Measuring the effect of acoustic treatments in the room

[ToTo Man]

Testing the audibility of rear wall reflections has inadvertently led me to discover a brand new (to me) listening position, about a foot BEHIND my speakers and facing the rear wall! (I'd removed the bass traps from the rear wall at this point and replaced them with a pair of Q7D diffusors). This is the first time I've tried this and I was expecting it to sound awful with rolled-off highs and a big hole in the middle of the soundstage, but it was unfathomably engaging.

The speakers completely disappeared and the room was filled with an incredibly wide, deep and spacious soundstage, and the tonal balance was unmolested. The only thing lacking was the ability to precisely locate the instruments across the stage, but the phantom centre imaging of lead vocals was superb. I've only listened to a few rock and jazz tracks in this position but I imagine it would work especially well with classical. It would seem that those who swear by omnidirectional loudspeakers have a valid argument!

I played a trick on my poor dad by leading him into the room blindfolded, spinning him around a few times to disorientate him, and sitting him in the new listening position between the speakers. I asked him where the sound was coming from and he was genuinely baffled (pun intended! ) and pointed in every direction apart from at the speakers! He also commented on how deep and spacious the soundstage was.

I also tried facing the front wall, keeping my chair in the same position between the speakers, but the illusion collapsed and the tonal balance became rolled-off in the highs. This supports my argument that the pinnae of the ear effectively block rear reflections at higher frequencies.

 

[tuga]

I think that I've posted this here before, a couple of snippets from "Acoustics 
Of
 Small Rooms" by Kleiner & Tichy:


Spaciousness and diffusivity

Localization of externalized single sound field components was shown to be fairly straightforward but dependent on many factors. Localization of sound field components that have identical sound levels at the ears will depend on further factors such as phase difference.
When sounds are correlated, such as a monophonic signal that is presented binaurally, the auditory event occurs inside the head, inside head localization (IHL). If the sounds at the ears are fully uncorrelated, such as two separate noise signals that are presented binaurally, there will be two auditory events, one at each ear.
An interesting effect can be heard when presenting a monophonic wide bandwidth noise signal in stereo (over loudspeakers or headphones) if the stereo signals are out of phase. The noise frequency components below 2 kHz are then perceived as spatially diffuse—having spaciousness— whereas those for higher frequencies are perceived as located between the loudspeakers (or for headphones, IHL occurs). The time difference in the low-frequency components provides phase cues that are ambiguous thus providing apparent sound field diffuseness, whereas the high-frequency sounds are analyzed by their envelopes and those will be identical at the two ears causing a located auditory event.
Similarly, when a wideband noise signal is provided over headphones to a listener and one of the headphones is fed with the signal delayed by a millisecond or more, the sound is perceived as diffuse.
What constitutes a diffuse sound field is thus different in the physical and psychoacoustic domains. In the latter, a diffuse sound field is that that provides non-locatedness of sounds or, alternatively phrased, that provides a sound that is located over all spatial angles (or rather upper hemisphere in a concert hall that has sound-absorptive seating).
In physics on the other hand, a diffuse sound field is defined as a sound field where all angles of sound incidence have equal probability, where the sound from each spatial angle is out of phase, and where the energy density is the same everywhere.
Obviously, the two ideas of what constitutes diffuseness are different in the two sciences. A physically diffuse sound field will also be psychologically diffuse but not necessarily the reverse. From the viewpoint of listening, it is of course the psychoacoustic properties that are of importance, not the sound field properties.


