Polk Audio continues its tradition of making brilliant-sounding speakers in stylishly designed packages — their Mobile Monitor (MM) Series Ultra Marine speakers look great and sound even better while meeting the challenges of sun and water to bring you superior sound on your boat.

The Polk MM651UM 6-3/4" marine speakers feature honeycomb-textured woven glass fiber composite woofer cones that resist and suppress unwanted resonances yet respond very quickly for a crisp and powerful sound. Large voice coils — over an inch in diameter — give the speaker high-power handling capability and extended reliability.

These sealed speakers resist damage from UV rays and salt water, so they'll provide great sound on your vessel for years to come. Each speaker utilizes an extra-small neodymium magnet that saves space and weight, 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 1" soft 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. Stainless steel terminal posts will not corrode or rust, keeping your speaker wires reliably connected.

Note: Polk describes this model as a 6-1/2" speaker. Most manufacturers designate this same frame size as a 6-3/4" speaker. For simplicity's sake, we list it as a 6-3/4" speaker.

Product Highlights:

  • 6-3/4" 2-way speakers (pair)
  • woven honeycomb glass fiber composite woofer cone with butyl rubber surround
  • 1" soft dome tweeter
  • designed and certified for marine use
  • stainless steel terminal posts
  • 4-ohm impedance
  • power range: 15-100 watts RMS (200 watts peak power)
  • frequency response: 40-25,000 Hz
  • sensitivity: 92 dB
  • top-mount depth: 2-5/8"
  • warranty: 1 year
  • Our 60-day money-back guarantee

Polk MM651UM Reviews

Average Customer Review:

4 reviews
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Great speakers

Written By Jesse T, Raleigh, NC on Saturday, October 20, 2012

These are some great marine speakers. Picked these up, along with their sister model that are split mids and tweets, and they are great in my boat. Big sound upgrade over the stock speakers. Can't go wrong with Polk.

They do the job.

Written By Dan O'Donnell, Newtown, CT on Saturday, July 28, 2012

Crutchfield Customer Review (What's this?)

I like the speakers. They sound great. Not cheap, but for a pontoon boat you need 4 x 100W so you can hear it with the engine running. These sound pretty crisp. I paired mine with a Rockford Fosgate 4 x 100w marine amp. One thing I am very impressed with is how much base this setup produces (with Rockford's base booster on the amp). I don't need to add a sub.

Pros: Crisp highs. Strong base.

Cons: Not cheap.

These speakers and this company rocks.

Written By NC BOY, Currituck NC on Monday, June 11, 2012

Crutchfield Customer Review (What's this?)

After reading all the reviews I decided to try these speakers and I must say they freakin rock. I wished I had more room for two more. The lows are great for a 6 1/2 mid speaker, outstanding! Put them on the boat and got rave reviews. As for as Crutchfield's, what can I say other than outstanding service, ordered on a Tuesday and was installing them Wednesday evening.

Pros: everything!

Cons: Wished it had a better grille to cover the speaker against damage, but the cone seems pretty stout so I don't think i will have any problems.

Great Speakers...very clear

Written By D08, Lake Geneva, WI on Wednesday, August 17, 2011

Crutchfield Customer Review (What's this?)

Did alot of searching for just the right speakers for the tower speakers for our wakeboarding boat... After talking to the experts at Crutchfield, I was sold.

They are cyrstal clear and a huge step up from the Kenwood speakers that we had. also the price here was as good as any place since I needed 2 pairs...

Hands-on research

What's in the box?

Download owner's manual

  • Two 2-way 6.75" marine speakers with built-in grilles
  • 8 Hex-head screws (3mm)
  • 8 Speed clips
  • Owner's Manual
  • Product registration information sheet

Features and Specs

Speaker Size 6 3/4
Design 2-way
Tweeter Design 1" Soft 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 40 - 25k Hz
RMS Power Range (Watts) 15-100
Peak Power Handling (Watts) 200
Impedance (Ohms) 4
Top-mount Depth (Inches) 2 11/16
Bottom-mount Depth (Inches) N/A
Cutout Diameter or Length (inches) 5 1/16

Product Research


Marine Certified: This Polk MM Series speaker is built to withstand exposure to moisture, salt water, high humidity, UV rays, and a wide temperature range so they work well in a marine environment; such as a boat. The speaker has passed rigorous hours of salt fog and UV testing. The speaker also features a completely sealed front design to prevent water intrusion. In addition, they include stainless mounting hardware for corrosion and rust resistance.

Klippel Optimized Components: Using a state-of-the-art Klippel Distortion Analysis, 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 Tweeters: The 1" liquid-cooled, silk/polymer composite tweeters provide clear, detailed highs.

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.

Built-In Crossovers: A sealed crossover assembly is built into the back of the speaker cover; no need to find room to hide an extra box.

Built-On Grilles: The speaker's built-on grille includes the tweeter mount which eliminates the traditional tweeter stalk.

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.