Jump to content

Why No Bandpass Filtering on Woofers?


Peter P.

Recommended Posts

We know that woofers can be damaged if they're over-driven at low frequencies, especially frequencies below their 3dB cutoff.

 

So why don't more manufacturers protect their woofers electrically with bandpass filters?

 

It may not be a perfect solution but I expect it would help.

Link to comment
Share on other sites

45 minutes ago, Peter P. said:

We know that woofers can be damaged if they're over-driven at low frequencies, especially frequencies below their 3dB cutoff.

 

So why don't more manufacturers protect their woofers electrically with bandpass filters?

 

 It must not be enough of a problem otherwise they certainly would to protect against warranty claims. Of all the woofers I've blown (there has been a few) or have seen blown up it was from either amplifier failure or they were just flat over-driven for too long. 

Link to comment
Share on other sites

1 hour ago, Crankysoldermeister said:

 1. A bandpass on that section of the network doesn't make sense. Besides, that's what the shunt capacitor does (if it exists).

 

2. You can also use inline fuses.

 

 3. Better yet, turn it down.

1. I would think a bandpass filter would make sense; it would roll off frequency response on the high end toward the tweeter or midrange, and at the low end it would provide some protection from frequencies below the designed low end from causing over-excursion of the woofer.

 

Replying to wuzzzer above; I blew a 6.5" woofer in a sealed cabinet, which was my first experience and why I brought up the issue.

 

2. The in-line fuses would be a great idea; I would expect speaker manufacturers to publish such tips in their owner's manual but they never do.

Or better yet; include the in-line fuses with the speakers as some companies did in the past.

 

3. Turn it down? You know, you said that on a KLIPSCH FORUM...!

Link to comment
Share on other sites

14 hours ago, Peter P. said:

So why don't more manufacturers protect their woofers electrically with bandpass filters?

Who is to decide the parameters of that bandpass filter -- cutoff frequency, filter order, filter type? Woofer excursion varies tremendously depending upon whether it's in a sealed, vented, horn, or other type of enclosure. Within each of those designs, excursion varies with enclosure size and alignment, horn design, number of drivers, and other factors. A "one size fits all" filter simply doesn't exist.

 

As for high frequency rolloff, that's the job of the crossover network -- not needed or wanted on the woofer.

  • Like 1
Link to comment
Share on other sites

1 hour ago, Peter P. said:

Replying to wuzzzer above; I blew a 6.5" woofer in a sealed cabinet, which was my first experience and why I brought up the issue.

 

3. Turn it down? You know, you said that on a KLIPSCH FORUM...!

 

 I think that's likely your problem right there, trying to get 12" performance out of a 6.5" woofer.. sounds like you need to step up your speaker game to accommodate your listening habits.

  • Like 3
Link to comment
Share on other sites

 My brother just traded down from KLF-30's to a pair of RP-600-II's and an RSL Speedwoofer 10 still running the same stereo Sunfire Pre with a Carver TFM-45 at 375 watts per channel I must have warned him a half dozen times the new stereo set up will not go as loud as the old one and to be careful these speakers do not distort even at high volumes so very easy to overdrive them. So far so good but he's only had the set a couple months we'll see I guess.

Link to comment
Share on other sites

Here is the frequency response of an Eminence Kappa 15" speaker. Notice it's natural roll-off characteristics at low frequencies.

 

The most problematic excursion issues I have seen were from record players creating low frequency oscillation. This is where a high quality preamplifier and it's internal filtering will take care of some of those issues even including a "rumble filter" which was on almost every preamp back in the days of when we only had records to listen to.

 

Basically just a low pass filter is all that is needed and the speaker itself rolls of enough as frequency is reduced. Which is why I and many also use sub-woofers with our setups because a woofer can't really produce excellent low frequency results without tuning the box and even then you should really only go down as far as the resonance which looks to be around 48Hz with the Kappa. The resonance may move a tad but it won't move down to 20Hz.

 

I do admit I'm more into electronics and I don't build or design speakers. Someone that does may jump in and explain it better.

 

spacer.png

Link to comment
Share on other sites

19 hours ago, Peter P. said:

We know that woofers can be damaged if they're over-driven at low frequencies, especially frequencies below their 3dB cutoff.

 

So why don't more manufacturers protect their woofers electrically with bandpass filters?

 

It may not be a perfect solution but I expect it would help.

 

Some Pro "subwoofers" have high pass filters.  The box tends to limit the LF excursion well enough, but at only 6 dB/octave.  Most music had very little energy below the 41 Hz low E of a bass, further limiting the need for a woofer high pass filter. 

