tuning aperiodic vents

[the full ranger]

I have to re-mount a JBL 2245H in an enclosure FAR too small for it and was thinking of using an aperiodically-vented approach. It's been so long since I have used this kind of loading that I forgot exactly how to tune the vent. It seems to me that I put a resistor (1 k ohm?) in series with the driver and measured the voltage drop across it as I changed the stuffing in the vent but I'm not certain of this. Any clues?

[pjpoes]

I'm not sure I will have a fully answer for you, but lets see if this helps. What you suggest would be no different than a method more measuring impedance, essentially. An aperiodic enclosure is essentially an under sized enclosure with an inappropriately tuned vent, thus it will have a pretty high resonant frequency. You then block the vent with a lossy material of sorts, which breathes, but poorly. This will progressively damp and lower the resonant frequency until you get to the desired level. This is measurable via the impedance, as the impedance will be very high at the zero motion point, or resonant tuning frequency. To tune this, you would simply find the resonant frequency, and keep adding until that frequency is lowered. However, I'm pretty sure it will both lower the resistance, at that resonant frequency, but also lower the Res. freq. and thus require that you readjust your test tone until you "refind" the res. freq. Thus a quicker method would simply be to use an impedance measurement device. A wallen jig or woofer tester would work if you have access to either.

[the full ranger]

The only test equipment I have is a frequency generator, oscilloscope and a digital multi-meter so that's what I'll be working with.

As I said, I've done this before with just a frequency generator and a voltmeter, but it was about thirty years ago and I've lost touch with the method I used.

This is a one-time procedure (I have only one JBL 2245H), so I'm not going to go out and get more test equipment.

[Ron E]

I would presume you were trying for the amount of stuffing that gives the flattest impedance without double humps. Double humps means it is acting like a vented box, and higher single peaks means it is acting like a sealed box.

Another way of testing is with a click box. Wire up a quiet-operating switch in series with a 1.5V battery wire this to the speaker terminals, click the switch on and off and you are looking for the click when it turns off to be similar to the click when it turns on. Leave the vent open and then seal it with something airtight to hear the two extremes. Experiment from there.

[Svante]

These curves describe what happens when damping in a bass-reflex port increases. I suppose that is what RonE meant?

[the full ranger]

Good idea, Ron. Sounds simple and effective.

Yes, you've got the idea. I'm trying to flatten the impedance as much as I can before it turns out to look like a camel - more like a dromedary!

[PB2]

Curious, Svante, what were the driver T&S parameters, box size, and port tuning for your example?

Pete B.

[Nanook]

(not me, ),

• start with 1/2 the cone area, and 1" thick uncompressed fibreglass or long hair wool (or Dacron)
• play test tones to discover or get to the Fs of the driver in enclosure
• use a DMM, set to AC Volts
• put a 10k, 10W resistor between amp+ and speaker+, play test tones
• add or subtract "stuffing" (flow resistance) until the flattest set of values occurs. These lowest set of values should be approximately 50% in amplitude compare to ported or sealed box

without speaker measurement software, it may be a little tedious, but worth the effort. For what it's worth, me and my audio partnerr are doing just this. But we didn't shrink the box and add aperiodic vents, we've left the closed box size alone and am using the aperiodic vents simply to reduce the impedance amplitude (and increase the output) at or near Fs

[Nanook]

(not me, ),

• start w/ 1/2 the cone area, and 1" uncompressed fibreglass or long hair wool (or Dacron)
• play test tones to discover or get to the Fs of the driver in enclosure
• use a DMM, set to AC Volts
• put a 10k, 10W resistor between amp+ and speaker+, play test tones
• add or subtract "stuffing" (flow resistance) until the flattest set of values occurs. These lowest set of values should be approximately 50% in amplitude compare to ported or sealed box

without a speaker software setup, it may be a little tedious, but worth the effort. For what it's worth, I am doing just this. But I didn't shrink the box and add aperiodic vents, we've left the closed box size alone and am using the aperiodic vents simply to reduce the impedance amplitude (and increase the output) at or near Fs

[francorovereto]

How many times I read that fiberglass is toxic and dangeros for lungs... for me, positioning a such material in a breathing vent is a stupid thing. In a couple days your room will be impregnated with microscopical glassy parts. And you and your family will breathe them.
Wool or sintetic stuff would be better.

Only my two cents.