Questions about hi-passing and dialling in a T48

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Mishkin
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Joined: Wed May 31, 2023 11:35 am

Questions about hi-passing and dialling in a T48

#1 Post by Mishkin »

Over these last two weeks I've been working on a pair of DR200s. I've been using glue drying breaks to sound test my T48 . For now i've been using a pair of old car 6by9 coaxials in rear shelf wedge type boxes as stand ins for the DR200s.

The Titan in question is a 2 x 12" loaded, 30" wide T48 thats being run at 2 ohm, It's being powered by a pioneer car amp (GM-D8701) and regulated by a Dayton 408DSP .
rat test.jpeg
First interesting thing to note is that the amp can put out a lot higher a voltage than we expected. When i play a 0db 40hz test-tone , If the source (a laptop computer), the amp gain, and the channel and master volume in the DSP are all brought to their maximum, i was getting 49volts from this amp. The amp is rated at 500watts @ 2 Ohms (continuous power output) - so thats 17 volts higher than we expected. I'll be limiting the sub at 31 volts. I guess this head room is a good thing, means the amp shouldnt run too hot, right ?

Annoyingly there is no dedicated brickwall Limiter in the 408DSP so I've had to limit by using test tone, setting everything to absolute max and dialling the gains or channel volumes back till i get the required voltage.

My first question is about hi passing. I am low passing at 100hz. If I choose the steepest curve filter available in the 408DSP ( Butterworth) and do my hi pass as instructed - set it at 40hz at 24db/Oct - I look at the curve and immediately notice that almost every part of my subs range is being to some extent attenuated. The only bit playing un-attenuated is maybe 59 - 69Hz ish. Would it be unsafe to set the hi-pass to only curve down from 40hz? or failing that is there a bit of leeway to get a bit more of the desired range to play un-attenuated ?
As i only have one sub ( for now), I'm desperate to get everything i can out of it.
dsp Sub.jpeg


My second question is does any one have any recommendations or advice for the EQ for either this T48 or the DR200s that will be working with it ?

Also been wondering about delaying the DR200s when it comes to using them. If this is recommended, by how much ? - i will experiment with all this with my own ears but any advice would be nice as guide lines. This is my first time setting a DSP and having this level of control

ALthough i haven't heard the sub with appropriate mid/tops yet , It sounds amazing already ... horns just sound very good :)

Mishkin
Posts: 53
Joined: Wed May 31, 2023 11:35 am

Re: Questions about hi-passing and dialling in a T48

#2 Post by Mishkin »

Ok after a bit of playing around today i understood a bit better how the Low and Hipass curve is then mediated by the EQing


I crossed over higher for now as my temporary mid/tops weren't really reaching down to the sub .
Ive boosted abit in various places to get the steeper curve i was talking about in the lowest end, i also delayed the tops a bit - just by ear.

Then I felt that in some tunes the kick drum was sounding a bit grim and overhwelming, remembered someone saying that horns are naturally a bit louder in the higher part of their range so i tried to locate what frequency this Kick problem was in and put a narrower dip in around there...

Any feed back on my approach much appreciated . As i said , im new to this :
WhatsApp Image 2024-03-03 at 21.53.11.jpeg

The sub does sound great to me! i really like it and it puts a smile on my face . I'm interested that even when playing at my limited max volume i dont feel any air rushing out the mouth with the bigger basslines... is this usual ? Or is it because the mouth is so biig ?

Keryn O'Shea
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Location: Brisbane, Australia

Re: Questions about hi-passing and dialling in a T48

#3 Post by Keryn O'Shea »

Hi mate, that's a weapon of a sub!
Mishkin wrote: Sat Mar 02, 2024 5:22 pm Annoyingly there is no dedicated brickwall Limiter in the 408DSP..
Best to arrange a limiter, it'd be a shame to torch your drivers with an accidental nudge :broke:
Mishkin wrote: Sat Mar 02, 2024 5:22 pm ..I look at the curve and immediately notice that almost every part of my subs range is being to some extent attenuated..
Yes it does seem this way, and that's how it looks on the curve, you're not missing out on anything to my knowledge though mate, that's how it's done.
Mishkin wrote: Sun Mar 03, 2024 5:21 pm ..i dont feel any air rushing out the mouth with the bigger basslines...
I wouldn't be sticking my head in there :) , yes this is normal
AT8 14", AT10 19", T30 28" LAB12, T48 36" 3015LF, THT 20" LAB15, SLA 4x3"/4x4", CArray 6x4", TLAH, TLAP, J10 FLat, J12 Flat, OT12 Flat, OT15 CD, SLAP 2x6"/4x6", WH8, DR280 Mld'd, XF210. Next J10L

Keryn O'Shea
Posts: 452
Joined: Sat Jan 26, 2013 5:50 pm
Location: Brisbane, Australia

