Evidence-based Speaker Designs

[andreasmaaan]

All that ctrl wrote above is true, and as far as I know there is little study around how the first 15ms of reflections affects stereo image in a stereo listening context. Most of the study seems to be around concert halls (or control rooms) and how clarity is determined (C50, C80 etc) because early energy versus late energy can impact on musical and speech intelligibility. But concert halls are completely different to stereo listening rooms, so I do not think much of that applies.

I searched long and hard, and the only study I could find concerning small rooms was the one mentioned in post #949.

 

[ctrl]

I searched long and hard, and the only study I could find concerning small rooms was the one mentioned in post #949.

I can't remember, but has this been mentioned before?
Imamura et al 2013: Influence of First Reflections in Listening Room on Subjective Listener Impression of Reproduced Sound

I do not have access to the original paper, but you can find summaries of it (THE EFFECTS OF A VERTICAL REFLECTION ON THE RELATIONSHIP BETWEEN LISTENER PREFERENCE AND TIMBRAL AND SPATIAL ATTRIBUTES - THOMAS ROBOTHAM):

Imamura, Marui, Kamekawa, and Nakahara, (2013) recently carried out an experiment to see how this directionality of early reflections within a sound field is perceived by a listener.

Using a dummy head, nine impulse responses were taken with various patterns of acoustic treatments using absorptive panels. These were then convolved with three music stimuli. The author notes that the difference of RT was almost equal throughout the patterns and therefore, specifically
focused on early reflections. Through headphones, subjects were asked to rate stimuli against each other using evaluative terms on individual listening impressions.

Terms used were: timbre brightness, width of sound image, envelopment, clarity, timbre naturalness, reverb suitability
and listeners’ preference. Out of these, width, envelopment and clarity displayed significant differences throughout the nine arrangements. It was concluded that as more first lateral reflection points are covered with absorption, width of sound image will narrow and also envelopment will lower. Vertical panels were not added individually, but simultaneously with front wall absorption thus the influence of early ceiling reflections cannot be solely assessed.
However, the author points out that absorption at these points not only decreased sound image width but increase clarity.

Imamura, Marui, Kamekawa, and Nakahara, (2014) further investigated perceived clarity based on previous results and simulated reflections via a lateral loudspeaker array varying directionality and delay times of reflections. The paper concludes by stating lateral reflections have a strong effect on perceived spatial clarity, apparent source width and listener envelopment. However this investigation did not incorporate vertical reflections, when previous results stated that increased clarity was also in conjunction with ceiling absorption.

In the linked pdf-file you will also find many interesting facts about the perception of vertical reflections and localisation - and of course more about early reflections.

Update:
As @PaulD has already pointed out, the requirements for a studio control room are completely different from those of a listening room at home.
The research by Imamura et al makes a good case for the fact that nobody wants to do without the early reflections when listening to music.

 

[PaulD]

I found the paper on an university site. I have not read it yet, but it looks interesting from glancing at it, and it cites Toole's work so that's good.

 

[andreasmaaan]

I can't remember, but has this been mentioned before?
Imamura et al 2013: Influence of First Reflections in Listening Room on Subjective Listener Impression of Reproduced Sound

Thanks, very interesting study, and actually somewhat at odds with most of the other research mentioned in the thread so far.

However, I'm a bit cautious to accept its findings without more research, as the stimuli were music samples that had been convolved with impulse responses taken from a dummy head in the listening room. Since the individual HRTFs of the test subjects were not used, it's unlikely IMO that the spatial information contained in the impulse responses translated correctly to the test subjects. This may explain IMHO why in this study there appeared to be no clear relationship between perceived spatial attributes and the direction(s) of early reflections.

Also, just to clarify, when I said that after searching long and hard I could only find one study, I was specifically talking about studies in which early reflections were the independent variable and phantom image localisation was the dependent variable.

 

[ctrl]

However, I'm a bit cautious to accept its findings without more research, as the stimuli were music samples that had been convolved with impulse responses taken from a dummy head in the listening room.

Toole and Olive have also used binaural recordings for some research. The use of binaural recordings via dummy head for easier evaluation is actually quite common practice.

Thanks, very interesting study, and actually somewhat at odds with most of the other research mentioned in the thread so far.

It is difficult to say whether it agrees in detail with other research, as the paper does not make any statement about the specific absorption behaviour of the glas wool panels used.

But it fits in with the statements and quotations of Toole in "Sound Reproduction":

For example, the perception described at threshold as “image shift or spreading” may seem like a negative attribute, but when it is translated into what is heard in rooms, it becomes “image broadening” or apparent source width (ASW), which are widely-liked qualities.

