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Author Topic: Use DSP to extend and flatten frequency response of sealed box sub?  (Read 18394 times)

Antone Atmarama Bajor

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Re: Use DSP to extend and flatten frequency response of sealed box sub?
« Reply #10 on: July 08, 2008, 02:56:23 am »

      I guess in the end it really depends on what type of music you're engineering.  Its amazing how few people actually incorporate any meaningful content below 40Hz.  I've heard some pop, electronic and some modern Jazz Fusion.

    Of course the explanation is probably as simple as, the average consumer doesn't have accurate sub bass reproduction capability, reasonable listening environment, or even have an awareness of what it really is.  So why waste precious head room on something that isn't going to be heard by the average listener.

    85dB is right at the threshold of hearing for 20Hz.

    Food for thought for Full scale cinema (exhibition houses) the surrounds and mains are mixed at 85dB with 20dB of Headroom and the LFE is mixed at 95dB with 20dB of headroom.(Movies actually take advantage of a large dynamic range compared to modern recorded music.)

    Although Cinema X curve is Far from Flat due to perf screen attenuation.  They shoot for SPL down to 35Hz and ignore things bellow, and have a 3dB per octave roll of from 2kHz to 8Khz, Khz mix against.

    Bag End:
I heard that Ron Wickersham (Wickersham Alembic Basses, and Greatful dead wall of sound fame)designed the ELF circuit, but had a falling out with Bag End, so they can't use his design anymore.

     So now Bag End have the Infra system.

I was shocked when I read about the ELF system being a Dual Integrator circuit.  An integrator Circuit is an elementary filter circuit with a 6dB per octave slope.  A dual integrator is nothing more than 2 of these circuits in series.  Probably the most complicated part of the circuit is the limiter, changing where the slope lines up with the box.

    I made an op amp circuit that sums a "dual integrator" with a High pass filter.  I think the Linkwitz transform is actually a  little simpler to build.  You may just want to build a simple Linkwitz transform for your sub, using the linkwitz lab spread sheet.  And put it in between the sub amp and the xover.  You may not have enough gain available in DSP unless you attenuate everything above the sub knee.  Hopefully you can work everything out in the wash.

     Ahh Bass.

Good luck.



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George Friedman-Jimenez

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Re: Use DSP to extend and flatten frequency response of sealed box sub?
« Reply #11 on: July 08, 2008, 09:05:12 am »

Thanks, Antone. The major focus of my music and my studio is Afrocuban rumba. This involves acoustic percussion, primarily congas plus claves, woodblock, and sometimes wood box drums call cajones. The bass content is important for 2 reasons. Most importantly, some very important parts of the rhythms are bass notes that are generally heard only by the musicians. This is because they are naturally softer and as you said the ear is much less sensitive to low frequencies. Sometimes the musicians inadvertently produce more bass sound than intended. I have heard commercial CDs in which these spurious bass notes have not been edited out and it sounds terrible. I suspect the studio's monitoring system was deficient in LF response. It is important to hear this in the mixing stage and edit these unintended bass sounds out as appropriate.

The effect of reduced perception of bass, especially at low SPLs, is huge. The Fletcher Munson curves are actually conservative in predicting the perceived effect. More recent curves, the ISO Equal Loudness Curves, are apparently more accurate than the Fletcher Munson curves and are more extreme in their predictions. Wikipedia has an internet accessible graph that compares the two, here: http://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_curves

As an example, for a 1000 Hz tone, 100 phon is the perceived loudness of a 100 dB SPL tone. The perceived loudness of a 40 Hz, 100 dB SPL tone is around 70 phon. At 20 Hz, your 100 dB SPL tone is perceived as having equal loudness as a 40 phon tone at 1000 Hz.

This has big implications for sound production. To produce a 40 Hz tone that is perceived as 100 phon requires about 117 dB SPL, interpolating from the graph. For a 20 Hz tone to be perceived at 100 phon requires 128 dB SPL. The situation gets even worse at lower SPLs. For a 40 Hz tone to be perceived at 80 phon requires about 106 dB SPL. For a 20 Hz tone to be perceived at 80 phon requires about 120 db SPL. The threshold of hearing for a 1000 Hz tone is at 0 phon but for a 20 Hz tone, the threshold of hearing is at 75 phon and at 50 phon for 40 Hz!

