Omnidirectional speakers. The future?

As Sean pointed out, most omni's aren't truly omni, since the drivers usually become directional at high frequencies. But the net effect can be the same as for a true omni - namely, a well energized and tonally correct reverberant field.

Just for the record, the Bose 901 was NOT an omnidirectional system. The array of 8 drivers on the rear of the enclosure was highly directional, in stark contrast with Bose's advertisements which depicted a very wide radiation pattern from the rear-firing array and a narrow pattern from the single front-firing driver. In fact, the opposite was the case! The rear-firing array's radiation pattern would have approximated that of a single driver roughly 8" tall by 16" wide (the 45 degree angle down the center of the rear baffle does help somewhat with horizontal dispersion, so maybe the net effect is more like a single 8" by 12" driver). The 901's forward radiation narrowed to a roughly 90 degree angle at about 4 kHz and continued to narrow progressively above that. The rear radiation narrowed to 90 degress at maybe 1.5 kHz, and of course it just got worse at higher frequencies. The selling point of the 901 was the psychoacoustically pleasing effect of a well-energized reverberant field. Note that assuming the equalizer corrects to give more or less flat power response, the on-axis first-arrival sound from that single front-firing driver will be quite tipped up. Bose got away with it because the reverberant energy was so much louder than that first-arrival signal that it dominated the perceived tonal balance. I'm not going into the other design tradeoffs Bose made at this point - they might sue me (I say that only half jokingly). But the 901 is an ingenious application of psychoacoustics, and I tip my hat to Amar Bose on that score.

The Shahinain Diapason on the other hand gets the tonal balance correct in both the direct and reverberant sound fields. The design uses two woofers, four cone midwoofers, two 3" dome mid-tweeters, two 1.5" dome tweeters, and six 1/2" dome supertweeters (the latter to maintain adequate energy in the reverberant field in the top octave). Designer Dick Shahinian takes into account not only the frequency responses of the drivers, but also their physical orientation and inherent radiation patterns in building a psychoacoustically intelligent loudspeaker.

The Ohm F, German Physics, Huff, MBL Radialstrahler, and Wolcott Omnisphere speakers are perhaps more elegant (though not necessarily better sounding) solutions than the Shahinians, as they use fewer and more exotic drivers, but their radiation patterns tend toward the doughnut-shaped at high frequencies rather than the truly omnidirectional. Bi-polar speakers from Mirage and Definitive Technology also do a good job with the reverberant field. Some planars (namely Beveridges and the big Sound Labs) also generate a tonally correct reverberant field, as do cornerhorns like the Klipschorn and Hartsfield, albeit with less reverberant energy (relatively speaking) than an "omni". But the principle of maintaining correct tonal balance in the reverberant field makes sense to me.

Apparently it also makes sense to Seigfried Linkwitz and Jorma (pronounced "Yorrrrma") Salmi. The latter is the designer of the Gradient Revolution, which is a very well thought-out system in my opinion (yup, I sell 'em). The Revolution uses a dipole bass system and a cardioid mid/tweeter module. At first glance it would seem that these very differently-shaped radiation patterns (the figure-8 dipole and heart-shaped cardioid) would give a very disjointed reverberant field, but the audio gods smiled on Jorma. The ear is very tolerant of arrival time differences in the reverberant field; timing is only critical in the first-arrival signal. And it just so happens that (assuming equal on-axis, first-arrival SPL's) the net energy put out into the reverberant field is the same for a dipole and a cardioid! So using fairly conventional drivers (and only a few of them) in a reasonably-sized enclosure, the Revolution has consistent tonality of both first-arrival and reverberant sound. And this matters because the ears take them both into account; a nice smooth on-axis anechoic frequency response curve is incomplete data, from a psychoacoustic perspective.

In case you can't tell, getting the reverberant field right is probably my favorite obscure loudspeaker design consideration. I find that often perceived loudspeaker characteristics are demystified when the reverberant field is factored in, and the most realistic-sounding systems I've encountered are ones that get the reverberant field right.

