[gkinberg]

Hello,

Basic question: In many pictures of peoples listening rooms, there are acoustical panels behind their main left and right speakers. What is the main function of acoustic panels in this location? Is it to absorb the rear port energy, reflections from the rear wall behind the listener or some other purpose?

My main objective is to tame what I perceive as some midrange harshness from what I believe is distortion from reflections. I assume that a panel behind my head will tame some of this but want to consider options.

Thanks, garth

[ACHiPo]

Garth,

You generally want to eliminate/diffuse first reflections.  There are techniques requiring a buddy with a mirror while you sit on in the listening position with a flashlight or laser pointer (or vice versa), and visualize first reflections.  Sometimes they're the side walls, but other times they are on the ceiling or front/back walls.  I'd suggest reaching out to the GIK folks.

The other reason for acoustic treatments, of course, is to tame bass nodes (no mean feat).  Again the GIK guys can get you started.

AC

[rbbert]

Absorption is definitely for more than just first reflections, they are just the place to start on a limited budget or to maximize your benefits.  There are all kinds of potentially destructive reflections occuring behind and between the speakers in most setups, where the sound can be improved by using absorption or diffusion.

[brother love]

Panels behind speakers on the front wall address Speaker Boundary Interference Response (SBIR).

GIK Acoustics article on SBIR: http://www.gikacoustics.com/speaker-boundary-interference-response-sbir/

It really depends on the room, speaker placement, other diffusion/ absorption used, equipment between speakers (rack, video display, etc.). I used it to good effect in one room of another house.  In current house/ room, measurements did not give as good a result.

I echo checking with GIK Acoustics for your particular application to see what they recommend. But ultimately if you get (2) or more 4" sound panels, you can experiment & see what sounds best to you.

[JLM]

An old (classic?) room treatment method is LEDE (Live End, Dead End).  The concept (as with any room treatments) is basically to reduce room effects.  In this case, the reasoning is that the closer the surfaces to the sound source the greater the benefits. 

Addressing low frequencies are much more difficult due to the amount of energy and size of the sound waves.  (In air sound travels at roughly 1,100 feet, or 343 meters, per second.  Size of sound waves can be calculated by dividing speed by frequency, so a 22 Hz signal is 50 feet long.)  This explains why diffusion is typically only effective down to roughly 1,100 Hz.  The size of sound waves also explains why low frequencies are omni-directional as the larger waves "wrap around" relatively small objects (like speaker cabinets). 

It's this omni-directional nature of low frequencies that makes sense of the effectiveness of acoustic panels behind the speakers.  But at higher frequencies (mid-bass on up) sound does behave like rays (versus waves) and it is for those frequencies that "first reflections", as ACHiPo references, are most effectively addressed.

But as anyone who has been in an anechoic chamber can attest, it's possible (not particularly easy) to overdue room treatments.  The human mind is conditioned to hearing some room reflections.  Floyd Toole, a noted acoustician, recommends limited use of treatments.  Another solution is the near-field speaker/listener setup (like studio professionals use).

[ssglx]

I've had good luck and fun with a Furutech RWL-1 panel. It can be used either floor standing or wall hung and primarily impacts the midrange. With just one I use it near the wall between the speakers. In my old room it smoothed out vocals a bit, affected the perception of mid-bass somewhat, and impacted the center image position.

My preferred position was about 2" off the wall. Moving it slightly forward would push the image forward in space and accentuate the midrange a bit. Pushing it back closer to the wall pushed the image back as well as the midrange and mid-bass balance.

Very cool to play with.

http://www.hifi-tuning.com/pdf/Magic%20RWL-1.pdf

 

[ACHiPo]

Panels behind speakers on the front wall address Speaker Boundary Interference Response (SBIR).

GIK Acoustics article on SBIR: http://www.gikacoustics.com/speaker-boundary-interference-response-sbir/

It really depends on the room, speaker placement, other diffusion/ absorption used, equipment between speakers (rack, video display, etc.). I used it to good effect in one room of another house.  In current house/ room, measurements did not give as good a result.

I echo checking with GIK Acoustics for your particular application to see what they recommend. But ultimately if you get (2) or more 4" sound panels, you can experiment & see what sounds best to you.

In my room I saw less benefit from massive bass trapping in the corners and boundaries that I'd hoped (from about +/- 12 dB to +/- 6dB bass modes and nulls), but heard an improvement that seemed to correlate to faster decay, from about 600 ms to 200 ms.  The lower decay added a lot of detail and spatial cues, expanded the soundstage in both depth and width, and also seemed to greatly help bass tonality, although the measurements suggest I've still got a pretty lumpy response below 200 Hz.  On my list of things to do is add a couple more subs (I'm running stereo Rythmik 12") strategically placed in the room, but it sounds so good right now I've been focused on other endeavors.

[mcgsxr]

I recently started treating my room.  I went with panels at the first reflection points and did corner trapping.

I prioritized those locations vs behind the speakers. 

Agree with comments about improved decay, better spatial cues and improved imaging. 

[Big Red Machine]

I like this case study as it shows the improvements in room response as treatments are added.

http://www.gikacoustics.com/room-setup-case-study/

Of course, it all sounds easy enough but then you second guess yourself if you are doing too much or too little. Like with stewed prunes - when is one enough and two, too many?

[BobM]

With dipoles, like Magnapans and others with tweeters on the back panel, it might be best to put diffusion panels behind them. for regular speakers absorption might work best. It's trial and error though, because nobody can tell you exactly what will work in your room, from this distance. You have to be there and give a listen.

[Hipper]

There are two types of absorbers people (like me!) use behind their speakers: bass traps, and other sorts of absorbent panels.

Bass traps are usually placed in corners and are relatively large to absorb bass frequencies.

The other panels - I use GIK 244 or 242s - are smaller and don't absorb as low. I think they have two purposes. One is to stop reflections off the front wall that arrive from elsewhere - the back wall say.

The second is perhaps a bit more controversial. It deals with sound that comes out of the speaker but hits the front wall behind it. It seems this can be frequency dependent and also on the shape of the speaker corners - rounded or square - and is called polarisation and edge diffraction.

Here are a couple of links with some ideas of what it's about:

http://realtraps.com/art_front-wall.htm

http://www.diyaudio.com/forums/multi-way/232834-speaker-cabinets-edge-diffraction.html

I can't really comment on this but what I do know, with my speakers, is that placing absorbent panels on the inside front wall of the speakers does have an improving effect.

[youngho]

Garth,

In order to answer your question, I'd suggest that you look at the concept of sound power response, which is basically the frequency response of the total sound radiated by a loudspeaker at different frequencies. In other words, speakers can produce sound even behind the speakers themselves, typically more at lower frequencies, but this totally depends on the design of the loudspeaker itself (controlled directivity designs would have much lower sound power response at higher frequencies whereas omnidirectional speakers would have a much higher response overall). You should then consider the design or directivity of the loudspeaker accordingly, as many listeners may absorb the back wave of dipolar speakers (though diffusion may be more desirable, accordingly to Siegfried Linkwitz) with absorption placed behind the speakers. The type of absorption may depend with the type of speaker, as well as the distance from the wall behind the speakers (with conventional speakers, closer proximity to the wall behind them tends to increase frequency response below 1 kHz, except for the cancellation effects that occur due to out-of-phase reflections, which tend to increase in frequency with closer proximity). Then there are port issues as you mention.