Help with amp choice for my DIY sub

[BrianC]

Howdy, I'm a couple weeks away from completing my Dayton HO 15" 4 cubic ft ported sub build. I'm amp shopping now, and trying to determine which amp is right for me. I'm likely to build another one if there are too many dips n dives in frequency responce, but that's at least a year away. My room is about 4200 cubic ft with an opening upstairs, so I'll likely need the 2nd to smooth overall response. I'm 50/50 music/ht.

When deciding on an amp, should I require an eq on the amp, regardless of the ypao eq already on my Yamaha 673? Also, the rating on the Dayton sub is 800 watts rms. Should I match that at a minimum? There are a couple nicely priced pro amps (qsc and crown) which have more than enough power for 1 sub, but come in around 500-600 watts per channel if I graduate to 2 subs. Is that underpowered?

Last, when determining the ports on my sub box, is there a big advantage to using 2 ports instead of 1? If I use 1, it's only a single 4"x10". If I do 2 ports, it's 2 4"x19"! Why would anyone complicate the build with dual ports? What is the advantage?

 

[rojo]

Having an amp with a built in DSP (like the Behringer iNuke DSP models) can be handy if you want to boost the house curve a bit or add a subsonic filter to prevent bottoming out below the tuning frequency. If you've got a measurement mic, it could be useful to straighten out room modes that YPAO didn't have the resolution to handle. But it's not crucial.

I don't think 500 watts will be underpowered. You're talking about a 3dB difference between 500 and 1000 watts, but adding a second sub should increase output by something like 6dB anyway. Just try it and add another amp later if it didn't work out.

2nd port is to add more weight to keep the sub from floating away in heavy bass passages... You know, because it's so light. Or it could be to prevent chuffing. I don't think you have much to worry about with a 4" port though.

Anyway, pics or it didn't happen.

Sent from my LG-VS980 using Forum Fiend v1.3.1.

 

[BrianC]

Having an amp with a built in DSP (like the Behringer iNuke DSP models) can be handy if you want to boost the house curve a bit or add a subsonic filter to prevent bottoming out below the tuning frequency. If you've got a measurement mic, it could be useful to straighten out room modes that YPAO didn't have the resolution to handle. But it's not crucial.

2nd port is to add more weight to keep the sub from floating away in heavy bass passages... You know, because it's so light. Or it could be to prevent chuffing. I don't think you have much to worry about with a 4" port though.

Sent from my LG-VS980 using Forum Fiend v1.3.1.

Cool, I'll stick with 1 port as it'll make the build much easier.

An inuke 1000 will be tons of power for 1 sub, but if I build a 2nd, will 500 wpc be enough? Or should I go bigger "just in case"?

Also, I am building this box with 1 1/4" thick mdf. Is that thick enough that I don't need to brace the box, or is bracing always wise? Box is externally 21" cube.

 

[rojo]

Some say that the walls of the sub enclosure will have their own resonant frequency, and bracing divides the walls into lengths that are resonant at frequencies above those which the sub plays. I couldn't tell you the resonant frequency of a 21", 1.25" thick square of mdf though. I'm not sure what to expect.

Sent from my LG-VS980 using Forum Fiend v1.3.1.

 

[rojo]

Re: wattage, you should be fine. I edited my other post while you were replying I think.

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[BrianC]

Lastly, any recommendations for best tuned frequency for 50/50 listening? 25,28, 32hz..?

 

[rojo]

What's the Fs of that driver? 24.25Hz? Tune the box at that frequency. That should put the impedance and excursion dips right at Fs. That'll result in the best sounding sub.

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[BrianC]

What's the Fs of that driver? 24.25Hz? Tune the box at that frequency. That should put the impedance and excursion dips right at Fs. That'll result in the best sounding sub.

Sent from my LG-VS980 using Forum Fiend v1.3.1.[/QUOT
Odele , señor Rojo! Thanks for your quick response. You da man! Go warriors!

 

[Verdinut]

Some say that the walls of the sub enclosure will have their own resonant frequency, and bracing divides the walls into lengths that are resonant at frequencies above those which the sub plays. I couldn't tell you the resonant frequency of a 21", 1.25" thick square of mdf though. I'm not sure what to expect.

Sent from my LG-VS980 using Forum Fiend v1.3.1.

Hi Rojo,
The resonant frequency of a 21" inch square panel is way higher than that of the subwoofer frequencies, so the question of standing waves is not a concern. However, when building a speaker box, some bracing is obviously necessary to decrease to a minimum panel vibrations, more so in the case of a subwoofer cabinet.

One good way is to glue 1" X 2" braces on edge to divide the panels in two sections. These braces can be made with either wood or MDF. A 1 X 2 belt can also be made around the panels which are perpendicular to the sub panel. Finally, I would suggest one or two additional braces joining the sub baffle to the back panel to make the enclosure more rigid. Of course, the volume of the box has to be increased by the space taken by the braces to maintain the proper internal volume.

The tuning frequency of the box is not necessarily that of the Fs of the sub. That was the old thinking of the 1950's. At that time, a lot of vented cabinets had a boomy response, just because the box was not properly tuned and some manufacturers were not too knowledgeable about the speaker/box interactions. After the works of Thiele and Small, things have changed.