Now the one justified criticism of omni's (and their wide-pattern cousins) is their typically non-pinpoint imaging, at least in comparison with a good two-way mini-monitor. I'm afraid that to a certain extent this is an inevitable tradeoff. Even from one concert hall to another, there's a tradeoff between precise localization of sound sources and enveloping ambience. The more energy in the reverberant field, the richer the ambience but the less precise the soundstaging. However, it is the early reflections that are the most detrimental to good imaging. By their very nature wide-pattern speakers have more energetic early reflections, and so with wide-pattern speakers it is especially important to treat the first reflection zones if imaging is a high priority.

Getting back to Stan's original post, I wouldn't say that I'm in favor of omnis just for the sake of omnidirectional radiation. Rather, I'm in favor of getting the reverberant sound to have the same (correct) tonal balance as the first-arrival sound - and an omnidirectional or quasi-omnidirectional system is one very effective way of meeting this criteria. And the test is this - with the music playing a bit louder than normal, walk out of the room. If it still sounds like live music through the open dooreway, then those speakers are getting the reverberant field right - much the same as real instruments do.

Duke

Opalchip, interesting points you bring up. I can't help but have a question come to mind: When you listen to a grand piano in a recital hall, the vast majority of the sound power that reaches your ears from the piano is reverberant energy, not direct energy. Would you consider this reverberant energy (which cannot possibly be time and phase coherent with the original signal) to also be "distortion"?

You see, the ear treats sounds arriving at different times in different ways. Different cues are extracted from reflections than are extracted from the first-arrival sounds. I believe the correct approach is to see them as two separate events, and to try to get them both right.

Twilo, I didn't hear the Hsu bipolar setup, but believe your description. However, note that the back-to-back speaker pair will probably have a deep notch in the response centered on the frequency where the path length difference from the two sets of drivers to the listening position is equal to 1/2 wavelength. Assuming the back-to-back speakers were each 8" wide and 6" deep the on-axis path length difference is 6 + 6 + (8/2) = 16 inches, so at approximately 420 Hz you'd have severe cancellation, along with partial cancellation at nearby frequencies. So back-to-back speakers may not be the ideal solution.

If I recall correctly, Mirage used a single bass driver on the front of a wide cabinet, and a rear-firing midrange and tweeter on the back of the cabinet crossed over higher than that 1/2 wavelength notch frequency. Definitive Technology patented a technique for using side-firing woofers along with forward and rearward facing mid/tweet arrays, once again to avoid that 1/2 wavelength cancellation notch.

Summitav, you make a very strong and I must say hard-to-dispute statement:

"First and foremost, "ANY" sonic artifact created "after" the recording process, in the reproduction space, cannot be reality, and can do nothing but degrade the original."

That sounds very convincing, but if it is true, then why are we not all listening to headphones? With absolutely zero degrading room interactions, wouldn't headphones be the holy grail - the "poor man's anechoic chamber", if you will?

Let me start out by noting that recordings are made to be listened to in a reverberant environment. If they were made to be listened to in anechoic chambers, they'd be mixed accordingly. (Some recordings are made specifically for playback through headphones, and I'm told that they sound wonderful through a good set).

So assuming those of us who have invested in loudspeakers are not insane (okay I know that's taking a leap...), the loudspeaker/room combination must be doing something good to the reproduced sound, else we'd all be saving up for a pair of Stax headphones.

The loudspeaker/room interface plays a major role in three critical areas: Spaciousness, image localization, and timbre. Let's take a look at each:

"Spaciousness is created by a large number of laterally arriving sound waves which are preferably delayed from the direct sound by more than 10ms. Only the reverberant field can possess this characteristic... In order to have the feeling of spaciousness, one must first be in a room location with a reasonably high reverberation level relative to the direct sound level." - Dr. Earl Geddes on sound perception in small rooms. So when it comes to spaciousness, reverberant energy is our friend.

On the other hand, early-arriving reflections are the enemy of precise image localization. And strong, distinct, laterally-arriving early reflections are unfortunately the worst offenders, tending to blur the image and even altering the tonal balance of the sound. So already we see a conflict here between the good things that reverberant energy can do, and the bad things it can do.