  • Like 1
Link to comment
Share on other sites

3 hours ago, captainbeefheart said:

Here is the frequency response of an Eminence Kappa 15" speaker. Notice it's natural roll-off characteristics at low frequencies.

 

Basically just a low pass filter is all that is needed and the speaker itself rolls of enough as frequency is reduced. Which is why I and many also use sub-woofers with our setups because a woofer can't really produce excellent low frequency results without tuning the box and even then you should really only go down as far as the resonance which looks to be around 48Hz with the Kappa. The resonance may move a tad but it won't move down to 20Hz.

 

spacer.png

So does what you're saying is in your example above mean that since the woofer's natural low frequency cutoff is 48Hz, that a recording with high energy below that frequency can't move the cone and damage the woofer? Seems to me if you're playing some rap or EDM, those frequencies are going to reach that woofer electrically, regardless and I would expect the woofer to try and reproduce it if the signal is there.

Link to comment
Share on other sites

1 hour ago, Peter P. said:

So does what you're saying is in your example above mean that since the woofer's natural low frequency cutoff is 48Hz, that a recording with high energy below that frequency can't move the cone and damage the woofer? Seems to me if you're playing some rap or EDM, those frequencies are going to reach that woofer electrically, regardless and I would expect the woofer to try and reproduce it if the signal is there.

 

Look at the plot and notice the impedance at the resonant frequency around 48Hz. It rises dramatically to 90 ohms!!

 

In a cabinet, and depending it's design this frequency will shift down lower.

 

An amplifier source impedance, assuming a voltage source is very low, voltage will remain constant vs impedance and current through load will change. At this resonant point the current dramatically gets reduced. For 10 watts at 8 ohms you'll have a little over 1 amp of current. At 90 ohms you'll end up with 1//10 the current at 100mA.

 

Also remember this is a motor. The plot shows greatly reduced sound pressure as the frequency gets reduced below it's Fs. It doesn't really matter if the woofer tries to reproduce the low frequency signal, it naturally puts out far less sound pressure meaning it's not moving in and out as far to push the air. So for the same voltage output for 10 watts (8.9v) into 8 ohms in the frequency region where the speaker is most efficient, say at 200Hz. With the same 8.9v at 20Hz sound pressure is down -28db. e.g. the woofer isn't moving in and out very much compared to the same signal amplitude at 200Hz with a sound pressure output of 100db.

 

So yes the speaker will reproduce those frequencies but with greatly reduced excursion moving less air and less sound pressure output.

 

How can it damage the woofer when; A- there is much less current B-there is much less movement produced in and out.

  • Like 1
Link to comment
Share on other sites

When I was with Altec-Lansing, I worked on a design project for a device that automatically adjusted the signal going to a woofer, based upon the design of the enclosure, the woofer parameters, a thermal model of the voice coil circuit, the history of power input, and the frequency content of the signal. (I wish that I could remember the name of the product.) The intent was to allow maximum output from the woofer without damaging it. This was 1995, so it's not a new idea.

  • Like 1
Link to comment
Share on other sites

6 minutes ago, captainbeefheart said:

So yes the speaker will reproduce those frequencies but with greatly reduced excursion moving less air and less sound pressure output.

That's only true for a closed-box woofer. In a vented box, once below the box tuning frequency the woofer and the port outputs are increasingly out-of-phase. So the woofer excursion increases even when the total radiated acoustical output is decreasing.

  • Like 1
Link to comment
Share on other sites

7 minutes ago, Edgar said:

That's only true for a closed-box woofer. In a vented box, once below the box tuning frequency the woofer and the port outputs are increasingly out-of-phase. So the woofer excursion increases even when the total radiated acoustical output is decreasing.

 

Excellent thank you!! As I said someone else that builds speakers will know more.

 

All my speakers except for maybe one pair are closed box not vented/ported. All my experience is with closed enclosures.

 

I think the crux of the matter is using the speaker within it's ratings. Your device you created was probably as much a protection device along with improvement. If you have someone with a very powerful amplifier cranking the EQ of the low bass up as high as possible and pushing the speakers beyond what they are designed to handle then yes you can easily damage the woofer.

 

A band pass filter just isn't necessary if using your speakers within their ability. If this was possibly a pro audio situation Edgar's device would be a much better approach than a band pass filter which was probably it's intended market?

Link to comment
Share on other sites

6 minutes ago, captainbeefheart said:

If this was possibly a pro audio situation Edgar's device would be a much better approach than a band pass filter which was probably it's intended market?

Exactly. This was intended for sound reinforcement loudspeakers, as might be used in a theater or performance hall. They face much higher power levels than the typical living room speaker.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...