tuga

Siegfried Linkwitz' thoughts on loudspeakers and domestic reproduction

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Has any of you read Linkwitz' thoughts on loudspeakers and domestic reproduction?

http://www.linkwitzlab.com/conclusions.htm

A few clippings which I concur with:

  • Loudspeakers
  • The best one can hope for with 2-channel sound reproduction is the illusion of listening into the recording venue. Physics does not allow the accurate reproduction of the original sound field with only two speakers.
  • Since sound reproduction is about creating an illusion it becomes very important to avoid or minimize any clues that would detract from the illusion. Such clues come from linear-distortions, such as frequency and polar response, and from non-linear distortions with their generation of tones and sounds that were not in the original.
  • Linear distortion - frequency response, polar response, resonance - affects primarily the timbre and clarity of a loudspeaker.
  • Non-linear distortion - intermodulation, harmonic, clipping - affects primarily the maximum tolerable sound pressure level.
  • When designing a loudspeaker it is essential to perform free-space measurements to see the effects of driver directivity and baffle shape on the important polar response.
  •  
  • Box loudspeakers
  • Typical box speakers have a generic sound due to their polar response, panel resonances, re-radiation through the cone and vented bass.
  • Bass from box speakers has more "punch" than from open baffle speakers, but is less "airy".
  • Vented bass speakers are resonant structures and store energy which is released over time. For accuracy, bass must be reproduced from sealed or open baffle speakers that are non-resonant.
  • Closed box speakers are best listened to from very close distance to minimize masking from an uneven room response.
  •  
  • Listening rooms
  • The room is rarely at fault. If it is comfortable for conversation and living in it, then it is also suited for sound reproduction. The problem is usually the inadequate polar response of the loudspeakers and their placement in the room.
  • Rooms should have lots of diffusive elements and not sound like a stuffed pillow if open baffle or omni speakers are used.
  • Placing absorbers at reflection points is the wrong approach. It only absorbs high frequencies and increases the difference between the direct sound and the delayed room response. It works against perceptually masking the room response as merely a copy of the direct sound.
  • Equalization for a certain response at the listening position is fraught with serious problems. DSP can do many things, but which acoustic inputs to take, and how to process them, is still very much at a research stage. It will change the sound you hear.
  • When I hear an unfamiliar loudspeaker in an unfamiliar room and it does not sound right, then I look for faults in the loudspeaker's design and placement long before I blame the room.
  •  
  • Listeners
  • People listen differently. Performing musicians and members of the audience are used to different perspectives and focus on different aspects of the sound. Both are valuable for analyzing a loudspeaker. People who only listen to loudspeakers and thus always compare loudspeakers are poor judges of accuracy.
  • Very few sales people of "high end audio" ever listen to unamplified life sounds. They are highly susceptible to marketing department suggestions.
  • Unbiased listeners have no difficulty recognizing accurate sound reproduction, even with hearing damage or with hearing aids.
  • Unfortunately, marketing departments and dealers think that bass and high frequencies need to be emphasized for products to sell. 
  • Some listeners prefer euphonic loudspeakers. Accurate, and thus neutral, loudspeakers are not that exciting unless the source material is.
  • I find it disappointing when loudspeaker manufacturers run extensive double-blind listening tests with trained and untrained listeners where they only compare loudspeakers to each other, but not to any live source. These are strictly preference tests within a given paradigm.
  •  
  • Source material
  • A loudspeaker can never do better than to accurately convert electrical signals into acoustic signals. Thus the source material determines ultimately how well an illusion can be created.
  • Recording is still an art, not a science. Two loudspeakers in a room cannot reproduce the original sound field. Surround sound could be science based, but today is far from it and mostly pan-potted mono.

He doesn't think much about the Toole conducted double-blind listening tests either... As I have mentioned before, they are about "taste" not accuracy.

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Here Linkwitz looks at Whatever happened to the quality of reproduced sound in the home?

+ On the plus side and on average the quality has improved due to CAD design & test .

