Polk Audio MM840

250W 8" Mobile Monitor Series Single 4 ohm Marine Certified Car Subwoofer

Product videos

• 500W Peak
• 250W RMS
• Single 4 ohm
• Polymer Woofer Cones
• Butyl Rubber Surround
• Carbon Composite Basket
• High Temperature Copper Voice Coil

Learn more about the mm840

Good news and a little bad...

This item has been discontinued

But you may be interested in the newest model that's available.

mm842svc thumbnail

Polk Audio MM 842 SVC (MM842SVC)

OUR PRICE: $149.00

View item

Product Highlights

• 500W Peak
• 250W RMS
• Single 4 ohm
• Polymer Woofer Cones
• Butyl Rubber Surround
• Carbon Composite Basket
• High Temperature Copper Voice Coil


Polk Audio brand icon

Authorized dealer

The Mobile Monitor Series by Polk Audio is the result of 3 years of research and development. The series uses high quality materials and the newest loudspeaker technology to bring the listener an unforgettable experience. The best part about the MM Series is the affordable price.

General Features

  • 8" Marine Single 4 ohm Mobile Monitor MM Series Subwoofer
  • Power Handling:
    • Peak: 500 watts
    • RMS: 250 watts
  • Impedance: Single 4 Ohm
  • Polymer woofer cones
  • Butyl rubber surround
  • Heavy duty input terminals
  • Carbon composite basket is lightweight and durable
  • Klippel optimized components for the best possible performance
  • 1-1/2" High temperature copper voice coil
  • Frequency response: 26-200 Hz
  • Sensitivity: 90 dB
  • Marine certified to withstand a harsh marine environment
  • db Series Subwoofer Grill Available
  • Authorized Internet Dealer
  • 1-year Manufacturer's Warranty

Product history:

Polk Audio MM842 SVC thumbnail
(newest, current model)
Polk Audio MM840 thumbnail
(previous model, replaced by Polk Audio MM 842 SVC (MM842SVC))
Polk Audio MM2084 thumbnail
(previous model, replaced by Polk Audio MM840)

Subwoofer Wiring Guide

Wiring Guide to Wire 1 Polk Audio MM840 Subwoofer

Mono Amp

2 Channel Amp - Bridged

Wiring Guide to Wire 2 Polk Audio MM840 Subwoofers

Mono Amp

2 Channel Amp

4 Channel Amp - Bridged

Wiring Guide to Wire 3 Polk Audio MM840 Subwoofers

4 Channel Amp

Wiring Guide to Wire 4 Polk Audio MM840 Subwoofers

Mono Amp

Mono Amp - Option 1

2 Channel Amp

2 Channel Amp - Bridged

4 Channel Amp

Specifications & Details

Subwoofer Specifications
Mobile Monitor Series
Subwoofer Size
Peak Power Handling
500 watts
RMS Power Handling
250 watts
Voice Coil(s)
Impedance (Per Voice Coil)
4 ohms
Frequency Response
26 - 200 Hz
90 dB
Xmax (millimeters)
20 mm
Subwoofer Design
Round / Circle
Main Cone Color
Woofer Composition
Woofer Surround
Butyl Rubber
Subwoofer Grille
Re-Cone Kit Available
Recommended Enclosure Specifications
Recommended Enclosure Type
Ported / Vented
Top-Mount Depth (Mounting Depth)
Cutout Diameter
Shallow Mount Installation
Warranty and Return Policy
Parts Warranty Duration
1 year
Labor Warranty Duration
1 year
Warranty Provider

Item Details


Model Number:




Internal SKU:


Internal Product ID:


Product and How-to Videos

  • Polk Audio Mobile Monitor Series Subwoofers | Product Overview
  • Car Subwoofer Size Buying Guide | What Size of Sub Should I Get?

Customer Ratings & Reviews

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Customer Reviews

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Don't Bother

Not enough bass for my Jeep JK Wrangler using the factory enclosure and an Alpine PDX-5. Lasted about 3 months and completely blown listening to mainly Phish and Jack Johnson, etc. Not even rap. Went with an RE Audio SL-10 and I am much happier!

By Dennis on July 19, 2010


found this helpful.

Great sub, great price, and very versatile.

This sub is a great sounding component which takes up little room and has good output and great sq. at its price point it's hard to beat unless you're looking more for crazy SPL.

By Donnie E on March 26, 2013


found this helpful.

Polk mm840

I purchased this subwoofer to use in a custom enclosure in the footwell of my Miata. The airspace for the box is about .25cf. I am powering it with a PPI p900.4 on the rear channels bridged. I estimate somewhere between 300-350w rms is going to this sub. It sounds great! It is very loud and responsive. My particular setup is peaky at about 65hz but and doesn't play down very low, but I attribute this to the airspace limitations of my tiny box. I imagine this sub would sound even better with a little more room to breathe. I would recommend this sub to anyone wanting to add some extension in the lower frequencies and take the strain off of their full range speakers. However if you are looking for super low, boomy bass for trap/hip hop/ etc you should go with either 10's or 12's in an appropriate enclosure.

By Brian on February 2, 2015


found this helpful.

polk mm840

Really great product as long as you know what you want. Polk is known for its crisp and sharp punches. If you're just looking to be absurdly loud and rattle all the metal in your car, go buy the Sony X or Kicker Comp 12's. I have 2 of these 8s in my car and they sound awesome. They don't take over all the music, but you still get that tickling feeling through your body. Definitely a great buy!

By Opie on November 25, 2011


found this helpful.


Great sound and Fit!

By James on December 14, 2011


found this helpful.

Great 8" subwoofer

I bought this woofer and the 8" angled box for my GMC S-15 PU. This is a great value and fits behind the seat. While listening, I am still at times surprised at how good this woofer sounds. I have not done any audio testing on it, but I can say I wonder why it took me so long to purchase this speaker. I am very satisfied with it.

By cliff on May 8, 2010


found this helpful.

Just enough bass

I initially bought this sub with the sole intention of having "adequate" bass to complement my JL C5s both front and rear. I have the sub powered by a JL 250/1v2 and the response is great for sq applications. The high sensitivity of the sub and low FS, compared to most other 8s, makes it very power efficient while having a wide frequency range of operation. The low FS gave it ability to play notes down to the 30hz range in a very composed manner. Having the sub in the recommended .35f3 sealed box makes it crucial that you provide the right amount of power, 250 rms min, in order to be able to hear/feel the bass in your music. The overall design of the sub is well thought out and the motor structure rivals subs costing two to three times as much. With having my bass in cabin, ext. cab pickup, the bass seems like it is enough most of the time, but if you like bass heavy music that goes well below 45hz or your sub is in the trunk, then I would recommend you look elsewhere. This is the perfect sub for people that have a limited amount of airspace available and require a somewhat shallow sub for their application.

By Alberto on November 28, 2015 Checkmark Icon Verified purchase


found this helpful.


yes I would let everybody know what a good speakers they are.

By jeffro on December 30, 2015


found this helpful.


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