Polk Audio continues its tradition of making brilliant-sounding speakers in stylishly designed packages — their Mobile Monitor (MM) Series speakers look great and sound even better. Each speaker utilizes an extra-strong, extra-small neodymium magnet that saves space and weight, allowing these speakers to fit into more places in more vehicles while still delivering a full sound with lots of kick. Their lightweight carbon composite frames provide strong, flex-free support for the speakers' motor assemblies.

The Polk MM5251 component system features 5-1/4" woofers with honeycomb-textured woven glass fiber composite cones that respond very quickly for a clean, crisp, and powerful sound. Large voice coils — over an inch in diameter — give the speaker high power handling capability and extended reliability.

Separate 1" silk dome tweeters add a sweet high end that blends perfectly with the low notes, and swivel mounts let you aim the tweeters to produce a superior soundstage in your car. The external crossovers direct the signal flow so all your music stays in perfect harmonic balance, from the fat thump of an electric bass all the way up to the "ping" of the cymbals. A tweeter attenuation setting allows you to tweak the high-frequency level to your taste.

Important Note: These Polk speakers use a low-impedance design to pull more power safely from your car stereo or amplifier. This design is compatible with virtually all factory and aftermarket stereos.

Product Highlights:

  • component speaker system includes two 5-1/4" woofers, 2 tweeters, and two external crossovers
  • woven honeycomb glass fiber composite woofer cone with butyl rubber surround
  • 1" silk composite dome tweeter with swivel mount
  • 2-way external crossover with 2-position tweeter level
  • certified for marine use
  • 2.7-ohm impedance for extra power (compatible with virtually all factory or aftermarket car stereos)
  • power range: 15-90 watts RMS (200 watts peak power)
  • frequency response: 50-25,000 Hz
  • sensitivity: 92 dB @ 2.83 V
  • top-mount depth: 2-3/8"
  • warranty: 1 year
  • Our 60-day money-back guarantee

Why install component speakers?

A component speaker system gives you the best possible performance from your car audio system. A system consists of separate woofers, tweeters, and crossovers, each designed to cover one specific range of frequencies. As separate components, the woofer can move freely to deliver more powerful performance, while the tweeter can be custom-mounted in a better spot up closer to your ears, revealing a whole new world of detail. Separate crossovers are far superior to the simple filters wired into full-range speakers, so the components work together for smoother, more focused sound.

Hands-on research

What's in the box?

Download owner's manual

  • Two 5.25" woofers
  • 2 Grilles
  • Two 1.0" tweeters (each tweeter has attached 18" speaker leads)
  • 2 Flush-mount cups
  • 2 Metal flush-mount clamps
  • 2 Surface-mount cups
  • 2 Crossovers
  • Two 41" lengths of speaker wire (each length of wire has female quick-slides on one end and tinned wire on the other end)
  • Two 39" lengths of speaker wire (each length of wire has male quick-slides on one end and tinned wire on the other end)
  • 10 Phillips-head screws
  • 8 Hex-head screws
  • 8 Speed clips
  • Owner's Manual (Eng/Span/Fren/Ger/Ita/Port)
  • Online registration sheet

Features and Specs

Speaker Size 5 1/4
Design 2-way
Tweeter Design 1" Dome
Tweeter Composition Silk/polymer composite
Woofer Composition Glass Fiber Composite
Woofer Surround Butyl Rubber
Parts Warranty 1 Year
Labor Warranty 1 Year
Sensitivity 92 dB
Frequency Response 50 - 25k Hz
RMS Power Range (Watts) 15-90
Peak Power Handling (Watts) 200
Impedance (Ohms) 3
Top-mount Depth (Inches) 2 3/8
Bottom-mount Depth (Inches) N/A
Cutout Diameter or Length (inches) 4 5/8

Product Research



Dynamic Balance: Dynamic Balance is a proprietary Polk Audio technology using laser imaging to determine what combination of speaker materials create the least amount of resonance. Eliminating resonance in speakers results in a wide, smooth response and low distortion.

Klippel Optimized Components: Using a state-of-the-art Klippel Distortion Analyzer, Polk engineers were able to optimize the woofer's motor structure, voice coil alignment and suspension for the best possible performance, even at extreme listening levels.

Woofer Cones: Woven glass composite cones feature tremendous stiffness-to-mass ratio. Lower mass means faster transients and finer detail.

Butyl Rubber Surrounds: For a smoother response and improved durability, the woofers use butyl rubber surrounds. The airtight rubber surrounds stand up to moisture, sunlight and extreme temperatures while allowing long excursion linear motion and years of reliable use.

