What exactly is beaming and why does it matter?

[paulspencer]

Simple question:

What exactly is beaming and why does it matter?

I hear it mentioned often, but what is it really?

Seems to me that it would actually be a good thing, to have less reflections, or do I misunderstand it?

[SY]

For a radiator of finite size, the dispersion pattern varies with frequency. At low frequencies with wavelengths large compared to the radiator dimension, the radiation is roughly omnidirectional. With increasing frequency (decreasing wavelength), the radiation pattern narrows and starts getting lobes. This is all due to classic interference effects and will happen even if the radiator is a perfect piston. If you look in a good acoustics text like Beranek, you'll see polar diagrams which illustrate this.

Now, is this a good thing? Like everything else pulled out of context, the answer is, "It depends." A very common situation where this is NOT a good thing comes when you try to mate a relatively large woofer with a relatively small tweeter. With practical choices of crossover frequency, the woofer's dispersion is narrowing, the tweeter still has very wide dispersion. So if you get the on-axis response flat, off axis response will have a dip in the upper part of the woofer's range and a peak in the lower part of the tweeter's range.

Because of that dip-and-peak, the reverberant energy will NOT have a flat response. So, OK, you adjust things to balance the reverberant energy. Oh-oh, the on-axis response isn't flat anymore. What you end up with is a speaker that either sounds colored everywhere or one that can sound fine as long as it is placed just so and the room is acoustically treated just so... The somewhat retarded chimp that's doing the review for a magazine notes that the speaker is sooooo good that it rewards every placement tweak that you can do.

There are many schools of thought regarding what an optimal polar pattern is, ranging from pencil-thin dispersion to flat on-axis but rolling off off-axis to full omni. What is never good is designing a speaker without first deciding on the target polar pattern and measuring the design against the goal.

[ThorstenL]

Konnichiwa,

Originally posted by paulspencer
What exactly is beaming

It refers to a change in radiation pattern with frequency, where higher frequencies are radiated in a narrower angle than lower ones. Or in other words, if you have a flat on axis response and you measure the response significantly off-axis you will observe a progressive lessening of SPL above a certain frequency.

Originally posted by paulspencer
Why does it matter?

Because unless your speakers is sufficiently "beamy" even at lower frequencies it will generate a room reverbrant field which dominates the sound, in other words the spatial signature (and to a lesser degree the son ic one) replaces that of the recording venue.

Of course false High End doctrine demands a wide dispersion (in other words an absence of beaming) which in turn then forces system owners to significantly invest in acoustic treatment.

Sayonara

[HeatMiser]

My impression is that it's a bad thing because the off-axis response will tend to vary with frequency, starting off even and wide and narrowing as frequency rises. So if you attempt to design for a smooth power response, the on-axis response ends up shouting at you, and if you shoot for smooth on-axis response the in-room response is biased toward the lower end of the frequencies in question.

Then there's the issue of power response at the crossover point, where if the mid/bass driver is beaming there can be a jarring change in power response when the tweeter takes over.


edit: oops, beaten and with superior expanations to boot. Ah well, back to the coffee.

[ThorstenL]

Konnichiwa,

Originally posted by HeatMiser
My impression is that it's a bad thing because the off-axis response will tend to vary with frequency, starting off even and wide and narrowing as frequency rises.

Surely this is a good thing?

It makes sure of two things:

1) With rising frequency the stereo separation improves and any impulses or transients, which tend to dominate imaging, are less distorted by room generated reflections as they are lower in level.

2) The off axis (room) sound will show the typhical "house curve" due to greater rolloff at higher frequencies, failure to do so produce invariably an overly bright sound.

If your speaker does not beam enough you in fact require acoustic room treatment to correct for the problems introduced.

This BTW illustrates the complete and utter idiocy of using wide dispersion high frequency systems (eg. dome tweeters).

Sayonara

[HeatMiser]

Originally posted by Kuei Yang Wang
Konnichiwa,



Surely this is a good thing?

It makes sure of two things:

1) With rising frequency the stereo separation improves and any impulses or transients, which tend to dominate imaging, are less distorted by room generated reflections as they are lower in level.

2) The off axis (room) sound will show the typhical "house curve" due to greater rolloff at higher frequencies, failure to do so produce invariably an overly bright sound.

