A few Newbie questions

[Joshua_R]

Greetings Everyone! I've been lurking for a few weeks while contemplating the design of a new sub for my home and thought I'd jump in and ask a few questions...

First off, I've read somewhere that polypropylene subs are less than desirable since plastic can flex and creep over time... Like a speaker cone made of tupperware. What are your feelings on this?

My intended design is an NHT 1259 (8 ohm) mounted in a 50 liter sealed box with "heavy" fill as recommended by BassBox6. According to BB6, this will give me a Qtc of 0.86 and an f3 frequency of 36.14 Hz. The 1259 has a polypropolyne cone...

Also another question... I will be driving this with a parts express 300-794 plate amp (150W into 8 Ohms) and it will be situated in a 14' x 14' x 8' room 12" from the left wall and the back of the sub about 5" from the front wall. Is this f3 too low based on how much room gain I will expect? I'd say the room is acoustically neutral as a guess.

The sub will be used for both music and home theater. The receiver is a JVC with a subwoofer output that (if I remember correctly) is crossed at 100 Hz. The main speakers are Boston Acoustics Lynnfield VR's (750's I think). Based on this description, what do you all think?

Thanks in advance for the input! Since this is my first speaker-building project, I want to make sure I do it right!

-Joshua

 

[AndrewT]

Hi,
I'm no expert here but no one else seems prepared to talk to you.
I fully expect some of my thoughts to be shot down in flames and you will have to filter the advice.

Q=.86 seems high. This will lead to a bass hump that gives apparently stronger bass but leads to a one note type of response. More suited to small speakers that desperately need bass help.

What is heavy fill? Why not settle for conventional filling like dacron fibre, long hair wool and other light weight fibres. They give a bit of damping and a small apparent increase in box volume.

In your smallish room that bass hump will sound worse and you are compounding it by placing the speaker in the corner (400mm is quarter wavelength of 200Hz).

If you choose a q<0.7 you may be able to live with corner placement.

Try to find a formula for room gain. It relates room volume to speaker size (Xmax*piston area?) You may have very little room gain with your chosen speaker unit.

Don't worry about polypropylene, many materials are used for cones and NHT have a fair reputation for good bass units. There is a good forum for sub-bass design that likes NHT, do a search I cannot remember the site.

 

[BassAwdyO]

hello,


AndrewT has some good points.

Qtc - .86 is a bit high, but not outrageous. Some people prefer high Q, some prefer low Q. From what I've read of may people's opinions a higher Q will tend to sound boomier and more "one note" bass. Low Q ~.5 will be tight and accurate, but perhaps a bit "dry" for some.

A f3 of 35 hz is pretty good and in room you should have response into the mid 20's probably.

The lower the f3, the better!

Corner placement will add a bit more SPL, but it excites room modes maximally, and unless you're really really really lucky you're going to have a few spots in your room which have hardly any bass, and a few spots which are basstastic(maybe too much so)

Sub placement is something you should play around with as much as possible to find a spot that you think sounds best. If you have a specific listening area, make sure the bass response is best there and dont worry much about the rest of the room. There will surely be a few modes.

Try it out. If you dont like the enclosure/placement you can always change it, or design and build another. Thats most of the fun anyhow!

 

[BobEllis]

I'll second the lower the Q advice - while it will sound a bit boomy. Get it down to .7 and it will probably integrate with the room gain better. Once upon atime, there used to be a generalization that rockers liked Q's around .9, classical fans prefer .5 and Jazz fans are in the middle somewhere.

You might consider building an ugly prototype box big enough to get you a Q=.5 alignment. With a box that is bigger, it is easer to get higher Q to experiment by putting bricks or other solid objects in the box to reduce the volume. Once you've found the Q that suits, build the finished box to size.

Have you checked Adire's drivers? The 1259 is a decent driver, but a bit dated. I haven't heard either though. I'm "stuck" in the stone age with a pair of JBL 2245H 18" subs.

Edit: composed while another vote was being cast, so make that a third for lower Q

 

[Joshua_R]

Follow-up

Ok... Here's a follow-up question: In order to get the Q value down, I go back into BB6 and increase the box size. What is the acceptable guide regarding the cone displacement and Xmax? For example, If I increase the box size to 100 liters and switch to the 4-ohm NHT driver since it has a lower Q, my Qtc drops to 0.606 and my f3 goes to 33.69 Hz. The problem I see is that the cone displacement exceeds Xmax at around 25 Hz with a 250W input. What is the acceptable frequency to allow the cone displacement to reach Xmax?

Any other drivers in particular you'd recommend? I've looked at peerless XLS, Dayton Titanic, Dayton DVC, Adire, Sonicraft, NHT, etc. As before, I'm looking to build a sealed sub for music and HT with a final volume of 50L or so. (using the PE 300-794 plate amp.)

Thanks!!!

-Joshua

 

[mike.e]

Re: Follow-up

Ok... Here's a follow-up question: In order to get the Q value down, I go back into BB6 and increase the box size. What is the acceptable guide regarding the cone displacement and Xmax? For example, If I increase the box size to 100 liters and switch to the 4-ohm NHT driver since it has a lower Q, my Qtc drops to 0.606 and my f3 goes to 33.69 Hz. The problem I see is that the cone displacement exceeds Xmax at around 25 Hz with a 250W input. What is the acceptable frequency to allow the cone displacement to reach Xmax?

most bass content is between 40hz-60hz. But HT will reach as low as your source will go,ie DC.

Any other drivers in particular you'd recommend? I've looked at peerless XLS, Dayton Titanic, Dayton DVC, Adire, Sonicraft, NHT, etc. As before, I'm looking to build a sealed sub for music and HT with a final volume of 50L or so. (using the PE 300-794 plate amp.)

Thanks!!!

-Joshua

Those drivers are all pretty much the same apart from the XLS which has flux shorting ring. see mfk-projects.com for measurements of several.

Lastly I wouldnt use sealed with HT.Or I would,with great care

 

[BobEllis]

Re: Re: Follow-up

snip

Lastly I wouldnt use sealed with HT.Or I would,with great care

I am curious. Why you say this? With HT often providing a signal approaching DC, it can easily get below the tuning frequency of a BR system, unloading the woofer, potentially leading to overexcursion. Sealed stays loaded, making overdriving the woofer more difficult.

If you are saying that BR will be punchier and give more satisfying LFE, I'll agree with you.

 

[lynchingacers]

it might not offer the low end tuning frequency u might want but it probably be less boomy depending on size i think