Bass-reflex to Aperiodic Conversion?

[Un-filtrd]

I've had to relocate a bass-reflex center channel speaker with twin rear-firing ports into an in-cabinet location and need to tame the resulting bloated bass response:

1. Would converting it into an "aperiodic" enclosure by plugging the ports with open cell acoustic foam be a viable approach?
2. If so, how do I determine the optimal amount of flow resistance to add?
3. Will adding port plugs cause excessive cone excursions at low frequencies? Can the woofer be damaged?

Thanks for any help!

[GM]

In the good ol' days of tube power, bloated bass was all too common due to the amp's high output impedance and near/at wall speaker positions required because they were so big, so the default suggestion was to 'stuff a sock in it' to ~aperiodically load the vent, converting it into a leaky sealed cab. Nowadays the cab is usually stuffed as required to get ~ the desired results and use vent damping to fine tune it.

To be truly aperiodic requires a ~complete flattening of the driver's impedance peak, so being able to measure it is required, but this is usually overkill, so stuff by ear till adding more starts to audibly lean out the bass too much.

Yes, if over driven, so keep an eye on it while testing and add a high pass if required. As a general rule though, if you have to roll it off due to excessive room/boundary gain, then this same gain protects the driver down low since it doesn't have to work as hard to reach the desired SPL at the listening position.

GM

[Shaun]

Firstly, high-Qts drivers suited for aperiodic (lossy) enclosures would not be used in a bass reflex enclosure, so I would not worry about getting the tuning "right". Just stuff a sock in the port. Your problem would not be related to low frequency extension, anyway*. It has to do with baffle step transition of the loudspeaker, which changes due to the way in which the shelf and surrounding objects effectively make the speaker baffle acoustically larger (vocals sound chesty, upper mids & treble receded). The proper remedy is to equalise or to redesign the crossover for this new placement configuration.

Edit: plugging the port or partially stuffing it would result in lower cone excursion, therefore safer than ported below port tuning frequency.

*If this is what you want to do, then perhaps just try a higher crossover frequency between centre and subwoofer.

[Un-filtrd]

Thanks for all the input! There are a host of issues resulting from the placement of the speaker within the center channel compartment of the A/V console. The most objectionable issue, however, is excessive mid-bass emphasis in my particular situation.

As luck would have it, some cylindrical foam cutouts from my telescope eyepiece case are exactly the right size and fit securely in the 1.25" diameter ports. After some trial and error, I settled for a plug length of 1.75". The mid-bass emphasis has been largely reduced and dialogue is much clearer now.

I've kept the center channel speaker size setting as "small", which has a fixed crossover point of 80Hz. This should prevent excessive cone excursions at very low frequencies. However, does plugging the ports typically cause excessive cone excursions in the 80-120Hz range? I ask since many film soundtracks have tremendous mid-bass dynamics.

Thanks again for your help!

[Shaun]

Originally Posted by Un-filtrd

However, does plugging the ports typically cause excessive cone excursions in the 80-120Hz range?

Not usually.

[infinia]

Originally Posted by Un-filtrd

Thanks for all the input! There are a host of issues resulting from the placement of the speaker within the center channel compartment of the A/V console. The most objectionable issue, however, is excessive mid-bass emphasis in my particular situation.

As luck would have it, some cylindrical foam cutouts from my telescope eyepiece case are exactly the right size and fit securely in the 1.25" diameter ports. After some trial and error, I settled for a plug length of 1.75". The mid-bass emphasis has been largely reduced and dialogue is much clearer now.

I've kept the center channel speaker size setting as "small", which has a fixed crossover point of 80Hz. This should prevent excessive cone excursions at very low frequencies. However, does plugging the ports typically cause excessive cone excursions in the 80-120Hz range? I ask since many film soundtracks have tremendous mid-bass dynamics.

Thanks again for your help!

No quite the opposite compared to BR all else being equal. As you know, the lower the frequency in the pass band is where excursion Xm comes to it's limits with a given SPL. So by plugging/stuffing the ports will tend reduce the lower bandwidth ie higher cutoff, but some folks may be tempted to increase the volume/SPL more in these cases reaching Xm again.

[Un-filtrd]

Thanks for the replies! Please note that my concern about excessive and potentially damaging cone excursions stems from information posted by David Fabrikant of Ascend Acoustics on his website. In designing port plugs for his Sierra-1 speaker, he has stated that:

Simply plugging the port of a properly designed ported loudspeaker is generally a bad idea. A woofer that is designed for a ported enclosure will act differently when installed in a sealed enclosure. The compliance of the woofer is not designed to see the internal pressures created by a sealed enclosure and the speaker then becomes overdamped, subjecting the woofer to greater excursions (often exceeding its mechanical limits) and pre-mature bass roll-off. You can also end up with a higher than normal impedance peak at system resonance.

The port plugs he designed for the Sierra-1 are hollow cylindrical foam tubes with end caps only 5mm thick. As these are not as thick as the ones I made, which are 1.75" thick, I want to make sure the additional flow resistance does not cause the problems he described.

[Shaun]

He contradicts himself. The only condition under which the woofer will see higher excursion is as Infinia said:

Originally Posted by infinia

...some folks may be tempted to increase the volume/SPL more in these cases reaching Xm again.

But, all things being equal, simply stuffing the port will restrict excursion, rather than increase it.

[Shaun]

What follows is a bit simplified:

A bass reflex enclosure is more efficient towards its lower band limit than a sealed enclosure. The port resonates at a frequency just below the pass band of the woofer, thus adding a bit of output at this frequency, and so extending the total pass band downwards. The sealed enclosure only radiates from the driver, and the lower cut-off point is higher, accordingly. Therefore, the sealed enclosure is less efficient at the lower frequencies. Also, the air mass inside the enclosure serves to damp the driver and so restricts cone movement in the lower frequencies, whereas a BR enclosure unloads the woofer below port tuning frequency (i.e., it is prone to free movement and overexcursion).

In order to get a sealed loudspeaker to output the same SPL as the equivalent BR, one has to add equalisation, essentially a bass boost filter (google Linkwitz Transform). In such a scheme the woofer must work much harder (therefore greater excursion) than before equalisation is applied. But, again, this is only so if the extra boost is applied.

[infinia]

I'm familiar with some plugs made with foam and they are a bit restrictive IMO to be called aperiodic, air should be able pass with a bit of pressure behind it to work in these cases. ie you could breathe thru it, if forced.
You are not capping it off so it's not a sealed system per se. Believe me the excursion won't be as severe as when it was a BR. You will be losing basically most of the ports contribution to the LF output.
What you want to do is stuff the port with is some loosely wadded poly-fill say 3-4 inches thick and then open a small passage way by pressing down on the poly fill close to an edge. Use a pencil or a small screw driver. You can experiment with compressing the wadding together more but keep a small opening. Heck in a pinch you could even use a rolled up pair of socks, darker colors of course.