  • But computer design models have limitations in how accurately they represent reality and in what they leave out.
  • As a result loudspeaker design has reached a plateau and is stuck in the box speaker paradigm

+ The potential in stereo sound has not been reached

- Insufficient attention has been paid to the radiation pattern

  • Vented box designs dominate. They produce a generic loudspeaker bass character.
  • Active loudspeakers have low customer acceptance though they give greater design freedom than passive crossover/equalizers and could improve the quality of speakers.
  • Practical listening room requirements have not been established. The audio industry has provided little guidance to the consumer on room acoustics and speaker placement.

+++ There is room for improvement!

+ The potential in the realism of stereo sound has not been reached

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Wow...this is one great post sir :)

Lots of things I had always taken for granted, seem to be challenged by Mr.Linkwitz.  Especially the room vs speaker interaction. And I was also considering getting my first reflection points treated this vacation. But Mr.Linkwitz seems to be suggesting against it. 

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3 minutes ago, newlash09 said:

Wow...this is one great post sir :)

Lots of things I had always taken for granted, seem to be challenged by Mr.Linkwitz.  Especially the room vs speaker interaction. And I was also considering getting my first reflection points treated this vacation. But Mr.Linkwitz seems to be suggesting against it. 

In my opinion, whether or not to do treat early-reflection zones depends on the type of speakers you are using as well as room width (distance to side-walls) and finally personal taste.

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@tuga

Great post on Linkwitz. I think the truth lies between Linkwitz, Geddes, Toole and Meyer (of Meyer Sound Labs).

I'd write up what those are, and where my philosophy is, but I doubt it would even fit on a page. 

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4 minutes ago, Metatron said:

@tuga

Great post on Linkwitz. I think the truth lies between Linkwitz, Geddes, Toole and Meyer (of Meyer Sound Labs).

I'd write up what those are, and where my philosophy is, but I doubt it would even fit on a page. 

Please do share. It will be interesting to see what the other masters think. I've got an entire weekend to read and digest all you can type sir :D

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7 minutes ago, Metatron said:

@tuga

Great post on Linkwitz. I think the truth lies between Linkwitz, Geddes, Toole and Meyer (of Meyer Sound Labs).

I'd write up what those are, and where my philosophy is, but I doubt it would even fit on a page. 

We ain't goin' nowhere. :whistle:

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Super Wammer

That the most sense I’ve read for a long time!

Great post @tuga!

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3 hours ago, tuga said:

We ain't goin' nowhere. :whistle:

Well, in short I agree with Toole on designing a speaker in an enclosure rather than a dipole. Linkwitz was spot-on with a lot of what you posted, but I think he conflates some things. For instance, he saw rooms as blameless to acoustic reproduction because we can talk in any room, or play a small orchestra in any room and it will be OK. Firstly, this is production not reproduction. He misses that recording takes place in one room and replay in another and as such there are affects of 2 rooms that we need to consider.

Second,  he believed that a pulsating sphere was the ideal speaker due to point source philosophy. Toole illustrates in his book that the sound fields from various instruments do not radiate uniformly in all directions. If we take a singer, they do not produce a sound that radiates backwards through their head as it does the front, and if we take a guitarist, the sound resonates in the sound box of the guitar and radiates forwards, but is mostly blocked from rearward travel by virtue of the guitarist. So the starting point for some of his philosophies on speakers is IMO, not right. I cannot think of a sound source that radiates in all possible directions evenly at once, except possibly explosive detonations at altitude and thunder.

Third, Linkwitz's comments on source material are spot on. We can only hope to reproduce the signal. But if a driver cone produces that signal in a forward direction, did the recorded medium dictate a rearward signal 180 degrees out of phase? It did not, but a dipole creates a rearward copy of the signal. He liked dipoles as they were relatively close to the omnidirectional concept (closer to the pulsating sphere), but able to be placed close to walls without creating excessive side-wall early reflections. While reducing side-wall early reflections is, I would argue, the correct approach, firing rearward is not, hence it is better to damp rearward sound in an enclosure. Rearward sound inevitably reflects off the rear wall and is typically the bass sounds due to their tendency towards omni-directional travel, so a listener will hear the direct bass from the forward excursion of a driver, and then if the speaker is 6ft from a rear wall, the backward radiated sound will arrive later smearing the bass and competing on loudness with later sounds travelling direct from the front of the speaker, which if on crossing paths will inevitably interact. To avoid phase issues in the bass, and blends of 'late' bass and 'current' forward it would be better to utilise boundary coupling like the D&D 8C with DSP to time align a rearward reflection wave with sound from the front of the speaker, or actually do what they do in studios: soffit mount the speakers so sound radiates from a wall. 

