Open vs closed back cabinets

[Cliff Swanson]

I have no experience with closed back cabinets, so I have no personal reference for the claims that are made regarding how closed backs can sometimes cause interference with cone movement and frequency response in a cabinet. I'm soliciting opinions and advice about using closed back, ported cabinets vs open back. I understand the relative differences in directional broadcasting between the two types. I'd like to know if any of you have strong opinions pro or con about closed backs and the basis for those opinions if you do.

Thanks in advance.

Cat

[Matt Farrow]

OK, here's the deal on cabinets - if I say anything wrong, someone correct me.

Newton's 3rd Law: for every action, there is an equal and opposite reaction.

Therefore, for every excursion your speaker makes, pushing air forward out the front of your cab, it pulls an equal amount of air in from the rear.

Also, for every incursion your cone makes, pulling air in from the front, your speaker pushes an equal amount out the back.

The ideal speaker box would be a plane of infinite height and length with the speaker mounted in it. There would be an equal amount of air on both sides of this plane, and no way for air to move beyond either side. You would be able to hear the sound of the speaker by itself. Oh, the plane would be immovable, too.

Well, that's never gonna happen, is it? So we compromise. In an open back cab, some of the air will make it around to the front of the cab, and if it is in phase with the air being pushed out of the speaker at that particular frequency, then the air from the rear will reinforce the air from the front and make the volume louder. Most of the time, however, the air from the back is out of phase on low frequencies and in phase at high frequencies, thereby cancelling out some low end. The solution?

Sealed cabinets. In a sealed cabinet, there is no way for out of phase speaker energy to cancel out the sound. However, depending on the cabinet design, driver type, and lots of other factors, the amount of air that the speaker can move is compromised. Remember Newton's 3rd up there? Well, if you run out of air in back, then you run out of air in front. So the cabinet can act to "damp" the speaker movement. The answer?

Ported cabinets. You take your standard sealed cabinet and cut some holes in it. If you tune the cabinet properly, you can get a boost at certain frequencies.

So that's it in a nutshell. I'm no acoustic engineer but that's the basics. It all comes down to cabinet design and your T-S (Thiele-Small) parameters as to what your cab will sound like. There's a copy of a JBL book on the web with cabinet designs that work with older JBLs, you could try that. I'll try to find that link in here and post it.

Matt Farrow

[Bobby Lee]

<SMALL>Ported cabinets. You take your standard sealed cabinet and cut some holes in it. If you tune the cabinet properly, you can get a boost at certain frequencies.</SMALL>

Normally, a closed cabinet is ported to counter the natural boost that happens at the cabinet's own resonant frequency. The size of the port (hole) as compared to the size of the box will have a damping effect at a certain frequency. Sizing the port properly helps to achieve "flat" response.

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<small><img align=right src="http://b0b.com/b0b.gif" width="64" height="64">Bobby Lee - email: quasar@b0b.com - gigs - CDs
Sierra Session S-12 (E9), Speedy West D-10 (E9, D6),
Sierra 8 Laptop (D13), Fender Stringmaster D-8 (D13, A6)
<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Bobby Lee on 08 December 2000 at 09:08 AM.]</p></FONT>

[Jim Smith]

I'm no acoustical engineer by any means, but my Stereo Steel cabinets sound great to me. I guess they fit the definition of "ported". They are closed back with four ~2" holes in the front panel.

[Jack Stoner]

One of the JBL cabinet design manuals, availble on line, has a forumula for determining the porting size. One interesting note on the porting is that JBL says it doesn't matter where it's placed on the speaker mounting panel (top, bottom, sides), or whether it's round or oval, rectangular, etc., and it can be several smaller porting holes as long as the total is equal to the disign number.

[Dan Tyack]

A closed back cabinet with a port is capable of more bass response and is more efficient than a typical open back cab. The main drawbacks to closed back ported cabinets that I have found are:
1) they have much less on stage sound dispersal, making it much less likely that the musician standing 20 feet to your side can hear you well.
2) most of the cabinets I have played through sound 'thumpy' and 'flat' to my ears (those are subjective not technical terms).

The reason for the latter is that most ported cabs are designed using calculations supplied by the manufacturer to ensure flat frequency response. The main problem with this is that the last thing you want with a musical instrument is a flat frequency response.

I have two ported cabinets built by THD Electronics of Seattle that are ported, but they are designed not to be flat, but to accentuate the frequencies that are most 'musical'. In appearence they look like an open back cabinet with a small opening. I love them, because they give me an increadible low end punch not possible with an open back cabinet, but yet they have pretty good stage sound dispersal.

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www.tyacktunes.com

[basilh]

Cliff,

I've been playing steel since 1952.....and have at one time or another, owned most of the commonly used amp/spkr combinations

OPEN BACK 15" JBL's Give me the sound and PROJECTION that I like

Baz

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Basil Henriques
Emmons D-10 1970
and
Emmons D-10 1970 "Anniversary"
1949 "Leilani"
1949 Dickerson
RICKENBACKER "Olde Uglie" Twin 8
"Fender 1000"

<SMALL>Steel players do it without fretting</SMALL>

http://homepage.tinet.ie/~basilhenriques/

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