Barron and Marshall (1981) evaluated “spatial impression,” concluding that it is strongly related to the proportion of sound arriving from the side, compared to that arriving from the front. This led to a “lateral fraction” method of examining sound fields. Since sound from the sides generates low IACC, the relation with the Ando results is established. Their results showed strong front-back symmetry; reflections from the front and rear were about equally effective at generating spatial impression (although not close to the medial plane). Those that arrived from the rear “were not perceived as coming from behind, but just produced a pleasant sense of envelopment” (p. 218).

When stereo listening tests were done in the two versions of the room, it was found that the condition with absorbing side walls was preferred for monitoring of the recording process and examining audio products, whereas reflective side walls (which reduced IACC, [=interaural cross-correlation]) were preferred when listeners were simply “enjoying the music.”

As might be expected, reflective side walls resulted in a “broadening of the sound image.” Adding absorption to the front wall, behind the loudspeakers, reportedly improved image localization and reduced coloration.

Memo for Listening room recommendations: add sound absorbing material to front wall.

 

[tuga]

In a control room the engineer needs to hear the early reflections in the studio and recording, so they need to suppress the early reflections in the smaller listening space (control room). It was about being able to accurately hear what was being recorded

So iI'm not the only one thinking that suppressing the early reflections or diverting them away from the listening spot results in a more accurate reproduction of the recorded signal.

 

[andreasmaaan]

Toole and Olive have also used binaural recordings for some research. The use of binaural recordings via dummy head for easier evaluation is actually quite common practice.

I agree it's common. I'd still primarily trust the results of direct testing though - in particular when the study concerns spatial perception.

To get a sense of the limitations, listen to a binaural recording taken on dummy head.

I am curious though, which Olive/Toole studies are you aware of?

It is difficult to say whether it agrees in detail with other research, as the paper does not make any statement about the specific absorption behaviour of the glas wool panels used.

It disagrees with other research in that it does not suggest that lateral reflections are any more likely to generate perceptions of spaciousness and ASW. In the direct studies, lateral reflections are shown to be far more effective at doing this, an effect which is attributed to their lower interaural cross-correlation.

The interesting thing about this headphone study is that perceptions of positive spatial effects seemed to correlate most with reflections that were delayed the most, regardless of angle of incidence. I can't help wondering whether the use of a dummy head and headphones tended to distort the spatial information to the extent that the delay alone becamethe determinative factor.

Ofc, I admit I'm only speculating here...

 

[tuga]

I've just started a new topic where I uploaded an interesting paper on this subject of early reflections:

The myth of "harmful" early reflections
Klaus Rampelmann

https://www.audiosciencereview.com/...ful-early-reflections-klaus-rampelmann.12853/

 

[ctrl]

I am curious though, which Olive/Toole studies are you aware of?

Toole, Sound Reproduction:

Olive et al. (1995) published results of an elaborate test in which three loudspeakers were subjectively evaluated in four different rooms. Figure 11.2 shows the rooms and the arrangements within them. The rest of this section is based on the account in Toole (2006)....

Binaural recordings were made of each loudspeaker in each location in each room, and the tests were repeated, only this time with listeners hearing all of the sounds through calibrated headphones. All tests were double blind. In each room, three loudspeakers were evaluated in three locations for each of three programs. The whole process was repeated, resulting in 54 ratings for each of the 20 listeners. These were the results from a statistical perspective:
■ “Loudspeaker” was highly signifi cant: p = 0.05.
■ “Room” was not a signifi cant factor.
■ The results of live and binaural tests were essentially the same.
A possible interpretation is that the listeners became familiar with—adapted to—the room they were in and, this done, were able to accurately judge the relative merits of the loudspeakers.

Olive, Binaural Room Scanning (a more sophisticated version of binaural recording )
http://seanolive.blogspot.com/2009/03/binaural-room-scanning-powerful-tool.html
https://seanolive.blogspot.com/2009/04/binaural-room-scanning-part-2.html

 

[localhost127]

The idea that all early reflections below 15ms are extremely bad for the stereo image and inevitably lead to sound coloration is often found in forums.
Sometimes it is also demanded to suppress the reflections up to 15ms completely.

it's really dependent upon end-state response and room requirements. a studio (control/mix/mastering room) has requirements to be an objective, critically-accurate reproduction space where mix/mastering decisions can be made without the bounded acoustical space masking or imposing distortion or changes in perception with respect to the direct signal - and thus where said decisions translate to a myriad of other reproduction environments and sources. ie, it is to be an objectively "neutral" room.

