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Lamont Sanford
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Tuning a port reflex cabinet

Outside of the universal calculations to determine the port length is there any way to measure what Hz the box is actually tuned? For example, if the calculations determine that the box should be tuned at 34Hz is there anyway to measure that other than just sticking the required length port tube in the cabinet? Is Figure 31 pointing me in the right direction?

http://www.stereophile.com/features/103/index5.html

Jan Vigne
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Re: Tuning a port reflex cabinet

The right direction toward what end? With current box programs a reflex ported speaker is the sum of the enclosure volume, the driver's TS parameters, and the port length and diameter or volume. You begin with the driver's TS parameters and you adjust the enclosure and the port volume to suit a tuning frequency the driver can support. There are plenty of freeware enclosure design guides available on the internet. Some are easier to manipulate and some give better results with more sophisticated mathematics. But any speaker design today begins with TS parameters of the driver you've selected. What exactly are you trying to accomplish? Do you know the TS parameters of the low frequency driver you're planning on?

Lamont Sanford
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Re: Tuning a port reflex cabinet

Thank you for that explanation. I have used various online calculators and manual calculations to no avail on getting a flat Qtc. The box is just too small. JA stated in the link in my first post that sometimes it is just best to go with a sealed enclosure. I went back to the drawing board and sealed the box based on the same methods used to try and quite down the Qtc and that seemed to work out fine.

On the link above, the orange line is the ported box. The purple line is after I sealed the enclosure (no ports). The blue line is the predicted performance of the sealed box. As you can see the sealed enclosure is preferable to the port reflex. I can actually make out the sort to speak bass tremolo towards the end of Hotel California whereas before it sounded sort of like one continuous single low note with the box ported. I'm learning as I go along. I made these cabinets back in the early 90s and they are constructed out of solid 3/4" red oak. If I remember correctly they were originally sealed and somewhere along the line I ported the box. That was a mistake due to my "booming" bad taste in sound. Below is the data I used to come up with something close to the Vb of the box by changing the Qtc input field. My box has thick fiberglass damping material on the left and right side to compensate for such heavy hard wood baffles so the actual Vb is sort of hard to determine but I must be close on estimating the actual Vb. They actually sound pretty good now.

Jan Vigne
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Re: Tuning a port reflex cabinet

And, therefore, your question is what?

Let's mention a few things first. I going to assume you know the TS parameters of the driver. Is that correct? When you do your calculations, you should end up with a significantly larger cabinet volume for the AS design vs. the BR enclosure. While stuffing a sock into the port on a bad speaker will change the Q of the system and reduce some boom, that is hardly the way to go about getting decent response from any given driver. Most drivers which operate well in a sealed enclosure do not perform well in a vented enclosure. The various forms of transmission lines tend to straddle the divide but still suit a driver with a particular Qts. Sealed enclosure drivers normally fall in a Qts higher (often much higher) than 0.3. However, vented alignments require a driver with a Qts within a certain range, According to The Loudspeaker Design Cookbook, a range between 0.2 and 0.5 works best in a vented enclosure. There is some flexibility there but the drivers which work best in one type of enclosure are typically not well suited to the opposite design. Suspension, driver mass and voice coils all tend to favor one sort of enclosure or another. If you force a driver to work in a particular enclosure, you will get a misaligned system and the result (with a vented system most obviously) will be the boom and ringing you had in your first (?) attempt.

At this point, it would seem best for you to simply buy a decent book on loudspeaker design. Have you done any research on designing the system other than plugging numbers into tables?

Sealed systems are generally best for the beginner but only if you have the correct driver.

What is the driver you're using?

Finally, just a few notes on your post. While you might find the solid wood attractive, it isn't how most good loudspeakers begin. MDF will give a less resonant cabinet and allow the driver to be heard above the din. It's also cheap and allows you to make a few trial cabinets before starting on the final project. The final cabinet can be constructed of veneered MDF if you want the look of real wood. The stuffing material is going to change the apparent volume of the enclosure and which sort of material you use will determine how much should be inside the cabinet. Typically DIY'ers stick with either Polyfill or fiberglass. The amount used with one type of stuffing will be incorrect for the other.

Where have you been finding your information on getting this project to work?

Lamont Sanford
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Re: Tuning a port reflex cabinet

This was the original question you never did answer Mr. Know-It-All:

"Outside of the universal calculations to determine the port length is there any way to measure what Hz the box is actually tuned?"

I'm not discussing Qts. It's Qtc. They are two different things.

.707 is the standard for Qtc and I wanted to get as close to that as possible.

Also, I think my measurements and changes correspond with JAs statement about sealed enclosures providing as good or better bass performance. Also, it is apparent that by using the standard using the calculated port length is not working on these boxes. They were originally designed as sealed enclosures. They should have stayed as sealed enclosures. I didn't use a sock to seal them and the thought never crossed my mind so you or one of your friends must of came up with that method. Also, the idea of using cheap material as ideal is ridiculous. There is nothing wrong with using the material you suggested but its use does not preclude using real wood from a tree. I'm not running a loudspeaker manufacturing operation here nor is this rocket science. You need to get real. This isn't a competition as well as a model airplane forum.

"Similarly, having measured many speakers with exotic LF alignments, ranging from the so-called "transmission line" to multiported, multidriver monstrosities, it is my considered opinion that in almost every case, the same or better bass performance could be achieved with an equivalent sealed-box alignment."

Jan Vigne
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Re: Tuning a port reflex cabinet

"This was the original question you never did answer Mr. Know-It-All:

"Outside of the universal calculations to determine the port length is there any way to measure what Hz the box is actually tuned?"

I'm not discussing Qts. It's Qtc. They are two different things."

To answer your original question, yes, there is. However, if you don't want to investigate the Qts of the driver, you will be stuck with the Qtc of the enclosure you've created. The two are interrelated. I believe Mr. Know it all will now drop out of this conversation. Good luck with that MDF model airplane.

Lamont Sanford
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Re: Tuning a port reflex cabinet

That is a good idea since you were never really on topic to begin with, which I accepted until you started your snob crap. And all the different terminology associated with box size, port or not to port, and so forth are all interrelated. You just wanted to show off your vast knowledge on cabinet making rather than help. Not at my expense.

mjalazard
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Re: Tuning a port reflex cabinet

Once you've got the proper port installed...fill 'em with rocks!!!
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Lamont Sanford
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Re: Tuning a port reflex cabinet

What?

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