Dispersion White Paper
- Last Updated on Wednesday, 30 January 2013 11:13
I started doing research and development work on ESLs in 1972. One of the major issues I had to address was "beaming" in large panels. Back then, I believed the common perception that a good speaker should have wide dispersion. So I worked on the problem.
I eventually solved the problem of beaming in 1979 when I invented a way of curving a free-standing, tensioned membrane. It was my design that Martin Logan called the "Curvilinear" ESL and produced starting in 1981.
After a lot of study and experimentation with the curved panel, I came to the clear conclusion that it was actually a poor design. I abandoned it in favor of a planar ESL. Why would I fail to use my own invention?
Here is where the physics get very interesting. I eventually came to understand that there are three serious problems caused by wide-dispersion in speakers. These are poor frequency response, poor transient response, and poor imaging.
Let's look at each of these issues in detail. For examples in this discussion, I will refer to two, imaginary speaker systems. For simplicity and to eliminate confusion, we will assume that both speaker systems are perfect in every way. The only difference between them is that one will have wide dispersion and one will have narrow dispersion.
Now let's examine what happens to frequency response in both of these speakers. We will hear the sound from the narrow dispersion speaker being beamed directly to us at the sweet spot. Therefore the frequency response from the narrow dispersion speaker will sound perfect (because we defined the speakers as perfect in every way).
But the vast majority of the sound from the wide-dispersion speaker will be sprayed all over the room rather than being beamed directly to our listening location. We will therefore hear most of the sound from this speaker after it has bounced off various surfaces in the room and is eventually reflected to us at the sweet spot.
Because the reflected sounds have to travel a greater distance to reach us than the direct sound from the speaker, the reflected sounds will be delayed by the speed of sound. Another way to say this is that reflected sound will be out-of-phase with the direct sound. The phase-angle will be determined by the amount of time delay between the reflected sound and the wave length of the particular frequency of interest.
For simplicity, let's examine just one of these reflections and how it interacts with the direct sound from the speaker. Let's assume that the magnitude of both the direct and reflected sound is the same (in real life, the magnitudes can vary all over the map).
If a 1 KHz tone arrives directly from the speaker at 80 dB, it will mix with the reflected sound (at 80 dB) at some phase angle depending on how much the reflected sound was delayed. Let's assume that this particular wave arrives 180 degrees out-of-phase. If so, the reflected sound would completely cancel the direct sound and you would hear nothing.
If the reflected sound arrived 90 degrees out-of-phase, it would reduce the direct sound by 50%. If it arrived 360 degrees out-of-phase, it would increase the sound by 100%, etc.
As you move up and down the frequency range, this particular delayed reflection would interact with each frequency differently (because the wave lengths are different so the phase angle would be different for each frequency). The result is that you would hear (and measure) the speaker's frequency response as consisting of severe, alternating peaks and troughs that look like the teeth of a comb -- hence we call such frequency response a "comb filter."
Now a comb filter sounds perfectly awful. Conventional, wide-dispersion speakers would be unlistenable if it weren't for the fact that there are thousands of delayed reflections in a room, and they are all random. As a result, there are thousands of comb filters formed and to a considerable degree, they can average themselves out so that the frequency response from a wide dispersion speaker is tolerable.
But this problem assures that the frequency response will never be perfect in a wide dispersion speaker. It is often necessary to move the speaker around to get satisfactory response, which is why speaker placement is so important.
The very short and intense reflected sounds from walls directly beside the speaker are particularly troublesome as they tend to dominate the sound. Hence, smart audiophiles have discovered the trick of using sound-absorbing "room treatment" near the sides of wide dispersion speakers to help achieve reasonable frequency response.
You can now see how the frequency response of a speaker is seriously degraded by room acoustics. This background information will make it obvious why transient response is degraded by room reflections as well.
For simplicity, let's examine a single, sharp transient (like a rimshot from a drum). The transient coming from the narrow dispersion speaker will be perfect (because we said the speaker was perfect). But what happens to the transient from the wide-dispersion speaker?
Once again, most of the transient sound is blasted all over the room where it bounces off various surfaces and eventually arrives at the sweet spot after being delayed by various amounts depending on the distances each reflection has to travel. So instead of hearing one, crisp transient, you will hear "popcorn." Like a pan of popcorn popping, we will hear a whole bunch of identical transient sounds separated by very short intervals.
These delayed transient sound are actually echoes. But the typical listening room is too small for the delayed sounds to be separated by a sufficient period of time for our brains to recognize them as distinct echoes.
It is a fact that the delayed sounds are distinct. We can see that on an oscilloscope. But we don't hear them that way -- as a multitude of distinct transients. We hear them as one sound.
Our brains have learned to understand that having a bunch of rimshots close together means that there was only one rimshot with room acoustics following it. So our brains do one of their psychoacoustic tricks and sweep all the delayed sounds together to form one rimshot.
But there is a "catch." And that is that the transient time of the rimshot now includes all the delayed sounds following it. This extends the transient time we perceive with the result that the transients are now "smeared."
