Why good bookshelf bass extension with sub crossover 80Hz? - AVS Forum
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post #1 of 14 Old 01-02-2004, 09:06 AM - Thread Starter
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A spec that often shows up when comparing several bookshelf-level speakers is their low frequency extension.

If we intend these speakers to be part of a home theater with a sub and the crossover at the ubiquitous 80 Hz, do we really need to care weather the bookshelf speakers in question extend to 60Hz or 70Hz?

Or is this measurement one that suggests better build quality and technology in other parts of the speaker.

From a theoretical standpoint, does a speaker's bottom end matter a whit when it's getting crossed-over 20Hz above where its bottom trails off?
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post #2 of 14 Old 01-02-2004, 09:29 AM
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I myself believe it speaks immensely towards build quality. Another thing to note is that if you crossover at 80hz many suggest having a speaker that will go to 40hz on the low end.

I'm not sure on this but I heard it somewhere on these forums:

Crossing over at 80hz still requires a speaker to be able to go lower than 80hz in order to achieve a seamless transition. Some suggest using speakers that will go twice as low (i.e. 40hz) while others suggest that as long as it can do 20hz below (i.e. 60hz) you should be fine.

For me personally its more a question of build quality.
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post #3 of 14 Old 01-02-2004, 09:36 AM
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Vlad is right. Do this:

Turn your sub OFF.

Put in an AVIA DVD and run the sound test for low frequencies (it plays a scale, starting at 100hz dropping to 20hz).

You'll hear your speakers drop off after 80HZ, but it's not like a lightswitch. It begins ROLLING OFF at 80HZ, but is still audible well below that.

The narrower the range a speaker has to provide, the better it can do it. If a "Woofer" in a bookshelf speaker has to produce 200hz "Punches" and 60hz bass, it will trip on itself. By being able to produce the lower lows without having to, it should be even more efficient when asked to only reproduce higher-lows, and will retain sound quality during the "roll off' under 80Hz.

Also, some people don't have subwoofers therefore haivng a full range is beneficial (like if you use them for Zone2).
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post #4 of 14 Old 01-02-2004, 09:47 AM - Thread Starter
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Any thoughts on ideal theoretical crossover frequency if my fronts are Paradigm Titans (50 Hz low freq extension and freq range is 60 Hz to 20 kHz)?

I'm hearing dips in the THX sweep tone when the crossover is set in receiver at 80Hz and sub's is turned up to the max (can't disable).
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post #5 of 14 Old 01-02-2004, 10:26 AM
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Frequecy respones of your speakers is measured in an anechoic chamber. This removes all the possible room effects on the frequency response. Once you take the speaker and hook it up in your room, sudddenly your room affects how the speaker responds. - And this may cause the problems that you are hearing.

My personal opinion is that low frequency response is largly a matter of box design, and crossover choice.
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post #6 of 14 Old 01-02-2004, 10:41 AM
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Quote:
Originally posted by Eyleron
Any thoughts on ideal theoretical crossover frequency if my fronts are Paradigm Titans (50 Hz low freq extension and freq range is 60 Hz to 20 kHz)?

I'm hearing dips in the THX sweep tone when the crossover is set in receiver at 80Hz and sub's is turned up to the max (can't disable).

On another post, Kevin C Brown said:
"even with small bookshelf speakers, remember that to pick a proper crossover freq, you need the -3 dB point of the speakers to be 1/2 to 1 full octave below that freq. For example, I have mains that go to 30 Hz, -3 dB. So I use a 60 Hz crossover freq (1 full octave up). You don't want the natural roll off of the speaker interferring with the slope imposed by the crossover."


In your case, you said your freq range is 60hz - 20 khz. 1 octave up would be 90hz. You likely can't set your receiver to 90hz, so use 100hz. You said yourself that you are hearing dips, so set your crossover higher. Also, if you haven't already, get an SPL meter to help you evaluate your levels during the sweeps.

