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What are the acoustic consequences of having a tall rack of equipment between two speakers?

The speakers themselves as far as these encounters or what about the rooms ?
We see the measurements showing they don't project just forward. Sound waves rebound when they encounter something. That's obvious. I am sure you are going after something else here, because I know you know it after having seen several of your contributions to this forum. Are you baiting for something? :-D
 
We see the measurements showing they don't project just forward. Sound waves rebound when they encounter something. That's obvious. I am sure you are going after something else here, because I know you know it after having seen several of your contributions to this forum. Are you baiting for something? :-D
You're overthinking things, was more interested in what I asked.
 
You're overthinking things, was more interested in what I asked.
In which case can you rephrase your question? I am not sure what you mean about "speaker encounters". They don't tend to show up at my place unannounced like good friends. :cool:
 
I think a lot will depend on the directivity of the speaker. But in general i think it's safer to put the equipment rack a bit behind the speakers (if there is space). Some of the best listening rooms i've been in did that.

But i don't think it's a big problem at all, your furnishing of the room (seats, tables, ...) will have a way bigger effect than the rack in the phantom center of the speaker setup. That is 90° off axis, where the signal mostly is already a lot weaker than closer on axis, and bass (where it gets less directive) won't be stopped by a stack of electronic equipment, it resonates trough it. But if you want to nitpick, put your center rack as far behind the speaker as possible.
 
Depends on the directivity of the speaker and the exact placement of the rack. But it can potentially create early arriving reflections, which will be detrimental.

I usually use a low rack or place it somewhere else. However, with my large horn speakers it doesn't matter because of controlled and narrow directivity low on in frequency.
 
I´m always asking that kind of question myself, when I see setups with diffusors behind / between the front speakers. Mainly bass/low frequencies are (also) distributed behind the speaker, where this diffusors have zero effect. Frequencies, affected by diffusors, are in a much higher range and become beamy forward- so these diffusors behind / between speaker can only have a positive impact for the very late frequencies (not first or second reflections), reflected a couple of times before. Which leads me to the question, for what reasons are these diffusors installed - I would expected big absorbers, affecting low frequencies, instead.

BTW: these sort of soundstage effects are difficult to see on a filtered IR plot, because the human hearing system is quite sensitive to nearfield reflections, but only if the direct/reflected ratio is still pretty high.
That´s something I want to learn a long time already - I played with acoustic elements behind the Speaker and tried to verify the effect - that I heard - via REW and IR measurements. But weren´t able to do that - so If you could give me some hints here, would be appreciated.
 
I have a 3rd full-size center speaker in between the L/R speakers and, when I play stereo, the phantom center is excellent. In fact, it seems even better than without the silent center because of its visual effect. :)
Having something eye-catching in the middle, the more if your brain expects sound coming out of it, goes a long way to create a perceived center image !
 
...But weren´t able to do that - so If you could give me some hints here, would be appreciated.

<tutorial>

First, the goal (stated clearly): finding early reflections in the (REW) measurements, and correlating how strong they are and where they are in-room.

Next, the most-used plot to find early reflections: in REW, the energy-time curve (ETC), which is found under the "Filtered IR" (Impulse Response) tab.

Below you will see "ETC" overlay curves for a couple of measurements of my center loudspeaker (tri-amped CP25 tweeter, K-510/K-69-A midrange horn/driver, Belle bass bin), using two different techniques:

Center JuBelle ETC measured at 1m and LP Overlay.jpg


Sort of a mess to look at. But essentially the top orange trace was taken at the listening position (LP--about 3m away in-room, centered on the tweeter axis) without any temporary absorption on the floor between the microphone and the loudspeaker. The bottom trace is taken at 1m with 2-3 heavy blankets laid out transversely on the floor between the microphone and loudspeaker. Why? The human hearing system is used to hearing floor bounce, but REW doesn't know what to do with it, so removing it temporarily for acoustic measurements is necessary.