If your speaker does not beam enough you in fact require acoustic room treatment to correct for the problems introduced.

This BTW illustrates the complete and utter idiocy of using wide dispersion high frequency systems (eg. dome tweeters).

Sayonara

I'll defer to your greater expertise in the area of speaker/room interactions as to whether the varying reflective nature of listening rooms tends to naturally correct a change in dispersion with frequency. The point it though that it does vary, and particularly in multi-driver systems that fact seems to suggest some design considerations.

[ThorstenL]

Konnichiwa,

Originally posted by HeatMiser
I'll defer to your greater expertise in the area of speaker/room interactions as to whether the varying reflective nature of listening rooms tends to naturally correct a change in dispersion with frequency.

Typhically early reflection points are not covered for diffusion or absorbtion, ceilings are always non-sound absorbing, the modern tendency towards laminate floors does also not help. So, at least for modern designs of room arrangements, nope, the envoironment does not help.

Originally posted by HeatMiser
The point it though that it does vary, and particularly in multi-driver systems that fact seems to suggest some design considerations.

Absolutely. Well controlled directivity with no abrupt swings in directivity (as are charateristic for common 2/2.5 Way speakers with cone midranges and dome tweeters) is a fundamental requirement for high performance speakers (the regular absence of said feature in proclaimed "state of the art" speakers nonwithstanding).

Sayonara

[ShinOBIWAN]

Originally posted by Kuei Yang Wang
This BTW illustrates the complete and utter idiocy of using wide dispersion high frequency systems (eg. dome tweeters).

Idiocy? Bit harsh but nevertheless...

IMO Room treatment is essential for any repectable hifi system regardless of dispertion. Some like me go steps further and use DRC and EQ. Purists will say that's wrong but you've got to consider which offers less distortion and I can easily answer that.

I've used ribbons, dome and current ring radiators for HF, all of the gained from room treatment.

[ThorstenL]

Konnichiwa,

Originally posted by ShinOBIWAN
Idiocy? Bit harsh but nevertheless...

Actually, I was being charitable.

Originally posted by ShinOBIWAN
IMO Room treatment is essential for any repectable hifi system regardless of dispertion.

A competently designed speaker (WRT Dispersion) is easily found to provide more accurate imaging and tonality in an untreated room than most conventional ones in moderatly treated rooms (eg. anything short of an anechonic chamber). This is not to say that sensible room treatment will not provide further improvement, merely that it becomes more of an option.

Originally posted by ShinOBIWAN
Some like me go steps further and use DRC and EQ.

Room treatment and EQ/DRC (DRC is really just a specialised EQ) work very different and on different issues. EQ cannot correct for badly designed dispersion or adjust RT60 etc, room treatment (and speaker design) can.

Mixing the two up is quite inapropriate.

Originally posted by ShinOBIWAN
I've used ribbons, dome and current ring radiators for HF, all of the gained from room treatment.

In other words you used devices that invariably have wide dispersion and are thusly singulary inaproppriate for reproducing music in acoustically relatively small spaces (ordinary living rooms for example).

Treating the poor dispersion control at the source much reduces the benefit of room treatment. Plus, not only tweeters are subject to overly wide dispersion.

It would seem ideal that a directivity index of 6db up to around 2...3KHz and then smoothly rising to 10db @20KHz seems the most desirable charateristic, even though it is difficult to achieve. Using Dipoles for LF we can reach 4.8db DI for low frequencies, suitable waveguides can produce 6db DI above around 300Hz without becoming unpleasantly large and having the DI smoothly increase above a few KHz is not particulary difficult either.

The speakers I currently use have 4.8db DI to around 800Hz and then increases smoothly to around 10..12db at the upper end of the audio range. This is achieved by the simple expedient of using a fairly large cone driver in a fairly large dipole and the addition of a narrow dispersion horn tweeter for very high frequencies. Room treatment past normal furnishings and room EQ have become largely unneccesary and are hence no longer in use.

Sayonara

[SY]

Without offering my own opinion, it should be said that there are various approaches to "proper" dispersion, all with their own adherents. The one thing all approaches have in common is people who claim that their paradigm is the one-and-only.

Where issues of competence and idiocy come in is whether dispersion was adequately considered and whether that consideration was followed up by appropriate and effective design.