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 MM521 5-1/4" speakers feature honeycomb-textured woven glass fiber composite woofer 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.

1" silk dome tweeters add a sweet high end that blends perfectly with the low notes so your music stays in harmonic balance, from the electric bass all the way on up to the cymbals. Swivel mounts on the tweeters allow you to aim the high notes to produce a superior soundstage in your car.

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.

Priced per pair

Product Highlights:

  • 5-1/4" 2-way speakers (pair)
  • woven honeycomb glass fiber composite woofer cone with butyl rubber surround
  • 1" silk composite dome tweeter with swivel mount
  • 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 replace your car's original speakers?

The speakers that originally came in your car were made from inexpensive materials, designed to create acceptable sound at the lowest possible cost to the manufacturer. If you love music, you deserve better. Name-brand aftermarket speakers deliver fuller, richer sound with considerably more detail and nuance, and they're capable of playing louder and cleaner than factory speakers. Connect new speakers to your factory radio and you'll get the best possible sound from your dealer-equipped system. Pair them up with a new car stereo for a level of enjoyment that can rival a high-fidelity home audio experience.

Hands-on research

What's in the box?

Download owner's manual

  • Two 2-way 5.25" speakers
  • 2 Grilles
  • 2 Passive crossovers
  • 8 Hex-head screws
  • 4 Phillips-head screws
  • 8 Speed clips
  • Owner's Manual
  • Product Registration information sheet

Features and Specs

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 at 1 watt
Frequency Response 50 - 25k Hz
RMS Power Range (Watts) 15-90
Peak Power Handling (Watts) 200
Impedance (Ohms) 3
Top-mount Depth (Inches) 2 1/2
Bottom-mount Depth (Inches) N/A
Cutout Diameter or Length (inches) 4 11/16

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.

Silk/Polymer Composite Swivel Tweeters: The 1" liquid-cooled, silk/polymer composite tweeters provide clear, detailed highs. The 15-degree swivel tweeter housings deliver direct, detailed imaging; instantly adjustable through the grille.

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.

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

Crossover: The speaker comes supplied with two separate 2-way crossovers; one for each speaker. Each crossover input 36" lead is terminated with tinned wire, and the two 20" output leads (one for the woofer and one for the tweeter) are terminated with female quick-slides. The positive woofer lead has a 0.250" female quick-slide and the negative woofer lead has a 0.187" female quick-slide. The positive tweeter lead has a 0.187" female quick-slide and the negative tweeter lead has a 0.110" female quick-slide. These quick-slides connect directly to the woofer and tweeter terminals on each speaker. The separate 2-way crossover network sends the appropriate frequencies to each driver for cleaner sound and better imaging. Dimensions: width = 3.515", height = 1.015", depth = 2.645"

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

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

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.

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 open the box, verify contents, 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 MM521 Reviews

Average Customer Review:

9 reviews
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MM 521 on Harley

Written By Anonymous, Indiana on Monday, April 02, 2012

Crutchfield Customer Review (What's this?)

Couldn't be happier. Product installed easily, sound awesome and look even better.

Pros: Great full range sound. WATERPROOF! Sweet grills.

Cons: Nope.

Suprisingly good for being poorly made

Written By Curtis, College Station Texas on Monday, December 27, 2010

Crutchfield Customer Review (What's this?)

I have some Polk MOMO speakers that are without doubt the best piece of audio gear i have ever owned, so when I needed to replace my rear speakers i figured the line that replaced them would be just as good as the MOMO's. These speakers have flimsy plastic baskets and miniscule magnets, and despite their 90W power rating, i've been feeding them ~80 watts (the 2.7 ohm resistance makes the actual number weird) on a 150 hz high-pass filter and both of them have blown. Crutchfield tech support has been amazing as always, but considering my MOMO's handled 65 watts over their rating (thats 180 watts RMS) with full-range, these are a poor replacement. Speakers don't need to be light, they need to work. That being said they do sound amazingly good for 5 1/4's while they're working with fuller bass than one would expect from such a small speaker and crisp highs.

Polk Audio MM521

Written By Josh, Oklahoma on Tuesday, November 02, 2010

I recently installed these in my '93 Suburban with a Pioneer 6200BT receiver, Alpine F300 amp (50x4), and MM6501's in kick panels. These speakers sound fine, but may just be too small to really keep up with the 6501s up front. I plan on continuing to tweak my settings, but even at moderate listening levels (classical music), these speakers were distorting slightly. I plan on adding a sub somewhat soon to fill in the low end where the 6501s and 521s just can't play. Overall nice, though I would go with a larger speaker if your car allows.

Great Stock Replacements For My Jeep

Written By Glenn, Mount Joy, PA on Monday, June 28, 2010

Crutchfield Customer Review (What's this?)

