Sub/Amp Heat Issues

[awesomebase]

I'm drawing out different designs for making a sub and was debating between a closed and ported design. I like that closed designs take up far less space, but I'm more concerned about heat issues. How does the heat produced by the driver escape without a port? Wouldn't it just cause the driver to weaken and distort the longer it is being played? Also, for plate-based amps, is there any need to build any vents for it to cool as well? I'm not talking vents that go through the sub's cabinet, but rather vents that only come out of the box around the amp pointing outward (probably above the top of the amp).
Also, is the volume of the port more or less important than the length? I realize that both are important, I'm just wondering if a 4" port that is 10" long would give similar measurements and tuning as a 2" port that is 40" long? or two 2" ports that are 20" long?

 

[MacManNM]

I'm drawing out different designs for making a sub and was debating between a closed and ported design. I like that closed designs take up far less space, but I'm more concerned about heat issues. How does the heat produced by the driver escape without a port? Wouldn't it just cause the driver to weaken and distort the longer it is being played? Also, for plate-based amps, is there any need to build any vents for it to cool as well? I'm not talking vents that go through the sub's cabinet, but rather vents that only come out of the box around the amp pointing outward (probably above the top of the amp).
Also, is the volume of the port more or less important than the length? I realize that both are important, I'm just wondering if a 4" port that is 10" long would give similar measurements and tuning as a 2" port that is 40" long? or two 2" ports that are 20" long?

With a closed design you are going to sacrifice sound below 30Hz, usually people that have sealed subs use an Eq to get that bottom end out of them. The other disadvantage of a sealed unit, you need WAY more power for them, a factor of 2 or 4 compared to a vented enclosure.

You don't need to worry about the heat generated by the driver. The movement of the cone and the sixe of the box wil take care of it (unless its extreemly small).

Plate amps don't require any additional vents.

Port length and width are equally important.
If your plans call for a vent that is 40in long, x 4in you can change the diameter of the ports, but not the length. Ie you can use two 2 inch by 40inports instead. If port length is an issue for you, you can always put them on the outside of the box, or use pvc pipe and snake them around inside. Just remember to account for the volume of the vent in your box calculation.

 

[awesomebase]

Good advice!

Thanks for the advice, I wasn't certain about the port volume, but your comments made it clearer. One question though; if a port is supposed to be quite long, then should there be any 90degree turns in it to fit it in the box? I need a high WAF for this project and doing the port outside the box is just not an option.
Also, I was wondering if you had any thoughts on whether the bottom frequency of a sub is as important as say, the weight of the magnet. In other words, is it better to go with a driver that can do 22Hz with a 90oz magnet, or is it better to go with a driver that can do 36Hz with a 160oz magnet? The reason I ask is I'm not sure whether or not ears can actually hear 22Hz. I've seen statements where it is claimed that we can hear between 20 and 20,000 Hz, some even claim that we can hear 16 Hz, but I've also seen claims saying that anything below 40 Hz is difficult to discern. I don't have the measurement tools necessary to actually see if I could tell the difference or not, so it is hard for me to say whether or not, in "real-word" practice, I can tell the difference between say 36 and 22 Hz.

 

[j_garcia]

You don't really hear 16Hz, but you do FEEL it. Not everyone hears the same, but the average person should be able to hear 20Hz. In my setup with the PB-10, you could hear 18Hz still, and 16Hz was pretty much just vibration, but that tactile rumble does add to the experience for sure. Below about 25Hz, you can't tell where the bass is coming from because it is everywhere, activating the air in the room because the wavelenghts are longer than most rooms. Real world, there is a definite, audible difference between 36Hz and 20Hz.

To achieve very long lengths for ports, you would use multiple ports sized appropriately to give the same effect as a single long port.

Without the tools to measure what you've built, I think you're going to have a difficult time designing this sub from scratch. I don't know if I recommended it to you or not, but check out these from GR Research. He'll provide you with plans for the correctly sized box; add an amp and you're good to go. No guesswork.

