to have an idea of what to do. I purchased this Sony MEX-GS600BT, a set of polk UM 5.25, a 10" kicker 2 ohm woofer and a soundstream Picasso p4.500 amp recently and have been trying to get it to sound the best I can. I have the 5.25 running at 2 ohms on 2 channels and the sub at 4 ohms on the other 2 channels. Thanks, Bradley
Bradley K on May 5, 2015
Polk Audio MM521
Polk Audio MM521
Sony MEX-GS600BT
Sony MEX-GS600BT
Soundstream P4.500
Soundstream P4.500
Kicker 10CVR82
Kicker 10CVR82
Best Answer: Hello Bradley,

When it comes down to it, equalizers are far too relative to make a proprietary video on. We plan on making an attempt, but even then it may not do you any good because your ears and our are Not alike, and therefore the outcome will very well be moot. Which brings me to the main point, there is no "right way" to set an EQ. If YOU feel there is Low End missing, then you bring up the Low End Frequencies. If you Feel there is TOO much Mid Range, then you bring that down. We can tell you how WE would tune the EQ to OUR preference, but in the end it may very well sound like "trash" to Your ears. So, play with the settings until it fits Your preference. There is no such thing as getting a system to "sound the best" it can, because it is all relative.
Reply · Report · Rick MStaff on May 5, 2015
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Sonic Electronix Wiring Guide

Wiring Diagram Legend

Warning: The image depicted shows the resistance change when wiring multiple subwoofer terminals. Please refer to your subwoofer's owner's manual for the proper wiring of its terminals. Sonic Electronix, Inc. is not responsible for damage caused to your audio system or vehicle due to improper installation. Please call tech support at 1-877-289-7664 if you require additional assistance.

Product Name: Polk Audio MM521