Regards,

Karl
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post #7 of 14 Old 01-02-2004, 11:25 AM
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Eyleron,

I think the ideal crossover is as much room and equipment (receiver / pre-pro) as it is speakers. I wrote all of the following to illustrate that a combination of equipment, speakers, room and seating location can cause what you would otherwise think should be the best choice, in reality is not the optimal choice. So answering your in theory question may not be optimal but a rull of thumb is to use a crossover setting that is twice the -3db point of your mains. So the numbers you posted would indicate that a 110 to 120 hz setting would be a very good place to start.

I have a Sony 4ES receiver and old acoustic suspension sansui speakers with 15" woofs. These aren't bookshelve speakers obviously but given the right situation I think some book shelf speakers could cause a similar situation as large speakers. I found it difficult by ear to judge which crossover setting was optimal. I went back and forth between 40 and 100 hz. I adjusted phase as necessary and recorded 1 /12 octave graphs (10 hz - 100 hz) of the different settings. I tried about everything I could think of including mains set to large + sub, Large only (to determine the in room extension capabilities) and small + sub.

I have been tinkering off and on for the past 9 months trying to get an optimal crossover and phase setting for my room. It really wasn't until I viewed my large + sub graph a little harder that I realized I should go beyond conventional wisdom and try something rather extreme. That is, I ran the crossover on the Sony all the way to 200 Hz which is the maximum setting the receiver will allow.

This setting actually produced the best graphs. Because my test tones stop at 100, I cross-referenced 100 hz to 500 hz with 1/3 octave pink noise to make sure nothing was out of whack up high. The pink noise test checked out fine and believe it or not I can not localize the sub. I also changed the set-up back to a conventional 80 hz crossover + sub and ran the pink noise up to 500 and it was basically the same as the 200 hz crossover setting.

The sub sits inches to the right of my front main speaker. I tried this setting with action DVDs and music at loud levels and found nothing unusual. If I ever get where I can localize the sub I'll try something else but so far this seems fine to me.

Why I think this works for me and my room? 1) The Sony seems to have a shallow crossover slope, probably 6 dbs an octave so starting at 200hz works a little better assuming the sub can't be localized. 2) Since the slope is shallow I was getting some minor (.5 db) to major (3db) cancellations below the crossover point between 35 - 75 hz. when using an 80 hz crossover point and the main speakers set to small 3) The main speakers have enough low end out-put to create cancellation and the slope was not getting them out of the way fast enough. 4) Sound quality wise, the sub is a much better bass producer than my mains.

The large + sub combo graph looked pretty good from 40 to 100 hz and would be okay with typical music but below 40 hz I got anywhere from 5 to 12 dbs of cancellation all the way to 17 hz. I made various phase adjustments to no avail.

What I would recommend is that you experiment as much as your internal crossover will allow with different crossovers and don't get caught up in the actual crossover hz setting. Most other receivers have a 12 db per octave slope and if that's the case for you then an 80 - 100 hz crossover may work as the speakers should be sufficiently out of the subs way beyond 40 hz. You will need an SPL meter and some test tones to quantify the results. Generally speaking, most sub woofers are better bass producers than the woofs in speaker cabinets, therefore the higher the setting you can go without localizing the sub the better off you may be. The crossover setting that causes the least disruption in frequency response while not being able to localize your sub will be your best setting.

Many audiophiles like to use a low crossover like 40 hz. Many of these individuals have a 24 db per octave sloped crossover so they can probably pull it off provided their main speakers can go flat to 40 hz. Using a low crossover eliminates the disruption in frequency response that can occur in the critical bass music frequency range when using an 80 hz crossover. Since I don't have a 24 db per octave crossover I went the other direction. Mentally I would prefer the traditional 80-100 hz setting but the reality of it is that the 200 hz setting graphs better and sounds fine to my ears so I will keep with using it until I find something that works better.
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post #8 of 14 Old 01-02-2004, 11:53 AM - Thread Starter
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Wow thanks for your great reply!

I hadn't thought about the receiver (which is a poor initial startup 5.1 to get me by for now) having perhaps a shallower crossover slope than other receivers. Again, something else I wish would be published!

