If this is related, then I've found it back
SIMULATING OPEN BACK CAB
- Jay M: I had an experience last night that serves as a textbook example what lots of open cab players hear when they talk about the "magic" of this kind of cab. I'm currently playing with a 75-yo bluesman who sings and plays harp and occasional guitar. His amp is a Fender Blues Deluxe. Last night we played in a barn of a bar, with a rectangular shape, high ceiling (~20 ft.) and a ~50 ft. distance to the rear wall. The front man sat in front of the drummer with his amp just to his left. I ended up standing to one side of his amp, exactly 90 degrees off axis. The room was extremely live, and the reflection from the far wall was clearly audible while we were playing. By contrast, the wall behind the bandstand had heavy drapes and fabric coverings and was not very reflective at higher frequencies. Here's the interesting part: in a room this size, the reflected sounds arrive beyond the Haas window (~15-20ms after the initial direct arrival), so you can easily localize them. The sound I heard from the front man's guitar was coming entirely from the back wall. I was hearing almost nothing directly from his amp. Standing next to a source of sound but hearing it entirely from an echo from a surface that is more than 50 feet away is a pretty dramatic effect. :shock:
I've pointed out a number of times before that an open back cab is an acoustic dipole. The primary characteristics of an acoustic dipole are cancellations of lower frequencies - how low depends on the size of the cab - and a "figure of eight" radiation pattern which has a near-perfect null (zero radiation) in the plane of the baffle. A player standing in a typical off-axis position will hear higher levels of reflected sound than direct. In a small room, the reflected sounds will not be localizable - because they arrive too soon after the direct sound for your ear-brain to be able to process them as separate events - and it is therefore difficult to impossible to identify exactly what is happening using unaided hearing. In a larger room such as the one we played in last night, the effect is dramatic and easily audible.
Here are some of my observations. I've offered these before, but last night's experience reinforces their relevance.
1. The effect of an open back cab is highly dependent on room acoustics. In many cases, such a cab will create sonic problems, but, because they primarily affect the audience, the player may be unaware of them.
2. The "magic" of an open back cab - when it happens - is experienced exclusively by the player and (possibly) others on the bandstand. The audience hears a mix of direct and reflected sounds that is as much a function of the room as the cab. They won't get the "magic," only you will.
3. Achieving a suitable balance is made more difficult by the use of open-back cabs. Because the player is often located in or near the null in the figure-of-eight directivity pattern, he may be playing much louder than he thinks.
4. Because the sound the player hears from an open back cab can consist almost entirely of reflected energy, his tone as heard by the audience may be radically different than what he hears.
As I have pointed out before, if you want to create the above effects with an FRFR system, you can do so with a second, rearward-firing cab - which need not be FRFR - operating in reverse polarity from the main system. With this rig, you could play games with the depth and orientation of the null and the figure-of-eight lobes that characterize an acoustic dipole. If you use open back cabs, an awareness of the above facts can be helpful in producing the best possible result.
- JM, 1/2012 TGP: A cabinet with an open back is an acoustic dipole. Its effect depends heavily on local (i.e., stage) acoustics, is not always desirable (in my case, never so for dry amp/cab sound), and is only experienced by the player and others on stage with him. If I want the effect of a dipole, I can (and do) easily create it with an FRFR system, either my W/D/W rig or a mono cab with an auxiliary rear-firing, reverse-polarity one. Personally, I've always preferred closed cabs (monopoles) to open ones for basic amp sounds, due to the more uniform directivity of the closed-back type. However, if I want a dipole effect, I can easily get it, and I can selectively apply it to time-based effects and orient the effect so that the audience experiences it as well.
