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Is mono testing biased under certain conditions?

TimVG

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I've been wondering about this for a while now. Many of you probably have read the results of the test organised by John Schuermann - comparing the Revel Salon2 vs the JBL M2, which is what first made me wonder about this.

Both of these loudspeakers are what can be considered excellent loudspeakers from an objective point of view, according to the comprehensive anechoic data available on both.

Yet in the blind test, the Salon2 was widely preferred across the board. It may be worth noting that while the difference in overall score was not extremely large - the Salon2 did take the lead on nearly all of the tracks played.

Reading through F. Toole's book it is stated that not treating the early lateral reflections is often preferred for loudspeakers with good (neutral) off-axis behaviour, and that the reason for this is because of the basic limitations of stereo reproduction. It helps with envelopment.

This makes me wonder if we can further conclude that this effect would only get exaggerated under mono circumstances - meaning that loudspeakers that have neutral off-axis behavior, but wider dispersion (stronger lateral reflections) would always have the upper hand compared to loudspeakers that are equally neutral, but have a more controlled dispersion pattern. Reading the comments from the participants it seems that the Salon2 'disppeared' more and 'imaged better' despite being played in mono.

Floyd Toole's comments said he felt it was statistical tie - he knows what the measurements look like. Yet in practice, one was preferred. So, for equally good (objectively speaking) loudspeakers, have we reached a point where mono testing has a certain bias installed?

I'd love to hear your thoughts.
 

aarons915

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I think mono testing is good and bad, it's definitely easier spotting differences in speakers in mono vs stereo but if you dig into the study that the mono testing was started from, the worst speakers were very close to the better ones in stereo, only in mono did they fall much lower in the preference scale. What that tells me is, in the real world, any speaker that measures decently is going to sound similar to a better speaker in stereo.
 

pozz

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Only thing I can think of is that mono testing likely won't apply well to speakers which don't have monopole-like dispersion patterns of bookshelves/monitors and floorstanders. Panel speakers come to mind.

And what about soundbars, which are like mini line arrays?
 
OP
TimVG

TimVG

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Listened to JBL M2 several times. Never impressed me. Subjectively, the highs sounded too “hot”. There is a plausible explanation for that:

https://www.reddit.com/r/audiophile/comments/3knjku/_/cw4hi5s

Could also be the result of using a 1,4" (1,5?) exit compression driver and making it flat on average over a 60° (30+/-) window up to 20kHz. Bound to be hot on axis above 10kHz. Having said that - haven't heard the M2 yet.
 

ernestcarl

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I wonder what the difference would be if one were to kill all/most direct/indirect reflections i.e. a dead(ish) room like in some studios.
 

Sergei

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Could also be the result of using a 1,4" (1,5?) exit compression driver and making it flat on average over a 60° (30+/-) window up to 20kHz. Bound to be hot on axis above 10kHz. Having said that - haven't heard the M2 yet.

Dr. Geddes explains very well what's going on inside a horn - provided the reader can follow his math. What follows is my grossly simplified, purely qualitative, interpretation of his teachings.

At a given frequency, a horn emits sound waves which come straight from the compression driver opening (Mode 0), and waves that are formed via reflections from the horn's walls. Mode 1 is formed by one reflection, Mode 2 by two reflections, and so on.

Only Mode 0 is desirable. If other modes are not suppressed, they introduce distortions perceived as delayed resonances (the characteristic horn's "ringing"). The amount of delay at a given frequency and mode, and strength of resonance, depend on the frequency and horn geometry. The higher the mode, the longer the delay.

One way to suppress the undesirable modes is to use the horn geometry that is "just right", so that there are no avoidable reflections: that's the rather non-trivial horn profile Dr. Geddes calculated. A contrasting illustration: an example of a "horn" with lots of avoidable reflections is a long straight tube.

Still, some reflections are unavoidable if we want the horn to be dispersing optimally. So, another way to deal with the reflections is to preferentially absorb higher modes: that's the reason Dr. Geddes recommends filling a horn with acoustic foam.

Qualitatively, this is how it works. Mode 0 passes through the foam just once and is attenuated. Mode 1 passes through the foam twice and thus gets attenuated significantly more than Mode 0. Mode 2 passes thrice and gets attenuated even more, and so on.