Auditory source width and image precision

As we listen to sounds, the apparent width of the auditory event, often called the auditory source width (ASW), will depend on many issues. To those listening to stereo or multichannel recordings of sound, it is quite clear that the width of the array of phantom sources treated by the recording or playback is determined by not only the layout of the loudspeaker setup in the listening room and the directional properties of the loudspeakers but also on the listening room itself. The more reflections arriving from the sides of the listening room, the wider will the ASW be. However, the ASW will be frequency dependent above 0.5 kHz and a 2 kHz sound arriving at ±45° relative the frontal direction will produce maximum ASW [38,39]. This is to be expected since the masking by direct sound is the smallest for this angle of incidence of early arriving reflections [16]. The ASW also depends on the low-frequency content of the signal, more low-frequency energy increases ASW [38,40,41]. Psychoacoustic testing shows that the spatial aspects of the early reflections are primarily determined by the reflection spectrum above 2 kHz [33].
Reliable data for sound reproduction in small rooms are difficult to find. A single omnidirectional loudspeaker judiciously placed close to the corner of a room may well create as large an auditory image as a conventional stereo loudspeaker setup placed out in the room as discussed in Chapters 9 and 11.
Using digital signal processing, the ASW can be made to extend far outside the bounds set by the stereo baseline. Sound field cancelation techniques


Symmetry

Early reflected sound will confuse hearing and make the stereo stage and its phantom sources appear incorrectly located or even blurred. As explained in Chapter 8 the listener’s placement of the phantom sources is dependent particularly on the transient nature of the sound that comes from the loudspeakers so it will be affected by the early reflected sound from the room surfaces. The early reflected sound will also affect the global auditory source width for an orchestra for example and may make it extend considerably beyond the baseline between the loudspeakers.
In asymmetric rooms where the walls on the left and right of the listener have different acoustic properties, the stereo stage may become biased towards the wall that reflects the most. The curve in Figure 8.23 shows the dependency more clearly for different levels of unbalance as applied to the center phantom source in a stereo loudspeaker system. The intensity will then be higher at that ear and the sound stage distorted. This distortion is usually compensated by changing the balance in amplification between the stereo channels.
At low frequencies in the modal region, symmetry may not be desirable since someone sitting in the middle of the room may be on or close to modal node lines. One way of avoiding such node lines is to make the room asymmetric in the low-frequency region.
This can be achieved by having an asymmetric rigid shell surrounding the inner room which is symmetric for mid- and high frequencies by suitably reflective side walls, ceiling, and floor. The inner room must be open acoustically to the outer shell at low frequencies, for example through ventilation vents, and similar large openings, for example at corners. In this way, one can have the desired listening position sound field symmetry for mid- and high frequencies while at the same time have asymmetric conditions in the modal frequency range. Bass traps to control the damping—and thus the reverberation times—of these modes can be placed between the outer and inner shell. It is important to remember though that noise transmission to the surrounding spaces will then be dependent on the sound isolation of the outer shell that must be physically substantial.

 

[ToTo Man]

Treating the rear wall with absorption

The treatment configurations included:

1) 2x GIK broadband Monsters centred on rear wall.
2) 2x GIK range-limited Monsters centred on rear wall.
3) 2x GIK broadband Monsters centred on rear wall with 2x GIK range-limited Monsters flanking.
4) 2x GIK broadband monsters centred on rear wall with 2x range-limited Monsters behind them.
5) 2x GIK range-limited monsters centred on rear wall with 2x broadband Monsters behind them.

Observations:
- The broadband Monsters, as expected, absorb the broadest spectrum of frequencies and reduce early specular reflections coming off the rear wall by the most.
- The range-limited Monsters reflect early specular reflections at mid and high frequencies at an attenuated level. Filtering the ETC by frequency will show more accurately what frequencies are reflected but you can get a pretty good idea of this by looking at the FR.
- The range-limited Monsters have more potent LF absorption than the broadband version but the difference is marginal. In my measurements there’s only a 0.4dB difference in absorption at very low frequencies, which is pretty disappointing. I am however only measuring two panels, I expect the performance delta would widen if I had a dozen of these in the room.
- Increasing the depth of the treatment by layering panels unsurprisingly increases LF absorption. Putting the range-limited Monster in front of the broadband Monster delivers better LF absorption than putting the range-limiting Monster behind. This confirms that the limp-mass membrane needs to be exposed to the wave front to do its job properly.