 

[awesomebase]

Design

Actually, I'm not so concerned with the cabinet design because I know many people that can help me with that including a rep with Parts Express. I know that it is VERY important to create a tuned cabinet and I'm certainly not undermining that. My concern was for the ported design because there are a couple of sub designs that show port lengts in excess of 4 ft with enclosures that do not have any dimension longer than 2 ft. It was not expressed whether the port's volume was more important or whether the length of the port was achieved by doing multiple 90 degree turns.
In terms of the actual sub cabinet, I'm sticking with some pretty standard designs though I may put my "personal" touch to it in terms of its finish, etc. The main concern here was whether I needed to create a internal "box" for the amplifier and whether I was going to go with a sealed or ported design. I'll be using Daytons subs combined with PE's 250W plate amps. I was just concerned that with all the heat generated by the driver and the amp, that creating a closed box might deteriorate the sound.

 

[j_garcia]

The amp sheds heat outside, so internal heat buildup should not be a problem - the heat will be drawn into the aluminum plate and shed via the heatsink on the outside of the cabinet. The new 300W bash amps at PE generate even less heat than a class AB amp. With a sealed sub, it might be a good idea to have the amp enclosed to make sure there sub's air space is truly sealed, but there are plate amps for sealed enclosures that are completely sealed themselves to prevent air leakage. As was mentioned, heat should not be a serious problem for the driver either; a driver intended for a sealed design will already have accounted for this.

 

[MacManNM]

You don't really hear 16Hz, but you do FEEL it. Not everyone hears the same, but the average person should be able to hear 20Hz. In my setup with the PB-10, you could hear 18Hz still, and 16Hz was pretty much just vibration, but that tactile rumble does add to the experience for sure. Below about 25Hz, you can't tell where the bass is coming from because it is everywhere, activating the air in the room because the wavelenghts are longer than most rooms. Real world, there is a definite, audible difference between 36Hz and 20Hz.

To achieve very long lengths for ports, you would use multiple ports sized appropriately to give the same effect as a single long port.

Without the tools to measure what you've built, I think you're going to have a difficult time designing this sub from scratch. I don't know if I recommended it to you or not, but check out these from GR Research. He'll provide you with plans for the correctly sized box; add an amp and you're good to go. No guesswork.

Multiple ports do not compensate for port length, only differences in diameter. So if a 40 inch long port is needed, then each of the ports in the box must be 40 inches.

As far as bends in the ports, you can have as many as you want. It can be a coil in the box and it won't hurt a thing. Just be careful, some bends actually reduce the diameter of the pipe so that will screw up the port volume.

 

[awesomebase]

Correct!

Yeah, I just confirmed the statement about the "bends" elsewhere as well. There is no compensating for the length required. However, like you said, it is better to ensure that the bends do not reduce the size of the port inadvertently.

 

[j_garcia]

You can use larger diameter, multiple ports to achive the same tune in the box or mutiple tuning points. If you design it so that the port length requres too many bends or too long of a vent, then the design isn't a good one, IMO.

I thought I read somewhere that you could use 2 ports to do the same as a single longer one, but I may have read that incorrectly.

 

[awesomebase]

True...

Ideally yes, I would like to have as few bends as possible. In fact there are some designs that put the port at an angle to gain a few extra inches instead of incurring a bend. There are a few designs I could use that would require significantly shorter ports and I'll probably go with those. If any bends were needed, I would like to keep it to just one and no more. Otherwise, the design should be changed.

 

[MacManNM]

Ideally yes, I would like to have as few bends as possible. In fact there are some designs that put the port at an angle to gain a few extra inches instead of incurring a bend. There are a few designs I could use that would require significantly shorter ports and I'll probably go with those. If any bends were needed, I would like to keep it to just one and no more. Otherwise, the design should be changed.

I have built boxes with what looked like a snake inside of it, the key is to make sure the port diameter is large enough to prevent huffing.

 

[awesomebase]

Huffing?

OK, so, what the heck is "huffing"? Maybe I know the effect by a different term, but that is new to me (though my memory may need to be jogged).

 

[j_garcia]

aka "chuffing" or port noise.

 

[annunaki]

Wow, there is a lot to go over here!

