School Project

[dreamofinsanity69]

Hey all first post here on the shack but I've taken the time to visit you guys much throughout the planning. I wanted to get some input on my final design. It will be made with 3/4" mdf, double thickness baffle, and will have an effective volume of roughly 8 cu.ft. I hope to make it a down-firing sub. I'm toying with a tune somewhere between 15-19 hz but i think i might settle on 17hz with two 4in ports 26.5 inches long due to inside dimensions only being up to 30". Im using the Dayton Reference series 15HF and hope to use it mainly for home theater but i would like music performance as well. I would be willing to tune lower if people thinks it's reasonable to use PVC bends in a port.

Any input welcome but box internal dimensions of 22x22x30 are practically set in stone. (pieces cut in tech as i had to get moving)

p.s i plan on using a rythmic audio 350 or.....350se?

and is 4-5 mm excursion over xmax a problem for short bursts at 25hz?

[Sonnie]

Hi Benny and welcome to the Shack!

We've got some guys who can help you with your project.

So this is a school project... is it high school or college? Are you going to be able to demonstrate the finished project for the class?

[brent_s]

First thing, down firing a driver is never really a good thing for the driver itself. The RS390s are particularly bad candidates...the HO has a sag of 4.7% and the HF is 5.4%...that's percentage of Xmax lost to gravity just from sitting there.

22x22x80 gives about 8.4 ft^3 gross...without doing any heavy lifting math I'd say your're looking at no more than 7.5 ft^3 net after driver/port(s)/bracing is subtracted. If the net ends up a little larger somehow, it's gravy as that will just lower your tuning point slightly for a given port length.

Is there a driver budget? The Tempest-X only has about 2.7% sag...with 27mm of Xmax, there's still plenty to work with. The DVC15 on the budget end of the scale, has 3.9% sag, but has been used numerous times in down firing designs.

Both look pretty good in 7.5 ft^3 tuned to 17hz. With 350 watts, output is virtually identical down to 15hz. That's about where the DVC15 will exceed Xmax for good if you don't use a rumble filter. The Tempest-X will keep on chugging down to almost 12hz before hitting Xmax without a rumble filter.

Where do you plan to set your rumble filter?

With a more realistic 7.5 ft^3 net, your dual 4" ports will need to be 28"...plus another 1" if you use PrecisionPort type flares. Many will probably disagree with me, but I'd just use one 4"x13.6" PSP including flares to preserve your net volume. Dual 4"x28" thin wall port, such as PSPs, will consume .46 ft^3 of your gross cabinet...standard PVC will bump that to ..52 ft^3. FWIW, I practice what I preach using a single 4" PSP with a 5.9 ft^3 DVC15...haven't noticed any port noise so far.

-Brent

[dreamofinsanity69]

is it high school or college? Are you going to be able to demonstrate the finished project for the class?

This is a high school project and I'll be able to use the Physics department's analog signal generator that goes down to 15 hz which might be kinda fun....


I remember now calculating that 5.4 % sag at one point and you're absolutely right Brent, as the RS390HF has low xmax to begin with. Well i guess driver selection is still up in the air but i'll be sure to look at the tempest-x and the dvc as alternatives.

The amp i'm almost positively using will be the rythmic audio 350 basic, is the 370 se worth the extra $$$?:

http://www.rythmikaudio.com/amplifiers.html

unless someone has a reason not too?


Unfortunately Brent you have some very good point about the ports and conserving my effective volume. However I'm almost certain a single four would be too little for the tempest x as at 30hz the air velocity is already 10 m/s and at 20hx its roughly 20 m/s. I'd predict much chuffing and port noise.

Any further advice is awesome as I'd love to make something i can use rather than just another project. I've always been somewhat of a bass head and love even the addition of a puny panasonic sub to any movie.

A few general questions aside;
- how are people able to upload response graphs from win isd...mainly how do you save it into a picture file from the program?
- can you use bends to make a longer effective port length

Thanks for the great pointers so far keep em comin

[arande2]

About bends in ports...

I don't know whether or you asking if bends make it longer, or if you can bend ports.

What I can say is that you can use bent ports (my JBL 4311s have bent ports) to increase length by fitting a longer length in. I'm not exactly sure how the bends themselves affect length.

I don't see a reason for the 370se. Unless you want more power (and you can get a lot more for the same amount of power on eBay), I'd stick with the 350, but it is your choice.

That's all I can contribute, really.