 

[the_bat]

Sorry if you've already covered this, but when I was trying to do some very rough measurements I found that moving the microphone a foot to so to the left and right of the central position gave significantly different results.

 

[ToTo Man]

Sorry if you've already covered this, but when I was trying to do some very rough measurements I found that moving the microphone a foot to so to the left and right of the central position gave significantly different results.

Yes, even moving it an inch can give different results, especially in the mid frequencies. I'm intentionally keeping the mic fixed in position for all these measurements so that the only variable being changed at the moment is the amount and position of the treatments.

 

[ToTo Man]

Now for the somewhat more sexy subject of rear wall diffusion.

I tested two types of diffusor on the rear wall, the GIK Alpha 6A and Q7D. The Alpha 6A is essentially a broadband Monster with thick veneered-MDF 2-dimensional binary sequence scatter plate on the front. It’s hard to tell what percentage of the scatter plate is open for absorption but eyeballing it I’d say it looks close to 50%? The Q7D is a traditional 1-dimensional QRD diffusor with N7 sequence.

As you can see from the FR graphs the Alpha 6A has identical absorption performance to the broadband Monster at low and mid frequencies. GIK ought to update their test data to show this because their own measurements imply that the Alpha has poorer LF absorption, which could put some people off choosing it over the Monster. The behaviour of these two panels begin to diverge above 2kHz, where the Alpha can be seen to begin reflecting/scattering HF back into the room and/or towards the listening seat.

Sadly I did not test the Alpha 6A units on their own without the range-limited Monsters flanking them, therefore you cannot use these graphs to directly compare their performance to the Q7D units.

When multiple Q7Ds are positioned side by side with no gap you are supposed to turn every other unit upside down to prevent lobing artefacts. The measurements show that turning one unit upside down does affect the ETC and FR, however I'm not sure my ears are golden enough to hear any difference? Regardless of how the Q7Ds are oriented they sound great to my ears and help to retain the liveliness in the room.

 

[Folkman]

Regardless of how the Q7Ds are oriented they sound great to my ears and help to retain the liveliness in the room.

Could you expand on that , did you play music to listen to what they do ?

This also could be of interest ....https://www.acousticsinsider.com/acoustic-measurements-some-bad-advice/

 

[ToTo Man]

Could you expand on that , did you play music to listen to what they do ?

This also could be of interest ....https://www.acousticsinsider.com/acoustic-measurements-some-bad-advice/

Gosh, I find describing the 'sound' of hifi equipment challenging enough but diffusion is on another level of complexity! This is only my second day of listening to music with the Q7Ds in place so I'm still getting acquainted with them. Also I literally only spent 10 minutes listening to both units top-side up before flipping one over, so I will at some point re-flip it to perform some critical listening in both orientations.

I was however previously using an 100cm x 100cm N23 diffusor that I had custom made by Bluetone Acoustics. This sat in the middle of what was my previous back wall (see pic in this post) and was to my ears preferable in sound to either an untreated wall or absorption.

I first treated my room with GIK broadband absorption in 2013 and did not take the plunge into diffusion until late 2021 as I was led to believe that diffusion is off bounds until all of your bass problems and first reflections are properly treated. I was also led to believe that my room was too small for proper QRD temporal diffusion because my listening position would be too close to them. I wish I'd have tried it sooner because I really like the effect they have on the sound of my room, regardless of whether or not I meet the 'minimum seating distance' criteria. If you sit REALLY close, like within a foot of it, then you can hear a colouration but I wouldn't say it's an unpleasant one. I really think diffusion is something one has to hear in person to appreciate what it does.

Learning from my own experiences and the insights I have gained from places like Gearspace, if I were doing the room treatment thing again, I'd have no hesitation in adding diffusion to my room from the outset. Also, I would not invest in off-the-shelf absorption products because they simply do not have the horsepower to deal with the most important bass issues (<100Hz) and you can end up sucking the life out of your room's sound above 200Hz by trying to buy enough units to take a small dent out of your <100Hz problems, especially if your starting out with a fully-carpeted room.