Now, if I were to design a speaker, there are a few requirements that I'd try to achieve based of the research from Toole, Linkwitz, Geddes, and Meyer that I do believe is accurate. I think of those men, Meyer is king, but much of his work is proprietary because he owns Meyer Sound Laboratories.

It's easier to explain my philosophy this way... sound works much like light, even for reflections, so we can understand speaker interaction with a room in the same way a light interacts. That is, we can light up the whole room (omni), a small area (spotlight), or use something that better controls the 'throw' of light to tailor where light (actually sound) goes.

So, imagine two torches towards the corner of your room. They are toed-in 30 degrees, but throw evenly 30 degress +/- the central axis. This means light/sound travels parallel to the near wall but does not interact with it. The same is true of the speaker on the opposite side. The light is strongest where it overlaps, but in the arc of coverage, each speaker provides even light over the area. We can equate this to SPL. So in essence we want even controlled dispersion of sound that avoids near wall reflections and yet provides a sweet spot from wall-to-wall. Now it's not perfect. Light from one speaker will reflect off the opposite side wall, but the delay here and amplitude drop-off from the reflection is enough that from an aural processing point of view in the brain, we know this is a reflection and don't use it for 'localisation' of sound, thus it doesn't interfere with the soundstage.

The closest speakers to this are cardioid speakers like the D&D 8C, or Kii Threes and pro equivalents like Barefoot's speakers. Why cardioid? Because it's a good way to ensure a rapid dropoff outside the angle you wish to disperse sound to. That is, I would want my speaker to maintain even SPL in the +/-30 arc, but otherwise fall off a cliff any wider than this. Therefore, any early side wall reflections benefit from perceptual masking by the greater SPL of the direct sound.

So that's the short form. 

Just to add... speakers must be sealed, not vented. As Tuga said elsewhere, this is a crutch and leads to undesired group delay and less accurate transient response. 

Yes, this design ethos practically removes your own room and makes it perceptually innocuous to playback. But then it would require that original venue ambient cues are recorded in the music, or at least faked at the mixing desk. A lot of music isn't recorded this way, so it might still not sound 'right', but that's where most of the problems lay. You can record for accuracy using processing to determine direct sound and remove the room, leaving the option of adding it at the mixing desk, or you can do a binaural recording to include reflections that are part of the venue's acoustic ambience and leave it alone. Both these approaches stand a chance of being faithfully reproduced on my philosophy, but music recorded outside these approaches (as is common) will never show the art of what is possible. But I suspect such a design will still sound better than most speakers on such program material.

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13 hours ago, tuga said:

Physics does not allow the accurate reproduction of the original sound field with only two speakers.

Damn I went to a gig and they had two speakers (well a stack of) each side of the stage - and according to this, physics does not allow me to accurately reproduce what I heard on two speakers with only two speakers.  Following that revelation I decided there was no point in reading the rest as I might have to much cognitive bias :doh:

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20 minutes ago, uzzy said:

Damn I went to a gig and they had two speakers (well a stack of) each side of the stage - and according to this, physics does not allow me to accurately reproduce what I heard on two speakers with only two speakers.  Following that revelation I decided there was no point in reading the rest as I might have to much cognitive bias :doh:

LOL. 

It's a bit OTT in my view. Mostly at gigs we hear sound from the front, not hearing the performers from the side or behind. Sure more speakers will create a facsimile of immersion if watching a film where it makes you feel part of the scene, but we don't actually need extra speakers for reproducing sound from a stage. It's one of the areas I think Toole over-eggs. It doesn't matter how many speakers we have, we cannot accurately reproduce each instruments true soundfield by speakers, only hopefully produce a better facsimile that fools the brain. Toole does say as much, but people focus on simply thinking more speakers results in better sound. Don't despair - stereo is still apt for most performances. Personally, I don't want surround sound doing the screams and shouts of an audience to make me feel 'there'. 

