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Sub woofer lacks punch...?

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Old 09-10-2018, 12:31 PM
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Default Sub woofer lacks punch...?

Installed: Mono sub amp JL MHX300/1 to a single RForsgate RM18D2 sub woofer...... I'm just not feeling the punch ( amp is now discontinued but seemed a good value at the time)

Please comment: Is it just the limitations of a 7" sub in a shallow mount? Or amp is too small?

Limited space and need for a 'shallow' mount sub woofer drove the selection.

Wired to a Fusion AV650 with Fusion 504 and 4 Fusion 7022 speakers. 7022's can take about 50% vol off this amp before they start to screech but overall good value.

I have adjusted and adjusted and again Fusion tech support was really good,
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Old 09-10-2018, 12:35 PM
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keep in mind that a 8" subwoofer is on the small end of subwoofers. Add to that it is probably free air mounted in an outside environment.....it can only move so much air.... A pair of 8s would be better but most agree that you need 10s or 12s - especially in an outdoor space like a boat....
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Old 09-10-2018, 01:30 PM
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I recently added a JL 600/6 to the 4 factory 6.5 speakers in my 21CC and bridged 2 channels to a 8in JL sub. My 2 channels are rated for 180w @ 12.5V while your amp is rated at 160w @ 12.5V. It certainly woke up my factory system but having had boats with twin 10's and big power this thing puts out noticably less. As mentioned your install location can also have noticable effect as well but I don't think you're going to "feel" the power of that small sub unless you sit on it! LOL (even changing the a 10" won't help without more power)
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Old 09-10-2018, 02:19 PM
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You are not going to get much "punch" out of that sized sub. Replace it with JL 10" and 250w and you will get some decent punch. I had that but wanted more, so changed it our for a JL 12" and 600w of power behind it. I can be running at 30mph and still here and feel the depth of the bass.
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Old 09-11-2018, 07:48 AM
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Hello, just as an aside, have you tried playing with the gain control on the amp?
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Old 09-11-2018, 08:09 AM
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Good point on the gain on the amp.

I assume the 8" sub is just mounted "free air" or infinite baffle and is not in a sealed or ported box? Where exactly is it mounted? Is the front separated from the rear? In other words if you mount the sub in the console and leave the door open some of the rear waves will cancel out the front waves when they meet. Mounting the sub in the liner is better so that the front air is better separated from the rear air......

Also check the voltage going into the amp - make sure you have 12-14volts - less voltage equals less power....

And finally how are you crossing over the signal to the sub? active sub out put from the head unit? active crossover on the amp? or passive inline filter on the speaker wire? Adjusting the crossover point a bit higher may help get some more midbass as smaller free air subs struggle to produce lower frequencies.
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Old 09-11-2018, 08:27 AM
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I wouldnít call a 8 inch speaker a sub woofer. Best you can hope for is good mid bass out of that size speaker. Especially in an open environment like a boat.
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Old 09-11-2018, 08:52 AM
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Check to make sure the polarity is correct. An out of phase subwoofer can produce weak lows.
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Old 09-11-2018, 11:52 AM
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Thanks for the feedback, I have checked all settings and also the head unit with Fusion and I think its all set properly. The amp states 300W at 2 ohm, the 'sub' is 2 ohm rated....so I thought I had 300W driving the sub at 2 ohm and that would be decent.

Can I live with the amp and upgrade to 10" sub and get some better depth and punch or, the amp is also too light ?

Shallow mount location only about 5" deep against a bulkhead wall are my limitations and I cant go 12" either.
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Old 09-11-2018, 12:13 PM
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if you have space, I would do 2 10" 4ohm subs wired in parallel to 2ohms. 300watts isn't a ton of power but it should be balanced against the rest of your system.
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Old 09-11-2018, 10:05 PM
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Check the Rockford site for wiring diagrams. I believe these 8" subs are sold in pairs. With a single sub you may have a higher impedance and inhibit the performance.
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Old 09-12-2018, 07:31 AM
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Tom S makes a great point. That RF sub is probably a dual voice coil in order to produce 2ohms. It may not be wired from the factory with jumper wires to the connectors on opposite sides of the basket. If not already jumped you need to jump them to wire in parallel and get 2ohms instead of only driving 1 coil at 4ohms.