Neodymium Magnets: The speaker's neodymium magnets offer exceptional strength to performance ratio and is extremely lightweight.

Large Diameter Voice Coils: Large diameter, 30mm voice coils increase the speaker's reliability and power handling capacity.

Space Age Materials: Space Age Materials reduce weight of components while offering rock solid reliability, advanced looks and ultra high performance.

Carbon Composite Basket: The speaker's carbon composite basket is lightweight and will not flex during music audio peaks.

Grilles: Strong ABS plastic grilles are included. These computer optimized grilles feature a spoke-wheel design.

Mounting: The speaker itself has 4 mounting holes for a standard 5-1/4" application.

Marine Certified Woofer: Polk MM Series speakers are built to withstand exposure to moisture and UV rays, so they work well in boats and cars. They also include stainless mounting hardware for corrosion and rust resistance.


Silk/Polymer Composite Tweeters: The 1" liquid-cooled, silk/polymer composite soft-dome tweeters provide clear, detailed highs.

Mounting: The supplied mounting cups allow you to surface- or flush-mount the tweeters. The tweeters swivel in the flush-mount housings, allowing you to adjust them for optimum imaging.

Tweeter Mounting Dimensions:

  Surface Mount Flush Mount
Mounting Height 1.065" 0.425"
Depth 0.625" (screw depth) 0.865"
Frame Diameter 1.995" 2.295"
Cutout Diameter   N/A 1.945"

Marine Certified Tweeters: Polk MM Series speakers are built to withstand exposure to moisture and UV rays, so they work well in boats and cars.


2-Way Crossovers: The MM5251 features two 2-way external crossovers with a 2-position tweeter level.

Tweeter Attenuation Switch: You can adjust the output of the tweeter to match a level appropriate to your vehicle or your system. On one side of the crossover, the tweeter attenuation switch can be set to 0dB or -3dB.

Electronic Tweeter Protection Circuits: Each crossover has a built-in protection circuit that guards the tweeter against accidental damage from high power.

Crossover Connections: The crossover inputs and outputs are screw terminals. The crossover housing is vented for adequate heat dissipation.

Mounting: There are two mounting holes in the base of each crossover.

Crossover Dimensions: Width = 4.25", Height = 1.25", Depth = 3.5265"

Our Product Research Team

At Crutchfield, you'll get detailed, accurate information that's hard to find elsewhere. That's because we have our own in-house Product Research team. They verify what’s in the box, check the owner's manual, and record dimensions, features and specs. We stay on top of new products and technologies to help people make informed choices.

Polk Audio MM5251 Reviews

Average Customer Review:

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Great sound quality

Written By BenS, Searcy, AR on Sunday, March 10, 2013

These speakers are truly amazing. I was looking for a sq oriented set of speakers that could also get loud as I listen to rock. And wow did these deliver. The mids will blow you away. My mid woofer are right at my knees in my car, I'm 6'5 so maybe not normal people lol. But when the mids hit my pants blow around from the air these move. And the highs are just as impressive. I only wish that the crossovers had a switch for the mids like it does the tweeters. But beyond that I couldn't ask for a better speaker setup.

Pros: Sq

Cons: Only has a two setting switch for tweeters and none on the mids

great speaker

Written By chuck, Alaska on Saturday, November 03, 2012

I put these in my 2000 GMC Sierra 1500. I was surprised how small they were and especially how light weight they are. The magnet is SMALL. but the sound improvement was HUGE.. Also installed the Pioneer KEH 80 PRS. I mounted everything in its stock location and am extremely happy with the sound of this system. Highs can be heard and those small speakers hit pretty hard. 100% improvement over stock.

Pros: Crutchfield adapters were perfect for the stereo and speakers. Installation was a breeze

Cons: I didnt do this a LONG time ago.

Very smooth and natural

Written By Ted, Michigan on Tuesday, February 01, 2011

Crutchfield Customer Review (What's this?)

I purchased these for my Mazda Protege and they sound amazing. I'm not running as much power as I would like to them (only 70 watts RMS per channel x 4), but they are very natural sounding compared to other components I've owned. The impedence is not standard, I'm thinking due to the x-over and dual drivers, but it was easy to balance with the inputs on my 4 ch amp. Running Alpine CDA117 into Alpine MRP-M450 amp. I have polk 6x9 in the rear DXi690. I also have a sub system. The polk MM5251 are my primary listening speakers and I couldn't be happier. I'm a pro-audio geek and mix live shows regularly, as well as lots of studio recording, reference monitors, etc. So - to say I'm picky is an understatement. I love these speakers. The only recommendation for improvement would be to allow the tweeters to swivel (they are fixed) for better "aiming", but their design is such that the dome is exposed well, and therefore the high frequency coverage is excellent regardless.

