Frontal reflections, depth of soundstage, and dipoles

[Keith_W]

There are multiple threads on ASR about reflections, including:

Perceptual Effects of Room Reflections - @amirm
Research on Reflections - @aarons915

However, it is mostly lateral reflections which are being discussed. There seems to be good science on lateral reflections, so we will NOT be discussing that here. Instead, what I would like to discuss are frontal reflections.

For a long time, I believed that frontal reflections are just like lateral reflections, in that within a certain window of time and amplitude, they create spaciousness. I simply transposed my understanding of lateral reflections to frontal reflections and used the same target - ETC should show a peak which is >-15dB with respect to the main impulse within the Haas fusion window of 20ms.

Then I came across this paper by Dr. Matthias Johansson (co-founder of Dirac). Although most of the paper is on mixed-phase DSP, he does mention this on page 7:

And on page 11:

Regrettably, Dr. Johansson does not cite a reference for his statement. So I did a literature search and came up with nothing. I looked up both Toole and Everest and whilst there is plenty of discussion on lateral reflections, there is no discussion on frontal reflections.

I sent @Sean Olive a message (given that he and Toole did a seminal paper on lateral reflections). He could not recall any papers on frontal research off-hand, and he theorized that:

So it appears that the experts think that frontal reflections are detrimental. Lacking in Johansson's paper is information about exactly how early and how attenuated the frontal reflections are.

Now I have a question: dipoles radiate sound front and back. The rear wave would produce copious frontal reflections. Linkwitz seemed to believe in dipoles, after all he designed the LX521. My personal experience of dipoles is that they seem to produce more soundstage depth than a monopole (and yes, I have heard dipoles and monopoles side by side in the same room, although it was an informal unblinded listening session).

I have also been doing my own experiments using bookshelf speakers as ambient speakers. Using DSP, the bookshelves were equalized and delays adjusted so that they are 15ms delayed and -15dB compared to the main speakers. I then compare the effects of spaciousness with the ambience speakers placed in different parts of the room. I found that if these speakers were placed so that they fire towards the front wall, there is no additional sensation of spaciousness. Instead, what I hear is smearing. On the other hand, if the speakers are placed to the left and right of the listening position, a remarkable sense of spaciousness and envelopment is the result.

If frontal reflections do not create the illusion of soundstage depth, then what does? And, in light of this, are dipoles a fundamentally misguided design?

 

[dweeeeb2]

I’ve always assumed soundstage is in the recording. Reflections are distortions of the original. Like valves some distortions sound good.
Frontal (edit) reflections would be at a set delay which could be at odds with what's in the recording.
EDIT: When you say depth of soundstage do you mean the speakers move back, or depth between frontal sounds and recessed sounds?

 

[DVDdoug]

I' don't know... Most people seem to prefer regular forward-facing speakers. It's what most people buy. But some people love their Magnepans, or whatever.

So, I'm just going to say... "Personal preference".

Studio monitors are forward-facing.

With dipoles, bi-poles, and omnidirectional speakers it seems like the room would have a bigger influence. You might like the sound in one room but not in another. The room always makes a difference but with more reflected sound it should make a bigger difference.

 

[Keith_W]

I’ve always assumed soundstage is in the recording. Reflections are distortions of the original. Like valves some distortions sound good.
Frontal (edit) reflections would be at a set delay which could be at odds with what's in the recording.
EDIT: When you say depth of soundstage do you mean the speakers move back, or depth between frontal sounds and recessed sounds?

What I mean is that the musicians appear to be coming from behind the speakers, sometimes even behind the front wall.

 

[Todd68]

I bet a lot of people haven’t experienced soundstage depth and 3D like center images because you need the speakers out in the room further than most people care to do. For whatever reasons, small room, WAF or don’t like the look of it.

Also a big TV in the center doesn’t help. Having Nothing in the middle of the speakers and some sort of dispersion panels in the center helps.

 

[Blumlein 88]

Most of my life I've had panel speakers. Not so far apart, well out into the room. Seemed like better depth than monopoles. I've heard extensively some Mirage speakers which are bipoles. Well out into a large room. Did not seem to have the depth like dipoles, but more spacious than monopoles. In one instance I had some panels across the long dimension of a long room, widely spaced, and angled toward the LP like 30 degrees because the short dimension of the room was pretty short. The wall behind was pretty reflective. Rear sounds bounced at that angle (at the right frequencies of course) then off the side wall and then some off the rear and some to the LP, but by then had traveled around an extra 20 feet or roughly 20 milliseconds delayed from the front sound. In some ways this was the most uncolored and least room dependent I had heard those panels (they were Acoustat Twos). There was a dispersed sense of space with the right recordings. So all anecdotal information for what its worth to your question. The speakers really disappeared in that setup at the narrow LP.

