Would this work? (Speaker poking through acoustic panel)

[-Matt-]

If you can't have your speakers more than about 1.5m away from the front wall then it is sometimes recommended to put the speakers as close as possible to the front wall instead.

With speakers > 1.5m from the wall the first cancellation dip due to SBIR will be at about 57Hz (below the typical 80Hz crossover - allowing the sub to fill in the dip).

For speakers that are 35cm deep, varying the gap between the wall and speaker, the first cancellation dip goes as follows... (I think - please check math)!

Baffle to front wall	First cancellation dip
35cm	243Hz
40cm	213Hz
45cm	189Hz
50cm	170Hz
55cm	155Hz
60cm	142Hz
65cm	131Hz

Having the speaker as close as possible to the wall pushes the SBIR dip to higher frequencies, making it easier to absorb the back wave.

The cancellation frequency is pushed quite a lot lower just by moving the speaker out far enough to put a panel behind. (This is in a frequency range where the efficiency of absorbtion changes quite rapidly with frequency).

Some also suggest that an air gap behind the acoustic panel can also improve absorption efficiency.

So how about this concept...
The speaker is right against the wall. The acoustic panel has a hole the same size as the speaker. (It is mounted sort of like a collar or almost approaching an in-wall type mounting). (Could also be achieved with two panels - one either side of the speaker).

Top view sketch...

Do you think the panel will still be able to do a good job of absorbing the back wave (after reflection from the front wall)?

There is a trade off between having the speaker as close as possible to the wall, and having enough room to physically install a panel. Where do you think the balance lies?

Anyone tried this, or something similar?

 

[Hipper]

From what I understand, the air gap behind an acoustic panel should not be bigger then the panel itself. Furthermore, I use acoustic panels by GIK and they seem to have air gaps within the construction.

The behaviour of speaker baffles is not something I know much about but I understand with my speakers, which have a rounded baffle, I will get sound bending round the sides and onto the front wall. Presumably having squared baffles, or no baffles (or other speaker drivers) will affect things differently. I will also get sound going directly to the back wall then reflecting onto the front wall and then to my ears. In addition bass frequencies seem to be flying everywhere so just stopping front wall bass frequencies will still leave all the other bass frequencies coming from elsewhere to be dealt with. For this reason I placed my speakers (and listening position - LP) more or less using 'The Thirds' principle and then dealt with the results at my LP with room treatment and an equaliser. My speaker drivers are 1.35m from the front wall.

One thing I theorised about was putting panels either side of the speakers so that I would only get direct sound at my LP. In fact I do something like that to stop side wall reflections from the higher frequencies. I also tried using panels to make a kind of hood around the listening chair but it wasn't very practical.

 

[alex-z]

Sure, it would absorb a good portion of the energy leaving the baffle and striking the wall. The listening position will experience less overall energy at low frequencies, but the response will have fewer peaks and dips, making it easier to EQ, and perform better in the time domain.

Although the better approach would be to build a rigid, false wall, stuffed with porous absorption to minimize bass leakage. That way there is no baffle diffraction or boundary interference in the first place. In-wall speakers have that inherent advantage, the only downside being that in order to aim the speaker, the wall itself must be angled. If the whole setup is designed on-axis, that isn't an issue.

@Hipper The baffle roundover behaves the same as a straight edge, it just smooths the ripple in the frequency response, because the diffraction itself is smoothed, even though the same amount is occurring. Placing absorption panels perpendicular to the speaker baffle may sound poor. Your direct vs reflected energy ratio will improve, but that isn't always ideal. You want a strong initial impulse, followed by a "dead" period, and then the room reflections arrive, and gradually taper off. This sort of decay pattern sounds natural, we are used to hearing some level of room interaction. The picture on this page shows a somewhat idealized room response. The perpendicular absorption panels would kill both the early reflections, but also the late reflections.

Sound Propagation in a Room

Sound Propagation in a Room

www.music.mcgill.ca

 

[raindance]

I have experimented with placing my speakers so that the SBIR null is equal to the primary mode in my room. In my case it is 46Hz so they land up too far into the room, however it does work and completely cancels the huge bass mode. Just not a practical solution! There are also new challenges with upper bass being recessed due to needing some boundary reinforcement, so a bit of EQ and wasted power is needed.

Placing the speakers close to the wall and absorbing behind them reduces the null due to distance, however the close wall placement gives you elevated low frequencies, so equalization is still needed. This is why you typically end up with the speakers away from the wall to reduce boom, but have to compromise on SBIR dips.

It's all compromises and you have to pick what works best for your ears.

 

[dasdoing]

Do you think the panel will still be able to do a good job of absorbing the back wave (after reflection from the front wall)?

why wouldn't it? it's not that the speaker will be a an acoustically transparent hole

 

[-Matt-]

Thanks for the replies.

I will have multiple subwoofers and will apply eq too, so should be able to use the main-sub crossover to avoid room boom potentially caused by having the main speakers near to the front wall.

I'm considering adding some form of room treatment to give the EQ an easier starting point - hopefully allowing it to achieve a better outcome.

I'm mostly trying to figure out ways of minimising the visual impact of the panels whilst maximising their effectiveness.

...related to that... I have my tv standing on a unit between the main speakers. There is a large area of wall behind the tv (not visible to me from the mlp). The tv is thin and the back of it is about a foot from the front wall. This means that potentially a large volume of acoustic material can be installed here with minimal visual impact.

Can absorption still be effective in this location given that it wouldn't be a path that results in a specular reflection to the mlp?

(Forgive the optics terminology - I mean that the angle of incidence and reflection for the sound would not be equal if sound re-radiated from this part of the front wall was to arrive at the mlp). Hmm this explanation isn't going too well so I'll try a diagram...