Amazing Speakers!

Written By Curtis, Topeka, KS on Monday, May 17, 2010

I have these installed in the dash of my 1998 Jeep Wrangler and the story goes pretty much the same as my review of the MM6501 speakers which I have installed in the overhead sound-bar. These are tough speakers which have stood up to the abuse of an exposed, filthy environment in a Wrangler. The tweeters deliver beautiful highs which aren't too harsh and the woofers hit hard enough to loosen screws all over the dash! They are definitely worth the money if you care at all about quality AND volume. With a PA500.4 amp these things will blow you away!

Polk Audio MM5251

Written By Matt, Baltimore, Maryland on Thursday, August 20, 2009

I had a pair of 10 inch Kicker L7's installed in my BMW 325 but that wasn't quite good enough so I had this Polk component system installed in the front and a pair of the Polk MM651 speakers on the rear deck. I can't believe how much better my system sounds. I have an Alpine-MRP 300 watt amp running the speakers and couldn't be happier. The amount of distortion on certain songs on my stock Harmon Kardon system was ridiculous. I have yet to hear a song with distortion and keep hearing things in songs that I didn't even know were there. I kid you not that these speakers sound better than my Bose noise canceling headphones. I highly recomend them.

Music taste: Rock, Rap, Metal, Reggae and old school Grunge.

Get everything you’ll need to enjoy your new Polk Audio MM5251

Reduce ambient road noise and your vehicle's "rattles," for better sound inside your car.
You'll need speaker wire to connect your speakers to your amp.
Keep your speakers clean and dry. They compress to fit behind your speakers.
The right tools will help keep your installation simple and clean.
If your stereo doesn't cut the deep bass from your full-range speakers, a set of bass blockers can help prevent distortion and give you cleaner bass too.
Dynamat 10415 Xtreme Speaker Kit

Dynamat 10415 Xtreme Speaker Kit

Two 10-in. x 10-in. Sheets

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Dynamat 10435 Xtreme Door Kit

Dynamat 10435 Xtreme Door Kit

With four 12" x 36" sheets — covers two doors

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Dynamat SuperLite Tri-Pak

Dynamat SuperLite Tri-Pak

Triple package — 3 sheets, 12 square feet (Model #10612)

Item #: 15410612


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T-Spec 16-gauge Speaker Wire

T-Spec 16-gauge Speaker Wire

Black/white jacket — priced per foot

Item #: 120SW16WB1


T-Spec 12-gauge Speaker Wire

T-Spec 12-gauge Speaker Wire

Black/white jacket — priced per foot

Item #: 120SW12WB1


Stinger PRO Series Speaker Wire

Stinger PRO Series Speaker Wire

Priced per foot (16 Gauge)

Item #: 541SPW516B


Boom Mat 5-1/4-inch Speaker Baffles

Boom Mat 5-1/4-inch Speaker Baffles

Protect your speakers
Regular and slim-line sizes (Slim-line: 2-1/2" depth)

Item #: 696B050321


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XTC 5-1/4

XTC 5-1/4" Speaker Baffles

Protect your speakers (Slim-line: 2-1/2" depth)

Item #: 237XT52


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Metra Crimp Tool and Connectors

Metra Crimp Tool and Connectors

Package contains crimp tool and over 150 pieces

Item #: 120CRIMPKT


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S&G Tool Aid 87615

S&G Tool Aid 87615

Window and Door Clip Removal Tool

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S&G Tool Aid 87650

S&G Tool Aid 87650

Door Trim and Panel Removal Tool

Item #: 18187650


Bojo Trim Panel Tools

Bojo Trim Panel Tools

Set of 6 pry tools — helps remove interior and exterior trim and door panels

Item #: 126CR6NGL


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Skewdriver Pro SPK4042

Skewdriver Pro SPK4042

Ratcheting straight and offset screwdriver set with 40 bits (Skewdriver Pro with ratchet handle, pictured)

Item #: 180SPK4042


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Spec Tools SQR-200a Squeeze Wrench

Spec Tools SQR-200a Squeeze Wrench

Narrow space ratchet kit with 18 sockets

Item #: 180SQR200A


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Install Bay 3320

Install Bay 3320

Auto-ranging digital multimeter

Item #: 1203320

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Bojo Trim Panel Tools

Bojo Trim Panel Tools

Set of 6 pry tools — helps remove interior and exterior trim and door panels

Item #: 126CR6LNGL


Add to Cart
Bass Blockers

Bass Blockers

Pairs (300 Hz cut-off)

Item #: 127BB300A2