high pass on low end of a woofer?

[patchwork]

Do you put a high pass filter on the low end of a woofer?

For example, if a woofer goes down to 50, do you put a crossover on it, so that it plays 50 and higher, or do you just leave it gradually roll off?

[kelticwizard]

A) Are you talking about an active or a passive filter?

B) Are you talking about a closed box, ported or transmission line?

[sdclc126]

"...if a woofer goes down to 50, do you put a crossover on it, so that it plays 50 and higher, or do you just leave it gradually roll off?"

Since the hypothetical woofer plays no lower than 50 hz, a crossover to attenuate it at the same point is unnecessary. You need nothing to stop it from playing lower - it does that by itself.

Think of a tweeter in the same way - you don't put a crossover up at its highest frequency - there's no point. Crossover go between drivers, not outside of them (perhaps there are some exceptions but not necessary for this explanation). You would put a crossover between a woofer and a SUBwoofer, but again it's between the drivers.

Now in car audio a subwoofer is often run on its own amp/channel, and if the satellites tend to "bottom out" on the low frequencies at high listening levels, you can install "bass blockers" (capacitors) on their lead wires to - you guessed it - block the bass frequencies. Most modern car amps can do this electronically too.

Clear as mud?

[kelticwizard]

If the woofer is in a Closed Box, it will cut itself off safely with no need for a high pass filter.

If the woofer is in a Ported or TL, a high pass filter is desirable, because once the speaker goes down past it's box range, the woofer starts undergoing a lot of movement and is subject to damage.

For instance, if your ported or TL enclosure is designed to go down to 50 Hz, the cone will vibrate rapidly if it is fed high volume signals at 40 Hz or below. So a filter is a good idea, although most people just take their chances.

However, because of cost of parts to make a passive filter that low, most often such a filter, if used, will be active-that is, in the amp itself or placed before the amp.

This type of active, low frequency high pass filter is frequently called a "rumble filter".

[djQUAN]

"rumble filter"

also known as subsonic filter.

[MatchASM]

If the woofer is in a Closed Box, it will cut itself off safely with no need for a high pass filter.

I heard that placing a highpass at approximately 30-35Hz will reduce distortion, even if the transfer of the woofer is inaudible at these low frequencies. Any experiences with this?

[lndm]

Originally posted by MatchASM
I heard that placing a highpass at approximately 30-35Hz will reduce distortion, even if the transfer of the woofer is inaudible at these low frequencies. Any experiences with this?

It has been used to increase the Q of a closed system without increasing fs, though it isn't exactly a free lunch.

[Geoff H]

There is an advantage. The cone moves with the lower frequencies without faithfully reproducing the signal causing the movement, adding distortion to that which we can hear.

Better off using a driver that will reproduce the material we choose to listen to.

Regards,
Geoff.

[roddyama]

Originally posted by MatchASM


I heard that placing a highpass at approximately 30-35Hz will reduce distortion, even if the transfer of the woofer is inaudible at these low frequencies. Any experiences with this?

In the 80's there were a few manufacturers like EV playing with 6th order ported box alignments. They would basically take a typical over damped 4th order ported box alignment which by itself has the response of a 4th order high pass filter and add a high Q 2nd order high pass active filter in front of it. The 2nd order filter had a peak in its response and was chosen to have an f3 5Hz to 10Hz lower than the 4th order alignment to effectively extend the FR of the woofer.

The woofer used in such an arrangement had to have the excursion to run at power in the lowest octave even more so in the 6th order alignment than in the standard 4th order alignments. You did get the benefit of the electrical 12db/octave "rumble" or "subsonic" filtering below f3. However, aroung f3 the cone excursion could be excessive when run at high levels.

[phase_accurate]

Hi Rodd

There are in fact tons of possible 6th order alignments. There is also the possibility of using a peaky reflex-tuning and flatten it by the use of a low-Q electrical highpass filter.

There is also a possibility to make 3rd order alignments accordingly using first-order highpass-filtering and closed boxes.

Regards

Charles