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Polk Audio's Mobile Monitor Series subwoofers bring out superior performance from lighter materials, so you'll get the impact you want without the extra weight of a conventional system. Advanced engineering and optimized components reproduce your sound with uncompromising accuracy, for low end that is rich and clear.

The MM840D 8" dual voice coil subwoofer handles up to 250 watts RMS. A copper voice coil and neodymium magnet motor drive the polymer cone with exceptional efficiency. The carbon composite basket features a "wagon wheel" design that resists flexing during heavy excursion, so your sub hits solidly without distorting. Dual 4-ohm voice coils offer options for wiring a multi-sub system or drawing maximum power from your mono amplifier.

This sub is certified for marine use, and Polk includes corrosion-resistant stainless steel mounting hardware.

Product Highlights:

  • 8" subwoofer with dual 4-ohm voice coils
  • polymer cone with butyl rubber surround
  • carbon composite basket
  • certified for marine use
  • optional matching grille complements the basket design
  • power range: 100-250 watts RMS (125 watts RMS per coil)
  • peak power handling: 500 watts
  • frequency response: 30-200 Hz
  • sensitivity: 85 dB
  • top-mount depth: 3-15/16"
  • sealed box volume: 0.35 cubic feet
  • warranty: 1 year
  • Our 60-day money-back guarantee

What's a component sub and why would I want one?

A component sub is simply a speaker that's designed to reproduce bass once it's installed in a compatible enclosure. Think about the impact of a movie soundtrack in a big theater or even in your own home HDTV setup. A component sub can add the same kind of power to the music in your car. And a car sub is not all about the boom that you hear on a street corner (although that's cool too) - you can pair a component sub with your choice of enclosure to get bass just the way you like it, from tight and punchy to fat and loose. And once you experience music in your car with a well-tuned subwoofer, you'll never go back.

Polk Audio MM840D Reviews

Average Customer Review:

3 Reviews

Great speakers for the size

Written by: Rustin from Houston, TX on May 10, 2012

Crutchfield Customer Review (What's this?)

  • "Coolness Factor"
  • Build Quality
  • Ease of Use
  • Sound Quality
  • Value

Let me start off by stating that these speakers ask for 250 rms per sub. the 180 listed on Crutchfields website isn't their error, Polk misprinted the manuals with an incorrect wattage. If anything it shows their responsibility in going through the manuals of new products. But aside from all that, these speakers rock. I haven't broken them in yet so I haven't really cranked the bass, but they hit spot on the beat and they hit hard. Now they won't give you respiratory problems but for their size they're very strong. I put two of these in my Jeep Wrangler in a custom box and since they require so little space, building an easily concealable box was no problem. I've got them lifted off the floor, aimed at the ground and they fit perfectly in between the seat and the tailgate which also keeps them from being removable without opening the back

Pros: Hit perfectly with the beat even when turned up Alot of bass from a small package Require very little sealed enclosure volume

Cons: none

Great liitle sub

Written by: TamN from Austin, TX on March 5, 2012

Crutchfield Customer Review (What's this?)

  • "Coolness Factor"
  • Build Quality
  • Ease of Use
  • Sound Quality
  • Value

I had a Kicker Comp VR8 dual coils (one driver) in my setup but not very satisfying with its performance. So I was in search for a better driver to replace and found MM840 DVC. This was chosen based on the limited space in my 2001 frontier and its relative shallow depth, combined with the positive review online.

Although not having a chance to listen the new sub perform enough, the first impression was incredible. Whenever it hit the low note, I can "feel" the thump vibrating through my skin. This "feel" I was not experienced with the Kicker, perhaps due to lack of require volume (Kicker requires upward to 1.8cf but it only had 0.70cf). I construct a custom sealed box and chose to go with 0.703cf, an upper limit recommend by Polk (0.68cf) and this is the volume MM840 requires. The sub seems to give a fairly flat response, tight control, and clean sound. My ears was really pleased with MM840 performance.

One note about Crutchfield, I purchase this from the dent/scratch category items and was not confident about the item's appearance. My unfounded worry was completely gone when I received the box. Not only the MM840 was in like-new condition, it is just new. In addition, the cost for the item was a sweet deal. Ya'll not to be scare to take advantage of deal like this. Thank you Crutchfiled.

Pros: Tight and clean sound. Goes lower than the Kicker.

Cons: The foam ring gasket is to thick for my application.

Sweet sounding Sub

Written by: J.T. from Virginia Beach, VA on January 30, 2012

Crutchfield Customer Review (What's this?)

  • "Coolness Factor"
  • Build Quality
  • Ease of Use
  • Sound Quality
  • Value

I have this sub run off of an Alpine M500 mono amp. This thing kicks in my 2011 Wrangler 2 door even in the stock enclosure.

Pros: Hits as hard as a 10" sub.

Cons: Wish the foam ring they sent was made of MDF instead.

Hands-on research

What's in the box?

Download owner's manual

  • One 8" dual voice coil subwoofer
  • 1 Foam gasket
  • Eight 1-1/2" Allen head self-tapping screws
  • 8 Speed clips
  • Owner's Manual (English/ French/ Spanish/ German/ Italian/ Portuguese)
  • Online registration sheet

Product Research

Enclosure Compatibility


Basket: The MM Series subwoofers use high-tech carbon composite basket structures for light weight and maximum rigidity. They will not rust or flex like stamped steel designs.

Materials: The new polymer woofer cone and butyl rubber surround will not break down over time and will survive the rigors of harsh automotive environments.

Dual Voice Coils: The high temperature, 1-1/2" diameter copper voice coils and polyamide former withstand the most rigorous use while maintaining extremely low distortion. The MM840DVC features dual 4-ohm voice coils; when wired in parallel the subwoofer presents a 2-ohm load, when wired in series the subwoofer presents an 8-ohm load.

Terminals: The woofer has heavy duty spring terminals that will accept single banana plugs, pin connectors, and bare wire up to 8 gauge.

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

Klippel Measurement Technology: State-of-the-art laser based Klippel measurement technology is used to optimize the woofer's motor structure, voice coil alignment, and suspension to improve performance, even at extreme listening levels.

Enclosure Compatibility

Sealed Box Volume: Polk recommends using this woofer in a sealed enclosure; the ideal internal volume is 0.35 cubic feet.

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.