Enclosure Stuffing

[Borat]

First, if I were going to modify the room to that extent I would just re-built it and do it right - damped walls etc. Next, the tiny main loudspeakers, are a joke, and no comprable center channel. This guy will of course tell you that he can't use bigger speakers because of WAF - I'm sure his wife loves all those lovely panels all over the place. Next, there should not be any absorption behind the listener, only behind the loudspeakers (lots of that)and the ceiling between the loudspeakers and the listener. With decent speakers you don't need to worry about wall reflections.

The owner of that room is probably very proud of it, but I pretty sure that I would be completely unimpressed with either the video - too small - and/or the audio - an obvious problem there. His money was spent in all the worng places.

if you're already rebuilding the room and ditching the speakers - why not flush mount the speakers into a wall and save the absorption *behind* them ?

 

[Borat]

Yes, there is velocity as well as pressure. The velocity in a closed box is zero (or nearly) at the walls, and maximum near the loudspeaker. So the foam should be as close to the loudspeaker as possible. Thats what I do.

but why are you trying to absorb sound which is not resonant ?

the first resonance will have maximum speed in the middle of the enclosure so if anything that is where the foam should go.

 

[Borat]

If you have trouble breathing through it then its not true open-celled foam.

well then its not *true* open cell foam. perhaps they aren't using the flame.

here is a picture of my friend using a saw to cut my Auralex foam block:

An externally hosted image should be here but it was not working when we last tested it.

the foam is so stiff it doesn't even flex as it is being cut.

 

[Borat]

skeptic43

I think we're both on the same page. But nevertheless the "theoretical calculated values" for the first mode are always quite exact even if the room (or enclosure) shape is irregular. The point was to show that for a subwoofer the first mode is way above the application. So why bother?

Best, Markus

because unless you are using state of the art technology your subwoofer will produce a significant amount of distortion components. these distortion components will fall into frequency range where ear is an order of magnitude more sensitive as compared to frequencies of their fundamentals. rising response and/or breakup of subwoofer driver will further amplify these out of band signals.

not to mention that if your lowpass filter is only 12db/oct then its not enough to even undo the fletcher munson equal loudness curves so in fact you don't have any lowpass at all ! ! !

but all that is besides the point because i am not talking about subwoofers here. what made you feel subwoofers were the subject of this discussion ?

i used plenty of damping material in my subwoofer - why ? because i can. but the optimal damping strategy that we are trying to develop in this thread is primarily for use in speakers, not subs.

in subs damping is optional. in speakers it is not.

 

[Borat]

And even if he did treat all of them I think the net result is to make you have to turn your amps up louder wasting energy.

let's keep Al Gore out of this discussion. there is plenty of energy to go around.

 

[Borat]

I think that the theory points in the other direction (bigger is better), but then that depends on whose "theory" you are using. Mine usually differ from other peoples, but I'm not too worried about that since mine are well founded in "first principles" and not folklore.

bigger speakers sound better but smaller ones sell better.

simple as that.

 

[Borat]

so Markus you realized that you were going to lose in an argument with me - is that why you decided to switch the topic to something you were more comfortable with ?

sorry, but the topic is still enclosure stuffing. in fact let me clarify for you - not subwoofers.

maybe the mods can help you split off a different thread from this one if you guys feel like the subject warrants further examination.

 

[Borat]

if you're already rebuilding the room and ditching the speakers - why not flush mount the speakers into a wall and save the absorption *behind* them ?

actually don't answer that. it has nothing to do with the topic at hand.

 

[markus76]

so Markus you realized that you were going to lose in an argument with me ... maybe the mods can help you split off a different thread from this one if you guys feel like the subject warrants further examination.

I try not to think in categories like "loosing" or "winning". That's just childish.
I'm fine with splitting the room acoustics part to another thread.

As to box stuffing all important factors of porous absorption were mentioned:

- Place at locations with air velocity (not at pressure locations).
- Have the flow resistivity between 800–2400 Pa s m^-1.
- Choose material (flow resistance) and thickness accordingly.

Best, Markus

 

[gedlee]

what do you suggest is the mechanism which makes absorption selective with respect to frequency ?

i can only think of 2:

1 - thickness of the foam used ( 1/4 wavelength thick or more for good absorption ) to ensure high air speed within the foam ( so that displacement maximum is located somewhere in the foam, which is always the case if your entire speaker is plugged with foam of course ).

2 - stiffness of the foam ( stuffing having zero stiffness can probably couple to the moving air at low frequencies reducing air speed relative to the stuffing and therefore reducing absorption )

but assuming the foam is not flexible and the entire volume of the speaker is plugged with it - why would it be less effective at lower frequencies ?