These perceived loudness curves are averages over small groups of study subjects that were included in the research studies, and I don't know how much variability of these parameters exists in the general population.

For this reason, seeking a subwoofer that is "flat" in its dB SPL frequency response seems very arbitrary, and it does not come close to ensuring that the perceived loudness is equal at different frequencies. A "flat" frequency response based on perceived loudness would sound massively different than what we are used to, aside from being technically unfeasible for bass frequencies. For mixing music, however, a "flat" frequency response in SPL gives us some basis to judge when the bass notes would sound softer or louder than the real life sound that was recorded, assuming the frequency response of the listener's stereo system or PA is also flat. To the extent that most people's stereos are not flat, our mixes will sound different on those systems, but there is nothing we can do about that. Also, some people choose to crank the bass and others cut it down.

So do you think that for purposes of monitoring recordings striving for a natural sound similar to the real acoustic sound, the small sealed sub I am proposing will be adequate?

If I need another 6 dB, I could build 2. I even thought of building 2 boxes 14" x 14" x 34" long, putting feet on the driver end and using them vertically as combined downfiring subs and speaker stands! Also, I already have the Peerless driver but the LAB12 driver has similar TS parameters and would give a similar response in the same box, with slightly higher sensitivity and double the power handling capability. I just don't know how clean (distortion free) the sound of the LAB12 would be between 20-40 Hz compared with the Peerless.
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Antone Atmarama Bajor

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Re: Use DSP to extend and flatten frequency response of sealed box sub?
« Reply #12 on: July 08, 2008, 04:56:16 pm »

     I may be the wrong person to ask about that.  I just love low bass.  But don't have a lot of personal experience mixing in a reasonably good environment.

    I have heard from an old engineer friend of mine that there is a trend almost a convention in the recording studios to EQ the LF in the monitors about 6dB Hot starting at either 80-100Hz.  I think some of this convention was started to help make sure tracks were recorded bass light to prevent to much bass to be put on a vinyl.  I also heard that it was sort of held on to because of the popularity of "Bass Boost" Circuits in consumer audio.  Because that slope is closer to what the average boom box owner is listening too.

    I'm sure you've taken an FFT of your average music track and seen the spectral density in the LF  I think on the average recording there is probably hotter 20dB+ in the 80Hz-100Hz range vs anywhere else in the spectrum, so if your 85dB spl mixing (That would be your spectral sum)  You are probably using most of your headroom in the bass area.

    Since you are trying to preserve the natural characteristics of the percussion perhaps take a profile of the drums spectrum with fft.  Then you can determine what sort of headroom in the range you are operating is required.  Not that Natural is often the desired in produced music.  If you have a huge 15Hz transient from a particular piece of percussion, which eats massive amounts of headroom, are you going to go to extraordinary lengths to faithfully reproduce it?

    I have built a Quad Lab12 and pushed them with about 3kW with some Car sub stress test tracks like "Bass I love you" it took quite a bit of Hype to make the tracks sound pleasing to me.  But they were pretty clean.  I may have been getting a bit of extra 2nd harmonic On the 6Hz tone I think maybe I heard a little mechanical noise.  My top speakers were little Blue sky monitors so for my subjective taste the sub bass balance was just about right.

    I think the Lab 12 in a smaller box would have less distortion due to better cone control.  You could have similar response in the Peerless, in a similarly small box, but your RMS power handling is less than 1/2.  And again in ~2cu' you could have a pair in push pull.

    Flat bass low is possible with all of the modern tools but normally only with a limited power bandwidth.

    Any engineers here feel free to chime in, I really am just a tourist in this world of sound reproduction.

