I have a moderate system in my bow rider/wake board/ booze cruise boat and recently the amp for the sub died. (Was old Fulton). Boat full range is 4 polks driven by older Fulton 200w and a set of electro voice dual 8" (4 total) on the tower driven with 300w Fulton. Sub is a jl audio 10" setup as free air. I recently bought a alpine class D (mrx-55d) and have it powering the JL audio 10". My question is the new amp has some settings that I am less familiar with. According to JL audio the sub has natural frequency of 46hz in free air environment.

Gain, I get this typically set HU to where rest of the boat speakers are nearly maxed out and adjust till sub or whatever set of speakers the amp is driving is same. Without an oscilloscope and all kind of crazyness is that my best approach? Should I download a source file, or is that getting above my skill set?

Cross over frequency- I assume set this to just above natural frequency (46hz in my case) but I am obviously open to advise from the pros

Sub sonic filter- this is new as compared to old amp, I take it is as a basically a low pass filter? Should I set it to some level less then resonance of speaker ? I found some stuff on Interweb about 1/2 or 1/3 octive below resonance of a sealed enclosure but no idea what to do for my free air application.

I assume since it is an infinint baffle in an open air environment the bass Eq (manual lists as 50hz boost up to 12db) knob should be set to zero ? Isn't "boosting " a open boat sorta like air conditioning the outside or peeing in the ocean ?


Thanks in advance everyone.
Mike

Not answering any of your questions, cause I'm clueless but setting gains has always been a weak point for me. I have buddies that do it by ear and swear by it. They set mine last summer and I thought it was an improvement over my ear setting method. I have also used JL's method of a voltmeter and voltage at the outputs. Never knew if that was accurate or not. Last fall I bought a cheap handheld Oscope $60?? off ebay and had it checked against a high quality one at work. It was dead nuts. When I hooked it up to my stereo to check against the setting my ear tuning friends did, I was amazed at how much clipping my sub and speakers were doing. So I reset using the Oscope using the gain cd, and the quality was way better I felt, though the loudness was less. And when I compared my new gain settings to the JL method, it was dead nuts.

Ive found setting up a boat for sound is by far different than setting up a home system. My approach is to set it up (cross over point's and gain's) in a way that compliment's the entire chain on paper...then comes the real tuning and that is by ear. To many time's i have set thing's up only to be annoyed by the fiberglass catching some sympathetic wave or resonance that blow's the whole thing...read the damm boat resonate's....Those low freq filter's are good for catching unwanted harmonic's.

I gave up on anything short of that due to my source music or recordings due to how much they vary....one cd or mp3 can sound tight and clean and the next just boom or shrill..

Mike,
When you say free air what do you mean? Isn't the speaker mounted to something? ? If not, it needs to be either in a sealed enclosure or a ported one. Otherwise you will have cancellation of certain frequencies making the sub less efficient. The crossover frequency should be around 100 Hz to 120Hz. The setting you have now is wrong, means that you are only giving the sub frequencies that it can't produce. The natural frequency you are talking about is most likely the free air resonance frequency of the sub. This is the frequency where the sub will start producing less volume. I would put the subsonic filter on. Otherwise the sub & amp are trying to reproduce sound below your hearing range and will eat up power of the amp needlessly. The gain I would set to a hair below maximum. This will not really matter much as long as the amp can be driven to full output. The bass boost is strictly a personal option. If you want accurate sound you would leave off or zero. It is very much like turning up the bass control. All this advice is without knowing the exact spec and speakers you are using but it will be very close. If you have any other questions give me a call John 224-610-6677

Good info, please keep it coming. I messed with it tonight but neighbors came out looking annoyed. (With tower speakers and tower and on a trailer it's quite loud.) I am going to launch boat tomorrow so will do more tweeting then. John, The sub is mounted ina vertical wall by rear seat in a fairly large compartment but the backside is not "sealed so I believe that is considered free air or infinite baffle.