Last edited by sclancy; 09-12-2018 at 01:12 PM.
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Old 09-12-2018, 11:25 AM
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https://rockfordfosgate.com/products/details/rm18d2/


Wiring Diagram #1 or #2....?

I went with #2. On the Wiring Wizard drop down.

Single mono amp to single woofer.
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Old 09-12-2018, 12:07 PM
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So simply switch it to diagram #1 and see what you get
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Old 09-12-2018, 01:21 PM
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Do NOT use wiring diagram #1 - best case is the amp will shut down. worst case you will fry the amp. very few amps are designed to run at less than 2ohms. The bad news is that you have an amp that can run at 2ohms but you have a dual voice coil sub that cannot since it is 2ohm on both sides. Run it parallel (as shown in #1) and the impedance is cut in half to 1ohm. Run it in series (as shown in #2) and the impedance doubles to 4 ohms. You can only get other impedance combinations with additional subs wired in various ways.

Ditch this sub and go to a 10" or 12" that is 4 ohm dual voicecoil so that it can be run in parallel at 2 ohms to maximize power from your existing amp.
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Old 09-13-2018, 04:06 PM
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Originally Posted by sclancy View Post
Do NOT use wiring diagram #1 - best case is the amp will shut down. worst case you will fry the amp. very few amps are designed to run at less than 2ohms. The bad news is that you have an amp that can run at 2ohms but you have a dual voice coil sub that cannot since it is 2ohm on both sides. Run it parallel (as shown in #1) and the impedance is cut in half to 1ohm. Run it in series (as shown in #2) and the impedance doubles to 4 ohms. You can only get other impedance combinations with additional subs wired in various ways.

Ditch this sub and go to a 10" or 12" that is 4 ohm dual voicecoil so that it can be run in parallel at 2 ohms to maximize power from your existing amp.

Thank you! I am headed in that direction now with a 10" DVC 4 ohm sub on the way. On the sub-amp, is the correct setting with the Low Pass filter on ? and then adjusting LP Filter pass thru at about 80 Hz ?? I read keep Bass Boost off or low and adjust Sub-sonic filter to about 35 Hz to minimize low end rumble? Input sensitivity I have at about 50% of the dial or 12:00 o'clock....( realizing this may need further adjustment )...I am looking for a good starting point with these settings so I can then tweak and avoid my usual CF.

Am I in the right neighborhood to get some punch and decent low end ?

Is my voltage setting on 'low' for the input setting? I set it at low figuring Fusion AV650 coming in at 4/6 W?

( I never thought this much went into a sub install ! ) So thanks.
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Old 09-14-2018, 05:45 AM
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What sub did you purchase?
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Old 09-14-2018, 07:33 AM
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Originally Posted by Dockholiday74 View Post
What sub did you purchase?
https://www.rockfordfosgate.com/prod...tails/rm110d4/

I hope this is a decent choice....I have infinite baffle situation and need a shallow mount at 10"....if its not a good option please let me know that, but 4.5" is about all the depth for the location, maybe 5" with a slight lift. I realize a sub woofer needs more volume behind it ideally. I just dont have it and the coax 7022 Fusions lack bass due to same shallow mounting which will be on both sides of the sub.

Thank you.



thank you.
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Old 09-14-2018, 08:52 AM
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Originally Posted by SeaCraftJ2 View Post
https://www.rockfordfosgate.com/prod...tails/rm110d4/

I hope this is a decent choice....I have infinite baffle situation and need a shallow mount at 10"....if its not a good option please let me know that, but 4.5" is about all the depth for the location, maybe 5" with a slight lift. I realize a sub woofer needs more volume behind it ideally. I just dont have it and the coax 7022 Fusions lack bass due to same shallow mounting which will be on both sides of the sub.

Thank you.



thank you.
its ok. Parameters are more of a sealed type sub. They claim itís good for everything and that canít really ever be the case.
I will tell you to be easy on it with 300watts. That will definitely get the most out of it but those RF subs are easily trashed with to much power.
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Old 09-14-2018, 08:54 AM
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Originally Posted by Dockholiday74 View Post


its ok. Parameters are more of a sealed type sub. They claim itís good for everything and that canít really ever be the case.
I will tell you to be easy on it with 300watts. That will definitely get the most out of it but those RF subs are easily trashed with to much power.
another idea and what we have to do often is put a ring in the front to add the clearance needed to make a better sub work.
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