If you doubt this, just think of the last time you heard headphones. I'm sure you will agree that the transients you hear in headphones are crisper and cleaner than what you hear from wide-dispersion speakers. This isn't because headphone drivers are so good (actually most are pretty bad), it's simply because there are no room acoustics in headphones to mess up the sound.
Now let's look at imaging. A holographic image will be 3-dimensional. It will not only have left/right information, but it will have depth.
Left/right position in the image is determined by loudness differences between the two channels. But depth is defined by timing information (phase).
The reason you hear the violins in a symphony orchestra to be in front of the brass is because their sounds reach your ears slightly sooner than the sounds from the trumpets and trombones. This phase information has to be preserved in the recording and then reproduced accurately by the speakers for you to get depth in the image.
By now, you probably know what I am going to say. The sound from the narrow dispersion speaker will supply proper phase information to your ears. The sound from the wide dispersion speakers will be blasted all over the room and the delayed sounds will completely confuse the phase information. As a result, the sound from wide dispersion speakers (and omni speakers are the worst) will have very diffuse and ill-defined imaging.
In summary, the frequency response, transient response, and imaging of a loudspeaker is ruined by room acoustics. So to achieve outstanding performance, a loudspeaker must eliminate room acoustics as much as possible.
My curved electrostatic panel failed to eliminate room acoustics. I found that a planar panel was far superior because it beams the sound directly to you and eliminates the room acoustics. So I abandoned the curved panel. Once you hear highly directional panels, you will immediately understand.
Many audiophiles believe that a good speaker should have a wide sweet spot. But this is a physical oxymoron.
The laws of physics dictate that all stereo speakers will have an infinitely small sweet spot, regardless of their high frequency dispersion. That spot is when you are exactly equidistant from both speakers. Only when you are equidistant from the speakers can the phase information arrive at your ears simultaneously from both speakers. Obviously, there is no hope of imaging well if the sounds from both speakers do not arrive at the same time as the phasing will be destroyed.
For a speaker to have a wide sweet spot simply means that the phase information from the room is confusing the sound so badly that you can't even tell when you are in the sweet spot and when you aren't. A wide sweet spot is a guarantee that a speaker has poor imaging and transient response.
I don't compromise. I want narrow dispersion in my speakers to minimize room acoustics so that I can get the best possible sound.
"Beaming" is not a fault. It is a huge advantage. It is the only way to achieve truly high performance in a loudspeaker.
Audiophiles sometimes say that narrow dispersion speakers require you to have your head in a vise. This is nonsense. You just sit in your listening chair and listen as you would to any speaker.
And what about the off-axis performance of a narrow dispersion speaker? Well, they sound just like wide dispersion speakers when you are off-axis.
That is to say that when you are off-axis, you hear the room acoustics, not the speaker. So my speakers sound just fine off-axis for casual listening. Of course, the image is diffuse and of poor quality, just like a wide dispersion speaker when you are off-axis. So for serious listening, you need to be at the sweet spot. This is true for all speakers.
As an aside, it is best to NOT absorb the rear wave from my speakers. Let it help energize the room for off-axis listening. That way the highs will be preserved off-axis.
Finally, the question arises, "Why doesn't the reflected sound from the dipole beams mess up the phase just like in a wide dispersion speaker?" The answer is that the reflections from a dipole radiator are only one rather than thousands.
Also this beam has to bounce off many surfaces before it finally reaches the sweet spot. In so doing, it is greatly delayed and attenuated. It arrives at the sweet spot so late and attenuated that our brains simply ignore it.
Of course, directionality only applies to midrange and high frequencies. Bass is omnidirectional in all speakers due to the long wave lengths involved. So bass room resonances still need to be addressed.
There is one perilous pitfall you need to be aware of with regards to delayed reflections in all speakers, even narrow dispersion speakers. This involves sitting close to a wall directly behind you, which is very bad.
Typically this occurs if a couch is used for a listening chair. Most of the time a couch is pushed up against a wall. If you sit in it, your ears are only a few inches from a wall that is perpendicular to your head. As the sound beam from a planar speaker arrives at your ears, some of it passes by your head, bounces off the wall, and returns to your ears after a very short delay.
This one reflection is very powerful. It is almost as intense as the direct beam and it will seriously mess up the frequency response and transient response of the sound.
To avoid this problem, ideally you should sit in a chair out in the room away from the wall. Even better would be to have the system on a diagonal so that the wall behind you is angled and reflects the rear reflection away from you instead of back at you. Diagonal room placement is also best for controlling the bass resonances from a speaker.
If your room decor makes this impractical, then there are several options you have. First, you can put sound absorbing material on the wall behind your head. The could be a simple as a very soft pillow that you set on the back of the couch so that it is behind your ears. This is not as effective as I would like because it will only absorb high frequencies. The midrange will still have a reflection.
Probably the best compromise is to have a movable listening chair. When not being used, the chair can be placed in the room wherever it pleases your wife. When you want to listen seriously to music, you move the chair out into the room where you have identified the sweet spot (usually with tiny pieces of tape on the floor). Then the sound can be spectacular.