Timbre relates to the harmonic structure of a sound; the same note sounds different on different instruments because of their differing timbre. Room reflections will inevitably influence the perceived timbre, as the reverberant energy's spectral balance is summed with that of the first-arrival sound by the ear-brain system. The rich, lively sound we so enjoy in a good concert hall (and find lacking at an open-air performance) is largely the product of a highly diffuse, relatively late-arriving and slowly-decaying reverberant field (Pisha & Bilello on live end/dead end room techniques).

So reverberant energy does some good things, and some bad things. Generally speaking, strong, distinct early-arriving reflections are likely to do more harm than good, while late-arriving, diffuse reverberant energy is almost always beneficial in a home listening room.

Now, using directional speakers in a fairly live (yet diffusive) room can work quite well. The directionality of the speakers reduces the number of early reflections, and that same directionality builds up the reverberant field more slowly than wide-pattern speakers would. But the key thing is, the radiation pattern should be as uniform as possible over as much of the spectrum as possible, and this is rarely achiecved with conventional loudspeakers. Most of what passes for "directional" is really "narrow pattern at some frequencies, wide at others, and omnidirectional in the bottom three or four octaves". In an extreme case, such a speaker may well sound best when listened to in the nearfield or in a semi-anechoic environment.

In my opinion the main advantage of wide-pattern loudspeakers isn't necessarily in the relatively higher ratio of reverberant to direct energy; rather it's in the much closer correlation between the tonal balance of the direct and reverberant fields. This promotes natural-sounding timbre. Although there is a trade-off relationship between spaciousness and sound image localization, by diffusing or (if necessary) absorbing the early reflections it's possible to get good results in both areas.

So while it makes intuitive sense to say that anything the room does to the sound is degradation, I'd argue that the room does some very good things to the sound: It adds spaciousness and timral richness and liveliness, hopefully with minimal detriment to image localization. Indeed when it comes to votes cast with our wallets, I think most of us have voted in favor of at least some room interaction. Most of us have studied and auditioned extensively to find the finest pair of speakers we could reasonably (ahem) afford, while relatively few of us have pursued headphones with anything remotely approaching the same passion and budget allocation (or budget-busting, as the case may be).

Opalchip, I gotta tip my hat to you for consistently holding to your convictions, even if they're very different from mine. I have a feeling your ideas are more the norm than what you're finding on this thread - I think you've stumbled into a hotbed of believers in planars and/or poly-directional loudspeakers (to borrow Dick Shahinian's term).

A comment about one of your arguments, if I may: While it is true that the microphone picks up hall ambience cues, microphones are usually placed much closer to the performers than listeners would normally be. So, relatively speaking, they pick up a much higher proportion of direct to reverberant sound than what a listener would hear in the same venue. This isn't always the case, but usually is.

Also, the direction from which reflections arrive make a difference in how they are percieved, and in most venues the reflections arrive from all around rather than from the exact same direction as the first-arrival sound. Reflections that arrive from the sides, and well delayed in time, are particularly beneficial in conveying a sense of ambience and acoustic space.

My father has done research in anechoic chambers, and he reports that music live or reproduced in an anechoic chamber has incredible clarity but also sounds dead and boring. I have not listened in an anechoic chamber, but having severely overdamped my listening room as an experiment let's just say I'm sure it wouldn't be my cup of tea.

My conclusion from fairly extensive research in the Journal of the Audio Engineering Society and other publications, and from my own crude experiments, is that the ideal would have the direct sound to arrive completely free from early reflections, then for the reflected energy to begin to arrive perhaps 10 or more milliseconds later, and then that reverberant energy would build up and decay over about 50 or so milliseconds.

However, if I understand your position correctly, you hold that all reflections are colorations - even those inevitably part of a live performance. So there is little point in me arguing that there's a right way and a wrong way for a loudspeaker to interact with the room if you see all room interactions as inherently detrimental. I doubt you and I will find much common ground here other than our passion for audio well reproduced, whatever that may mean. Hey, that's enough for me. I'd love to hear your system some day, and if you're ever in New Orleans give me a holler and come hear mine.

Cheers,

Duke

Hi Summitav a.k.a. John Casler,

Thanks for taking the time to reply. I'm not going to try to rebut your individual points, as I think we've both put sufficient effort into stating our positions.