Good luck-

Antone-
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John Chiara

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Re: Use DSP to extend and flatten frequency response of sealed box sub?
« Reply #13 on: July 08, 2008, 11:40:36 pm »

George Friedman-Jimenez wrote on Tue, 08 July 2008 01:01

Thanks, John for the tip. I looked at the Bag End website and the Infra MXB is their balanced I/O subwoofer processor that has replaced the ELF Integrator. It seems to be limited to a 12 dB per octave slope, which would work better for some sealed driver/box combinations than others, depending on the TS parameters, box size, stuffing, etc. Also, the MSRP for the Infra MXB is over $1k (I didn't find a street price on the web), while the Driverack 260 on the web is around $800, so I don't think the cost would be substantially different. The Bag End processor also seems much less flexible and adaptable than a DSP unit like the Driverack 260 or the Xilica 3060, which have crossovers, many different filter options, and delays. The disclaimer on the Bag End website even says not to use with subs other than the Bag End Infra series, although that is probably CYA language for folks who aren't paying attention to Xmax.




The thing is..the ELF unit is designed to do what you want to do. I have a homemade sealed box and it works fine. Getting enough level for rock shows is gonna be tough. It sounds like you want to reinvent the wheel. They already figured it out..and it works. Unless you have a lot of free time...
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George Friedman-Jimenez

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Re: Use DSP to extend and flatten frequency response of sealed box sub?
« Reply #14 on: July 09, 2008, 01:05:15 am »

Thanks John for your thoughts. SPL is not a major concern, I am not doing rock shows, so a single 12" sealed sub will probably be fine in a small recording / rehearsal studio. If I am wrong, building a second box and driver is not that expensive, I will already have the amp and whatever EQ system I decide on.

I am not clear on how adjustable the filters on the ELF are, were you able to adjust your homemade sub for a flat frequency response? What are your sub driver, box dimensions, and stuffing?

I am definitely not in favor of reinventing the wheel, but for a homebuilt sub, I would have to do all the frequency response measurements, filter adjustments, crossover adjustments, optimization of physical placement, and level adjustments from scratch with the Bag End ELF anyway. Would it really take that much more time and effort to do it with a generic DSP like the Xilica or the Driverack?

I don't think there will be much cost difference, the new model Bag End Integrator looks like it sells for almost $1k. Do they still sell the original ELF unit? I couldn't find it on the web.
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John Chiara

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Re: Use DSP to extend and flatten frequency response of sealed box sub?
« Reply #15 on: July 09, 2008, 01:23:36 am »

The idea is to run the sealed box below it's normal cutoff..which I think is supposed to be a very linear rolloff..therefore easy to compensate for. I have a 2x12 box that I had built..and they guy did it not to my specs. The bos size doesn't matter that much..I don't think. I power it with a 500 watt amp..and it is very good for my control room setup..with Mackie 824's. It might be easy to emulate this...it's not really my field. Bag End also still makes the Infrasub..I believe..self contained..plug in and go. I really like my setup and using it as a reference has really helped in managing low frequencies in a life setting as well as recording/mixing/mastering.
Good Luck whatever you settle on.
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"mix is a verb, not a noun" Sooo, as Aunt Bea would say.."Get to it!!!"

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Silas Pradetto

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Re: Use DSP to extend and flatten frequency response of sealed box sub?
« Reply #16 on: July 09, 2008, 09:17:43 am »

Ivan stated that the Xilica's 6 PEQ's would probably be enough. Based on that, 4 wouldn't be enough, so I wanted to mention that. Since the PEQ's can also be shelving EQ's, I'm sure you could make do with 4 or 6. Actually, the less you actually use the better.
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Ivan Beaver

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Re: Use DSP to extend and flatten frequency response of sealed box sub?
« Reply #17 on: July 09, 2008, 09:55:01 pm »

Silas Pradetto wrote on Wed, 09 July 2008 09:17

Ivan stated that the Xilica's 6 PEQ's would probably be enough. Based on that, 4 wouldn't be enough, so I wanted to mention that. Since the PEQ's can also be shelving EQ's, I'm sure you could make do with 4 or 6. Actually, the less you actually use the better.


I didn't say it would take 6, only that 6 were available IF NEEDED.

I do agree that less is often more in audio.
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