t-man Posted January 22, 2010 Share Posted January 22, 2010 I got a pair of RB5s on the way, but due to the rear port, I don't have enought room to let them breathe. I have a very good SVS sub to take care of the bass, so I was thinking I could fill the port with a bung - use socks or foam. Will this be better than doing nothing? Speaker will be about 6" at most from the back wall. Will blocking the port screw up the sound or just free up the RB5 more from having to produce deep bass (muddy next to wall) and allow the subwoofer to do it. I always set my speakers to small, BTW. Thanks! Quote Link to comment Share on other sites More sharing options...
jtnfoley Posted January 22, 2010 Share Posted January 22, 2010 The following is my understanding, from a lot of reading. While I'm an engineer, I've never been active in the audio world, so take the following with a pound of salt! The air column in the port is a spring, not a vent... this is why ports are pipes rather than simple holes in the box. The air column compresses a little as the woofer(s) retract, increasing their efficiency when the compressed air pushes back. Port length and diameter, and number, are all parameters that speaker designers play with to tune the efficiency of a design given the specific performance of the drivers. In short, leave the ports as designed and 6" from the wall is fine! Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted January 22, 2010 Share Posted January 22, 2010 If the speakers are crossed over above the tuning point of the speaker, it may not even matter. Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 22, 2010 Share Posted January 22, 2010 The air column in the port is a "mass" or more specifically an acoustic inertance. The air contained in the enclosure volume is an acoustic compliance. Together they form a second resonance element of the system. At the resonance frequency, Fb, the port volume velocity is at a maximum whilst the impedance of the driver is a minimum. Putting it another way, the air volume in the port is not a spring but the air in the enclosure minus driver, port volume, bracing etc, is. The impedance minimum at Fb is fascinating wouldn't you say? And so armed with this understanding what are you trying to accomplish stuffing the port? or more precisely do you know "why" you want to stuff the port? Quote Link to comment Share on other sites More sharing options...
Groomlakearea51 Posted January 22, 2010 Share Posted January 22, 2010 Short version is try them both ways, including variable distances (within what you have to work with) before blocking the rear ports. Quote Link to comment Share on other sites More sharing options...
Mr. RF62 Posted January 23, 2010 Share Posted January 23, 2010 The air column in the port is a "mass" or more specifically an acoustic inertance. The air contained in the enclosure volume is an acoustic compliance. Together they form a second resonance element of the system. At the resonance frequency, Fb, the port volume velocity is at a maximum whilst the impedance of the driver is a minimum. Putting it another way, the air volume in the port is not a spring but the air in the enclosure minus driver, port volume, bracing etc, is. The impedance minimum at Fb is fascinating wouldn't you say? And so armed with this understanding what are you trying to accomplish stuffing the port? or more precisely do you know "why" you want to stuff the port? I may be having a mind absent day but is there a way to measure what the resonant frequency is of a port? Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 23, 2010 Share Posted January 23, 2010 I may be having a mind absent day but is there a way to measure what the resonant frequency is of a port? You measure the resonance of the "system" with a given port size. Load the enclosure with a low Fs driver and obtain the impedance plot. Fb is the local minima in the impedance (the vc barely moves at Fb so the impedance is near the DC resistance of the coil). A wee bit more precise method is to use the phase angle modulus. Fb is a function of the net internal enclosure volume and the volume contained within the port. It can be modified by changing port volume, stuffing enclosure with adiabatic material, added/removing volumes and sealing leaks (speaker to baffle, speaker mounting bolts thru Tee-nuts). I'll plot some examples for you later today! Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 23, 2010 Share Posted January 23, 2010 Here's an example. The local minima in the response at 40Hz is approximately equal to the Fb of the system. The peak to the left is Fl (=26Hz) and the peak to the right is Fh (=78Hz). For very large woofers (high power capacity, 4-6" diameter voice coils) this approximation will underestimate the actual Fb of the system (by 5-10%). The actual Fb for this case is determined by acquiring one additional plot and that plot is the impedance of the system with the port blanked off. The peak in that response is Fc. Using Fc, Fl and Fh, Fb for large inductance woofers can be determined. Quote Link to comment Share on other sites More sharing options...
Mr. RF62 Posted January 24, 2010 Share Posted January 24, 2010 Thanks for that John, I'll have to do some more reading up on the subject and add in the information you have given. [] Quote Link to comment Share on other sites More sharing options...
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