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-   -   to vent or not to vent ? (http://www.diyaudio.com/forums/full-range/137108-vent-not-vent.html)

Bigun 22nd January 2009 07:02 PM

to vent or not to vent ?
 
I have been thinking about a Fonken for an HT set up. Today, I realized that my HT set up will very likely incorporate a sub, which means I don't need to stretch the LF performance of the main speakers. In fact I don't really want them interfering with each other do I.

I'm new to this game so my thoughts may be off-track - here goes:

Venting allows me to get more LF with some tradeoffs. Most of the tradeoffs are ok, but I am lead to understand that a possible issue is that the cone is left 'unloaded' at low frequencies where excessive signal would drive the cone hard against it's xmas and produce distortion in the mid frequencies. This could be a scenario with an HT set up if I'm not there to supervise !

So I go to thinking that maybe an FE127e sealed box would be a better approach as it keeps the cone better loaded at LF and the HT sub fills in for the quick fall off at 100Hz or so?

I understand that one may have chosen a different driver for such an enclosure type, but the driver is already purchased.

Perhaps 100Hz is too high a cut off for the sub in that it may be too directional and not 'disappear' as effectively as the more traditional 80Hz cut off ?

I'm not sure if the resistive nature of the Fonken ports provides more effective loading at LF than a 'regular' BR box and perhaps represents a compromise that is the best of both worlds ?

I'd welcome opinions on the above ?

p.s. I'm still looking for a free excel based vented box calculation spreadsheet that shows me the frequency response of the speaker whilst allowing me to specify the box volume.

p.p.s. I try to be diligent in doing a 'search' before I start a new thread, but I find the search engine digs up too many results without a good match to my search expression, am I the only one ?

germpod 22nd January 2009 07:40 PM

I have the same problem with the "search" function. I do my best to add words to get it narrowed down, but I still fail at it quite a bit.

chrisb 22nd January 2009 07:58 PM

Re: to vent or not to vent ?
 
Quote:

Originally posted by Bigun
I have been thinking about a Fonken for an HT set up. Today, I realized that my HT set up will very likely incorporate a sub, which means I don't need to stretch the LF performance of the main speakers. In fact I don't really want them interfering with each other do I.

I'm new to this game so my thoughts may be off-track - here goes:

Venting allows me to get more LF with some tradeoffs. Most of the tradeoffs are ok, but I am lead to understand that a possible issue is that the cone is left 'unloaded' at low frequencies where excessive signal would drive the cone hard against it's xmas and produce distortion in the mid frequencies. This could be a scenario with an HT set up if I'm not there to supervise !

So I go to thinking that maybe an FE127e sealed box would be a better approach as it keeps the cone better loaded at LF and the HT sub fills in for the quick fall off at 100Hz or so?

I understand that one may have chosen a different driver for such an enclosure type, but the driver is already purchased.

Well, you could certainly find no shortage of suggestions for different drivers - including Fostex (i.e. the FE167), but you could certainly do worse.

Quote:




Perhaps 100Hz is too high a cut off for the sub in that it may be too directional and not 'disappear' as effectively as the more traditional 80Hz cut off ?

that could depend on your room, but quite possibly so - one reason why a lot of folks recommend (at least) 2 powered woofers. I've had plate amps with XO's as high as 120 or more - "sub" is a bit of a misnomer at these frequencies

Quote:



I'm not sure if the resistive nature of the Fonken ports provides more effective loading at LF than a 'regular' BR box and perhaps represents a compromise that is the best of both worlds ?



I think this has been discussed in another thread, but the short answer to this one is "yes, in fact quite effective controlled loading" - based on some quickie experiments with a small FE127 design, I'd tend to say - "not much of a compromise, actually..."

sreten 22nd January 2009 08:18 PM

Hi,

http://home20.inet.tele.dk/kou/ubdwnld.html

As long as your HT set up does high pass filtering (set to small)
for the speakers venting can be used to increase power handling
and maximum levels around the cut-off, low frequency unloading
will not matter with the hp filter. Fostex driver excursion needs
all the help it can get, Qts is usually too low anyway for sealed.

:)/sreten.

Bigun 22nd January 2009 09:19 PM

Thanks guys.