A Gearspace contributor has taken a particular interest in quest for a better sounding room and thinks that the best way to achieve my goal of preserving as much of my speakers' power response as possible whilst treating my modal and SBIR issues is to build DIY bass traps that absorb below 200Hz and reflect above 200Hz. This will basically entail treating my entire wall with a sufficient depth of earthwool and then covering 80% of its face with plywood sheet, with an open slat every 5.65ft which is the length of a 200Hz wave. I'm not sure if I have the stomach for such a drastic course of action!...

 

[ToTo Man]

I had thought about recording audio clips with the UMIK-1 as I played about with different treatments but decided it would add too much to my workload. Also I'm not sure just how useful it would be given the captured audio would be in mono as all spatial cues would be lost. I also suspect it would make the room sound 'wetter' than it is, as I remember recording an audio clip of my room several years ago when it was more heavily treated and when I played it back over headphones it sounded very vague and distant, which is the antithesis of what the room sounded like in person. I suspect audio clips would be better served by a stereo pair of mics specifically designed for performance capture.

 

[tuga]

Typical UK sitting rooms are quite small but many have a fireplace and those alcoves to the sides of the chimney breast could be used as built-in bass-traps.

I had thought about recording audio clips with the UMIK-1 as I played about with different treatments but decided it would add too much to my workload. Also I'm not sure just how useful it would be given the captured audio would be in mono as all spatial cues would be lost. I also suspect it would make the room sound 'wetter' than it is, as I remember recording an audio clip of my room several years ago when it was more heavily treated and when I played it back over headphones it sounded very vague and distant, which is the antithesis of what the room sounded like in person. I suspect audio clips would be better served by a stereo pair of mics specifically designed for performance capture.

In my experience we are unable to relate to recordings of audio playback of our own room let alone an unknown one. I find them worthless to be honest.

 

[AndyU]

@ToTo Man What are you trying to achieve?

 

[ToTo Man]

@ToTo Man What are you trying to achieve?

A smooth response and short decay times in the bass whilst retaining life and sparkle in the mids and highs, the banishing of the 75Hz SBIR issue I seemingly cannot escape from, plus an immersively wide and deep soundstage within which the speakers completely disappear, all in a room measuring just 4.1m x 3.8m. Is that too much to ask??!!

FWIW I suspect many of these problems would be solved if I simply moved back into my old, cold, north-facing room that measured a positively capacious 7m x 4.75m, but it would take me at least a year to clear out all the sh!te that's currently being stored in there!

 

[ToTo Man]

Absorbing the front wall

I was curious to hear the effect of putting broadband absorption at the first reflection point on the front wall, especially with regards to imaging and soundstaging. After listening to a few familiar tracks I can’t say I heard much difference for better or worse in this respect.

Unless I'm reading the ETC graph incorrectly, the front wall reflection arrives around 23ms after the direct sound which, AIUI, is long enough delayed from the early reflection ‘danger zone’ of 6ms not to cause significant problems with masking/blurring. It is also already significantly attenuated in level compared to the direct sound, which again makes the case for absorption questionable. It does reduce reverberation times, but a reduction in RT60 would arguably be better achieved by absorbing earlier reflections, such as those from the side walls and ceiling. Perhaps the front wall would be a more suitable candidate for first reflection absorption if I were to move my speakers closer to it, but the way I see it just now there are no upsides to using broadband absorption here.

 

[ToTo Man]

Absorbing the side walls

Using broadband absorption at the early reflection points on the side walls is by far the most audible of all the surfaces I’ve tried treating. The soundstage narrows and becomes confined to the space between the speakers. It reaches no wider than the outer (i.e. wall side) edge of the speaker baffle, unless the recording contains special processing to intentionally make sounds appear beyond the lateral peripheries of the speakers.