It would be fun to turn that up and rattle the classroom.

[avaserfi]

dreamofinsanity69 wrote:

A few general questions aside;
- how are people able to upload response graphs from win isd...mainly how do you save it into a picture file from the program?
- can you use bends to make a longer effective port length

If you want to upload the pictures I would print screen the graph and copy it into paint then upload to photobucket or whatever you use to post pictures.

As far as bends in ports you can use them, but try to use as gradual a bend as possible so that air flow is not constricted and distortion isn't added such as port chuffing. Have you thought about using a slot port? They add a little complexity to the build, but allow for longer ports in certain situations and if done properly you will have no port noise of course.

[brent_s]

What alignment and software are you using? WinISD Pro for Tempest-X in 7.5/17 with 350 watts shows 14 m/s @ 30hz, 29 m/s @ 20hz. Remember, that's a sustained power simulation...real world listening is dynamic. Drop the power to 200 watts which is still probably more than your steady state power on real content and also a less than 3dB reduction in SPL and the numbers become 11 and 22 respectively. I also recommended the PSP dual flared port, which will have lower air speeds than the straight ports that Unibox/WinISD simulate.

While lower velocity is better, even 29 m/s is less than the 34 m/s recommendation that Peerless published in a white paper that I can no longer find. Earlier versions of WinISD and other s/w used 34 as the suggested limit, probably based on the Peerless work. I know Collo's done some testing where the upper limit has been cut in half to 17 or so, but you should take your real world listening levels into account in addition to the volume lost/velocity tradeoff.

SVS uses a 3" PSP port in their PB10. If you sim a Dayton Titanic10, a fairly high excursion 10", in 2 ft^3 tuned to 20hz with a 3" port an 300 watts, you'll see almost 49 m/s at 20hz. That's going to be a pretty close approximation of the PB10's port behaviour, yet none of the reviews I've seen have complained about port noise in real world use. I'm sure you can cause noise if you try, but it's not been identified as an issue.

Same with my DVC15. In room at Reference level, I've never noticed any port noise with a single 4" PSP. Maybe it helps that I made the port down firing.

As far as the Rythmik amps, never used them myself, but never read anything bad about them. If you go with the Tempest X and the budget allows, you might want to look at the PE HPSA500 that's currently on sale for $240 and free shipping or the similar OAudio 500 watter...a little more power and a single band of PEQ on board. My biggest concern with the Rythmik's is no crossover bypass...doesn't seem to actually bother folks who've used them and other amps with the same "feature", but it's not something I'd choose.

-Brent

[dreamofinsanity69]

Alright so its an O Audio Bash 500 amp that is coming in the next few days but im still at a loss for driver choice. The rs390 15hifi seems to have very conservative xmax ratings and i hear that on top of it's 14.1mm xmax there is just as much until it bottoms. What does a driver start to do past "xmax" but before it bottoms?

- as far as the tempest-x goes...it likes a much large box then my 215l and seems to roll off really quick at 24hz which is undesirable.

any further driver suggestions guys? (budget limit 175)

One last thing...do i want more then 105-110 db to match a pair of technics tower speakers with 12" woofers from the 80's? what does that level feel like? Is there a driver that would provide output more like 115db in this boxx with 500w?

[avaserfi]

Having a driver exceed its rated xmax is one way in which distortion can be added to the system. In general I try to keep max cone excursion within 10% of rated xmax to avoid adding unwanted distortion.

[brent_s]

Xmax is intended to mean linear travel. The other figure you refer to is Xsus for suspension travel. The RS series are reported to have Xsus=21mm.

You will continue to get increased output beyond Xmax up until you go CLANK at Xsus, however, as linear implies, it's not as predictable. Distortion will rise both as excursion increases and the motor loses control beyond it's linear travel.

105-110dB measured how? I find 105dB peaks , not average, in room to be plenty loud and generally watch movies more in the 90-95dB peak range. WinISD sims for groundplane...essentially outdoors with no boundary reinforcement or room gain added in. For comparison, a typical gasoline push mower is mid 90dBs at 1m outdoors. If you've got an SPL meter, you could do a little measuring of every day events to get an idea of how loud a particular number is.

-Brent

[dreamofinsanity69]

Awesome thanks alot guys it's been very helpful in making decisions and i have a better understanding of many things after all of this research I hope i've made a good choice with the o audio amp and i believe i will do my best to acquire a tempest-x for my enclosure. Wish me luck!
Ben