# Beaming: theory and experiment

876 Views 15 Replies 6 Participants Last post by  robabeatle
I am thinking a lot more about beaming lately and wonder what rules of thumb you follow, interesting theory, or real world results (RTA before and after modifying axis)

I'll start with this in depth video about theory which is interesting but not so much about applicability:

Erin's Audio Corner: Beaming
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This thread was supposed to be for me to be lazy schooled!
Beaming is the point where wavelength is equal to cone diameter. With a 2-way setup you would be concerned about beaming from the mid. The obvious answer is crossing it below that wavelength but that requires a tweeter that can play comfortably lower.
The shape and material of the cone seem to also effect beaming.
I look at speaker measurements for the point where on and off axis responses diverge.

Here's the Purifi mid I'm using. At about 1.5KHz the response at 60° starts to fall off. So I wanted my xover to be around this point below the beaming frequency. Finding a tweeter to play down to that point can make this work. Then it's just a matter of trying different xover types, points and slopes in that range to find what works best.
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So what benefits do we get when we keep the speaker out of beaming ? Won’t the response be much different when the driver is mounted ?
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Beaming is a physics problem that the only variable is speaker diameter. When you're below beaming. The nearfield and far field frequency response match. This can be graphed as polar response. The rate of roll off is fixed regardless of frequency response. Mounting doesn't change that. What can change with mounting and install is nearfield on axis frequency response.

For example, on Bills graph, as measured the speaker is -9db @ 3khz. If, due to the install location, 3khz was boosted by +9db, then the off axis response would still be -9db @ 3khz.

"That's great!" you say. "I can now cross the speaker over higher for better integration with the tweeter". The unfortunate part is the environmentally induced reflections.

If the speaker is crossed over below beaming then all the room reflections will have the same frequency response as the primary signal. If you cross it over above beaming then the reflections will have a boosted frequency response equal to the roll off of the speakers off axis response. The dissimilar responses then cause problems with imaging as they will drag the stage around depending on the intensity and delay of the reflection vs original signal.
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So what benefits do we get when we keep the speaker out of beaming ? Won’t the response be much different when the driver is mounted ?
Think of it this way. If you have varying output from the source (speaker) that is frequency and axis dependent that makes an already reflective environment worse. The beaming frequency is around 2KHz in my case. By crossing below it, the response is more linear.
You can mount your speaker 30° off axis. Beaming frequency will be ~2KHz for a 6.5" which is also around the frequency off axis response becomes erratic. The 6.5" wavelength means the majority of what you hear is reflected. So the axis of the speaker has little to do with the reflections you inevitably hear. You hear (or measure) the combined response at the listening position because the wavelength is much shorter than the path length and your listening to a stereo in a fishbowl.
Beaming is a problem to avoid because there are enough problems to deal with in a car. Why make it worse?
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Basically, keeping a speaker below beaming will make it sound more similar regardless of listening position, so the passengers will have better sound without requiring a specific tune for them. Beaming isn’t the end of the world, but if you can keep a speaker out of those frequencies the sound will be more similar throughout the entire listening environment than if you are pushing a speaker into beaming and tuning around it.
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So I always kept my drivers in what I call the usable “pass band” or where it stays most linear. So the upper end where it starts to go to sh## is the area it starts to beam correct ?
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Correct
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I recently traded out my SI M25 (1.5 inch diameter) tweets for the AF gs25 (2.5 inch tweets). This allowed me to run a much lower crossover so beaming from the 6.5 midbass is no longer an issue but now with a 2.5 tweet, I have to consider beaming above 5k or so. I have them mounted in a prototype mount to get them more on axis. Frankly, I am the only one in the vehicle that really cares about sound so a driver's tune is good enough. I think I do not have them on axis enough at this point even though the RTA looks good. If I scrunch down and get my head more on axis, the upper end brightness returns. I definitely notice that much of the human vocal passband is no longer just sounding good in a small region around the drivers head, but if I stay two way, it will be that the upper end sparkle is restricted to that point.

I wonder if three way would just be better, boy this has me spinning in circles and I am tired of mounting and retuning and whatnot for now. I prolley could mount both the gs25 and the m25 flush to the door panel right next to one another, without any angled mounts. What is holding me back is the eventual loss in global filters; I have the amp channels, dsp channels, and wiring ready to go though.

rambling....
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Im using the Bliesma 1.5" tweeter. That was the first piece of my system I selected. I have a 4th order xover at around 1.7KHz. They are pretty linear out to 20KHz so no shortage of sparkle and play low enough to avoid beaming from the 6.5"
Most of what I'm trying to accomplish is limiting reflections and stabilizing imaging. Those results have been pretty interesting, I can open the window or door without imaging reality changing. Having no speaker in the door obviously helps but the xover points really are an important part of that. Getting good coherence is important no matter how many speakers you're running. With my install underlapping seems best, my mid is at 1.1KHz.
I recently traded out my SI M25 (1.5 inch diameter) tweets for the AF gs25 (2.5 inch tweets). This allowed me to run a much lower crossover so beaming from the 6.5 midbass is no longer an issue but now with a 2.5 tweet, I have to consider beaming above 5k or so. I have them mounted in a prototype mount to get them more on axis. Frankly, I am the only one in the vehicle that really cares about sound so a driver's tune is good enough. I think I do not have them on axis enough at this point even though the RTA looks good. If I scrunch down and get my head more on axis, the upper end brightness returns. I definitely notice that much of the human vocal passband is no longer just sounding good in a small region around the drivers head, but if I stay two way, it will be that the upper end sparkle is restricted to that point.

I wonder if three way would just be better, boy this has me spinning in circles and I am tired of mounting and retuning and whatnot for now. I prolley could mount both the gs25 and the m25 flush to the door panel right next to one another, without any angled mounts. What is holding me back is the eventual loss in global filters; I have the amp channels, dsp channels, and wiring ready to go though.

rambling....
I’ve heard people getting great results with a wideband. The wide band has to be on axis though to get the full benefit.
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I’ve heard people getting great results with a wideband. The wide band has to be on axis though to get the full benefit.
I have read the same and it makes sense. My gs25 is not quite on axis (on the drivers side) yet. I will try fabbing up a new prototype and see where that gets me. It is quite an improvement in sound when I move my head about a foot lower than driving position. All this is said with a mediocre, quick tune and not even really sure where I want to cross the 6.5 to the 2.5 at this point. I am trying a 800 Hz acoustic crossover at this time, but maybe makes sense to go even lower.
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Here are comparisons between my 6.5 mounted flush in the lower door with a standard plastic mount. Then I built an angled mount that also brought the driver a bit away from the door, mdf mount with plastic angled part. Let see how they compare in raw measurements. Sorry that I have a bunch of other drivers in the photo. In the first image, focus on the green which is the left woof.

Now here is the left woof with new mount. What say you?

I almost dont believe my eyes at how much better this looks.

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