Fine Tunes #30

I love a simple tweak—one so elegant that I bang my forehead in recognition, moaning "Why didn't I think of that?!"

Such as this little gem for smoothing out the bass and getting rid of those lumps down below. I must credit this one to Victor Tiscareno, he of Red Rose/AudioPrism, out Washington State way.

"Jonathan," he e-mailed me, "I have a simple tweak that may work for some people, and for some it won't." Victor's a forthright guy. "Most of us loony audiophiles have a pair of speakers that are not in use, hanging around the house like albatrosses," he went on. "So here's a great tweak—drop the extra pair of speakers in the corners of the room behind the speakers in use. The acoustic effect of the woofers in the pressure zones will cancel out some of the room-resonance bass-frequency nodes!"

Of course. So wickedly clever. (Did you hear the sharp report caused by my slapped forehead?) But not only that! There's more!

"The attenuation can be changed by playing with shorting straps—both with and without—across the terminals. If the speaker is a bass-reflex design (ie, ported), then stuffing the port will change the tuning. I suppose that a small 1, 2, 4, or 8 ohm resistor across the terminals will also change the damping.

"I want you to know I'm always prepared for a demo room's bad acoustics without a Tube Trap in sight," Victor continued. "I've used this technique before, and I've turned a terrible-sounding room into a reasonable-sounding one, saving it from 'Worst Sound at Show'!"

I love Victor's tweaks. They're always so...grounded.

"Port stuffing"—or Port Noise Complaint, as Jeff Joseph of Joseph Audio eruditely puts it—is nothing new. Some manufacturers even include a port stuffer if their speaker's bass in-room response is lumpy or boomy. The Sonus Faber Amati Homage comes with port plugs, and ProAc was famous for its port-stuffing straws. Another port tidbit: Passive radiators are considered port substitutes, wherein the mass of the membrane and its suspension compliance take the place of the slug of air in a reflex design's port and must be factored into the equation. too.

In a reflex cabinet, the magnitude and frequency of the port resonances are affected by the cabinet volume, and by the length—and even the diameter and shape—of the port. If you look at the measurements sections that accompany Stereophile's speaker reviews, you can see that the port resonance is set just below the frequency where the woofer begins rolling off, extending the speaker's frequency response downward.

So altering the dimensions of the port or restricting the flow of air within the port affects the speaker's bass response, and might help you "tune" a room that's "beating" to some bass or midbass frequencies and thus muddying the sound. Plugging a port will reduce the bass extension but also reduce the slope of the rolloff. Foam, a sponge, or clean socks work as well, with different effects based on their individual acoustic transparencies. But try not to drop your old shorts into the speaker cavity. Have pity...

"Port stuffing is frowned upon for two reasons," according to Jeff Joseph. "A plug wipes out the low bass—the port is presumably already tuned for the best low-frequency response. And higher up, it creates excursion problems for the woofer, because at whatever point you tune the port, it stops the woofer from moving at that frequency. Resistively loading the ports with open-cell foam-type materials also truncates the lowest bass and reduces the Q—the resonance—of the box (footnote 1). Now that may be good or it may be bad; depends on the ear of the beholder!"

Don't forget that repositioning your speakers is another free tweak that can do wonders for eliminating annoying bass peaks. Jeff's Trick to Click is to play heavy bass material out of one speaker at a time, to identify which channel is giving you (more) trouble.

Move the speakers around, even if only by an inch or two; plug their ports, if that's your pleasure. But be aware, as Joseph warns, that you might hear more port "chuff" when restricting the airflow, especially if the port fires to the front. For the same reason, you might even experience a bit of dynamic compression. "If the driver alignment was done correctly, changing the port's tuning may cause a misalignment of the woofer. That is, if it was done correctly." JJ smiled his evil smile. "And there's the noise; a properly designed port reduces noise by its shape."

You're actually looking for the ideal PTA—the Port Tuning Angle for your room! (Port joke!) If you stuff your port too much, you'll know it. Then just back off and listen again. What counts is what makes you happy. Trust your instincts. Trust your hearing. Experiment and listen for the results of the changes. At some point, like the French Chef I know you to be, you'll step back, cry "Voilè!," and it'll be done.

For now. (J-10 laughed his evil laugh.) Happy listening.

Footnote 1: Mix the electrical and mechanical Quality Factor or Q of the driver to derive the QTF, drop the driver in a box, calculate its Q, and the result is the Q of the box, or QB.
Share | |
Site Map / Direct Links