However, since you dispute the quote I included, just for the record let me say that Dr. Earl Geddes is a long-time loudspeaker industry professional, author of numerous research papers and several books, wrote his doctoral thesis on small room acoustics, is probably the world's foremost authority on waveguides, holds seventeen patents with seven more pending, and recently gave a loudspeaker design seminar at ALMA in Las Vegas and will be doing so again in Europe this summer for the Audio Engineering Society convention there. I did not take his remarks out of context - he was referring to the reproduction of sound in a small room ("Premium Home Theater, Design & Construction", page 95). You might want to check out his website, www.gedlee.com - not because his website supports any of my points, but because he's on the frontier in many areas, such as refining our understanding of what kinds of distortion matter to the ear and what kinds don't.

By the way I have listened to my stereo outdoors (well, a stereo I used to have), and it did sound better than inside my room. Timbre was more natural, and imaging and clarity were much improved. But those speakers were poorly designed from a room interaction standpoint, and poorly setup within my room (zero attention paid to minimizing early reflections, for example). In all fairness I have not heard a genuinely high quality home stereo system outdoors - that would be an interesting experiment that I hope to try one day. I have turned my living room into a virtual padded cell by means of panels of thick open-cell foam on frames leaning against the walls, and I did not like the results at all - very precise, but lifeless. In my opinion, the best in-room reproduction I have heard has been from setups where (among other things) care has been taken to establish the kind of late-arriving, well-energized, diffuse reverberant field I've described above.

Duke

If anyone is interested in seeing two industry professionals comment on in-room reflections, yesterday a couple of very interesting posts were made over at audioroundtable.com (I just stumbled across them). Follow this link to a post by Wayne Parham of Pi Speakers, and be sure to read the reply posted by Dr. Earl Geddes:

http://audioroundtable.com/HighEfficiencySpeakers/messages/1684.html

These guys' comments tend to support my "the reverberant field matters" and "reflections can be our friends" approach, so foes be warned! ;)

Cheers to all - those into bouncing off the walls as well as those into padded cells!

Duke

Hi Billhound,

In general, a large room is desirable because it introduces a greater time delay between the first-arrival sound and the onset of significant reverberant energy. This is probably more true for directional speakers than for omnidirectional ones, but both types would probably benefit from a large room in this respect.

The larger the room, the more the power response (summed omnidirectional response) tends to dominate the perceived tonal balance. This is because as you move farther away from the speakers, the loudness of the direct sound falls off more rapidly than does the loudness of the reverberant energy. With conventional speakers, the tonal balance is likely to be audibly degraded once you move back far enough for the power response to dominate. With omnis or other consistent-pattern speakers, the tonal balance will hold up better in a large room.

To given an extreme example, once upon a time I hauled my then-pair of big home-brew speakers (1" tweet, 7" mid, two 15" woofers) to an informal dance held in a church's activity room. To my shame and horror, my beloved behemoths sounded sluggish on bottom, harsh in the mids, dull on top, and overall quite fatiguing. I was mortified, as I'd been letting the locals know what a kick-butt speaker builder I was. Now I understand that those were the wrong speakers for that application, as the power response totally dominated the perceived tonal balance in that large space and their power response was quite poor, having never been given a second thought by their designer (ahem). That's why people use horns or horn/woofer hybrids at dances - they have a much better power response, even if up close and in the "sweet spot" conventional speakers would sound better. High output omnis like the Wolcott Omnisphere also work well in this sort of application.

Getting back to your 1600 square foot room with 16 foot ceilings, I would definitely argue in favor of a speaker that generates a tonally correct reverberant field over one that does not. You may or may not prefer a very wide-pattern speaker like the Shahinians; if you do, they'll sound great in there. In a medium-wide but still consistent pattern speaker, you have full-range planars like the Sound Labs and big Maggies, and dynamic dipoles from Gradient and Audio Artistry. If you like a narrower pattern speaker, then for a large room you might consider horn-based systems like the offerings from Edgarhorn, Classic Audio Reproductions, eXemplar, Pi Speakers, Zingali, SP Technologies, and the Klipsch Heritage line. [disclaimer - I sell two of the brands mentioned here]. But dealing with a 1600 square foot room with 16 foot ceilings is a nice problem to have.

Well, them's my thoughs.

Duke