We are looking for reflections in-room from 0 ms (the shifted time to peak of impulse response) to about 6 milliseconds-ms). These are the most troublesome in terms of disturbing the soundstage/imaging of loudspeakers. Measuring one loudspeaker at a time (only--not two loudspeakers at once).

We see three peaks in the orange trace at 2.5 ms and 3.65ms within the first 5 ms. How far are these reflections from the tweeter's exit? The speed of sound in air at room temperature is ~345 m/s. Multiply the speed of sound by the delay seen, 0.002 x 345 = 0.69m and 0.00365 x 345 = 1.26m (that is, ~27 inches and ~49.5 inches for US-based readers).

What was at this distance in-room? The floor (in this case, bare areas of a tile floor on either side of a dog bed between the loudspeaker and microphone). That's why you don't see those same three reflections in the blue trace, because they were suppressed by the blankets.

What does this tell you? It's difficult to see. Why not try another view type to see what's happening? Spectrogram view. First the blue trace (suppressed floor bounce with microphone at 1m):

Center JuBelle Spectrogram measured at 1m.jpg


Now the measurement with floor bounce taken at the listening position (LP) of 3m:

Center JuBelle Spectrogram measured at 3m (floor bounce).jpg

Now we have something that is much easier to pick out. Time on the horizontal axis (just like the ETC curve), but now we have frequency (including the acoustic size of the reflected object) vertically, and acoustic (SPL) intensity in color.

We can see the two floor bounces at 2.5 and 3.65 ms from just above 2 kHz up to 10-11 kHz. Is that information that you can use? Yes. It tells you the thickness of the absorption on the floor that you need to absorb those floor bounces. Thin is okay here.

What else can you see in the spectrogram plot? (A lot...) It shows you a lot of spread out energy from 600 Hz to 1 kHz. Also below 300 Hz.

Now let's look back at the 1m spectrogram with absorption on the floor to control floor bounce: those extra areas of acoustic energy are not there. Same loudspeaker, different microphone distance, with and without floor absorption.

Are you beginning to see the pattern between comparing these two spectrograms? It tells you a lot. It also tells you the approximate size of the acoustically reflecting object (i.e., a half wavelength fits across the dimensions of that reflector corresponding to frequency). Then you can go on a Sherlock Holmes expedition to find the objects in-room that cause early reflections.

You can also do this with a coffee table between you and the loudspeaker, or a rack between two stereo loudspeakers (with and without temporary absorption on over the rack to absorb early reflections). You can do this with large TV screens (with and without a blanket hung over the screen area). The key here is to also listen carefully to each case and correlate what you hear to those conditions. Some absorption will be preferred, others, not to much.

</tutorial>

Chris
 
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I should also point out that in the floor bounce case, above, there were areas of acoustic reflections that were at lower frequencies that roughly corresponded to the higher frequency bounce areas, but the resolution of the lower frequency bounces don't permit you to see them nearly as well.

Also, you can put down absorption over the floor and any other acoustically reflective surfaces (temporarily) and then take another measurement, then do it again with thicker or thinner absorption used. This will of course tell you a lot about the thickness of absorption needed for different areas of reflections.

You will see things that will stretch your imagination on what is actually occurring, but remember that you have control over what absorption is there.

Rule of thumb for absorption: you need a thickness of ~1/5th wavelength to be effective at the frequencies of interest. If thinner than this, the absorption effectiveness sharply decreases.

Diffusion: most people don't have diffusion panels with differing dimensions of blocks or reflectors (if using randomized block panels), but you do have control if you use curved diffusor panels along the side walls and ceiling: just introduce more or less curvature in the reflective diffusion panel, then take another measurement.

Chris
 
@Chris A thank you - using the ETC and specto is well known for me , as I used it for placements of absorbers. I was specifically referring to the front reflections between the speakers, where you said (and I agree) it's hard to see differences with those two methods. I weren't able but yet.
 
