250Ti - effect of stuffing the port?

[Andyoz]

Hi,

My 250Ti's were moved to a new listerning room 4 months ago. The room is big and boomy (i.e. lots of heavy masonry and plasterboard surfaces).

I am limited as regards placement and they will have to stay about 200mm from the rear wall. Before I start experimenting with equalisation, etc. a quick question:

1. What is the effect on the low-frequency response if I stuff the reflex ports so they become a sealed enclosure?

I know this is heresy but at the moment the bottom end is overpowering everything else and I am not enjoying them.
I will obvously lose extension and efficiency(?) but these things are rarely cranked up and I want a more balanced "in-room" response at low-to-moderate listening levels...

[Andyoz]

In my youth, I sort of understood some of the speaker enclosure alignment parameters, etc. but I will plug them up and see what happens. I like to know a bit of theory about what the changes will be though.

I can measure the in-room response but that involves taking my PC, sound card, microphone downstairs and I really am getting lazy lately!

Will just try it and see...

[Robh3606]

Run it in WinIsd and see.

http://www.linearteam.dk/default.aspx?pageid=winisd

Rob

[boputnam]

Run it in WinIsd and see.

+1

[JBL 4645]

I’d guess it’s peaky somewhere between 35Hz.

Join HTS and then get REW for free run a frequency sweep of left and right separately then compare the two graphs to see where peaks are.
http://www.hometheatershack.com/forums/downloads-area/19-downloads-page.html

You don’t need to run them at full throttle the peak will still exist virtually at any level.

[rdgrimes]

The 250s are VERY strong in the 40-60 Hz range in my room. They need attenuation of up 6-10db for flat response in that range.

[scott fitlin]

OR, you could look into a decent parametric EQ, to tune the in room response. Used properly, I think this will yield better performance.

You can also go to this site, www.marchandelec.com/wm8.html. He mAkes a correction EQ unit for sealed speakers that he sas works well to tame the peakiness and boominess of vented speakers, too.

The Marchand products are available in assembled or kit form, are economically priced, compared to many other brands available, and sound very good. but, most of all, this page offers some decent reading on exactly what your talking about.

[Andyoz]

I tried stuffing the ports last night.

Quite a massive difference really in my particular rooms. The really low "thump" I was getting is tamed and that allows the upper bass to come through with more clarity. That's good.

I will take the measuring equipment down when I get a chance but it appears to have done what I wanted.

Overall bass output is reduced, but that was never an issue in my room

[duaneage]

Just be advised that going from a bass reflex to a sealed system changes the woofer behavior near resonance. The woofer will excursion will be greater at the resonant frequency than if it was vented.

Vented boxes typically use speakers designed for vented applications while sealed box systems conversely use drivers designed for that purpose.

[Andyoz]

Yes, I was reading about that....the port at resonance gives rise to a high load on the woofer diaphragm and reduces excursion. I only play at low to moderate levels and the 250's are barely coasting along to be honest. The bottom end definitely sounds tighter in my particular room - less extension of course though..

One thing I never really understood was the issue of woofer excursion at frequencies below port resonance. As I understand it, the woofer is essentially unloaded (the port acts as a hole in the enclosure) and can be subject to high excusrions if the source has those frequencies present and the amp is beefy enough to drive the woofer. Without a high-pass filter in the system, how come there is not woffer damage?

[grumpy]

There certainly could be woofer damage if there was enough music power in the
frequency region sufficiently below resonance to do the deed. Often, there is
not. You -will- notice warnings/cautions to use high-pass filters on many ported
pro systems for the precise reason you've stated.

A poorly isolated turntable system can cause low-frequency feedback and
fully "exercise" a woofer's suspension before the owner notices... but this
is at a frequency below normal music (certain genres and test recordings
excluded).