Question about enclosures

[davidlzimmer]

It has come to my attention that speaker manufactuers recomend less volume for a sealed encloser than a vented encloser for the same speaker.

I'm not fighting it. I accept it. But, it just seems to me that a sealed enclosure would require more volume because of the compression. I remember some old "infinite" baffles being huge. Obviusly, I'm clueless.

Can anyone explain why this is in "dummy" terms. I'm talking mainly about mid woofers.

Just curious.

Thanks,

[sreten]

Hi,

Download WinISD and play with it, you'll soon see,

(Basically a reflex needs the driver and box if sealed to have
a lower Q than a sealed box = a bigger box. Drivers with Qts
over 0.6 are almost impossible to reflex sensibly. Drivers with
very low Qts, less than 0.3, generally need to be reflexed.
Try a driver in WinISD with Qts around 0.4)

/sreten.

[richie00boy]

Adding to what sreten said, the compression in a vented box at the vent frequency is higher than in a sealed box.

[davidlzimmer]

I said DUMMY terms. :-)

[sreten]

OK,

(the following is not strictly true, but the truth is equivalent)

At port resonance the driver and port move in the same direction,
the port moves further roughly displacing as much air as the unit.

So its like having two units in the box, so a reflex needs ~ twice the box volume.

/sreten.

[davidlzimmer]

sreten,

Ok. I think I understand that.

Thanks,

[davidlzimmer]

So, a ported box is more effeciant than a sealed. Correct?

Then why would one choose a sealed over a ported?

[dfdye]

Cause it sounds better!!!

Seriously, there are those of us who don't like how the port resonances sound a little "sloppy" for lack of a better term. It's way past my bed time, so I'll leave it for others to elaborate on, but there is a definite coloration added to the sound of a set of drivers when they get close to the port frequency IMO (of course, like anything, YMMV and different systems exhibit this to a different extent, etc.). I personally have had better luck with sealed enclosures, with the notable exception of my set of aperiodically damped home theater speakers, but that's a discussion for a different day.

Remember too, that the efficiency "benefits" only come at resonant frequencies, meaning that often the boxes are "tuned" to amplify the bottom end of woofer response. This can sometimes leave the impression that the resulting bass is more boomy than tight/accurate. Like I said, somebody will elaborate on the details, I am sure, but these are some of the qualitative tradeoffs I have noticed.

[BobEllis]

Originally posted by davidlzimmer
So, a ported box is more effeciant than a sealed. Correct?

Then why would one choose a sealed over a ported?

Efficient isn't the right word. You can (if the driver parameters are appropriate) get better bass extension without equalization (which adds to the power required) at the expense of box size. The standard bass extension - sensitivity - small box trade off. Pick any two.

dfdye, As for boomy reflex boxes, perhaps your experience has been limited to boxes tuned for flat anechoic response down to somewhere in the neighborhood of 30 Hz. In most rooms, interaction with room boundaries will cause a lift in bass response. So, if you put an anechoic flat to 20 Hz speaker in a real room, chances are you'll have rising response on the bottom end, sounding boomy and unnatural.

The solution is to design the ported box as part of the box-room system and tune it a bit lower than the standard QB3 alignment proposed by most design programs. As Sreten has proposed many times, a few dB droop starting at 100 Hz or so sounds more natural than anechoic flat response.

Most unequalized sealed boxes start rolling off relatively high and room lift takes care of adding extension.

Of course, there is the ported boxes have greater group delay issue. But, as far as I know, the jury is still out as to the audibility of group delay at low frequencies. My EQ'd flat in room to <20 Hz subs sound quite natural and reproduce the 1812 overture cannons quite well despite group delay peaking at 50 ms. I'm slowly working my way towards a sealed/Linkwitz transform box to see if it can be improved but current performance isn't a real issue.

[davidlzimmer]

dfdye,

That's the answer I was "hoping" for. I've always thought that sealed boxes seemed more real or alive sounding.

When I was very young (before most on this forum were born), I had an opportunity to compare side by side the two types of enclosures.

The sealed encloser was a Bozak (no NOT Bose!) and the bass reflex was a Jenson system. Not really a fair comparison since they were different speakers. Anyway, we recorded an ensemble using an Ampex recording system and played back on a McIntosh 240.

Little did I realise this would be one of the best systems I would ever hear in my life! Anyway, I voted for the sealed encloser. It seemed to play back exactly like the original sound.

Ahh the 60's!

I lost the vote and the school bought the bass reflex box.