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Posted (edited)
8 hours ago, uzzy said:

Damn I went to a gig and they had two speakers (well a stack of) each side of the stage - and according to this, physics does not allow me to accurately reproduce what I heard on two speakers with only two speakers.  Following that revelation I decided there was no point in reading the rest as I might have to much cognitive bias :doh:

I wasn't referring to gigs/amplified sound or studio productions. With this kind of recording home reproduction may have the edge because PA is focusing mainly on loud and is a lot less accurate and unrefined. Studio productions are created on and for speaker reproduction. But neither the recording of a gig nor your pair of speakers will be able to recreate the original soundfield as listened from the audience (I'm sure you've noticed that the crowd clapping and cheering noise is not coming from where it should?).

Perhaps I should have written that Physics does not allow the accurate reproduction of the original sound field of a live unamplified musical event with only two speakers. This is so because, unlike speakers, instruments and humans radiate sound in every direction depending on the frequency/musical note that they are producing:

rbEfwoR.png

dKAPRrZ.jpg

And this sound is reflected by the venue's surfaces/boundaries and reaches the listener/audience from every direction:

GxEYV1B.jpg

.

To capture such a soundfield one needs a spherical mic array and to reproduce it one needs to sit inside a sphere of speakers in a preferably near-anechoic space:

Commercially-available-spherical-microph

OHczghT.jpg

20180921094408_BangOlufsenLab-GenelecFro

Edited by tuga

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Posted (edited)
9 hours ago, Metatron said:

Well, in short I agree with Toole on designing a speaker in an enclosure rather than a dipole. Linkwitz was spot-on with a lot of what you posted, but I think he conflates some things. For instance, he saw rooms as blameless to acoustic reproduction because we can talk in any room, or play a small orchestra in any room and it will be OK. Firstly, this is production not reproduction. He misses that recording takes place in one room and replay in another and as such there are affects of 2 rooms that we need to consider.

I agree with this.

9 hours ago, Metatron said:

Second,  he believed that a pulsating sphere was the ideal speaker due to point source philosophy. Toole illustrates in his book that the sound fields from various instruments do not radiate uniformly in all directions. If we take a singer, they do not produce a sound that radiates backwards through their head as it does the front, and if we take a guitarist, the sound resonates in the sound box of the guitar and radiates forwards, but is mostly blocked from rearward travel by virtue of the guitarist. So the starting point for some of his philosophies on speakers is IMO, not right. I cannot think of a sound source that radiates in all possible directions evenly at once, except possibly explosive detonations at altitude and thunder.

I'm would also disagree that the pulsating sphere is the best speaker for stereo but Toole is wrong too (see previous post, and if you can read french see also this slide presentation by Le Cleac'h -> http://www.melaudia.net/zdoc/distorsion_de_phase.pdf )

9 hours ago, Metatron said:

Third, Linkwitz's comments on source material are spot on. We can only hope to reproduce the signal. But if a driver cone produces that signal in a forward direction, did the recorded medium dictate a rearward signal 180 degrees out of phase? It did not, but a dipole creates a rearward copy of the signal. He liked dipoles as they were relatively close to the omnidirectional concept (closer to the pulsating sphere), but able to be placed close to walls without creating excessive side-wall early reflections. While reducing side-wall early reflections is, I would argue, the correct approach, firing rearward is not, hence it is better to damp rearward sound in an enclosure. Rearward sound inevitably reflects off the rear wall and is typically the bass sounds due to their tendency towards omni-directional travel, so a listener will hear the direct bass from the forward excursion of a driver, and then if the speaker is 6ft from a rear wall, the backward radiated sound will arrive later smearing the bass and competing on loudness with later sounds travelling direct from the front of the speaker, which if on crossing paths will inevitably interact. To avoid phase issues in the bass, and blends of 'late' bass and 'current' forward it would be better to utilise boundary coupling like the D&D 8C with DSP to time align a rearward reflection wave with sound from the front of the speaker, or actually do what they do in studios: soffit mount the speakers so sound radiates from a wall. 

My experience is limited but I tend to agree as it seems quite logical, more so if we consider that two-mic (real) stereo does not allow the accurate reproduction of the original soundfield and so we should avoid introducing even more interference that will hinder the illusion.

9 hours ago, Metatron said:

Now, if I were to design a speaker, there are a few requirements that I'd try to achieve based of the research from Toole, Linkwitz, Geddes, and Meyer that I do believe is accurate. I think of those men, Meyer is king, but much of his work is proprietary because he owns Meyer Sound Laboratories.