One thing these box calculators exclude is the affect of the 'Onken resistance' in the performance of the system. Given that the Base Reflex calculation is based on a Helmholtz oscillator it should be a simple matter of adding a resistor (R) to the equivalent circuit diagram and comparing the results with R=0 and R>0 to see what happens to the frequency response and phase information. If I was one of those people who had a spice simulator and knew how to use it we might get some quick data.... of course it's not that simple to determine the correct value of R for a given vent configuration but at least it would reveal the general behaviour ?

Call me lazy, but I haven't played with Spice for 20 years and so I'd have to start from scratch as well as find a free version for my iMac. Perhaps somebody out there would like to pick this up? OR maybe it's just wasting time when I could be experimenting - I heard that some mail hs arrived at home for me today - hopefully my Fostex drivers, so one less excuse for building something !

chrisb 22nd January 2009 10:02 PM

Gareth:

to follow up on my final point on post #3 above:

http://www.diyaudio.com/forums/showt...02#post1719202


so in other words, sometimes the math only answers part of the question, and only at specific pressure levels - the highly dynamic nature of musical signals can mean that the enclosure's tuning might only be "perfect" at a particular volume level

Bigun 22nd January 2009 10:40 PM

2 Attachment(s)
Hi Chris,

so all these graphs and plots and calculators and discussions about ripples and curves and bumps... gets us maybe only half way there :bawling:

but on a practical note, I see the danger of going around in cirles here, the key things I need to take away are:

a) there's enough benefit in providing vents that I should do so
b) the vents will extend the LF enough to avoid placing more difficult constraints on the sub, will improve the power level and is probably more suitable for my chosen driver
c) the Onken style vent is the optimal method for venting in this case and I can employ some HF filtering (either via the receiver or more directly) if high LF levels cause problems
d) unless following a proven design, don't be shy to the idea of doing a bit of post-build 'tuning' on the ports
d) the music will speak louder than the maths


And having opened the box but 20 minutes ago !....

rjbond3rd 23rd January 2009 12:22 AM

Hi Bigun, in terms of simple, easy newbie-friendly tools, there are lots. have you tried WinISD or Unibox?

The search function works flawlessly for me -- are you choosing "Show results as posts" rather than the default of "Show results as threads"? Within a very long thread, there's also a "Search within this thread" at the bottom of the page, right-hand side.

Bigun 23rd January 2009 01:42 AM

2 Attachment(s)
HI,

That's a very handy tip on the search feature, I believe that will answer to the problem.

Couldn't resist playing with the calculations. I also found something helpful at :http://micka.de/org/en/index.php#ideal

I extracted the dimensions from published data for the planet10 Fonken and mFonken which use the same drivers I have. Looking at the Fonken, with Vb=13L, port length of roughly 26cm and an effective port equiv. diameter of 7.8cm I find that the frequency response looks very close indeed to the recommended vented box frequency response for this driver using the calculator on that web site I just quoted. This makes sense, confirming that the Fonken is set up like a BR.

Then I used the data for the mFonken, Vb=4.5L, port length roughly 16cm and a port equivalent area of roughly 4.3cm. I tried varying the port dimensions to convince myself that this was the optimal in terms of flatness of response and I think it is. So again, a nicely designed box set up like a BR.

However, with the smaller box of the mFonken there are limitations and the frequency response doesn't offer a 3dB point much lower than a sealed box. I'm not convinced that the venting of this box provides much benefit in real life. I haven't read much about what people think of the sound from the mFonken. It looks a very tight box with this driver. I do read many good things about the Fonken, but with that design, the box volume really allows this thing to perform.

Here's a plot showing my attempt at modelling the mFonken (and what I learn is that for small HT system I may not want to vent after all, or if I do then the box volume should be made large enough - which I think I have space to accommodate)

chrisb 23rd January 2009 05:13 PM

Gareth - just for kicks, you could build a pair of the mFonkens* ( a far simpler build than the Prime), and report back on how what you hear in the real world relates to what the modeling shows in cyberspace.

You could find it's not exactly "set up like a(n average) BR". Is there any software capable of modeling with the type of highly dynamic and spectrally complex signals that we actually listen to?


* if you do, please note that the driver bracing is important, and the box is small enough that lining with damping material is probably easier if you install the back panel last.

As for not reading much about what people think of the sound from the mFonken, certainly as compared to the "Prime" - that's likely as there have been few, if any, of the small ones yet built (other than our own the 3 or 4 pairs), while at last count there are probably over a dozen pairs of the Fonkens by DIYers. Why not be the first report your impressions on the little guys?


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