early reflection of sidewalls: absorption or not?

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I am planning a music listening/living room as an addition to my new home renovation. The room dimensions L20’x W14’8”x H11’ (6.1x 4.47x 3.35m) were developed using room analysis software to fit into the existing building restrictions. That room will have a single entry door on the long sides 40” (1m) from the front wall. My speakers (DIY bending-wave thus fairly wide dispersion) could be placed 1m from the front wall but I an thinking it would be more interesting to experiment with corner baffle speakers (built in the front wall corners). That setup is used in many monitoring systems (e.g.:Moulton Laboratories :: The Real World of Project Control Room Monitoring) but almost never in domestic listening rooms and I wonder why.

This corner baffle placement would largely eliminate any 1st reflection from the front wall but sidewall refection will still occur with both version of speaker placement. I think for proper stereo imaging symmetric room behavior is essential but on one side wall there needs to be a door. Just in my internet searches I found differing opinions about side wall reflection. I found a lot of proponents of damping the area of 1st side reflections (often by ones which sell room treatments) but especially Floyd Toole says that first side wall reflection are necessary for proper sound staging. So what is right? Or does it depend on how the recording is made (including the room sound of the recording venue or not)?

I guess for my room damping 1st side reflections would be easier by using microperforated panels on the door and have an equal sized panel on the other side wall. Achieving equal reflective surfaces may more difficult.
 
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Hi, a starting point could be to understand how soundstage is important to you. Sometimes, listening to musicians, I remember it and close my eyes ...
Acoustic treatments can be expensive but an old mattress can give you an idea at no cost, IMO, where I'm playing with acoustics, early reflections of midbass compromise listening.
 
I think that soundstage per se is not the most important aspect of music appreciation but it is related to the separation of instruments which is. Many recordings have anyhow an artificial soundstage in an exaggerated form one can not ever experience in a live acoustical music performance.
 
I'm here, why do you quote? I'm not sure what you mean. Sometimes I follow errant violins or guitarists flying in the room. For that I think it is good to break down first reflections, and not to cultivate them. And scatter the late ones. My problem now is that a lot of good guys here use narrow directivity for the same / best result.
 
As I mentioned in my initial post,Toole, Olive etc. like sidewall reflections but others do not. It is also how much room sound one wants to have in a listening room vs recorded room sound. My specific issue is that a door on one side makes it difficult to have identical side reflections left and right.
 
Hello,

It is your room you can do what you want.

First reflections are important in crafting the sound stage. Your choice, controlled beamwidth and or Acoustic Panels.

Harmon and others work hard to manufacture Controlled Directivity speakers that sound good in average rooms round the world. These rooms are without dampening of side wall reflections.

Thanks DT
 
The point of interrupting a first reflection is to prevent reflections that arrive at your ears too soon.

Early reflections are bad. If a reflection arrives at your ears soon after the direct sound does, your brain interprets the reflection as being part of the direct sound and you perceive the sound as muddied.

Your brain interprets later reflections as the sound of the room, so they don't harm the clarity of the direct sound. Late reflections are good because they give you a sense of space and they reinforce the direct sound.

If you want to interrupt a first reflection but you want to keep its sound energy in the room, use a diffuser instead of an absorber.
 
There is already an image in the recording, does it need to be crafted?

AllenB,

You will likely tell me that the exception is the rule. I say not.

Is there not already an image in the recording? I say maybe to some extent, but not so much.

The recording is mixed. The illusion of the center channel is in the head of the person doing the mix. Localization and space are due to speaker placement and reflections; they are not in the recording, only in the head of the person doing the mix.

Now output this mix/recording and play it back over the best set of headphones that you can find. The center image, the localization and sense of space that the person doing the mix experienced are now gone, they are not in the recording.

Thanks DT
 
Hi.

I'm 17 years into my career as an acoustical engineer. I've designed auditoriums and other critical listening spaces.

