With open back, you have less bottom end, more open sound, not quite as loud in front, and anything behind them affects the tone.

The answer to the question of which sounds better is totally subjective. It is the entire system (hands, pick-up, cables, strings, preamp, power amp, room, etc.) that must be taken into account. Although, I think that the speaker itself has the most to do with tone, (all other things being the same). The cabinet really needs to be designed around the speaker. I think that using the accepted Thiele-Small parameters for porting is not what you want to do for a musical instrument cabinet. Maybe for bass guitar it's OK but for steel it's all about shaping the low frequency tone, not reproducing it in a linear fashion.

The closed back cabinet will only reduce high frequencies behind the cabinet. Mids and lows will radiate around to the back. The best steel tone I think I have ever heard was directly behind Paul Franklin's closed back Marrs cabinets. Really solid lows and mids with just enough highs. Awesome.

I like how the low frequencies sound in closed back cabinets (tight and punchy) and how the highs sound in some open back designs (smooth and silky). The jury is still out on mids. I think what I like about the highs in the open back design is the cancellation of some mid and high frequencies when they bounce off of a rear wall and interact with the direct sound.

Believe it or not, electro-acoustics is really complicated stuff and I am really enjoying the quest to understand it a little better.
TC

Exponential horns and piezo's (for example) don't need it because their "backsides" are enclosed.

The most popular "nearfield" monitor in the world is the Yamaha NS-10, you know, those black speakers with a white paper woofer that are in just about every studio. Those also are not ported. They have awful bass response. To use them correctly, you either have to touch the cone with your hand to feel the subs, or you need to add a subwoofer. Many people will use the NS-10 to get the mix right, and then use a more full range system to hear the deep lows and the more detailed highs. Most other "nearfield" monitors will likely be smallish speakers but with ports to extend the bass response.

Nearfields are good at creating a nice, accurate sound image at a close distance, like 2 to 5 feet away. Midfields or Farfield monitors are generally larger and need to be placed at a greater distance from the listening position to create a good and useful image.

As far as bass guitar cabinets go, it only makes sense to port them to extend the response, or else it would take HUGE cabinets to get the deep bass.

As far as open vs. closed back for steel, I've become a convert to a degree. I used to swear by open back cabinets, but in recent months I've had the pleasure of checking out some of TC Furlong's SPLIT cabinets. The models I've been messing with are closed back, front ported, 12" cabinets. The sound is really good and even, and from the sides and behind it's pretty balanced too. Big fat, punchy bass response. I've noticed that my sound from stage to stage is more consistent with the sealed back cab. It seems that an open back cabinet puts out so much sound out the back, that the acoustical character of the stage becomes a far more significant variable in my tone, where the sealed cab is pretty consistent no matter where I use it. I still like both, but I'm definitely hot on the sealed thing.

Brad

[This message was edited by Brad Sarno on 20 June 2006 at 01:03 PM.]

The typical open backed instrument speaker is a compromise between size and efficiency. It’s folded-back wall is big enough to eliminate most, but not all, of the wrap-around cancellation, which mainly effects the lows (so not a big problem for guitar or steel). Also, almost half of the sound is going out the back of the amp. That sound going out the back is strongly affected by walls, curtains, space, etc. behind the amp. Also, there is no air pressure to dampen and protect the speaker from long bass excursions. This is okay for a guitar amp, as long as the speaker handling capacity is matched to the amps power. It gives a room filling sound in small rooms, and the sound can be heard behind as well as in front of the amp. Open back guitar amps were originally designed to be placed in front of the musician. This arrangement minimized feedback from a mic or the early archtop electric guitars. And the open back acted as a monitor, so the musician could hear himself even though the amp was facing away. Also, an open-back combo is cheaper to make, because you don't have to seal off the tubes and chassis from the sound pressure buffeting they would get if they were inside a closed-back cabinet.

The bass range of a bass requires so much power that efficiency becomes a major concern, so all wrap-around cancellation (which strongly effects bass notes) must be eliminated. Also, those low notes are the ones that will destroy an open back speaker. Therefore, a closed back cabinet is better for bass. It needs to be big, so the pressure inside that protects the speaker will not greatly lower the efficiency. One or more ports on the front preserve enough air pressure to protect the speaker, but kick the sound inside the cabinet out the front, thus increasing efficiency. After bouncing around inside the cabinet, the highs and mids are not coherent enough for any cancellation to occur. Putting in some baffling material on one of each set of opposing sides helps eliminate “boxy” sound that might result from midrange reverberation. The low resonance that results from the free resonance frequency of the speaker, combined with the resonant frequency of the inside of the cabinet, and path size of the structure leading to the port, can be tuned to match the shoulder where the speaker’s bass response begins to fall off (the opposite of cancellation). The bigger the speaker, the lower the shoulder will be, and the bigger the box must be to get a low enough resonance to enhance the shoulder. Placing a “reflex” tube or passageway behind the port or slot both minimizes any bass frequency wrap around and further drops the cabinet resonance frequency without adding cabinet size. Therefore, a closed-back reflex ported design (Thiele design) is a good compromise that is highly efficient, and is big but not huge. The trick to tuning these is to reinforce the shoulder smoothly, without creating excess resonance at any particular frequency. We’ve all heard that one note that booms out with cheap bass rigs.

A closed-back bass reflex or Thiele design for guitar or steel does not have to be as big as a bass guitar cabinet, simply because the strings are shorter, and not as low frequency. Therefore, the fundamental of the lowest string is not as near the shoulder of the speaker response. Reinforcement of the shoulder is less of a concern, so you don’t need a huge cabinet with a low resonance frequency and long reflex path. That’s why something like the Marrs cabinets work for steel, even though they are smaller than bass cabinets. The closed-back models protect the speakers, eliminate wrap-around, and kick all the sound out the front with good efficiency. Some closed-back steel and guitar speaker cabs just have port holes in the front baffle, with no reflex tube. I have found that putting tubes on those eliminates some bass wrap around and improves the bass response a little. Another trick I have used is to take the amp chassis out of a combo (to put in a head cab), put a closed back on the old combo cab, and use the open chassis slot as a port. That’s a pretty big port, with substantial bass wrap-around, so I use a 6-8” wide board to make a shelf below the port and leave an inch or two clearance between the back of the board and the cab back. This creates a reflex path that eliminates some wrap-around and noticeably improves the bass response. My reflex ported Marrs cabs and converted reflex combo cab are not precisely tuned. So they may have non-optimum shoulder response or uneven resonances for a bass, but if so, these problems must be below the lowest string fundamental on my uni pedal steel, because I don’t notice anything wrong down there. These cabinets are very efficient and have great lows for C6 or uni. With two Marrs type cabs, you don’t have to worry about directionality – you just angle the two speakers any way you want.

I don't know the meaning of "near field" and "far field," but it sounds like it has to do with how far you intend the listener to be from the speaker.

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Student of the Steel: Zum uni, Fender tube amps, squareneck and roundneck resos, tenor sax, keyboards

[This message was edited by David Doggett on 20 June 2006 at 05:43 PM.]