Someone please explain enclosure tuning (1 Viewer)

[James Edward]

After reading numerous subwoofer reviews, I am intrigued by something that some of them talk about. It seems that at certain tuning frequencies for vented woofer enclosures, the woofer cone hardly moves at all, and the port is handling most of the output, and vice versa. How does this occur? I always thought that the air coming from the port was a direct reflection of the woofer back pressure.
Can anyone explain or point me to a site for an explanation? Thanks.

 

[Scott Quick]

James,
The tuning frequency of the port is essentially a resonance frequency of the specific port length/diameter with a given box volume. Excursion of the woofer is somewhat "normal" above the tuning frequency, but as it gets closer to the tuning frequency (usually about an octave above), the excursion begins to decline exponentially to almost nothing at the tuning frequency. Here the port takes over almost completely. This is a great advantage of ported alignments. The problem is that as frequency decreases below the tuning frequency, driver excursion climes *EXTREMELY* fast... one of the downfalls of ported alignments. This is why its fairly important to choose a driver that can support a low tuning frequency - specifically for LFE channel. With a dedicated music system, it's not quite as crucial... unless of course you like to crank up an organ piece such as Also Sprach Zarathustra like myself

Hope this helps,
Scott

 

[Ryan Schnacke]

On a related note:
I know that with a ported box you tune by adjusting the length and diameter of the port as well as the size of the box. With a passive radiater you tune by adding weights to the PR and adjusting the size of the box. With a closed box, you tune by adjusting the size of the box. But with an infinite baffle, what's left to tune? Do you just get-what-you-get for that specific driver? Is the final tuned frequency just equal to Fs of the driver?

 

[Jack Gilvey]

Do you just get-what-you-get for that specific driver? Is the final tuned frequency just equal to Fs of the driver?

Yes, basically, It allows the driver to behave "perfectly" as if in free air (without the backwave cancellation, of course). The Fc is as low as possible for a given driver, as is the Qtc.

 

[Ryan Schnacke]

Thanks Jack. I guess that just seemed to easy. Just pick your driver and go.
The other thing that was confusing me was that I've seen people post about Tempest and Shiva subs tuned below their Fs: Tempests tuned to 16Hz, Shivas tuned to 18Hz. But those were ported applications. And you can't do that with a sealed sub.
What about power handling? Seems like with the "infinitely" large enclosure the driver no longer has to compress and decompress the air inside the enclosure as it moves. So would it take less power to reach its limits of excursion? And this means more efficiency? Any idea what the power limits would be for, say, a Shiva or Tempest in an infinite baffle?

 

[Julian Data]

IBs. prolly 100W that's my guess but ThomasW might be able to take a visit in here as I call him the God of Shivas.
And that he runs 12 Shivas in an IB config.
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[ThomasW]

Single driver IB's have slightly lower power handling than single drivers in "boxes". But that's usually not a major problem because very few use just a single driver for an IB.
Here are links to the "Cult of the Infinitely Baffled" and the IB photo gallery page
[Edited last by ThomasW on July 13, 2001 at 08:35 PM]

 

[Jack Gilvey]

quote: I guess that just seemed to easy. Just pick your driver and go.[/quote]
I think that throws people. IBs really are the simplest of subs, you just need lots of space behind them, which not everyone has.
quote: What about power handling? Seems like with the "infinitely" large enclosure the driver no longer has to compress and decompress the air inside the enclosure as it moves. So would it take less power to reach its limits of excursion? And this means more efficiency? [/quote]
You're exactly right. Smaller boxes have better "power handling" because it takes more power to reach Xmax at a given frequency. In a sealed sub, the max SPL at a given frequency is determined by the displacment of the driver, and the driver doesn't care if it's being driven to Xmax with 100 watts or 500 watts. (Actually, it might. Since an IB requires less power for a given SPL, there might be less chance for non-linearities being introduced due to voice coil heating. I'm not sure, though, if this heating is primarily caused by the voltage through the coil, or by the excursion of the coil itself.)
In practice, a driver in a small box may be able to "handle" (read: not bottom) certain program material "better" than an IB when calibrated to equal levels, but that's only because the smaller box has caused such inefficiency with decreasing frequency that those tones that are causing max excursion in an IB are reproduced much lower in level with the small box. So, by avoiding those frequencies, "power handling" appears to be better. Hardly a trade I'd make.

And, as Thomas points out, IB installations lend themselves to multiple drivers, as the spaces people tend to use as their "enclosure" are rather large volumes. The "manifold" really only needs to be big enough to mount the drivers on, and it's hidden.
[Edited last by Jack Gilvey on July 13, 2001 at 09:35 PM]

 

[Ryan Schnacke]

Hmmm. Now displacement volume and resonant frequency seem a whole lot more important than power handling. Thanks again.

 

[ThomasW]

Remember that the driver mounted in any type of cabinet(sealed, ported, IB) is a "circuit". So what's important is creating the optimal circuit for the given installation.
Also it's getting the right mix of T/S parameters that makes a good woofer. So it's not a good idea to fixate on just a couple parameters. Yes Fs is important, but so are Qts, Qme, Xmax, Vas, etc.

But with an infinite baffle, what's left to tune? Do you just get-what-you-get for that specific driver? Is the final tuned frequency just equal to Fs of the driver?

Sorry I missed this earlier. The answer is yes, no, and not much . The Qts of the driver/drivers are indeed the Qtc of the system. The Fs of the drivers help determine the Fb of the system.
If you look at the 12 Shiva's output listing (it's after all the pictures) you'll see that the system plays far lower than the Fs of a Shiva.
An IB can't be tuned like a ported one. It's really a sealed system, where the box size is equal to or greater than 10 times the Vas of each driver used. As a result proper choice of drivers is important