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Why No Bandpass Filtering on Woofers?


Peter P.

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On 9/4/2022 at 2:33 PM, 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.

 

Woofers roll off at their low end, just as tweeters roll off at their top end.  A woofer is a simple machine.  It doesn't "try" to do anything.  If you send it really low-frequency music, if it can't play it, you won't hear it.  That's why we have subwoofers:  because nearly all affordable (costing under, say, $30,000) speakers can't play the bottom octave or two.  The sub lets us hear that part of the music that was missing when played through the speakers only.

 

Rolled-off bottom end was not such an issue in the days of vinyl, because they have rolled off deep bass, for a couple of reasons.  Remember rumble filters?  They would cut off the lowest frequencies.  Now we have full-range music, so we need full-range sound systems.  Enter the subwoofer.

 

This makes me think of those who write that they set their speakers as Small so that the subwoofer can "spare the speakers the load of playing deep bass", or so the sub can "do the heavy lifting".  The speakers' woofers do what they can, no more than that. As another member pointed out, if you turn on the Loudness button or turn the bass control all the way up and then play the system at high volume, then you're asking the woofers to do more than they can, and damage can occur.  A little common sense with the volume control goes a long way.

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My guess is 2 main reasons:

 

1) Cost - Even though it would be minimal, the accounting department would go crazy!!!!  The same reason many speakers don't have a simple cross brace or two in a larger cabinet.

 

2)Manufacturers couldn't boast how low their speakers play over another brands speaker.... even though it may be at -10dB!!! (Same reason some companies boast that their speakers play up to 40khz!)

 

I used to DJ back in college and do weddings and school dances, etc. afterwards to supplement my great pay as a teacher. I ran a three way system, but always a 4 way crossover.  I would cut my subs (high pass) at 40hz.  I would still get that chest pounding bass without worrying about over excursion on my subs.

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6 hours ago, Chief bonehead said:

Can someone figure out the values for two pole highpass filter with a -3dB at let’s say 40 Hz for let’s say a 4 ohm load?  

That's another good argument against the idea -- inductors for low cutoff frequencies get very large, very fast. Furthermore, you're trying to design a passive filter for a load whose impedance is varying wildly near the system resonant frequency, so the filter that you design won't be the filter that you actually get. If such a filter is to be used, then it should be placed in the low-level signal chain, i.e., before the amplifier.

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2 hours ago, Islander said:

If you send it really low-frequency music, if it can't play it, you won't hear it.

No, but it will still be moving considerable amounts, in response to those low-frequency signals that it cannot play loud enough for you to hear, adding modulation distortion to what you can hear.

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^^^ These last three posts are Gold.

 

Most popular loudspeakers are bass reflex / ported enclosures, those unload the woofer below the port tuning frequency and it just flaps around under that: Modulation distortion and possible speaker damage are the results.  Easiest way to deal with fixing this is at the signal level, before it hits the power amplifier & steeper than 12dB/octave.

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On 9/5/2022 at 10:50 PM, Chief bonehead said:

Can someone figure out the values for two pole highpass filter with a -3dB at let’s say 40 Hz for let’s say a 4 ohm load?  

Big and Grande (expensive) are values that come to mind...much easier to put a high pass filter in at the line level stages.

Like @John Warren

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On 9/3/2022 at 10:01 PM, 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.

Anyone doing modern Home Theater with subs is using bandpass filtering on the woofer section from 40-80 Hz. on up by way of Subwoofer Crossover with "Small" speaker setting on AVR or Pre Pro. Technically it's High Pass filtering on the the whole passive full range speaker, where the passive part rolls off the top end of woofer to midrange in a 3-way, or Tweeter range in a 2-way.

 

It's not the manufacturer's job to high pass their woofer sections, it's the user's. Unless of course they are modern DJ type "speakers on high stands" that use XLR signal input to Active Bi-Amping, which will likely contain bandpass filtering.

 

So I guess it is NOT as rare as you claim. Also there is not such thing as a "perfect" solution. Only compromises!

 

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On 9/6/2022 at 8:29 AM, Edgar said:

That's another good argument against the idea -- inductors for low cutoff frequencies get very large, very fast. Furthermore, you're trying to design a passive filter for a load whose impedance is varying wildly near the system resonant frequency, so the filter that you design won't be the filter that you actually get. If such a filter is to be used, then it should be placed in the low-level signal chain, i.e., before the amplifier.

I did bandpass filtering on my Quarter Pie horn by using an affordable Behringer Ultra Drive Pro on the bass horn only, with passive on the top horns (35-350 Hz.). This allowed a sharper slope at the top end of the woofer to match the midrange/treble section with added 4-6 db signal level boost across the band. It was one of the lowest distortion setups I ever had.

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On 9/6/2022 at 1:50 AM, Chief bonehead said:

Can someone figure out the values for two pole highpass filter with a -3dB at let’s say 40 Hz for let’s say a 4 ohm load?  

Too expensive for production and too lossy to be practical (Toroidal chokes the expensive exception to losses).

 

I'm going with 23 mH Toroid inductor with 12 Awg wire (0.2 ohms?) in series with the woofer with 700 uF worth of capacitors shunting the woofer. You might also need a separate cabinet if you use Film Capacitors instead of Non Polar Electrolytics.

 

When I asked PWK and Jim Hunter in 1985 about high pass filtering. PWK handed me a huge, iron core, inductor that he deemed to be a "good enough" high pass filter. AFAICR, the thing weighed about 5-7 pounds!!!

 

Either way, Bean counters still rule.

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