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Polk Audio MM840D

Mobile Monitor Series 8" sub with dual 4-ohm voice coils

10 Reviews | 2 questions - 2 answers

Item # 107MM840D

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Our take on the Polk Audio MM840D

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.

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.

Priced per speaker

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 in the Box:

  • 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

Awesome Sub! DVC and solid performer even at 8 inches! grills are a bit tough to find but have found a great metal aftermarket that works great.

Scott, Gastonia, NC


Polk Audio MM840D Reviews

Average Customer Review(s):

(10 Reviews)

GREAT sound!!!

Scott from Gastonia, NC on 4/11/2017

Crutchfield Customer Review (What's this?)

Awesome Sub! DVC and solid performer even at 8 inches! grills are a bit tough to find but have found a great metal aftermarket that works great.

Pros: Amazing performance

Cons: No Grills

Polk Audio MM840D

Anonymous from Springdale, AR on 4/3/2017

2015 Subaru WRX
Crutchfield Customer Review (What's this?)

Exactly what I needed from a sub, sounds great in the sound ordinance bass bunker and I'm really surprised by the low notes this thing hits. My trunk has so much room for activities

Pros: Doesn't need much airspace, recommends .35, sounds bigger than an 8"

Cons: None

Polk Audio MM840D

Anonymous from Forest Hills, NY on 3/20/2017

Crutchfield Customer Review (What's this?)

the product is very well made it looks impressive But it is a bit under the specs. I put this speaker in a 0.35 cubic feet box like it recommends end it was not butting out like I expected it It sounded muddy at times this box was a sealed box. therefore its going back.



Best Buy!!!

Raul from Ponce, PR on 1/30/2017

Crutchfield Customer Review (What's this?)

I Like it and the Crutchfield service was excellent!! If I need any other item, I will buy with Crutchfield Again!!

Pros: Good Size, Nice Finished and Very Good Price!!

Cons: None...

Polk Audio MM840D

Anonymous from Ocala, FL on 1/7/2017

Crutchfield Customer Review (What's this?)

Great addition to my Lexus!! This sub woofer is the final piece to replacing all of the original stereo in my SC 430 Lexus what a difference! The price of every piece was good enough so I could replace all of the OEM equipment in my car. I should have done this much sooner, I didn't know what I was missing.



Polk Audio MM840D

Pasquale from BayShore, NY on 10/14/2016

2002 Lexus LS430
Crutchfield Customer Review (What's this?)

Great speaker sound and Price.

Pros: Great Price and great sound quality!


Polk Audio MM840D

Anonymous from Davenport, IA on 8/15/2016

Crutchfield Customer Review (What's this?)

Great Sound, fits great (14 JKU)



Great speakers for the size

Rustin from Houston, TX on 5/10/2012

Crutchfield Customer Review (What's this?)

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

TamN from Austin, TX on 3/5/2012

Crutchfield Customer Review (What's this?)

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

J.T. from Virginia Beach, VA on 1/30/2012

Crutchfield Customer Review (What's this?)

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

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.

Q & A

Customer Q&A

Crutchfield asked  
Why did you buy this?

polk audio componets have always performed great for me [ john  Mar 30, 2017 ]
I have been a Polk Audio fan for several years. It's been a long time since I put a system together and this will probably be my last, it was Polk or nothing... [ Gregory  Mar 24, 2017 ]
product is supposed to be able to withstand additional environmental abuse (and will pair nicely with my soundstream nano amp power wise), since I will be using in an offroad desert truck that will constantly be jarred and vibrated to death. :) Plus, it fits in the stock mounting location of factory one. :) [ Ryan  Mar 18, 2017 ]
Needed a small sub for a utv to mach a set of mm series 5.25". To bad they don't make the mm series any more, they are awesome. [ Danen  Mar 10, 2017 ]
Specs were better than most shallow mounts, which is what I had to use in my regular cab ranger. Xmax was higher and required less space (.35 cu ft) compared to most that are around half a cubic foot. [ MATTHEW J  Jan 23, 2017 ]
I like the sound [ Adam  Dec 25, 2016 ]
Dual Voice Coil and high sensitivity and along with a good history based product [ Brian  Nov 06, 2016 ]
Price [ raymond  Aug 10, 2016 ]

2 questions already asked

Thanks for your question. It's almost an inch. 0.95" to be exact. Hope this information helps. [ Kenny  Jan 21, 2017 ]  Staff
Thanks for your question. Weight is 9.5 pounds. Hope that helps. [ Kenny  Nov 29, 2016 ]  Staff