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Author Topic: subwoofer eq approach  (Read 4907 times)

boburtz

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subwoofer eq approach
« on: September 04, 2014, 08:56:45 pm »

Recently I had a discussion about equalizing a dual 18" reflex subwoofer box with a person who is a renowned speaker and amplifier designer and was surprised at what he told me. Bear in mind, this person's designs are in current use in the wild and are some of the most powerful systems I have ever heard, so it stands to reason that he knows what he is talking about, I am just looking for a second opinion before putting this idea into practice with my own inventory. I've been running the same drivers for years without failures, but could certainly use a little more low frequency extension for the edm stuff. If I can get it without consequence, the there is no reason not to.

The basic idea was that you can boost the eq at the box tuning frequency up to 8 or 9 db across a fairly wide (~1 octave) bandwidth, as long as you put a high pass with a steep slope (24db/oct) a cycle or two under the boost, and the drivers are voltage limited to their rms capacity. So if my box is tuned (meaning, has its lowest impedence) at 36 Hz, then boost 36Hz 9db, and set the high pass at 34 or 35Hz.  It seems to me that this would cause over excursion of the driver at full power but I am not an expert on this matter. If it makes a difference, the boxes are QRX218s powered with PL380, limited at 69 volts, with 2dB of headroom. Any thoughts?
« Last Edit: September 04, 2014, 09:05:03 pm by boburtz »
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Ivan Beaver

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Re: subwoofer eq approach
« Reply #1 on: September 04, 2014, 09:11:00 pm »

Recently I had a discussion about equalizing a dual 18" reflex subwoofer box with a person who is a renowned speaker and amplifier designer and was surprised at what he told me. Bear in mind, this person's designs are in current use in the wild and are some of the most powerful systems I have ever heard, so it stands to reason that he knows what he is talking about, I am just looking for a second opinion before putting this idea into practice with my own inventory. I've been running the same drivers for years without failures, but could certainly use a little more low frequency extension for the edm stuff. If I can get it without consequence, the there is no reason not to.

The basic idea was that you can boost the eq at the box tuning frequency up to 8 or 9 db across a fairly wide (~1 octave) bandwidth, as long as you put a high pass with a steep slope (24db/oct) a cycle or two under the boost, and the drivers are voltage limited to their rms capacity. So if my box is tuned (meaning, has its lowest impedence) at 36 Hz, then boost 36Hz 9db, and set the high pass at 34 or 35Hz.  It seems to me that this would cause over excursion of the driver at full power but I am not an expert on this matter. If it makes a difference, the boxes are QRX218s powered with PL380, limited at 69 volts, with 2dB of headroom. Any thoughts?
Think of it this way.

It does not make any difference if you simply turn up the input level by 9dB or boost the eq by 9dB.  The resultant voltage (power) to the driver is the same.

The down side is that while at lower levels the boost will sound nice.  But at higher levels the low freq that is boosted will limit first-and could reduce the overall apparent SPL of the cabinet.
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David Morison

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Re: subwoofer eq approach
« Reply #2 on: September 05, 2014, 08:18:21 am »

The basic idea was that you can boost the eq at the box tuning frequency up to 8 or 9 db across a fairly wide (~1 octave) bandwidth, as long as you put a high pass with a steep slope (24db/oct) a cycle or two under the boost, and the drivers are voltage limited to their rms capacity. So if my box is tuned (meaning, has its lowest impedence) at 36 Hz, then boost 36Hz 9db, and set the high pass at 34 or 35Hz.  It seems to me that this would cause over excursion of the driver at full power but I am not an expert on this matter.

Ivan's points do give you the bottom line, the resulting boost will make you reach your system limit sooner if you do have significant content coming through at the boosted frequencies.

The concept replies on the fact that near the box tuning frequency there is a reduction in excursion, with it being higher either side - so if you confine your boost to that region you have less risk of causing over-excursion.

It sounds very similar to what EAW seem to be doing for a lot of their subs - they're quite rare in showing you the unprocessed box response, the processor response and then the resultant system respone all on their spec sheets, so you can take a look at some of those to see how it works.

