Why Choose Crutchfield?
Great gear and expert advice
Our experts help you pick the ideal gear for your needs.Meet our advisors
Free tech support for the life of your gear.Read about our one-on-one expert help
You may return items that are complete and like new for any reason within 60 days.See the details of our return policy
Earn one point for every dollar you spend and use those points to save on future purchases.Get full details on Crutchfield Rewards
Deep discounts on car installation gear
We offer vehicle-specific installation instructions, dash kits, wiring harnesses, and speaker accessories at deeply discounted prices with car stereo and speaker purchases. Available for most vehicles.Learn more
Trusted since 1974
Our customers have trusted us with their business, and they’ve helped us win awards for our service.Check out our most recent awards
The manufacturer’s warranty always applies.See more details
Polk Audio MM840D
Mobile Monitor Series 8" sub with dual 4-ohm voice coils
Our take on the Polk Audio MM840D by Crutchfield's Gabrielle Thorndyke
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
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
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
- 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
Polk Audio MM840D Reviews
Average Customer Review(s):
Thanks for submitting your review
Features and Specs
|Impedance||2 ohms or 8 ohms|
|Surround Material||Butyl rubber|
|Ideal Sealed Box Volume (cubic feet)||0.35|
|Ideal Ported Box Volume (cubic feet)||N/A|
|Port diameter (inches)||N/A|
|Port length (inches)||N/A|
|Dual Voice Coil||Yes|
85 dB at 2.83 volts
|Frequency Response||30 - 200 Hz|
|RMS Power Range (Watts)||100-250|
|Peak Power Handling (Watts)||500|
|Top Mount Depth (inches)||3 15/16|
|Bottom Mount Depth (inches)||4 11/16|
|Cutout Diameter or Length (inches)||7 1/8|
|Parts Warranty||1 Year|
|Labor Warranty||1 Year|
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.
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.
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