 

[Todd68]

As far as “the speakers disappear” I had better luck with open baffle designs. I needed to experiment with the positioning of the speakers and room treatments. But once I heard what’s possible regarding soundstage and imaging it’s hard not to want that quality.

The way my system is setup and sounds is not as accurate as speakers or systems that measure better, but I don’t care about that so much. It’s about enjoying the music, no right or wrong as long as you’re happy with the results.

 

[Duke]

...are dipoles a fundamentally misguided design?

I agree with your observations, and imo dipoles are particularly demanding when it comes to set-up, and not just because they like to be fairly far out from the front wall.

Ime dipoles benefit significantly from sufficient toe-in such that the backwave energy mostly (if not entirely) bounces at an angle towards the sidewall, rather than directly towards the listening area. We tend to think of toe-in as aiming the front-firing energy but for dipoles and bipoles it aims the rear-firing energy as well.

Ime good dipoles set up well (which imo includes a nice long reflection path for the backwave enregy) often do a very good job of conveying soundstage depth.

 

[dualazmak]

Just for your possible interests, let me refer to two of my post maybe/possibly related to the topics discussed here...

- Not only the precision (0.1 msec level) time alignment over all the SP drivers but also SP facing directions and sound-deadening space behind the SPs plus behind our listening position would be critically important for effective (perfect?) disappearance of speakers: #687

- A new series of audio experiments on reflective wide-3D dispersion of super-tweeter sound using random-surface hard-heavy material: Part-4_Provisional conclusion to use Case-2 reverse reflective dispersion setting in default daily music listening: #929

 

[sigbergaudio]

I bet a lot of people haven’t experienced soundstage depth and 3D like center images because you need the speakers out in the room further than most people care to do. For whatever reasons, small room, WAF or don’t like the look of it.

Also a big TV in the center doesn’t help. Having Nothing in the middle of the speakers and some sort of dispersion panels in the center helps.

This is not true, I have a huge soundstage including depth, speakers are 15cm from the wall.

 

[Keith_W]

Thanks @Duke. I agree with your observation that dipoles should be toed in so that the backward energy is reflected towards the side wall. I don't currently own dipoles, but I have found that well set up dipoles tend to have (1) a lot of distance between the speaker and the front wall, and (2) are toed in.

But this does not explain what creates soundstage depth. Some people say it is in the recording, but some speakers are better at reproducing this than others. I have often found that well set up dipoles that are particularly good at this. If it is true that frontal reflections are detrimental, I have to throw my current understanding out the window. Then what explains soundstage depth? Phase coherence?

Also, does anybody know where Dr. Johansson got this from. Is it from a formal study, or is it his personal opinion? The studies on lateral reflections were performed in anechoic chambers with two speakers spaced horizontally, with the second speaker delayed and attenuated to simulate a reflection. Surely it wouldn't be too difficult to place the speakers one behind the other and repeat the experiment to simulate frontal or rear reflections. In fact I did simulate this myself by placing my ambience speakers behind the main speakers - I did not get increased depth, I got smearing.

If this is true, then it has implications as to how we should be treating our listening rooms - the front and rear walls should be treated with broadband absorbers to minimise frontal and rear reflections as much as possible, and dipoles should be avoided. In particular, we should avoid placing the listening position hard up against the rear wall.

 

[Duke]

If frontal reflections do not create the illusion of soundstage depth, then what does?

... But this does not explain what creates soundstage depth. Some people say it is in the recording, but some speakers are better at reproducing this than others. I have often found that well set up dipoles are particularly good at this. If it is true that frontal reflections are detrimental, I have to throw my current understanding out the window. Then what explains soundstage depth?

At the risk of oversimplifying, imo soundstage depth has to do with getting the spatial characteristics of the recording to dominate over the spatial characteristics of the playback room, which in turn can mean suppressing or manipulating the early in-room reflections.

 

[Emlin]

At the risk of oversimplifying, imo soundstage depth has to do with getting the spatial characteristics of the recording to dominate over the spatial characteristics of the playback room, which in turn can mean suppressing or manipulating the early in-room reflections.

That's not oversimplifying. It's totally right. If people want to listen to their rooms, fine. But if they want to hear the recordings then they need to minimise the affects of the room.

 

[Sokel]

If of any use,one of my requests to the people that fixed my room was exactly that,depth.
I had experienced it in good rooms and I wanted it.

So two things (three,including the recording which is the most important)
- enough toe in (even with monopoles)
- really treated front wall (both absorption and diffusion) with the speakers at a very small distance window to it (1m to 1.2m is the freedom they gave me,no more,no less) .

From the little I got,it's not only the reflections angle but both their amplitude and phase at a certain freq range (mid and up I guess) .