the Inter Signal Delay gap (ISD, redefined from Manfred Schroeders "ITD" from concert hall research) - is an effectively anechoic time-period where-by only the direct signal is allowed to be processed, and is usually defined from 15-25ms in length. other constraints (such as a tracking room ISD, which should be 2-5ms shorter than the control room's ISD) may be the primary constraint on this value, but not always. the absesnse of a tracking/recording ISD does not imply the reproduction room's ISD is no longer relevant.

what's missed in many of these discussions is that this defined ISD defines the psycho-acoustic size of the bounded space. a smaller room that imposes earlier high-gain first-order reflections will be perceived to be a "smaller space" than that of a room who imposes first indirect energy at a later point in time. therefore, delaying in time (via absorption or redirection) when the first indirect energy impedes the listening position will impart perception of an acoustically larger space than the room's natural boundaries allow. this, from a reproduction standpoint, will also allow one to "hear more into" the tracking/recording room (or digital reverb fx) of the source material than would otherwise.

we know from toole's research that loudspeakers with poor horizontal off-axis response should have sidewall/lateral sparse indirect first-order specular reflections attenuated (ie, with broadband treatments such as porous absorber) - and for those with good off-axis/CD polar response, he states "it is a matter of taste" whether one prefers a (to some) subjectively pleasing apparent source widening (at the expense of image accuracy) or to absorb if one has requirements for an objectively accurate reproduction space in terms of imaging, localization, and intelligibility.

but the key is the loudspeaker characteristics in terms of polar response and the impedance of the boundaries - something that is rarely defined at the start of the conversation and of which can help decide someone of "what to do".

I think that a specification that is intended for studio recordings was transferred to the hifi world. For studio recordings the requirement is to avoid early reflections up to 15ms, because otherwise the recordings are "blurred" (without an increase in spatiality).
This is somehow plausible, since early reflections always occur when listening in living rooms, it is certainly not an advantage if the music signal contains additional early reflections.

typical home residential living rooms are naturally (physical and acoustically) small. the first-order reflections arrive 1) earlier in time and subsequently, 2) higher in gain. attenuating these first-order reflections is a relatively benign and effortless way to increase the perceived size of the reproduction space - and thus allow the ear-brain to more easily hear into the indirect sound-field present within the recording (if applicable).

this will also have positive effects on the frequecy domain (low-mid band), as the higher magnitude reflections constitute to larger peaks/nulls within the subsequent comb-filter interference pattern generated by the superposition of the direct and first-order reflections as viewed within the frequency response.

A very important aspect of early reflections in living rooms is their masking. The negative effects of early reflections like comb filter effects or roughness and beating are masked by other reflections.

if you want to "fill in comb filter effects", sparse specular reflections from bare sidewalls are not optimal. instead, diffusers (such as reflection phase gratings) will convert single sparse reflection into many reflections both spatially and temporally dispersed. the magnitude variations of the comb-filtering thus can be greatly reduced, because of the phase difference of the array of delayed reflections (due to varying depths of the wells) and also their attenuation due to the spreading (diffraction lobes) across a given hemisphere.

diffusers actually create "more reflections" which in turn (superposition with the direct signal) create "more comb-filters" which is overall beneficial and better "well-mixing" of the indirect sound-field over a given area in 3space. comb-filtering isn't inherently bad - just sparse comb-filtering. diffusers exist to create more and denser "comb-filters".

This means that a reduction of the reverberation time, for example, can lead to a stronger perception of the negative effects of early reflections.

a concert hall has sufficient volume (and mean free path) as to support the development of a statistically random-incidence reverberant sound-field. small acoustical spaces such as home residential rooms lack the volume to support "reverberation time". in small rooms, there exist no reverberant field that is high enough in gain to mask the early reflections (contrasted to a typical concert hall).

Also very negative are single very early (little attenuated) reflections, such as those caused by a mixing console in the studio or by the living room table at home.

and these are highly destructive due to their very-early (1-3ms) arrival time, which can have drastic image-shift effects (especially apparent with binaural source material). further compounded by the fact they are colored due to size of desk/console with respect to wavelength.

The whole context is much too complex for a simple rule like "avoid early reflections up to 15ms" or "...never feed it after midnight".

requirements and real-world loudspeaker characteristics drive the recommendations. per toole, a loudspeaker with poor off-axis response will almost always want sidewall reflections absorbed due to the spectral changes of the reflection and subsequent tonal changes imposed on perception of the direct signal. a well-behaved off-axis loudspeaker opens to the recommendation to allow small room natural sidewalls to impose sparse early first-order reflections, if subjective preferences call for such (at the expense of accuracy).