That may explain that prior to SPL measurements (waiting on the DVE to arrive), judging by ear the sweep tones, I had fewer dips at 150 Hz, even though I wanted to set it at 100 Hz. If it's a shallow slope, setting it at 100 Hz may have been eliminating many frequencies!

It sounds like I should make a graph of the front speakers low frequency performance without a crossover myself, and then again with the receiver's crossover turned on.

Thanks again.
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post #9 of 14 Old 01-02-2004, 02:44 PM
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TRC,
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Most other receivers have a 12 db per octave slope and if that's the case for you then an 80 - 100 hz crossover may work as the speakers should be sufficiently out of the subs way beyond 40 hz
The problem isn't that the mains are out of the way, but that they are actually creating the frequency holes with their too steep acoustic rolloff (unless they are THX certified or sealed).

Many ported mains have a 24-36dB/octave rolloff (unlike your sealed speakers 12dB/octave rolloff) combined with most receiver's electrical 12dB rolloff it really creates a steep high-pass that the sub and receiver's low-pass just can't blend with correctly.

Currently I use a symmetrical 24dB/octave high-pass/low-pass setup with an active external crossover @60Hz from mains with an f3 of 32Hz to the sub.

Most subs probably won't be too happy at 200Hz (very high distortion). I find it hard to believe you can't localize frequencies @200Hz, I know I can.

Eyleron, the best you can do is to try the different options provided by the receiver. If the receiver is THX rated, the 80Hz for sure will be a 12dB/octave slope.

BruceD


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post #10 of 14 Old 01-02-2004, 03:22 PM
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"I find it hard to believe you can't localize frequencies @200Hz, I know I can. "

Well so far I can't, but only time will tell. I only started measuring and using this set-up in the past few days. I never considered using anything higher than 120 hz because my mains were large. And believe, my I re-checked my measurements several times. Like I said, mentally it does not seem like the right thing to do. Doing the Avia subwoofer set-up is far easier now because the subwoofer noise doesn't cause the needle to jump around as much as it did with the 80 hz setting. It still swings just not as much.

At 6db an octave slope and maybe -3db down at 200 Hz to start, the first octave goes all the way to 100 hz. So playing filtered pink noise without the sub on shows quite a bit of output in the mains at 120-150 hz. The sub really doesn't kick in sufficeintly above the mains until I breach 100 hz. Due to room layout, my mains are near corners of the room so their bass is being reinforced more than they probably should be. If I could pull them out from the walls and away from the corners a bit. I think even if I could I would need to go up to a 120 hz crossover with this rig.

And yes my mains did have a few holes, confirmed by the Large only frequency sweep, in th 75-90 hz range and going higher on the crossover helped fill in those holes. But the crime of it all was the -3db or so cancellation effect I got below 40 hz on some of the tones when I used the standard 80 hz setting.

I could still use an eq. And if I had an eq., I probably would and could get the 80hz setting to work for me. But in the interim I will stick with this unless I get to where I can localize the sub.

One thing I haven't tried is the mains set to large + sub at the 200 hz setting. The mains set to large and 80 hz yielded a terrible response below 40 hz so it will be interesting to see how they graph out at this new setting.
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post #11 of 14 Old 01-02-2004, 04:54 PM
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TRC,
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At 6db an octave slope and maybe -3db down at 200 Hz to start, the first octave goes all the way to 100 hz. So playing filtered pink noise without the sub on shows quite a bit of output in the mains at 120-150 hz. The sub really doesn't kick in sufficeintly above the mains until I breach 100 hz.
Not quite sure I understand what you are doing.

Are you using the low-pass from the receiver @200Hz for the sub output?

Are you also setting a specific XO on the sub itself?

At the crossover frequency of 200Hz, the receiver's high-pass signal from the mains is already down by 3dB. Another octave down ( at 100Hz) means 6dB more attenuation for a total of 9dB and should be quite noticable.

Most of the receiver's low-pass XO use a 24dB/octave slope for the sub, do you know about yours? Doesn't sound like it would be a good match for your setup.

Yours certainly is the stragest setup I've heard of, except for Maggies which are typically XO in the 100-200Hz range.