Free air installs need a subsonic filter..... Set that on. Crossover should be set to blend the sub with the other speakers. Check the frequency response of your others speakers. My guess 100Hz or so. The free air is going to have a hump at 46 since that is the natural resonance. Set bass boost at 0 then fine tune for preference. Gains should also be set at unity to start then adjusted from there. You don't want the gain turned up too far or you will introduce distortion. Hard to explain on the internet but easy to show in person. Without listening or using a meter it would be impossible for me to tell you exactly where it should be set. I can only tell you where to start.

Forgot to say this... Set you gains first then crossover then bass boost last. If you set things up out of order you won't be able to fine tune properly. Example if the gain is not setup right then it won't matter what the crossover is set at...

Hey Mike,

In my experience, in regards to the amp, don't know if Alpine and JL are the same in their manuals, but JL actually provides you with a voltage reading of what your amp gain should be set at to match the RCA input voltage... In their format, you typically set all your HU crossover and bass boost and loudness to 0 or neutral, if you have a remote bass knob, you set this to 1/2-2/3 of the way up, then you run a constant tone either recorded on a CD or in my case my iphone, so subs, they typically use a 50hz tone, obviously unplug your + speaker wire so no sound is coming out, and unhook your RCA's to your other amps so the tone doesn't go through... You then turn your volume knob on the HU 3/4 of the way up, then take a voltage reading at the Speaker Output of the Amp, and dial the Gain until you hit the correct voltage. Depending on ohm load, 2, 4, 1.5 etc, the voltage will be different, but this is all defined in the manual for setting up... Again, don't know if Alpine provides this, but double check... If not, the typical way should be similar, and you turn the volume up until it starts to distort your other speakers, then dial in the sub amp to the same sound... The problem is if you over gain adjust, you tend to get the hiss of the amp as a result of the gains being turned to high which is a nuisance.

For my 10's sealed, I run around 80hz as the crossover, some of the guys are saying 100hz, ultimately you will have to play with it to see what sounds best to you, my Q factor, if you have it is around 1.1. You'll notice are you go into the 100hz range, you will start to loose your low end bass, but again, with free air, you have to keep the speaker happy, and if the lowers sound distorted lower than 100hz, then you have to move it up, but again, it to your liking...

The subsonic filter was originally designed for ported boxes where as the resistance from a sealed box would not allow a sub to enter into a sub sonic state which can damage the woofer, typically this is anything lower than 30-32hz... For your 10" free air I would think 35-40hz for a safe bet since you said natural is 46hz...

Hope this helps

This is just a general way to setup gains. Put the head unit at about 3/4, then turn your amplifier up until you start to hear distortion then turn it back down a bit. While this is not a perfect gain match, it allows for when you have songs that are not as loud as others.

The crossover frequency is so that the amp only amplifies the signals in that frequency range. On subwoofers in most boats that will fall in the 100 to 120 hz range. You should have a crossover on your other speakers that would be set at 100 to 120 hz as well. You want the subs doing the lower range and the other speakers doing the higher range. Adjusting this setting higher or lower will have no effect on the lower bass output. It is just the point that the subs and the other speakers divide in frequency coverage. As others have stated you just set it to what sounds best to you.

This is a setting to stop frequncies from going to the subwoofer that it can not produce. With ported enclosures below the tuning frequency of the port the sub basically has no control and can be damaged by these lower frequencies. With a free air sub they are designed differently and are more able to deal with the lower frequencies. Still it is a good rule to set it at about 30hz. There really is not much music down below that point and the subs wont really produce much output anyway. So save the amp power.

Basically that is telling you that the amplifier has a bass boost centered around the 50hz frequency. You can adjust this to what sounds good to you, but don't go all the way to 12db full boost.

Again these answeres are general rule setups. Written as basically as possible for someone who really has no experience in the setup. I would actually reccomend taking it to a shop and let them do the adjustments to get the best sound, or spend a lot of time researching what you are dong and playing with it. As long as the amps are not putting out distortion and sounding crappy you can basically set it how ever it sounds best to you.

I have also used a powered Radio Shack speaker to listen for clipping. Pretty slick and cheap, but the $60 oscilloscope is so easy and fool proof. Setting the gains with my current head unit at 3/4 wasn't any good. With a range of 0-50, it doesn't clip till 48 using the o scope. So if I set gains to the generic 3/4 volume method at 38, it left me way short.