Interesting question and one that I was curious about some years ago. It turns out that in a spherical enclosure one can solve the wave equaltion exactly with an assumed filler which has a complex speed of sound. This is the closest that one can get to an analysis of the problem at hand and so looking deeper into it does provide some insight. The details of this solution can be found in my book.

As the frequency goes lower and lower starting at some relatively high frequency where the response is modal, the damping effect gets smaller and smaller, but its effect on the lowest mode and below is the most interesting. Thats because with this solution you get a resonance shift at the lowest mode. This shift is downward and a direct result of the damping no thermal considerations at all. This corresponds to an apparent increase in the boxes volume, just as we note in reality, but the explaination is NOT thermal, as you indicated earlier and as widely believed in the folklore. Perhaps both things happen, I don't know, but I do know that the isothermal/adiabatic thing never set right with me.

So in a nut shell what is happening is that below the first mode the absorption acts more to slow the wave speed than it does to disipate it. Hence there is always an apparent efect from the damping material, but it effect changes phase if you will going from exponential real to exponential imaginary.

 

[gedlee]

I try not to think in categories like "loosing" or "winning".

Best, Markus

Borat - I agree with Markus here, that "winning and loosing" is an immature point of view. If you take on that kind of attitude with me I will simply stop discussing things with you because I am not here to "win or loose" and I find that people who are will do anything to "win", no matter how unprofessional or impolite. So "win or loose" discussions are a non-starter with me.

 

[pos]

Talking about suitable stuffing material I like to use polyester fibre filled pillows. The really cheap ones from IKEA come with a very light outer fabric - ideal for our application: air passes through, fibres stay in.

Best, Markus

Just bought 4 of them. Thanks for the advice Markus.
Did you use them in your subs? How many pillow per cu ft did you use?

 

[Key]

This is basically where the discussion within the audio community stopped nearly 3 decades ago.

Best, Markus

Thanks, read most of it last night. So far I find it a little incomplete - concentrating mainly on equilateral stereo which I find to be an incomplete picture of what is available to be projected from a stereo source. But still some good stuff once you sort through all the redundancies and verbosity. I do think that the discussion has been taken further it's just most people aren't involved since they are stereo only and stubbornly sticking to that model.

 

[markus76]

pos,

I used 2 long pillows in a 65l box. Can remember the exact weight. Place them in the box so nearly the whole volume is covered. Internal braces help keeping them in place or use a stapler. Don't overly compress. Make sure they don't touch any moving parts.

Best, Markus

 

[markus76]

And you always will.

I still hope to find some answers at least on a subjective level once my Smyth Realiser ships.

Best, Markus

 

[Key]

I still hope to find some answers at least on a subjective level once my Smyth Realiser ships.

Best, Markus

What's that? Headphones that shift imaging when you move your head? Where does the 8 channels surround come in?

 

[markus76]

The Smyth Realiser emulates any real room with up to 8 channels/speakers over headphones. See Smyth Research

Best, Markus

 

[Key]

Hmm so it's a headphone system and the 8 channels are to send to your surround sound system. It then takes a measurement and matches your surround sound - I have a feeling it wouldn't do what I wanted it to in this regard. And they even have my least favorite deficiency of headphones covered - tactile response. Seems cool in theory let us know how well it works for you.

 

[markus76]

I already had the chance to hear it. I was unable to tell if the speakers were playing or the headphones. That's the device I was waiting for the last 20 years.

Best, Markus

 

[Borat]

Interesting question and one that I was curious about some years ago. It turns out that in a spherical enclosure one can solve the wave equaltion exactly with an assumed filler which has a complex speed of sound. This is the closest that one can get to an analysis of the problem at hand and so looking deeper into it does provide some insight. The details of this solution can be found in my book.

As the frequency goes lower and lower starting at some relatively high frequency where the response is modal, the damping effect gets smaller and smaller, but its effect on the lowest mode and below is the most interesting. Thats because with this solution you get a resonance shift at the lowest mode. This shift is downward and a direct result of the damping no thermal considerations at all. This corresponds to an apparent increase in the boxes volume, just as we note in reality, but the explaination is NOT thermal, as you indicated earlier and as widely believed in the folklore. Perhaps both things happen, I don't know, but I do know that the isothermal/adiabatic thing never set right with me.

So in a nut shell what is happening is that below the first mode the absorption acts more to slow the wave speed than it does to disipate it. Hence there is always an apparent efect from the damping material, but it effect changes phase if you will going from exponential real to exponential imaginary.

so you're basing this on a mathematical model. have you or anybody else tested this model in the physical world ?