On the right I'm trying to show that the direct reflection path would bounce off the front wall out beyond the side of the tv (the diagram isn't to scale).

On the left I'm showing the possible path for some sound re-radiated from the front wall (which could potentially be absorbed by panel behind the tv).

I know that sound, especially at low frequencies will be pretty omnidirectional, not really like a light beam, but with analogy to the Huygens-Fresnel principle... Presumably the SBIR dip is due to destructive interference between the direct sound and the coherent sum of re-radiated sound (integrated across the whole front wall). The integration across the front wall combines to form a "beam" at the specular reflection angle but maybe you can still attenuate that "beam" by absorbing at some distance from the main axis of propogation.

What do you think will it be effective there?
Anyone tried it?

 

[hex168]

For the frequencies concerned, think of a ripple tank rather than optics.

Sometimes, when an idea is as easy to implement as the one you've described, might as well try it and see what happens!

 

[-Matt-]

Yes, a ripple tank is a really nice mental picture for this.

Of course, ripple tanks are also used to demonstrate principles of optics!

Maybe we can agree on this slight modification...

For the frequencies concerned, think of a ripple tank rather than ray optics.

As for the "suck it and see" approach, I could, but it is not completely without cost. The panels can be fairly expensive. Hence asking the questions here to try to determine the best approach before forking out!

 

[tmuikku]

Specular reflection is correct term for sound as well, like angle of incidence and reflection, works like light. Specular reflection happens and then some amount of diffusion, transmission to the wall and what not. Put your panels to specular reflections. If room modes are a problem you probably need another solution for those due to very long wavelengths, like tuned resonators.

I'm not sure though how much acoustic material you need to reduce sbir on the low frequencies, perhaps too much. Try get 10db attenuation for meaningfull effect, to reduce the lowest null. To get the null out completely you'd need much more attenuation though. SBIR is not just the first null although this is the simplest thing to imagine. Interference is not just destructive but also constructive in between and continues all the way up untill the speaker is directive enough, >20db less sound towards the reflection than towards listening spot. Basically size of the baffle (or construct) determines at what low frequencies diffract around the speaker towards front wall without much attenuation (bafflestep), above which sound mainly reflects forward and less gets towards front wall. Eventually smallest transducer on the speaker beams above some wavelength and practically no sound radiates towards the front wall. Its the path length difference with relative level towards the specular reflection point, that makes the combfilter, "problems".

ps.
It is counter productive to use speaker with wide coverage angle and then attenuate with acoustic treatment. Much better to direct sound where its wanted, free sensitivity, with narrower coverage angle speaker. Altgough, usually we have to work with something else so acoustic treatment it is. Anyway, big things are needed to control long wavelengths.

 

[ROOSKIE]

If you can't have your speakers more than about 1.5m away from the front wall then it is sometimes recommended to put the speakers as close as possible to the front wall instead.

With speakers > 1.5m from the wall the first cancellation dip due to SBIR will be at about 57Hz (below the typical 80Hz crossover - allowing the sub to fill in the dip).

For speakers that are 35cm deep, varying the gap between the wall and speaker, the first cancellation dip goes as follows... (I think - please check math)!

Baffle to front wall	First cancellation dip
35cm	243Hz
40cm	213Hz
45cm	189Hz
50cm	170Hz
55cm	155Hz
60cm	142Hz
65cm	131Hz

Having the speaker as close as possible to the wall pushes the SBIR dip to higher frequencies, making it easier to absorb the back wave.

The cancellation frequency is pushed quite a lot lower just by moving the speaker out far enough to put a panel behind. (This is in a frequency range where the efficiency of absorbtion changes quite rapidly with frequency).

Some also suggest that an air gap behind the acoustic panel can also improve absorption efficiency.

So how about this concept...
The speaker is right against the wall. The acoustic panel has a hole the same size as the speaker. (It is mounted sort of like a collar or almost approaching an in-wall type mounting). (Could also be achieved with two panels - one either side of the speaker).

Top view sketch...
View attachment 218176
Do you think the panel will still be able to do a good job of absorbing the back wave (after reflection from the front wall)?

There is a trade off between having the speaker as close as possible to the wall, and having enough room to physically install a panel. Where do you think the balance lies?

Anyone tried this, or something similar?

Dude you are still going to have SBIR from ceiling, floor, side walls.
No need to hit the front wall with the back of your mains. Been there.

Plus absorbers are not linear, who knows if you will like what happens. Unless you need to sing in there try judiciously applied PEQ.

Position the mains so the big three Side,Front,Floor distances are all different.
Floor bounce is also a thing and not the same as the SBIR floor cancellation.

The effect of SBIR is less with multiple woofers and larger woofers due to large and varying radiating area. (At least in the 100-200hrz zone) below that maybe not.

I sometimes overlap the mains and subs 1/2-1 octave and PEQ the levels. Evens things out well.
Or try putting the subs right against the wall and the mains right were they get hit at 80-90hrz and cross the subs in at 100. With some care there is zero chance of localization unless you have rattles or chuffing of perhaps something else that is not the subs fault. Even 150 is hard or won't be able to be localized in a good setup.

Anyway remember your room is not a recording studio (or is it, guess I should ask 1st)so reflections and even cancellations are not horrible things vs losing a sense of spaciousness and naturalness if you will.

 

[hex168]

Yes, a ripple tank is a really nice mental picture for this.

Of course, ripple tanks are also used to demonstrate principles of optics!

Maybe we can agree on this slight modification...


As for the "suck it and see" approach, I could, but it is not completely without cost. The panels can be fairly expensive. Hence asking the questions here to try to determine the best approach before forking out!

Modification gladly accepted! Just to try it, you could use some denim insulation. No expensive panels necessary.