Speakers not fit for an avr with xover???

[Deleted]

Hi guys. I know there is tons of knowledge here so wanted to post this weird comment form a poster on AVS who has been in he last very knowledgable. But this one has me scratching my head...

His referring to my speakers from psa that specs extension to 79hz

A speaker that rolls off at 80Hz is unfit for use with an AVR that provides a crossover.

Thoughts?

[klinemj]

Odd comment! Have you asked him why he thinks that?

Mark

[Deleted]

Yes and he just makes comments that I just don't understand how things work.

[ansat]

can you link to the post?

I would like to see the rest of the context.

Tony

[ansat]

nevermind -- I found it.

[Boomzilla]

The issue probably stems from the THX crossover point of 80 Hz. Second order crossovers (12 dB per octave) normally assume that the speaker and subwoofer will both be flat for at least one octave beyond the crossover point. This would mean that the subwoofer should be flat to 160 Hz and the main speakers should be flat to 40 Hz. If the speaker itself already rolls off at, let's say 6dB per octave (typical of sealed speakers), and if the speaker begins that roll off at the 80 Hz crossover point, then the electronic crossover's slope ADDS to the speaker's inherent roll-off. Therefore, the speaker is actually rolling off at 12+6 = 18 dB per octave below the crossover point.

So what? Well, in a word - PHASE. An 18 dB per octave roll on the main speakers does not work properly with a 12 dB roll on the sub. There will be partial cancellation at the 80 Hz crossover point and there will be a dip in frequency response at that frequency.

Now if the main speakers are ported, their acoustic roll off is typically 12 dB per octave. In that case, the mains will roll off at 24 dB per octave, and the sub at 12. This will NOT cause any acoustic cancellation at the crossover point, but will reduce the amplitude below the crossover point - another dip.

Crossovers work best if both elements roll off at the same slope. There are exceptions to this statement, but not to get into them here...

So the guy on AVS is mostly right. Speakers that start their acoustic roll off at about 80 Hz should be crossed over at 160 Hz by the AVR.

Cheers - Boom

[ansat]

Aug 16, 2015 8:02:40 GMT -5 Boomzilla said:

The issue probably stems from the THX crossover point of 80 Hz. Second order crossovers (12 dB per octave) normally assume that the speaker and subwoofer will ...

Cheers - Boom

You beat me boom. I agree with your assessment.


Tony

[DYohn]

People misunderstand loudspeaker "roll off." All it means is that the system has a natural reduction in output as frequency drops below the system's resonance frequency due to impedance of the woofer voice coil rising. It is not a crossover, and it does nothing to affect phase in the way a crossover does. It's simply that the driver cannot output as high a dbSPL as it can in it's efficient band. It's added resistance - it's as if a volume control had been used. So a 12 db/oct crossover applied during the "roll off" of the driver remains a 12 db/oct slope on the signal. The output level will roll off at the added db/oct of the two effects, but phase does not shift further than the 180 degree signal phase shift imposed by the crossover.

You can compensate for the effect (right up to the thermal limits of the voice coil) by adding signal strength along a curve that is inverse to the rising impedance. See Linkwitz Transform.

There is nothing wrong with using a system that rolls off at 80Hz in an HT setup.

[garbulky]

A guy who says you don't understand, it's smart to leave his learned genius be and talk to smarter people who can explain their thoughts.

[Boomzilla]

I resemble that remark!

[KeithL]

Actually, that's not correct.

Inductive reactance is lower at lower frequencies... so the impedance of the woofer's voice coil actually drops as the frequency gets lower.

The reason the output of a woofer in a sealed cabinet drops off at lower frequencies is that there is in fact a mechanical filter present. The combined spring of the spider, the suspension, and the air pressure in the cabinet acts as the "capacitor", and the acoustic impedance of this capacitor rises at lower frequencies; the mass of the cone acts as the "inductor", and together the two comprise the mechanical equivalent of an electrical L-C high-pass filter. When you add the response of this mechanical filter to the response of the crossover, including the electrical characteristics of the driver itself, you get the "overall response of the system". And, yes, you do get phase shift from the "mechanical equivalent filter" just as you would from an electrical version.