Rather than being snarky and slapping up a link, show me where I'm wrong.

I will speak to two places that you do not have it correct. This is like a True/False question on the final exam.

True / False; First reflections are always bad? If there is one exception the answer is false.

First, you have forgotten who the client is. You seem intent to sell him a large live performance auditorium with adjustable sidewall diffusers. What your client has is a small 3000 cubic foot playback/listening room.

Secondly, early or first reflections are not necessarily bad or good, they are just the nature of the beast. You can manage and control them to your advantage. You do not need to put a stake in their heart to kill them.

One school of thought is to use Controlled Directivity speakers to aim or direct the speaker beamwidth where you want it to go. This approach limits unintended frequencies from just bouncing, without purpose, around the room. The right ear will receive direct sound from the right speaker. The left ear is in the sound shadow of the head and torso and will hear sound from the right speaker as a first reflection from the near wall. Vice versa for the left speaker, direct sound at the left ear, right ear is in the head torso shadow and the first reflections arrive at the right ear. Our brains are amazingly effective at extracting perceived soundstage information from this setup.

Thanks DT

I am a consultant of more than a few years. In this context my thing is more about noise and vibration. One School of Medicine research lab constructed of Ductile Moment Seismic Resistant construction shook so bad when a delivery truck drove down the street the field of view on the Scanning Electron Microscopes shook so bad that the building turned into facility offices and junk storage. A new replacement building of cast in place concrete (same thin brick veneer) was built next door in the parking lot.
 
I wrote an article comparing two speakers with very different directviity indexes. You can really hear the difference the room sound adds from the wider directivity speaker, as I made binaural recordings of each speaker: KEF LS50 (David) Versus JBL 4722 Cinema (Goliath) Speaker Comparison with Binaural Recordings - Reviews - Audiophile Style

Since both sets of speakers in the room are setup along the long wall, and fire across the short dimension, the are no side wall reflections, as they are several feet away from the speakers. However, on the binaural recordings, one can clearly hear more room with the wider directvity speakers (i.e. KEF LS50) versus the much narrower directivity (JBL 4722) speakers.

While not directly related to sidewall reflections, it can determine ones preference for more direct versus more reflected sound. Which I believe is what is boils down to. A preference neither right or wrong...

In the case of measurements, and specifically an ETC measurement, the rule of thumb is that the first reflection and subsequent reflections should be -15 to -20 dB down from the main peak, within the first 20 to 40 ms of early reflection analysis.

May I humbly suggest in you specific case, the only way to find out is to listen to your setup with no treatments and then with enough acoustic treatment to make a difference, and listen again. Then you can determine your preference for more room sound or more direct sound, wrt sidewall reflections.

I spent +10,000 hours in control room chairs recording and mixing music. I prefer the in the corner or in the wall mounting of speakers over monitors sitting on the mixer bridge. Gives the cleanest sound minimising early reflections and to my ears produce a cleaner, more coherent sound field or auditory scene. The point I am trying to make is that it is you personal preference that matters most, as there is no right or wrong...
 
I am planning as mentioned initially to built speakers in a corner baffle. I can see that this would generate very little 1st refections from the front wall (where the speakers are located) and would also make a more space saving room setup as there this eliminates the space behind the speakers. But your -15-20 dB from the sidewalls would be not possible without absorptions. I guess some empirical experimentation will be required but some theoretical guidance would reduce the cost of this!
 
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Building speakers to properly fit the room corners without modifying the room gives a large delay for the early reflections, but building this way is challenging. Consider treating what you have if it falls into the early category.
 
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Now output this mix/recording and play it back over the best set of headphones that you can find. The center image, the localization and sense of space that the person doing the mix experienced are now gone, they are not in the recording.
Not even the reflections within the booths, or panning?

Irrespective, the room adds a constant but unrelated effect which is why this tends to be referred to as spaciousness rather than image.
 
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