Once again, this is a grossly simplified explanation. An accurate one given by Dr. Geddes involves beautiful, brilliantly derived, yet sophisticated math, which requires some college-level education and persistence to fully comprehend.

JBL M2 neither uses Dr. Geddes optimal horn profile, nor employs acoustic foam to suppress the undesirables modes. Because of that, it doesn't sound like a neutral studio monitor or a smooth hi-fi speaker.

What is M2 good for? My take: it shall be very good for mixing music and broadcast material which will be predominantly delivered over typical PA systems, used at stadiums, concert venues, houses of worship etc. Such systems share the M2 general design outline.

I consider M2 a high-quality, gently-distorting PA speaker. Not distorting so much that mixing for a long time would cause fatigue. Yet distorting enough to give a proper feedback, so that the resulting master doesn't sound hugely distorted on real PA systems.
 

GrimSurfer

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Good explanation, @Sergei. Taking the foam approach, however, negates one of the strengths of a horn: high efficiency.

Agree wrt your points about PA speakers.
 

KozmoNaut

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The M2 should sound similar to JBL's other speakers using a similar or derived horn/waveguide design, from the humble 3-series up, just with significantly more power and dynamic capability behind it.

I would love to hear a well-setup set of M2s, to find out how similar they sound to the PA speakers I've played with. I'm the kind of person who would use a set of JBL PA speakers with dual 15" woofers and a set of 18" PA subs indoors, for that live rock concert experience.
 

eliash

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I've been wondering about this for a while now. Many of you probably have read the results of the test organised by John Schuermann - comparing the Revel Salon2 vs the JBL M2, which is what first made me wonder about this.

Both of these loudspeakers are what can be considered excellent loudspeakers from an objective point of view, according to the comprehensive anechoic data available on both.

Yet in the blind test, the Salon2 was widely preferred across the board. It may be worth noting that while the difference in overall score was not extremely large - the Salon2 did take the lead on nearly all of the tracks played.

Reading through F. Toole's book it is stated that not treating the early lateral reflections is often preferred for loudspeakers with good (neutral) off-axis behaviour, and that the reason for this is because of the basic limitations of stereo reproduction. It helps with envelopment.

This makes me wonder if we can further conclude that this effect would only get exaggerated under mono circumstances - meaning that loudspeakers that have neutral off-axis behavior, but wider dispersion (stronger lateral reflections) would always have the upper hand compared to loudspeakers that are equally neutral, but have a more controlled dispersion pattern. Reading the comments from the participants it seems that the Salon2 'disppeared' more and 'imaged better' despite being played in mono.

Floyd Toole's comments said he felt it was statistical tie - he knows what the measurements look like. Yet in practice, one was preferred. So, for equally good (objectively speaking) loudspeakers, have we reached a point where mono testing has a certain bias installed?

I'd love to hear your thoughts.

Since I am reading Toole´s book currently (half through, your issues where at least partially covered), I reflected also what I was doing in regard to room treatment and how it fits together with what he stated. When I moved my floor stand tower Dynaudio Focus220 speakers to my hut-shape attic, I had an awful lot of audible problems originating from first order room reflections, also due to the asymmetrical room properties but mainly because of the additional influence from the tilted ceiling.
Basiscally concentrating on what your topic is about, I found that damping first order reflections (these ones in visible mirror positions) made the stereo "picture" much clearer (for my perception) and also kind of "dry". The ultimate test though was the center image stability for voice and instruments during mono replay (both speakers fed with the same signal). With these melamin-resin foam absorbers the image became much more stable centered and often annoying "break-aways" from the center at certain frequencies became rather seldom. Also the mentioned "beyond the physical stereo width" perception became seldom.
I guess Toole is right by stating that the off-axis radiation and reflection plays some role in the overall frequency response, perceived in the listening position. So some decent accentuation of bass to lower mid frequencies (where the absorbers don´t work yet) can be explained by this kind of room treatment.

After all I can state for myself, these absorbers where a good investment, since my listening prefence is indeed more towards kind of an analytical "monitoring type" rather than "entertainment type" enjoying additional envelopment from reflections. Anyway, many records also bring their amount of envelopment or ambience with them, which is not lost thereby but accentuated...
 