 

[localhost127]

It is an extremely unreliable measurement.

the ETC is a time-domain measurement. how exactly is the Envelope Time Curve "extremely unreliable measurement" when analyzing the time (and thus corresponding total flight path distance) arrival of indirect specular energy?

you could have electronic propagation delay (in the soundcard, for example) - but that is appropriately addressed by utilization of a simple loopback. something any competent operator is aware of.

You can have two ETCs that look the same but be from very different sounds. Dr. Toole shows that in his book:

View attachment 58420

i'm not sure where or how this would be relevant? acoustic engineers don't use the ETC to identify sub-500hz band-limited reflections of speech in small acoustical spaces (studios nor home residential reproduction rooms). the ETC is commonly used in small rooms to identify spurious indirect energy arrivals that can subsequently be traced back to their incident boundary where subsequent treatment can be placed to achieve a pre-desired set of design requirements (both time and frequency domain).

or perhaps to verify the time-arrivals and gain of the indirect reflections from both Left and Right loudspeakers are equal. or perhaps to verify precense of alignment issues in a multi-component loudspeaker. etc...

ETC should only be used when you know what you are looking for, not the other way around.

the ETC is used to 1) identify spurious sources of indirect energy, 2) verify end-state time-domain response once all treatment and/or room construction has taken place (just as you would verify frequency domain via the 2d frequency response against your initial requirements). i'd be interested to hear your process of using the ETC to "know what you are looking for" when treating a myriad of rooms to a wide range of design requirements. what is your experience here?

i mean you can go to home depot and stand next to the specialty tools section and proclaim at the top of your lungs how "this tool isn't needed for you" and "if you use this tool to do X instead of Y it will be unreliable" - but what exactly is the purpose?

it's simply a tool. the operator must always be smarter than the tool and thus understand its use, limitations, etc.

 

[Ilkless]

10/5/20 - Added ME Geithain and Sausalito Audio/Grimani Systems entry (edit: and GGNTKT).

Look at these quasi-anechoic spins, I've never seen anything like it:

Would be a treat for such a speaker to be tested on the NFS. Sausalito Audio/Grimani Systems are clearly engineering-focused people making wide-directivity loudspeakers, befitting the spirit of this thread. Hence the addition of them into OP, together with ME Geithain for their popularisation of cardioid loudspeakers.

I also just found out about Ocean Way's large waveguide active speakers, which are surprisingly affordable for what they are. However, they will not be put on the directory in OP unless I can find decent measurements of their directivity and FR.

 

[q3cpma]

Thanks for continuing to curate this list for newcomers. Should add the recently discussed GGNTKT.

 

[Ilkless]

Thanks for continuing to curate this list for newcomers. Should add the recently discussed GGNTKT.

Thanks, slipped my mind. Added. Tagging @roland{at}GGNTKT in this thread too, so he knows his speaker has an entry here.

 

[q3cpma]

Thanks, slipped my mind. Added. Tagging @roland{at}GGNTKT in this thread too, so he knows his speaker has an entry here.

I don't think it's inspired by the LSR waveguide. It's mostly a biradial horn, like some older JBLs.

 

[Ilkless]

I don't think it's inspired by the LSR waveguide. It's mostly a biradial horn, like some older JBLs.

The diffraction slot and wider directivity seems to suggest otherwise though, I thought biradials were around 90-degrees.

 

[Eurasian]

10/5/20 - Added ME Geithain and Sausalito Audio/Grimani Systems entry (edit: and GGNTKT).

Look at these quasi-anechoic spins, I've never seen anything like it:

View attachment 62813
Would be a treat for such a speaker to be tested on the NFS. Sausalito Audio/Grimani Systems are clearly engineering-focused people making wide-directivity loudspeakers, befitting the spirit of this thread. Hence the addition of them into OP, together with ME Geithain for their popularisation of cardioid loudspeakers.

I also just found out about Ocean Way's large waveguide active speakers, which are surprisingly affordable for what they are. However, they will not be put on the directory in OP unless I can find decent measurements of their directivity and FR.

I'm sending an HR5 to Amir. I am very interested in the Klippel data as Ocean Way makes some pretty bold claims about this loudspeaker. I like the mini-Ubangi look, too.

 

[Icboschert]

I'm sending an HR5 to Amir. I am very interested in the Klippel data as Ocean Way makes some pretty bold claims about this loudspeaker. I like the mini-Ubangi look, too.

Really pumped for this one

 

[Eurasian]

Me too! I hope we're not disappointed.

 

[AudioStudies]

I haven't read the whole thread yet, but can anyone tell me if Focal speakers have been discussed or will be discussed? They make some passive radiator models, both two-ways and three-ways that use aluminum inverted dome tweeters and a kind of flax material in the woofer/mid.