HTH,
David.
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Art Welter

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Re: subwoofer eq approach
« Reply #3 on: September 05, 2014, 10:32:11 am »

The concept replies on the fact that near the box tuning frequency there is a reduction in excursion, with it being higher either side - so if you confine your boost to that region you have less risk of causing over-excursion.
Granted, a large boost at FB won't result in over excursion, but impedance is lowest at Fb, and with excursion at minimum, vent cooling is also at minimum, so heating is maximized. It is more common to burn PA sub voice coils than to cause damage from over excursion.

The "Q" of the vent varies with the alignment, some may be as wider than an octave, some may be only 1/3 octave or so.

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Art Welter

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Re: subwoofer eq approach
« Reply #4 on: September 05, 2014, 10:57:04 am »

I've been running the same drivers for years without failures, but could certainly use a little more low frequency extension for the edm stuff. If I can get it without consequence, the there is no reason not to.

 If it makes a difference, the boxes are QRX218s powered with PL380, limited at 69 volts, with 2dB of headroom. Any thoughts?
I had used EVX drivers since their introduction, never burned one even using an amp delivering around 126 volt peaks. Sold a pair to a DJ playing mostly hip-hop. He decided to use just one (I told him always use the pair) and burned it on the first gig, using an amp rated around 600 watts.

69 volts is around 600 watts per driver, the AES rating of the EVX-180B speakers used in your cabinets. The AES rating uses a pink noise signal with 6 dB crest factor. A fair amount of EDM has LF content with as little as 3 dB crest factor, same as a sine wave. To be safe with that type of program, you need to limit the long term average to around half to 3/4 of the AES rating, especially if you boost at the impedance/excursion minima. EDM really needs separate peak and average limiters.

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Dan Hoppin

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Re: subwoofer eq approach
« Reply #5 on: September 05, 2014, 11:05:08 am »

The basic idea was that you can boost the eq at the box tuning frequency up to 8 or 9 db across a fairly wide (~1 octave) bandwidth, as long as you put a high pass with a steep slope (24db/oct) a cycle or two under the boost, and the drivers are voltage limited to their rms capacity.

I have been experimenting with a similar approach to this over the summer.  I have been using the technique mentioned above in situations when I have pre-recorded music in small to mid-sized rooms, and I don't need ridiculous SPL, more the "room rattling" type of serious low-end response.  I have been very happy with the results of this.

I have done just the opposite though for outdoor gig's with live bands, trying to maximize SPL 50Hz and above.

I build a lot of my own subwoofers, and there is some great speaker design software available that allows you to "see" the relationships of tuning frequency/excursion/EQ Boost/Hi-Pass/SPL.  That has really opened up my eyes to how these relationships work and how they affect each other.

Ivan and David, I understand the points you have both made, and you comments make sense, but I believe the technique Bob mentions actually works.  At tuning frequency, speaker excursion basically drops to zero.  In both speaker modeling software, and in actual physical testing, I will push a subwoofer to XMAX.  I will apply the type of boost the Bob mentioned (typically using 2-6dB of boost) with the appropriate hi-pass filter.  It appears to me that I am getting appropriately increased response boost at the tuning frequency, without any SPL reduction that you have both mentioned.  I am trying to wrap my mind around how this actually works, so any further comments you can offer would be appreciated.

"Old Man" Welter, I agree with you comments.  It's funny that you commented while I was posting this, as you are actually partially responsible for me doing these experiments.  Your PPSL design over at DIYaudio.com inspired me build some of my own Push-Pull prototypes this summer.  This technique works very well with a variation of your PPSL design I built.  I have had good results, but not quite as noticeable response increase, with some other Tapped Horn and BR type cab's I have.

I have just began experimenting with using dynamic EQ to make these adjustments automatically according to the SPL.  Basically I am trying to automatically compensate for the Fletcher-Munson type response of human hearing.  I haven't gotten my setting fully dialed in yet, but I am please with what I have come up with so far.