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Wombat

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Listened to JBL M2 several times. Never impressed me. Subjectively, the highs sounded too “hot”. There is a plausible explanation for that:

https://www.reddit.com/r/audiophile/comments/3knjku/_/cw4hi5s

Individual over-sensitivity to HF is a common hearing condition. I would like to see this discounted before placing the blame on loudspeakers.

There are those who subjectively find such speakers(M2s) as realistic and many 'audiophile' speakers muted, me among them.

Measurements in the environment are the best constructive starting point.
 
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oivavoi

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Dr. Geddes explains very well what's going on inside a horn - provided the reader can follow his math. What follows is my grossly simplified, purely qualitative, interpretation of his teachings.

At a given frequency, a horn emits sound waves which come straight from the compression driver opening (Mode 0), and waves that are formed via reflections from the horn's walls. Mode 1 is formed by one reflection, Mode 2 by two reflections, and so on.

Only Mode 0 is desirable. If other modes are not suppressed, they introduce distortions perceived as delayed resonances (the characteristic horn's "ringing"). The amount of delay at a given frequency and mode, and strength of resonance, depend on the frequency and horn geometry. The higher the mode, the longer the delay.

One way to suppress the undesirable modes is to use the horn geometry that is "just right", so that there are no avoidable reflections: that's the rather non-trivial horn profile Dr. Geddes calculated. A contrasting illustration: an example of a "horn" with lots of avoidable reflections is a long straight tube.

Still, some reflections are unavoidable if we want the horn to be dispersing optimally. So, another way to deal with the reflections is to preferentially absorb higher modes: that's the reason Dr. Geddes recommends filling a horn with acoustic foam.

Qualitatively, this is how it works. Mode 0 passes through the foam just once and is attenuated. Mode 1 passes through the foam twice and thus gets attenuated significantly more than Mode 0. Mode 2 passes thrice and gets attenuated even more, and so on.

Once again, this is a grossly simplified explanation. An accurate one given by Dr. Geddes involves beautiful, brilliantly derived, yet sophisticated math, which requires some college-level education and persistence to fully comprehend.

JBL M2 neither uses Dr. Geddes optimal horn profile, nor employs acoustic foam to suppress the undesirables modes. Because of that, it doesn't sound like a neutral studio monitor or a smooth hi-fi speaker.

What is M2 good for? My take: it shall be very good for mixing music and broadcast material which will be predominantly delivered over typical PA systems, used at stadiums, concert venues, houses of worship etc. Such systems share the M2 general design outline.

I consider M2 a high-quality, gently-distorting PA speaker. Not distorting so much that mixing for a long time would cause fatigue. Yet distorting enough to give a proper feedback, so that the resulting master doesn't sound hugely distorted on real PA systems.

This is probably the best common-sense explanation I've ever read of HOMs. Thank you!

You need to hand it to dr. Geddes for intellectual honesty though: His business idea was making waveguide speakers, yet noone has done more than him to document the inherent challenges and problems with forcing sound waves through a waveguide.

But he thinks the trade-off is worth it, particularly with the foam hack, as he believes more narrow directivity is conducive to better imaging.

Going back to the topic of the OP, he has apparently done a study on stereo imaging, which shows that narrow dispersion is good for imaging. Which may indicate that there are some things that mono testing doesn't catch.
 

Hephaestus

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This is probably the best common-sense explanation I've ever read of HOMs. Thank you!

You need to hand it to dr. Geddes for intellectual honesty though: His business idea was making waveguide speakers, yet noone has done more than him to document the inherent challenges and problems with forcing sound waves through a waveguide.

But he thinks the trade-off is worth it, particularly with the foam hack, as he believes more narrow directivity is conducive to better imaging.

Going back to the topic of the OP, he has apparently done a study on stereo imaging, which shows that narrow dispersion is good for imaging. Which may indicate that there are some things that mono testing doesn't catch.

It seems that there is something about this narrow dispersion / stereo imaging "phenomenon". Professor Edgar Choueiri has been demonstrating his BACCH SP this year with Manger speakers which have pretty damn narrow HF dispersion (MSW dispersion narrows down relatively smoothly with two bumps @ 1.7kHz and 7kHz + some lobing effects over 3.5kHz)
 
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