The box should be tuned based on the sub's Thiele/Small parameters, mainly its Fs, it's Qts which depends on the moving mass/magnet coercivity ratio, and the possible projected low frequency curve. A software such as BassBox is very useful for the design of a properly tuned enclosure.

 

[Verdinut]

Having an amp with a built in DSP (like the Behringer iNuke DSP models) can be handy if you want to boost the house curve a bit or add a subsonic filter to prevent bottoming out below the tuning frequency. If you've got a measurement mic, it could be useful to straighten out room modes that YPAO didn't have the resolution to handle. But it's not crucial.

I don't think 500 watts will be underpowered. You're talking about a 3dB difference between 500 and 1000 watts, but adding a second sub should increase output by something like 6dB anyway. Just try it and add another amp later if it didn't work out.

2nd port is to add more weight to keep the sub from floating away in heavy bass passages... You know, because it's so light. Or it could be to prevent chuffing. I don't think you have much to worry about with a 4" port though.

Anyway, pics or it didn't happen.

Sent from my LG-VS980 using Forum Fiend v1.3.1.

For a 15" sub, it is preferable to use two 4" ducts than one. With only one, there will be less low frequency output at the box tuned frequency and more possibility of chuffing. Longer ducts will be required to get the same low tuning frequency with two ducts though. The volume taken by the ducts has to be accounted for in calculation of net box volume.

When I build enclosures, for ducts, I always use ABS pipes which are light, easy to work with and very rigid.
For ecological and health reasons, I glue them with the No More Nails stuff.

 

[rojo]

The tuning frequency of the box is not necessarily that of the Fs of the sub. That was the old thinking of the 1950's. At that time, a lot of vented cabinets had a boomy response, just because the box was not properly tuned and some manufacturers were not too knowledgeable about the speaker/box interactions. After the works of Thiele and Small, things have changed.

The box should be tuned based on the sub's Thiele/Small parameters, mainly its Fs, it's Qts which depends on the moving mass/magnet coercivity ratio, and the possible projected low frequency curve.

I'm not sure I follow your logic. From my understanding, there's an impedance and cone excursion valley at the box tuning frequency. If one tunes a box below the Fs of the driver, then the driver plays at its resonant frequency at longer excursion, resulting in some form of sadness and despair. Just a wild guess, but I imagine the result is a less than optimum BL(x), and by extension, greater harmonic distortion. Tuning above the Fs isn't as big a deal since the sub bottoms out below its port tuning too quickly for the loss of symmetry to matter, but tuning below is not a good practice.

Hopefully @TLS Guy or @shadyJ will correct me if I'm mistaken about that explanation. I'll be honest with you that I haven't seen any graphs indicating the problems resulting from too-low tuning, so I am open to being proven wrong.

For a 15" sub, it is preferable to use two 4" ducts than one. With only one, there will be less low frequency output at the box tuned frequency. Longer ducts will be required to get the same low tuning frequency with two ducts though. The volume taken by the ducts has to be accounted for in calculation of net box volume.

When I build enclosures, for ducts, I always use ABS pipes which are light, easy to work with and very rigid.
I glue them with the No More Nails stuff.

When I model subs, I try to model the vents with an eye on air velocity, limiting the peak to 20 m/s at 125 watts. Of course two longer vents are better for avoiding chuffing than one, but the designer ought not build the box bigger and heavier than it needs to be simply to accommodate an extra vent that doesn't improve the output of the sub. As long as the vent air velocity limit is not broken, there's little advantage to overkill on the vents.

 

[Verdinut]

I'm not sure I follow your logic. From my understanding, there's an impedance and cone excursion valley at the box tuning frequency. If one tunes a box below the Fs of the driver, then the driver plays at its resonant frequency at longer excursion, resulting in some form of sadness and despair. Just a wild guess, but I imagine the result is a less than optimum BL(x), and by extension, greater harmonic distortion. Tuning above the Fs isn't as big a deal since the sub bottoms out below its port tuning too quickly for the loss of symmetry to matter, but tuning below is not a good practice.

Hopefully @TLS Guy or @shadyJ will correct me if I'm mistaken about that explanation. I'll be honest with you that I haven't seen any graphs indicating the problems resulting from too-low tuning, so I am open to being proven wrong.



When I model subs, I try to model the vents with an eye on air velocity, limiting the peak to 20 m/s at 125 watts. Of course two longer vents are better for avoiding chuffing than one, but the designer ought not build the box bigger and heavier than it needs to be simply to accommodate an extra vent that doesn't improve the output of the sub. As long as the vent air velocity limit is not broken, there's little advantage to overkill on the vents.

As I was mentioning to you, tuning has to be based on Thiele/Small parameters. Usually for best results, a woofer with a Qts of less than 0.38 requires a box tuning above its Fs and, and with a woofer with a Qts higher than 0.38, there could be exceptions, but usually you need to tune the box to a frequency lower than the Fs of the cone.

I know that fact and I have been building speaker enclosures for many years and I am well documented. I know quite a bit about bass reflex enclosures and their designs. I also have all the equipment to measure Thiele/Small parameters, test speaker frequency response and I design passive crossovers as well. Moreover, I had my own speaker company up to recently before I retired.

I am assured that TLS Guy will agree with me on the comments which I posted to you.