My immediate reaction was one of disappointment having lost this lateral width. I also noticed a reduction in ambience and shortening of spatial cues in the high frequencies, as if the air in my room had suddenly got heavier. There were, however, also some positives…

Image precision improved (both the clarity of the instruments and their placement in the lateral plane), as did resolution / low level retrieval. I also noticed an improvement in bass weight and definition. The bass sounded more even handed, especially in songs featuring D2, D#2 and E2 notes. I ran a frequency response sweep to check if my 75Hz SBIR null had magically been filled in. Alas, it hadn’t, in fact at first glance the low end FR looked almost identical to how it was before. However, by zooming into the area of interest, I could see that 70Hz-85Hz had improved by almost +2dB, so my ears weren’t deceiving me!

As I continued to listen with the 244 absorbers covering both near and farside speakers’ early reflection points I became more accustomed to the narrower and more focused presentation. When I then removed the panels a few hours later I experienced another slightly jarring acclimatisation period as my brain adapted to hearing more of the room.

So, where does this leave me? Good question! I hoped the addition of broadband absorption to the side walls wouldn’t be to my liking, as it would be nice to benefit from the view and natural light provided by my window, but I do think there is some merit in managing side wall reflections, just not with broadband absorption as it narrows the soundstage too much for my tastes.

That leaves me with the choice of polydiffusion (a reflective flat surface that's curved into a half cylinder to scatter equally in all directions) or, as I think @tuga suggested to me in my previous room treatment thread, angled reflectors to deflect the early side wall reflections away from the listening seat instead of absorbing them. I wasn’t in a position to try this before because my door was the early reflection point for my right speaker. However, now that my speakers are on the other wall I have ample space for deflector panels. The question is what to construct them from, as they need to be large and thick enough to reflect all the way down to the lower mid frequencies, but not so heavy that I cannot move them! Suggestions??

PS - I also took the opportunity to measure with a) only the nearside, and b) only the farside speaker reflections being absorbed, and have included those measurements below, however I’m not sure how much sense this makes as a reflection management strategy!

 

[tuga]

My immediate reaction was one of disappointment having lost this lateral width. I also noticed a reduction in ambience and shortening of spatial cues in the high frequencies, as if the air in my room had suddenly got heavier.

You should try angled reflective panels on the side-wall early-reflection zones (deflecting the reflection towards the back of the room) to create a reflection-free zone yet keeping a degree of ambience.

 

[ToTo Man]

You should try angled reflective panels on the side-wall early-reflection zones (deflecting the reflection towards the back of the room) to create a reflection-free zone yet keeping a degree of ambience.

I'm struggling to get my head around how angling the side walls prevents all early side wall reflections from reaching the listening seat. A wide-dispersion speaker 'sprays' its sound in a fan-like shape, so while angling the side wall may successfully redirect the early reflection that was previously hitting the listening seat, does it also not cause a new early reflection at the listening seat? I've tried to illustrate what I mean in the diagrams below:

 

[tuga]

I'm struggling to get my head around how angling the side walls prevents all early side wall reflections from reaching the listening seat. A wide-dispersion speaker 'sprays' its sound in a fan-like shape, so while angling the side wall may successfully redirect the early reflection that was previously hitting the listening seat, does it also not cause a new early reflection at the listening seat? I've tried to illustrate what I mean in the diagrams below:

You'll need a more accurate drawing tool than that.

D'Antonio, who created the RFZ layout, is probably the top studio designer. The BBC also produced a study which looks not too different in terms of re-directing the early reflections to the back wall:

 

[Nero]

How does it sound?

 

[tuga]

@ToTo Man , are you using this early-reflection zone calculator?

https://www.acoustic.ua/forms/calculator4.en.html

 

[Tony L]

There can be some advantages in a totally unconventional room layout if you are really hitting problems/don’t like the sound as-is, e.g. setting it up as a diamond, even trying a totally asymmetrical layout. Not something for me, I just couldn’t deal with the aesthetics, but maybe worth trying as an experiment given how open things are at present. It may help identify where the nodes/cancellations etc are.