Magico - Wide vs. Narrow

Quoting @tomic601 (replying to my post that rear-firing tweeters can correct the spectral balance of the reflection field) :

"While destroying the timing information that the ear brain is much more sensitive to. "

There are conditions under which a rear-firing tweeter can can be detrimental. Imo we do not want the output of the rear-firing tweeters to arrive too early nor be too loud, and we want its power response to be correct for the application. These characteristics are perceptually intertwined. Hence the "implemented correctly" stipulation in my previous post.

@tomic601’s follow-up post:

"my decision references are unamplified acoustic instruments in reverberant spaces captured w simple microphones, so the temporal data is in the recording."

Imo effectively presenting "the temporal data in the recording" is precisely the scenario in which a correctly-implemented rear-firing tweeter is most beneficial.

You see, in the playback room there is a "competition" between two sets of spatial cues: The venue spatial cues on the recording, and the "small room signature" inherent to the playback room. It is desirable for the venue spatial cues on the recording to be perceptually dominant if the goal is a "you are there" presentation.

Painting with broad strokes, the ear judges the size of an acoustic space by three characteristics: The time delay between the first-arrival sound and the first reflections; the temporal "center of gravity" of the reflections; and the decay of the reverberation tails.

It is in the effective presentation of the reverberation tails on the recording that a well-implemented rear-firing tweeter is most beneficial. Briefly, the in-room reflections act as "carriers" which deliver the reverberation tails on the recording from all around.

The ear looks at the spectral balance of incoming sounds to judge whether they are reflections or new sounds, and the overtones are especially critical to the ear making this determination. If the overtones are not loud enough for the ear to correctly identify the reflections as such when delivered by the in-room reflection field, that energy ceases to be "signal" and becomes "noise". But if the overtones are still loud enough, the ear will hear the reverberation tails arriving from all around as they decay, delivered by the spectrally-correct in-room reflections. This spectrally-correct delivery of the reverberation tails by the in-room reflections effectively presents the natural decay characteristics of the reverberation tails on the recording, which in turn are a significant contributor to a "you are there" playback experience.

And a well-implemented rear-firing tweeter corrects the spectral balance of the reflection field, restoring its typically-too-weak overtone levels, without introducing other problems. 

That being said, this is a complex topic and this post is an incomplete look at just one aspect of it.

Imo and ime.

Duke

But do we really want an increase in our sense of space? Is this illusion correct or an aberration?

@tomic601: To your list of RIP geniuses I would ad Siegfried Linkwitz. By the way, Linkwitz himself "believed" in narrow front baffles, and in his open baffle models used electronic means of compensating for the dipole cancellation that is inherent in open baffle loudspeakers.

Danny Richie of GR Research is also a proponent of both open baffle loudspeakers and narrow front baffles, but uses a different method to deal with dipole cancellation: He makes his front baffles just wide enough to house the driver(s) being used, but then uses side wings to manipulate the frequency at which dipole cancellation begins. The greater the distance between the front and rear of the driver(s) in an open baffle design, the lower the frequency at which dipole cancellation occurs. Danny’s use of side wings provides the front-to-back separation dipoles require (in the GR Research OB models), while at the same time allowing him to use a narrow front baffle.

And by the way, in the Infinity IRS and RS models, Arnie Nudell used curved wings on either side of the EMIT and EMIM drivers those models used for the same reason Danny Richie does: 1- To prevent front baffle diffraction, and 2- To lower the frequency at which dipole cancellation begins to effect the frequency response of the loudspeaker.

In his Eminent Technology LFT-4 planar-magnetic dipole loudspeaker (out of production for many years), Bruce Thigpen did as Danny Richie does: he incorporated a sloped wing on either side of the panel the LFT drivers are mounted on, the two wings facing each other. The wing is only 2" wide at it’s top (and 41" off the floor), gradually increasing to 12" at it’s bottom. I assume the shape of the wings was chosen to create the desired frequency response.

There are several GR Research YouTube videos in which Danny Richie discusses and explains this topic in great detail, showing in measurements the effects different wing sizes and shapes have on the response of different drivers. Well worth searching for.

@erik_squires wrote: "That’s an interesting POV. I would have thought that the slot [the area between back of a speaker with a rear-firing tweeter and the well immediately behind it] would act like a severe low pass filter."

Sorry I over looked responding to this earlier.

I manufacture a bass guitar speaker cabinet which uses a 3" cone unit for the top end. It’s gently highpassed around 1.5 kHz or so. Behind the cone is a wide, shallow isolation chamber which extends laterally the full width of the cabinet, with generous openings on the top and sides. So, it’s a slot of sorts. The openings allow enough of the 3" cone’s backwave energy to escape out the top and sides that the bass player can clearly hear his overtones even if he is virtually atop the cab, and I’ve gotten feedback that the other musicians can better hear what the bass player is doing as well.

I’m sure there are some losses in the slot behind the 3" cone, but it’s still making a worthwhile contribution, so I don’t think the net effect is a particularly severe lowpass filter.

Duke

@audiokinesis Thanks for that input. See I keep thinking about Snell’s downward firing cone and the behavior that the 3-4" high slot would have. Not that I can replicate it in my current listening environment, but still find very few speakers with that sense of the speaker dominating the air pressure in a room like that.

I had thought that the slot loading was a kind of low pass filter, and when you mentioned the tweeter and close wall placement I had to rethink what I knew.