Thank you,
DH



 
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boburtz

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Re: subwoofer eq approach
« Reply #6 on: September 05, 2014, 11:39:37 am »

A fair amount of EDM has LF content with as little as 3 dB crest factor, same as a sine wave. To be safe with that type of program, you need to limit the long term average to around half to 3/4 of the AES rating, especially if you boost at the impedance/excursion minima.
I don't understand why a less dynamic wave would require a lower limit threshold. It seems to me that the only difference between a 3dB and a 6dB sine wave, at a particular rms voltage, would be lower peaks on the 3dB crest factor sine wave. The rms voltage would be the same, no?
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Ivan Beaver

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Re: subwoofer eq approach
« Reply #7 on: September 05, 2014, 01:05:13 pm »

As far as heating and excursion is concerned-69 volts is 69 volts.  No matter whether it comes from an eq boost or simply driving the cabinet harder, the strain on the driver will be the same.

The difference is what THE REST of the response is-as to whether it makes a difference or not sonically.  The
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Kevin McDonough

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Re: subwoofer eq approach
« Reply #8 on: September 05, 2014, 02:28:33 pm »

hey

Ivan is right in that voltage to the driver is voltage to the driver, no matter whether its from an EQ boost or by turning the volume up.


However it seems that, least from my hazy memories, people are missing the point of this technique.  It's not really for pushing the driver lower, as far as I remember the main point/advantage was to get a smaller box for the same low frequency.


Say for example you have a sub box that is 300 litres and tuned to give a good response with a tuning frequency of XXhz. You can then shrink the box a certain amount (there's a ratio if I remember right) but keep the tuning frequency the same so that in theory the low response suffers a bit, but you make it back up with the low boost.  The low boost doesn't cause the driver any extra stress over what it would have had in the originally designed box and the general consensus that the sound is slightly tighter and sounds a little better because its a smaller/tighter box and a marginally better impulse response and group delay.

So you're not using it to push the drivers lower than where they'd usually play, but using it to get a smaller box with the same response and potentially slightly better sound quality (subjective obviously).

Google a B6 alignment, should give some info I'm sure there's a Don Keele paper on it.


K


EDIT:  The paper in question....

A New Set of Sixth-Order Vented-Box Loudspeaker System Alignments

A new and useful set of low-frequency assisted alignments contain Thiele's sixth-order Butterworth (B/6) alignment as a central member. The new alignments provide the same low cutoff with moderate amplifier boost (+6 dB) and low out-of-band driver excursion as the assisted B/6 alignment 15 of Thiele. The method of alignment generation is based on shifts of driver suspension compliance.


http://www.aes.org/e-lib/browse.cfm?elib=2690




« Last Edit: September 05, 2014, 02:31:03 pm by Kevin McDonough »
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Art Welter

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Re: subwoofer eq approach
« Reply #9 on: September 05, 2014, 03:24:32 pm »

I don't understand why a less dynamic wave would require a lower limit threshold. It seems to me that the only difference between a 3dB and a 6dB sine wave, at a particular rms voltage, would be lower peaks on the 3dB crest factor sine wave. The rms voltage would be the same, no?
You will notice that EV has not used "Watts RMS" for ages, watts "RMS" would be a sine wave. A 69 volt sine wave (or music with only 3 dB crest factor) measuring 69 volts RMS has twice the average power as the (old) AES pink noise signal with 6 dB crest factor having 69 volt peaks.
And a true square wave has twice the average power as a sine wave, the reason we don't want to hard clip amps.

If you run a 69 volt sine wave (or droning EDM with a 3 dB crest factor) at FB into your EVX 180B, the Kapton formers will start to blister and drag after a relatively short period. Give it a try, you can fix the blisters by cutting off the dust cap, putting in some emery sandpaper, running the driver at 20 Hz to sand the blisters flat. Then clean out the gap, glue the dust cap back on.

If you run "normal" pop music with a 10-12 dB dynamic range, the voice coil will be fine with twice that voltage, four times the peak power, but only a fraction of the average power.
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Re: subwoofer eq approach
« Reply #9 on: September 05, 2014, 03:24:32 pm »


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