Any issues with 2 speakers slightly facing each other? Phase?Noise?Feedback?other?...

[Steve Conner]

So, which Olson book says not to build a guitar cabinet with the speakers angled towards each other? I have "Music, Physics and Engineering" and it's not in there.

You'll find endless argument on the forum about the efficiency of multiple speakers. I think we agree that when two speakers are coupled, the efficiency of them is boosted by 3dB, and that this is a separate effect apart from any constructive interference. Coupling only happens at frequencies such that the distance between the speakers is small compared to a wavelength.

Interference effects don't affect the total radiated sound power, but they can boost the SPL if you happen to be standing in a lobe. That is how PA line arrays work after all. And if you're not careful they most certainly can affect the frequency response, as the pattern of lobes changes with frequency.

I've used one of those Fender cabinets with the big inverted pyramid horn and I seem to remember it sounded fairly crap.

[woodyc]

I don't think anyone is trying to disqualify the merits of acoustics here...

Well the funny thing is, as I mentioned above, the dude can answer his question in less than 5 minutes without any knowledge of acoustics or carpentry. Like this...

[jmaf]

Well the funny thing is, as I mentioned above, the dude can answer his question in less than 5 minutes without any knowledge of acoustics or carpentry. Like this...

Some rather nice gear you have there....

[Chuck H]

Woodyc, you didn't answer my first question. Instead you chose to respond in a derogatory way to my suggestion that the OP build a mock up. Which still seems like sage advice to me. I'll tell you why. See if you can follow along and pay attention to every part of the post instead of just one.

First, with guitar amps we are intentionally coloring the sound. The unpredictable result of experimentation is often a desireable one.

Second, the acoustic theory required to understand the complexities of a V front open backed cabinet with an amp stuffed inside are beyond my skill, probably yours too.

Third, even if you could parameter everything about such a cab (phase relationships of two speaker backs angles slightly toward each other and this interacts with the front of the cab, etc...) how can you know that the target result will sound good for a guitar amp? IME the acid test for any guitar amp related parameter is to hear it. Often as not a targeted parameter will need to be adjusted by ear.

Fourth, with such a cab, because of the phase relationship between the open back and the front, the cabinets placement in the room, the size of the room and the surfaces in the room will greatly affect the final result.

In the end it's just too many variable to achieve a predictable result with a paper design. I also don't think angleing two closed back cabs is an accurate enough simulation for what the OP intends. But what do I know. I'm not an acoustical theorist.

I don't think you have any business trying to make me out for a fool by suggesting that I would replace acoustical theory with carpentry and I'll thank you to consider your attitude.

[woodyc]

...Interference effects don't affect the total radiated sound power, but they can boost the SPL if you happen to be standing in a lobe. That is how PA line arrays work after all. And if you're not careful they most certainly can affect the frequency response, as the pattern of lobes changes with frequency...

I totally agree. But wouldn't you say that the overall SIL is of more interest than on-axis-SPL (or other interference effects) in guitar stuff because usually about 0% of your audience's ears are on-axis?

I don't have an opinion one way or the other on angling speakers. My only point was that it produces a bump in the mids which is not a simple interference effect.

[jmaf]

My only point was that it produces a bump in the mids which is not a simple interference effect.

Are the blonde and the black tolex on that photo speakers in electrical phase? BTW I think the OP meant in angling them reflex angle / \ not acute \ /

[Chuck H]

The OP has asked about both during the course of the thread. But your Q about whether those two cabs are in phase is a good one. Original Jensen speakers had their polarity marked opposite from other brands. Since Fender used many different brands of speaker it could be that those two cabs are out of phase with each other to begin with. In which case facing the toward each other would increase efficiency rather than decrease it.

[Steve Conner]

I totally agree. But wouldn't you say that the overall SIL is of more interest than on-axis-SPL (or other interference effects) in guitar stuff because usually about 0% of your audience's ears are on-axis?

I don't have an opinion one way or the other on angling speakers. My only point was that it produces a bump in the mids which is not a simple interference effect.

This is controversial, particularly in the hi-fi world. Hi-fi speakers are (with a few exceptions) designed for flat on-axis SPL, because that's all the designers can get their heads around. But the SWL is not flat because of frequency-dependent dispersion. So when the audiophile takes them home to his living room, which is not completely anechoic, he hears a contribution from the off-axis response, and lo and behold they all sound different.

So, I agree that in guitar amps, the diffuse field is important. One example is that open-backed amps have that big rear firing lobe that lights up the room behind them and makes them sound "airy". But I still like to think in terms of on-axis SPL, and believe a guitar amp should have smooth, wide dispersion, so everyone in the crowd hears a similar tone, and moreover, similar to what the guitarist hears. I have been at small gigs and had my face burnt off by combo amps placed on the stage floor. The high end just beams past the guitarist's legs and he can't hear it, so he turns it up too much.

Parting thought: If only SWL mattered, Marshall stacks would work just as well facing backwards, or pointing at the ceiling.

[woodyc]

...Parting thought: If only SWL mattered, Marshall stacks would work just as well facing backwards, or pointing at the ceiling.

Well on-axis superposition isn't the only reason closed backs are directional -- by design they radiate mostly in a hemisphere, except at low frequencies. Even so, they do work just as well backwards as long as you aren't outdoors and the rear wall and/or ceiling is fairly reflective. Believe it or not, I've seen people gig both ways (backwards and up) with Marshall 4x12s... but not stacks.

[Steve Conner]

The motivation there is presumably that the thing is too loud for the gig, and they are trying to reduce the SPL on stage by pointing the axis the wrong way.

So, the argument still works in my favour: if total radiated sound power were all that mattered, then pointing it backwards or skywards wouldn't help, it would be as obnoxiously loud on stage as ever.

And, if SWL were all that mattered, Jim Marshall could have made his famous stack smaller by putting 4 speakers on the front and the other 4 on the back. But he wanted the directivity gain, to make a directional weapon for playing big gigs before large PA systems were invented.

About the "midrange bump" by mounting two speakers facing each other- Surely it's a lift of all the frequencies in the driver's piston range, due to coupling, so bass ought to get boosted too. There will also be a loss of high end on axis, because the axis of the system is no longer the axis of either driver. Of course this is just the desired widening of dispersion seen in a different way.

I have seen midrange speaker systems with a front cavity, forcing the speaker to fire through a vertical letterbox-like slot. A TOA PA speaker and a Trace Elliott combo of some kind. While there is a speaker expert in the house, do you know what the reason for that is? I thought it was to widen horizontal dispersion.

[jmaf]

This is one of the more successful hi-fi speaker systems ever...

http://i232.photobucket.com/albums/e...c/IMG_0751.jpg

Maybe it inspires the OP to experiment

[Steve Conner]

Yes, and oddly enough, that pic shows the back of the Bose 901! It has 8 drivers in the back and only 1 in front. Dr. Bose obviously agrees with woodyc.

[woodyc]

The motivation there is presumably that the thing is too loud for the gig, and they are trying to reduce the SPL on stage by pointing the axis the wrong way...

No, it was just to avoid the nasty beaming.

[Gtr_tech]

Sunn did that inward facing speaker thing in the 70s. Never really caught on....

http://i141.photobucket.com/albums/r...ott/6x1201.jpg

http://hillamplification.com/sunn.jpg

[jmaf]

Sunn did that inward facing speaker thing in the 70s. Never really caught on....

http://i141.photobucket.com/albums/r...ott/6x1201.jpg

http://hillamplification.com/sunn.jpg

Never had seen those. Thanks for sharing.