Sealed enclosures can work very well at very low frequencies. They roll off at 12 db/oct. as opposed to 24 db/oct. with a ported enclosure. Sealed enclosures typically require about 2x the power to reach similar output levels of ported enclosures. The also require more excursion capability from the woofer itself than their ported counterparts. None of this means that they cannot play down or below 20 hz. It simply means you will need a dynamic amplifier to do it at a reasonable output level. A large subwoofer (15") would definitely help in that department. Sealed enclosures are usually 25%-50% smaller than ported with the same woofer.

Sealed enclosures will not have problems with heat if the driver is well designed and has a decent cooling system.


Ported enclosures are very sensitive to changes in port length and diameter. For a given port surface area, the length must remain the same to achieve a desired tuning frequency. Meaning a single 4" (12.57 in.^2) circular port would be equal to a 1" x 12.57" (12.57in.^2) slot vent or 4-2" (12.57 in.^2) circular ports. As long as all are the same length, they all tune to the same frequency.

Bends in a port are fine as long as the width or diameter does not change. Precautions should be taken when doing 90 degree bends in slot vents, such as rounding the edges of the bend with a sander. Rounding the openings on both ends will also decrease turbulence. Using "aero ports" also works well in circular porting applications.

Ported enclosures, especially those tuned very low, require large enclosures. Usually the extra space is to make up for the port's displacement. Many enclosures tuned down to 20hz or lower require extremely long ports, not to mention large diameters (surface area) to avoide port noise

The T/S parameters (Thiele/Small) can easily dictate what type enclosure the woofer will most likey perform best in. If you can get me the T/S parameters of the woofers in question I can help you out some. Also, in reference to you question about resonant frequency and magnet weight, the Fs, or resonant frequency is more important when designing a home theater subwoofer.

Hopefully this will help you out some.

 

[awesomebase]

Reading Up

Thanks! Yeah, I'm reading to learn more, refresh my memory, and catch up with speaker design techniques. It has always been a desire of mine to do this, so it is also rewarding. I'm going through the "Speaker Design 201" book and it has some much needed formulas and calculations.
In terms of your question with regard to the speaker choices, I've been evaluating a few, but have narrowed it down to Daytons for the 5 or 6 inch woofers, Daytons or MTXs for the subs (I know MTX is better for car subs and their range does not go down as far as the Daytons, but I'm still evaluating), and I'm looking at a number of different tweeters to use going from Daytons to Morels to some of planar/ribbon type tweeters.
I like the Dayton MKIIIs in terms of being a very good all around subwoofer. I am leaning towards those, but ultimately, I have to discuss the design with my wife as these need to have a high WAF. I want to do a couple of dual-driver subs. I want them to match nicely with the rest of the speakers I will be building as well, so size and design are critical in this case. The advantage of using the non-ported cabinets is exactly as you mentioned... it reduces the size needed. However, I do want enough low-end sound, so, it will be a matter of balancing out these two requirements and making a decision on what will work best.
In any case, the reading material is certainly helping me out a lot and at the same time I can eliminate ideas that were poorly thought out in favor of those that are reinforced by the material I've been going over. Hopefully I can settle on a design by the end of January and start the project in the spring. If you have any thoughts on the performance of the drivers I mentioned, please provide them. Thanks!

 

[annunaki]

A dual driver subwoofer would work well if you do not want to go with a ported design. You will need a substantial amplifier to drive two woofers instead of just one.

Two subwoofer drivers are also going to require more space as well though. It is going to be all about comprimises.

I just modeled up a Dayton Titanic MkIII 12". It requires a 1.72ft^3 sealed enclosure per woofer to yield a .707 Qtc. To do two of them in one enclosure, would require a rather large box. You could probably go down to 1.25ft^3 per woofer and still have very good results. Though the box would still be on the big side, say 16.75" x 16.75" x 16.75" figuring displacement.

Porting this woofer is not a very good idea unless you can yield about 3.5ft^3!! Not to mention having to tune it to 20hz to avoid and nasty bumps in frequency response. It appears to have good respose if one has the space, otherwise, stay sealed.