JimP
Ex Member
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Hi guys,
The problem of "unloading" of the woofer below it's resonant frequency can be an issue, but likely only if you are using a somewhat powerful amp. Below the horn cut-off frequency (35-40 hz) there is no load on the woofer, so if there is a signal applied, at say 25 Hz, the excursion can get extreme. The Imperial's large chamber helps with this, or at least that's what some of our modelers have reported.
Would you hear this unloading? Most likely you might notice some severe intermodulation distortion, especially if you are using the Imp full range. You may not hear it, but certainly your woofers are feeling it and it could result in damage. At a few watts, the woofer probably won't be flying enough to cause damage, but at higher power (50 watts from Gaz's example) the woofer could flap.
A low-cut filter can help this and I use a 30Hz roll-off filter when we are using the Imp as a PA (very high power) to avoid problems. Not an audio purist set-up, but it saves woofers. It's not much different from the rumble filter you would use when playing vinyl. A good cartridge can easily produce 5 Hz tones (like from warped records) and those can really wreck your day and your woofers. Most woofers are "unloaded" at that frequency.
In practice, I have not had any noticable problems with unloading, but both of my woofers Fs are around 35Hz, so I think it's not a big issue with my set-up.
Jim P
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