BruceD


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post #12 of 14 Old 01-02-2004, 09:32 PM
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Yes, it is strange but it seem to be working okay. I don't have the specs on the crossover but an infomal testing with the Rat Shack meter produces a reading that would indicate a -6db slope and about -3db down at the crossover for a total of -9db for the first octave. I'm not using any sub crossover, strictlly receiver.

To mimic a 12 db per octave slope I think I would have to set the receiver crossover to 160 hz in order for my mains to be down the same db at 40 hz when compared to a receiver that uses a 12 db per octave slope at 80 hz. So the real issue to me becomes, can the sub handle it and can I localize the sub. And so far the answer is yes and no. I'll give it the Barry White test tommorow to see what happens.

Down -9db at 100 hz is noticeable but what I was trying to say is that because the first octave is so large, 100 hz, that at say 150 hz the mains are only down ~4db. One other thing I forgot to mention, is that my main speakers produced a nasty peak in the 150-200 hz range when playing the pink noise. The higher crossover setting forces the 150 - 200 range down enough on the mains that this area is much smoother than before.

My mains are old and I don't have the specs on them but it would appear that the mids crossover to the woofer somewhere in the 250-300 hz range. I tested this with pink noise and at 250 the woofs were producing most of the sound and at 300 hz the mids and woofs were about 50/50.

I'm not endorsing my set-up as the norm, that's for sure. But I do think people that are unsure about their crossover setting could benefit by actually measuring the frequency response curves using higher than normal settings. Like me, they may be suprised at the results. I agree that bass frequencies in the range I am working can be localized. But as long as the the mains are kicking out enough sound up there, I think it will be hard to pinpoint the sub.

By the way my LFE cut is at 90 hz. I can set my speaker crossovers at 200 hz and have the LFE cut out at 90 hz. I verified this using Avia's 20-200 LFE signal sweep. So I am strictly talking crossover settings for non digital or LFE settings. There's no way I'd run a crossover setting higher than 90 hz for a dedicated sub channel. As a dedicated sub signal would certainly be localized.

I don't rely totally on measurements. I let my ears have the final say. When listening to music I swiched from the 200 hz setting to 80 hz on the fly and I must say that the 80 hz setting tendended to the muddy the sound. The bass with the 200 hz crossover is a little better since there is less cancellation.
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post #13 of 14 Old 01-03-2004, 04:40 AM
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So one should always try to match the roll off of the mains and receiver whether it is 12db or 24db?
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post #14 of 14 Old 01-03-2004, 09:13 AM
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Quote:
So one should always try to match the roll off of the mains and receiver whether it is 12db or 24db?
The general idea is that a receiver/prepro has 2 XO slopes to consider:

1) The processor's low-pass slope which is typically 24dB/octave (attenuating all higher frequencies sent to the sub) with the sub's own internal XO either turned off or set to the maximum value which must be higher than the receiver's.

2) The processor's high-pass slope which is typically 12dB/octave (attenuating all lower frequencies sent to the various main, center, and surround speakers). Speakers themselves have a natural acoustical rolloff in the lower frequencies which then adds to the receiver's slope. In the case of THX certified speakers this is 12dB/octave acoustic rolloff slope at 80Hz. With sealed speakers it is typically 12dB/octave acoustic rolloff at a specific frequency, but with many ported speakers it is a crapshoot and can be anywhere from a 12dB-36dB/octave acoustic rolloff slope at a specific frequency.

The general idea is to have both slope curves be a mirror of each other thru the XO frequency (for 24dB/octave slopes or 4th order as they're called), so that SPL levels remain stable across the XO boundary.


As an example, a THX certified speaker 's 12dB/octave acoustic rolloff slope added to a receiver's 12dB/octave electrical high-pass slope equals a 24dB/octave slope for the high-pass XO. This will match nicely with the receiver's 24dB/octave XO slope for the low-pass going to the sub.

With mismatched XO slopes, you typically get different SPL levels on either side and through the XO creating a playback signal with mismatched frequency levels. IMHO one of the many reasons why it's difficult to get good subwoofer integration.

BruceD


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