At this point you have two choices: You can arrange things so the mechanical filter falls in the same range as your electronic crossover, and include it in your crossover calculations, or you can make sure the mechanical filter cutoff point is an octave or so below the electrical crossover point, in which case it won't have much effect on those calculations, and so can be more or less safely ignored.

Tuned port designs are somewhat more complex because the speaker system acts as a tuned system (band-pass filter) rather than a high-pass or low-pass filter. At the tuned frequency, the impedance rises sharply (often to four or five times the stated impedance), but the mechanical efficiency also rises sharply. If the designer calculates everything correctly, the result is that the gain due to the mechanical resonance is slightly more than the loss due to the speaker drawing less power due to its higher impedance, and the net result is that the slight bump caused by the tuning pulls up the frequency response where it would normally be starting to drop off, resulting is a response than goes flatter lower than it otherwise would, but drops off more sharply below the tuned frequency. With tuned designs, you also typically have very little woofer movement near the tuned frequency, but the woofer is very poorly controlled below the tuned frequency - which is why you really don't want to send a lot of bass below the tuned frequency to a bass reflex speaker - as it will result in lots of cone movement and very little actual bass output.

Aug 16, 2015 9:22:31 GMT -5 DYohn said:

People misunderstand loudspeaker "roll off." All it means is that the system has a natural reduction in output as frequency drops below the system's resonance frequency due to impedance of the woofer voice coil rising. It is not a crossover, and it does nothing to affect phase in the way a crossover does. It's simply that the driver cannot output as high a dbSPL as it can in it's efficient band. It's added resistance - it's as if a volume control had been used. So a 12 db/oct crossover applied during the "roll off" of the driver remains a 12 db/oct slope on the signal. The output level will roll off at the added db/oct of the two effects, but phase does not shift further than the 180 degree signal phase shift imposed by the crossover.

You can compensate for the effect (right up to the thermal limits of the voice coil) by adding signal strength along a curve that is inverse to the rising impedance. See Linkwitz Transform.

There is nothing wrong with using a system that rolls off at 80Hz in an HT setup.

[Boomzilla]

Well said, Mr. L. I agree, point for point.

The "poorly controlled" woofer movement below the port frequency is why I like to have ported speakers fitted with a rumble filter (or have a crossover at least an octave above the roll off frequency). This prevents the cones from flapping around (and yes, they seriously "flap") below the port tuning frequency.

Boom

[stiehl11]

Aug 18, 2015 6:45:15 GMT -5 Boomzilla said:

Well said, Mr. L. I agree, point for point.

The "poorly controlled" woofer movement below the port frequency is why I like to have ported speakers fitted with a rumble filter (or have a crossover at least an octave above the roll off frequency). This prevents the cones from flapping around (and yes, they seriously "flap") below the port tuning frequency.

Boom

My old Pioneer ST-300 speakers used to do that.

[Boomzilla]

Yeah - I once had a buddy who had some ported 15" speakers. He showed me how "good" they were by holding a candle in front of the port. playing some Bach organ music, and having the air coming out of the port blow out the candle. Of course, the speakers weren't actually producing organ fundamentals - but the cones (no longer loaded by the port) were flapping furiously enough to create some serious air velocity coming out of the port.

I didn't have the heart to tell him that the candle trick had nothing to do with speaker quality, so I just acted suitably amazed and complimented him on the system. Some years later, we discussed those speakers and came to the realization that maybe they weren't actually so good, even if they could blow hard. LOL He's still a good friend.

Boom

[klinemj]

The cooler trick would have been to make the candle flame up...like, well, you know....the old college trick with a lighter and flatulence? How cool could that have been for a speaker in college?

Mark

[Darksky]

It pleases me when I can pick the flaming fart guy out in a room. Mark, you did. Not disappoint.

[Boomzilla]

I foresee serious safety issues! But yes, a speaker that belched fire would have, indeed, been a hit (at least, for the frat boys...).