I've never heard of Meyer before. :oops:

Could you post a couple of links?

9 hours ago, Metatron said:

It's easier to explain my philosophy this way... sound works much like light, even for reflections, so we can understand speaker interaction with a room in the same way a light interacts. That is, we can light up the whole room (omni), a small area (spotlight), or use something that better controls the 'throw' of light to tailor where light (actually sound) goes. 

So, imagine two torches towards the corner of your room. They are toed-in 30 degrees, but throw evenly 30 degress +/- the central axis. This means light/sound travels parallel to the near wall but does not interact with it. The same is true of the speaker on the opposite side. The light is strongest where it overlaps, but in the arc of coverage, each speaker provides even light over the area. We can equate this to SPL. So in essence we want even controlled dispersion of sound that avoids near wall reflections and yet provides a sweet spot from wall-to-wall. Now it's not perfect. Light from one speaker will reflect off the opposite side wall, but the delay here and amplitude drop-off from the reflection is enough that from an aural processing point of view in the brain, we know this is a reflection and don't use it for 'localisation' of sound, thus it doesn't interfere with the soundstage.

The closest speakers to this are cardioid speakers like the D&D 8C, or Kii Threes and pro equivalents like Barefoot's speakers. Why cardioid? Because it's a good way to ensure a rapid dropoff outside the angle you wish to disperse sound to. That is, I would want my speaker to maintain even SPL in the +/-30 arc, but otherwise fall off a cliff any wider than this. Therefore, any early side wall reflections benefit from perceptual masking by the greater SPL of the direct sound

I agree entirely. I use 3D modelling lighting simulation and rendering software professionally and one can easily simulate how different boundary surface materials and surface colours affect the final result.

The first problem that we have with any of these topologies is crosstalk, or the overlapping of the beams at the listening spot which creates auditory confusion and acoustic interference. The solutions are headphones, a partition between the speakers and right up your nose, or DSP, but none of them really works in my opinion:

8FhOhbu.jpg

.

The second problem is that a conventional box speaker radiate differently depending on the frequency:

U06lVIj.jpg
And this is why, as you've mentioned, the D&D cardiod bass approach makes sense.

.

Horns, which produce a well controlled and narrow dispersion are a good option above that range:

unofig4.jpg

Fig.4 Avantgarde Uno, lateral response family at 50", from back to front: responses 90 degrees-5 degrees off-axis, reference response on design axis, responses 5 degrees-90 degrees off-axis.

unofig5.jpg

Fig.5 Avantgarde Uno, lateral response family at 50", normalized to response on design axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.

Read more at https://www.stereophile.com/content/avantgarde-acoustic-uno-series-two-loudspeaker-measurements-part-2#ZbqZr5g2B8bXdov5.99
Edited by tuga

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Posted (edited)
11 hours ago, Metatron said:

Yes, this design ethos practically removes your own room and makes it perceptually innocuous to playback. But then it would require that original venue ambient cues are recorded in the music, or at least faked at the mixing desk. A lot of music isn't recorded this way, so it might still not sound 'right', but that's where most of the problems lay. You can record for accuracy using processing to determine direct sound and remove the room, leaving the option of adding it at the mixing desk, or you can do a binaural recording to include reflections that are part of the venue's acoustic ambience and leave it alone. Both these approaches stand a chance of being faithfully reproduced on my philosophy, but music recorded outside these approaches (as is common) will never show the art of what is possible. But I suspect such a design will still sound better than most speakers on such program material.

I agree with your assessment that studio productions (akin to a photographic collage) do, for a lot of people, benefit from room reflections. I think that Toole's research, which is based on tasting, does support this in a way.

What I don't agree is that close-mic'ing (needed to remove venue sound) can accurately capture the original event because the excessive proximity will change tonal balance and exaggerate mechanical or mouth noises (I know some audiophiles love them but you just can't hear them live, not from the first row let alone the best seat in the house).

.

A couple of images to illustrate the difference between a documental approach to recording:

DSCF2204.JPG

.

And a studio production (a fabricated collage consisting on a mix of several "takes"):

QjqWz3U.jpg

Edited by tuga

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