(that's for a real example if of any value)

 

[Emlin]

What I mean is that the musicians appear to be coming from behind the speakers, sometimes even behind the front wall.

I don't know how far your speakers are away from the front wall, but I would say always, not even sometimes.

 

[Emlin]

If of any use,one of my requests to the people that fixed my room was exactly that,depth.
I had experienced it in good rooms and I wanted it.

So two things (three,including the recording which is the most important)
- enough toe in (even with monopoles)
- really treated front wall (both absorption and diffusion) with the speakers at a very small distance window to it (1m to 1.2m is the freedom they gave me,no more,no less) .

From the little I got,it's not only the reflections angle but both their amplitude and phase at a certain freq range (mid and up I guess) .

(that's for a real example if of any value)

Almost all recordings, even mono ones, have a wealth of depth, so the recording is most certainly not the most important part. Getting to hear the recording is.

Toeing in speakers reduces side reflections. Always good in this context.

Room treatment may have the same effect, depending upon how it's done.

But start with speakers the have even dispersion characteristics, probably cardioid too.

 

[Keith_W]

At the risk of oversimplifying, imo soundstage depth has to do with getting the spatial characteristics of the recording to dominate over the spatial characteristics of the playback room, which in turn can mean suppressing or manipulating the early in-room reflections.

Thank you. This is a lightbulb moment for me.

So does this mean we should be acoustically treating the front and rear wall to get rid of detrimental reflections?

And, what implication does this have for dipoles and omnis? Fundamentally flawed in conception? Should we all be aiming for monopoles or cardioids?

 

[Emlin]

Thank you. This is a lightbulb moment for me.

So does this mean we should be acoustically treating the front and rear wall to get rid of detrimental reflections?

And, what implication does this have for dipoles and omnis? Fundamentally flawed in conception? Should we all be aiming for monopoles or cardioids?

The rear wall won't have as much affect, as the brain knows that the reflections are coming from behind and therefore have little to do do with the original sound.

Cardioid, yes.

 

[Emlin]

Also, there seems to a presumption among many that soundstages should be as wide as possible. That's why they set up their systems to sound as wide as possible, and are actually listening to the acoustic properties of their rooms rather than the recording. In doing so they also sacrifice depth.

Properly, all recordings have different soundstages and depths thereof, and a decent system should reflect that.

Would you want a recording made in a cathedral to have the same soundstage as one made in a toilet cubicle?

But that's what many seem to strive for.

 

[Duke]

Thank you. This is a lightbulb moment for me.

So does this mean we should be acoustically treating the front and rear wall to get rid of detrimental reflections?

And, what implication does this have for dipoles and omnis? Fundamentally flawed in conception? Should we all be aiming for monopoles or cardioids?

In my opinion, the problem with absorbing the backwave of a dipole or omni is that that energy is now GONE and cannot arrive as beneficial spectrally-correct later reflections. Most absorptive treatment is not broadband so it degrades the spectral balance of the reflections, removing the shorter wavelengths moreso than the longer ones, which imo is undesirable. Sometimes absorbing the backwave is the best we can do under the circumstances, but imo it would be better if that energy could arrive later and spectrally intact (or close enough), and from significantly different directions than the direct sound.

My unproven hypothesis is that the in-room reflections act as "carriers" for the reverberation tails on the recording, ideally delivering them spectrally intact (or close enough) from many directions, and thereby effectively presenting the ambience information on the recording. At the same time, imo it is desirable to minimize the "small room signature" of the playback room. There are different approaches to doing so, one of which would be pushing the arrival of the first strong horizontal-plane reflections back in time further than they would normally be in a room that size.

Ime success in this area is not limited to a specific loudspeaker enclosure type, but good off-axis behavior helps a lot. So imo cardioids are not inevitably superior, BUT designers who go to the trouble to use a cardioid enclosure are probably paying a great deal of attention to getting good off-axis behavior, and are using cardioid loading to extend that pattern control down lower than they otherwise could in a given enclosure size.

... there seems to be a presumption among many that soundstages should be as wide as possible. That's why they set up their systems to sound as wide as possible, and are actually listening to the acoustic properties of their rooms rather than the recording. In doing so they also sacrifice depth.

Properly, all recordings have different soundstages and depths thereof, and a decent system should reflect that.

Agreed. Ime there is a trade-off relationship between soundstage width enhancement via strong early same-side-wall reflections on the one hand, and soundstage depth & image precision on the other.

I find it immensely enjoyable when the sensation of acoustic space changes dramatically from one recording to the next. When that happens seems to me it must be the recording's spatial cues, rather than the playback room's spatial cues, which are perceptually dominant, because the playback room's acoustic signature does not change from one recording to the next. Hardcore subjectivist Harry Pearson advocated for pursuit of the "you are there" experience, and I think he got that right.