I was looking for an upgrade in my '03 Wrangler, and these Polks do the job nicely. I plan to add an amp in the near future, but for now, I'm very pleased with the improved sound quality these little components provide. I was a bit concerned when I opened the box, and realized that I needed to find a home for the crossovers while doing the install, but there's plenty of room in the sound bar, so space was never an issue. With the aftermarket harness adapters, the install was a breeze. The Polks are an exact fit. I'm old school, and always assumed that big fat magnets mean big fat sound. Even though these speakers seem light in magnet size and weight, the sound that they crank out is pretty impressive. Be sure that you have enough depth in your mounting location, because they're not shallow by any means. The folks at Crutchfield can help you with the details. If you're looking for smooth sound, and great value in a component speaker with multiple installation configurations, you'll be happy with the Polks.

Nice little speakers from Polk

Written By 1coole60530, Daly City, Ca on Wednesday, April 14, 2010

Crutchfield Customer Review (What's this?)

Nice speakers with crossovers.

Polk Audio MM521

Written By Steve, Ames, IA on Sunday, April 11, 2010

Crutchfield Customer Review (What's this?)

The stock speakers in my 2009 Honda Civic EX Coupe sounded awfully muddy and weren't very efficient, so I upgraded them (front and rear) with the Polk Audio MM521's. When I removed the stock speakers that came with Honda's "premium" sound system, I noticed the woofers were nothing more than coated paper and the speakers were pretty weak.

The sound difference between the stock speakers and these MM521's is just incredible. Bass is surprisingly good for 5 1/4 inch speakers (better than stock), midrange is crystal clear, and the highs are incredible. Also, very little distortion at maximum volume. All of this with the stock head unit. Supposedly these are supposed to sound even better as they "break in."

The only downside was the installation in the rear deck...not so easy but it's doable. You need to be patient and take your time. Also, these speakers have external crossovers which add an extra step.

Excellent speaker purchase

Written By Gary, East Tennessee on Thursday, May 14, 2009

Crutchfield Customer Review (What's this?)

I asked Brian at Crutchfield to help me set up a system for my 03 wrangler and he pointed me toward the Polk Audio MM521 speakers for my sound bar to go with the Polk Audio MM416's in the dash and an Alpine MRP 300 amp, It sounds excellent using a Kenwood KDC-HD942U head unit (also suggested by Brian). I was blown away when I first heard the clarity and power. I have been listening to the system now for a few weeks and it just keeps getting better sounding over time. I also went with an Alpine MRP 500 amp to run my 2 10" Jensen subs. Using Crutchfields vehicle specific instalation guide made this install a breaze. I couldn't be happier .

Outstanding Speakers

Written By Steve, Russellville, AR on Monday, March 30, 2009

Crutchfield Customer Review (What's this?)

I ordered these speakers for the sound bar of my '05 Jeep Rubicon Unlimited. They are paired w/ 4x6 Polk Mobile Monitors in the dash and a 10" JL Audio sub driven by a Kenwood amp @ 200W RMS. I'm running the mobile monitors off the head-unit (JVC KD-PDR50) and they sound incredible. I can push them to loud enough volume levels, with no distortion, to hear everything crystal clear, even with the top off the jeep.

Polk Audio MM521

Written By Slee, Fort Smith, AR. on Sunday, March 29, 2009

Crutchfield Customer Review (What's this?)

I installed 2 pair of these in my Jeep Wrangler, one in the dash with the help of an aftermarket bracket to replace stock 4x6 plates, and a pair in the Mod Pod speaker boxes also sold by Crutchfield in the kickpanel location. Even just after the initial install I was impressed with the clarity of the tweeters. Very neutral , no harshness or tinny sound,easy to listen to. The mids were very neutral also, but I have noticed that even after maybe 6 or 7 hours of playing time they are becoming richer, and I would expect that trend to continue.
I initially purchased these because of the marine grade properties of the material in the environment of my Jeep,which is topless most of the summer months and has been in more than one summer shower or heavy dew. I had hopes that they would sound decent compared to the Poly Planar marine speakers I had in the soundbar and I will say that they have greatly exceeded any expectations I would have ever had. Simply said, they are excellent speakers and well worth the price, particularly when if you buy one, the second set is half off!!

Get everything you’ll need to enjoy your new Polk Audio MM521

Reduce ambient road noise and your vehicle's "rattles," for better sound inside your car.
You may need speaker wire to connect your speakers to your amp or receiver.
Keep your speakers clean and dry. They compress to fit behind your speakers.
These speaker connectors are handy if you're running new speaker wire or if your speakers have oversized terminals.
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

Item #: 15410415


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Dynamat 10435 Xtreme Door Kit

Dynamat 10435 Xtreme Door Kit

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

Item #: 15410435


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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


Add to Cart
XTC 5-1/4

XTC 5-1/4" Speaker Baffles

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

Item #: 237XT52


Add to Cart
Metra Female Quick Slides

Metra Female Quick Slides

An assortment of female quick slide connectors for connecting to speaker terminals

Item #: 120FQS10PK


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 Wire Cutter/Stripper Tool

S&G Wire Cutter/Stripper Tool

Cuts and strips 12- to 22-gauge wire

Item #: 18119100


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S&G Tool Aid 87615

S&G Tool Aid 87615

Window and Door Clip Removal Tool

Item #: 18187615


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


Add to Cart
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

Add to Cart
Install Bay IB35400

Install Bay IB35400

Premium door panel and trim tool

Item #: 12035400

Add to Cart
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