Infinite Baffle Loudspeaker
Since I started being interested in audio, I’ve seen all kinds of speaker designs, weird arrangements and weird geometries. From all those speakers, in terms of extremely low frequency reproduction the best ones where the infinite baffle subwoofers. Those setups go easily down to frequencies like 10Hz. You may say these figures are useless with a musical program and normally i would agree but if we are in a quest for perfection i would say it’s mandatory. Even if our music doesn’t have much information below 27Hz (lowest note on a grand piano, though pipe organs go even lower) it is important to have the roll off region in frequency response at least an octave lower.
The reason for this is that the speaker and the cabinet box form a high pass filter and like with all filters it will add lots of delay in the slope region which may affect the quality of the reproduction. For Home Theater applications response down to teen frequencies becomes a necessity. If you look at the spectrogram of a .50 Barret rifle shooting sound you will see the most information is in the 10 to 20 Hz area. You may not hear the 10Hz but you will certainly feel it, much like if you would be standing next to the rifle.
The principle of IB (Infinite Baffle) is to install the woofers on a panel that is wide enough so that the waves coming from the back of the woofer never meet the waves coming from the front of the woofer. Looking at the wavelength of a 10Hz sine you will see you need a very big panel. However IB can also be done if you put the woofer in a box big enough so that it has zero damping. Custom HT rooms have the woofers mounted on a sealing or floor and have the attic or basement act as that big cabinet box.
In this project I’m presenting the author build a speaker on a panel to fit a door way and have an entire room to isolate the back waves from the woofers. He is referring to the Hoffman’s Iron Law that says: having efficiency, low frequency response and box volume one must sacrifice one of the three to have the other two. IB of course sacrifices box volume. There are some who argue this Law, because it actually holds true if you place the subwoofer in an open environment. Closed spaces have gain that will your low frequencies even if your speaker loses efficiency, and here by speaker i mean the woofer plus cabinet. Woofer’s efficiency is constant.
Another thing is nowadays Hoffman’s Iron Law can easily be defeated with powerful woofers and equalizers. There are woofers that can take tons of power and have huge excursion and can go really low in small enough boxes. Returning to the IB setups you will also need high excursion woofers and high pass filter on your amplifier with a corner frequency between 5 and 10Hz, sometimes higher depending on the woofer your using. You will need high excursion because the woofer won’t be damped at all so it will be free to move. At high powers and low frequencies it will move allot.
In this project the author didn’t built a subwoofer but a full-range infinite baffle speaker. I don’t see the point of this expect for a center channel. He used 8 x 12″ woofers for the low frequency part. If the woofers aren’t such quality it’s better to use many of them, this way the stress will be divided between them. When i first saw the baffle i thought it is too thin to hold the eight woofers. But then i saw the bracing on the back, and that will make it more rigid for sure.
The tweeters are piezo which in my opinion are not that great. Also the crossover i think could use more work. I always liked higher orders crossovers because they give you allot more dynamic range. The whole system reaches 96dB/W/m, which is very nice. Is a nice and fun build, and even if it won’t live up to your expectations at least you will have a sonic weapon.
September 21st, 2009 at 3:53 am
I’m not too keen on piezo tweeters either. I want to like them but I think that because the piezo element is often mounted on a plastic base. It resonates and sounds plasticy. A piezo element mounted on a nice lump of lead with a light diaphragm might just outperform magnetic tweeters perhaps?
September 24th, 2009 at 7:19 pm
I doubt it. because of their nature its hard to find a piezo with something that resembles flat frequency response. Plus they wont go as low as a magnetic tweeter and their impendance wont be as friendly for the crossover.
July 27th, 2010 at 5:56 pm
I have built a 9 cubic foot speaker system that was a bass reflex system i employed a passive radiator instead of the tune port pipe
with the radiator being more compliant than the low frequency audio transdocer I basically had the equivalent of an air suspension system
I lost it during a divorce. It went down to 19 hz I used the QTC of .707 to buld the box when it came to standing waves I played with it, ended up with a 30% ainsulated box. it was a fourth order network system crossing over at 800 hz and 5k
it was colorful and tight not only would it move the earth underneath your feet, all of the transients were there the ripples, subtle pulse.
I am fixing to build a infinite baffle system using of all things a goldwood 15 inch with of all things a 20oz magnet. one box model that I have run this thing on tells me it is going to go down to 10 hz.
the infinite baffle’s “one size fits all misnomer” when I put that woofers parameters into several software programs looking for the cubic foot requirements needed for its optimum performance, I end up with a 14 cubic foot box.
when I make the box larger the forcasted response shows a higher f3 meaning it doesn’t go as low.
Every low frequency driver has a perfect box size and every one of them needs one.
if you put that driver in the right size box and you got it in your living room and the bass is pounding, boxing your jaws, moving the earth underneath your feet.
How big is that room? what is the cubic footage of that room? lets say that that driver requires a 23 cubic foot box and that is the size box that you put that driver in that you got blarring in livingroom.
if it’s putting that much pressure on you in that huge room, what’s going on in that 23 cubic foot box?
the excited air in that box reinforces the driver giving it the necassary mechanical dampening that it needs in order for it to perform optimally.
with out it it will exhibit the same qualities found in systems where standing waves are not dealt with
you will have a loss of transients brought about because of a lack of mechanial dampening that is needed to keep the voice coil exactly where it needs to be within it’s magnetic field.
July 29th, 2010 at 11:43 am
Hello jonathan,
allow me to make some clarifications
“with the radiator being more compliant than the low frequency audio transdocer I basically had the equivalent of an air suspension system”
That is not correct. Acoustic suspension is when air inside a closed box has a linear sprigness and thus damps the high compliance of the woofer. What you have with a passive radiator is clearly not the case. A higher compliance of the PR will only result in a lower Fs and so a lower tunning frequency of the system.
“when I make the box larger the forcasted response shows a higher f3 meaning it doesn’t go as low. ”
in a closed box, Fb will never be lower than woofer’s Fs. Making the box bigger will lower Fb (up to a point) but will lower Qtc as well. This tends to be interpreted as less output in low frequency. You have to consider the electrical model the software uses!
“you will have a loss of transients brought about because of a lack of mechanial dampening that is needed to keep the voice coil exactly where it needs to be within it’s magnetic field.”
that is also incorrect. In IB setups the woofer’s electrical damping dominates all mechanical damping. This is why the woofer must be chosen carefully.
May 21st, 2012 at 7:21 am
Great information
November 15th, 2012 at 5:38 pm
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