I've used a couple of other methods, including moving the microphone around then looking at RT30 values in-room (which surprisingly moves the RT30 curves around a lot dependent on mike location). I've done this approach to add just enough panels to the sides of the center loudspeaker (nowadays a K-402-MEH) and above/below on either side until I stop seeing changes in RT30 and spectrogram changes. I also hear the effects of things like temporarily covering up the screen and placing absorption on the floor, etc. These early reflections can be seen in the first 3-5 ms of the spectrograms, usually at higher frequencies.

The most effective treatment was adding absorption panels (3-high) on the side walls just next to the horn mouth exits). These turn out to be broad-band acoustic reflectors and result in reflections within 1.5 ms of the direct arrivals from the loudspeaker (This I think is key to answering your above question.) These three 2'x2' (0.6x0.6m) panels gave me back my center imaging that I had been missing from the corner horns, almost alone. These are the reflections that most strongly affect loudspeaker imaging.
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[Some background information to understand my answer(s): the reason why I linked the "Corner Horn Imaging FAQ" above was that I found this method (looking at reflections inside 2-3 ms from direct arrivals) and the strongest indicator of soundstage/imaging issues to control.

Most people are not familiar with corner horn loudspeakers. One thing that you might not be aware of is that corner horns have full-range horizontal directivity control...down to ~100 Hz (depending on the exact loudspeaker design) and therefore can maintain very high direct/reflected acoustic ratios down to and below the room's Schroeder frequency (the point where directivity control no longer has any meaning). This means that, with just a little absorption in the extreme nearfield, you can have all the advantages of anechoic soundstage without the disadvantages. I found some very interesting characteristics of such a configuration: Subconscious Auditory Effects of Quasi-Linear Phase Loudspeakers that allow for audibility of soundstage performance and difficult-to-express subjective acoustic performance that comes with nearly linear phase output that is basically unsurpassed. (JMTC.)

I've even seen some try to pull the corner horns out away from the room corners by more than 18" (0.45m) to avoid having to put absorption tiles on the side walls. Of course, this completely destroys their bass response below ~100 Hz, and significantly raises resulting modulation distortion levels (after having to boost EQ dramatically to regain on-axis amplitude response). I found that putting a minimum of absorption tiles on the side walls to treat the midrange early reflections completely precludes the need to pull the corner horns (i.e., full-range directivity loudspeakers) out of the corners. This is primarily why I wrote that article many years ago. In the process, however, I learned a whole lot more.]
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So when I talk about soundstage effects acoustically reflective objects between the listener and the loudspeakers, the above experience goes much further than just treating the room for little "monkey coffin" loudspeakers that generally lose directivity below 1-2 kHz.

In the case of these little loudspeakers that can't control their midrange polars (which curiously, most "audiophiles" don't believe has any merit...quite inexplicably), there is little advantage to controlling early midrange reflections from these type of loudspeakers since the loudspeakers themselves are painting nearfield walls and furniture/equipment with lots of acoustic energy. You see quite a few loudspeakers-on-a-stick that purport to solve this problem. What these designs end up doing, however, is limiting the performance of the loudspeakers to below 70-80 dB/1m. For instance, the KEF LS50 design is used by many "audiophiles" that basically can't use their setups to listen to anything near actual music transients or acoustic (unamplified) performance levels, much less anything in the louder music genres that use only amplified instruments/vocals.

Chris
 
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Having something eye-catching in the middle, the more if your brain expects sound coming out of it, goes a long way to create a perceived center image !

Certainly plausible.

FWIW: my diffuser is placed upon my big Home Theatre Centre channel, which is behind my two channel stereo speakers, and placed low on the ground. The entire surrounding area is covered in black velvet.
All the loud speakers surrounding the screen are covered in black velvet and are invisible with the lights dimmed.

I often place a sheet of black velvet (very thin and acoustically transparent) over the centre channel. This means that I can see nothing beyond my two channel speakers, except my projection screen wall, under normal listening conditions.

And yet the diffuser effect is still there in terms of focus and palpability (except that when I cover it with the thin black velvet, it seems to take some of the highs out of the reflected sound - without that black velvet covering diffuser, the diffuser tends to “ brighten” the sound of my system a bit.
 
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