Re: Questions about hi-passing and dialling in a T48

#4 Post by Keryn O'Shea »

Mishkin wrote: Sat Mar 02, 2024 5:22 pm ..does any one have any recommendations or advice for the EQ for either this T48 or the DR200s..
There is an "EQ Settings" subheading in the forum, you should be able to find your info there.
Have you read the "Sub Placement Rules" thread in the FAQ?
Mishkin wrote: Sun Mar 03, 2024 5:21 pm The sub does sound great to me! i really like it and it puts a smile on my face..
Excellent :D
AT8 14", AT10 19", T30 28" LAB12, T48 36" 3015LF, THT 20" LAB15, SLA 4x3"/4x4", CArray 6x4", TLAH, TLAP, J10 FLat, J12 Flat, OT12 Flat, OT15 CD, SLAP 2x6"/4x6", WH8, DR280 Mld'd, XF210. Next J10L

Mishkin
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Joined: Wed May 31, 2023 11:35 am

Re: Questions about hi-passing and dialling in a T48

#5 Post by Mishkin »

Hi Keryn,

thanks for your reply.

i have read the sub placement guide, but had missed the EQ settings section. will check it now !

Mishkin
Posts: 53
Joined: Wed May 31, 2023 11:35 am

Re: Questions about hi-passing and dialling in a T48

#6 Post by Mishkin »

Keryn O'Shea wrote: Mon Mar 04, 2024 3:05 am Best to arrange a limiter, it'd be a shame to torch your drivers with an accidental nudge :broke:
do you think it is :

A. not safe to limit in the way I have been, even before my attempt at EQing?

B. not safe to limit in the way i have with the boosting i then did in the eq in the low end ?

or C.do you mean a nudge like someone was to adjust the the DSP settings? ( this shouldnt really happen bcos its via a pc plugged into the dsp which has no manual control, so i guess itd only happen if i decided to start fiddling mid event which im going to force myself to avoid)


If i could find a affordable standalone limiter for 12v id buy it! otherwise only option i know of- without breaking the bank- is to sell the dayton DSP and buy the supposedly lower quality t.racks DSP 4x4 Mini. How frustrating that the Dayton doesnt have one ! :wall:

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Seth
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Re: Questions about hi-passing and dialling in a T48

#7 Post by Seth »

In my research and experience over the years, a tuned response curve similar to the one shown below in orange provides a very pleasing listening experience with heavily compressed recorded content and also works really well with live sound too.

Image

What I'm going to do here is explain how to use the crossover filters instead of EQ to get us super close to the final response curve were looking for. It doesn't have to be spot on perfect. It seems even relatively close is often times fantastic on the first try. And you can always make changes and final EQ to suit your personal taste and ear. There are several strategies and methods in manipulating the signal, but here's how I like to do it.

Looking at the sub's response bandwidth in the chart above, it's essentially a straight line from about 45Hz to 125Hz, with 40-45Hz at +15db above 125Hz. To be honest, +15db is sometimes a little too much in the bottom and I end up pulling a little after listening. Anyway, back to it. If we then look at the response of the subwoofer, we can then start planning our strategy.

Since we don't have an actual response graph for the actual combination, a 30" wide T48 loaded with two 12's (Fountek FW322 4ohm), let's just use something close. Below is the measured response of a 15" wide single loaded with a 3012LF, which I've added 6db to the trace to account for there being two to equal the cab in question. Simply adding 6dB doesn't accurately portray what actually occurs when doubling cab count. Typically the curve flattens and the corner frequency is pushed lower. And the difference in drivers will have slightly different overall response curve shape. But, for what we need, this should get us in the ballpark in leu of not having measurements of the actual cab combination.



I put some markers on the chart denoting what the sensitivity is at 40Hz and 125Hz, as well as where the desired response would be at 125Hz after tuning, in relation to the sensitivity at 40Hz.

At 40Hz we're looking at roughly 99.5dB. So, if we subtract 15dB from that number we get the desired 125Hz response level after tuning, 84.5db. If we subtract that figure from the native 125Hz response of 114dB, we know we want to attenuate the signal at 125Hz by about 29.5dB.

Every doubling of frequency is an octave. So, 40Hz to 80Hz is 1 octave and 80 to 160Hz would be another octave. But, we only need to get to 125Hz. So, to figure out how deeply into the second octave 125Hz is, simply subtract 80 from 125, giving us 45. 45 divided by the total bandwidth of the octave in question, 80, gives us 0.5625. So, from 40Hz to 125Hz is 1.5625 octaves.

The total desire attenuation at 125Hz (29.5dB) divided by the number of octaves (1.5625) will give us a slope... 18.88dB/octave.

That get's us a rough idea. We can fine tune it a little to get a little better result. Let's run the numbers for a 50Hz Butterworth filter which is down -3dB at it's specified cutoff frequency.

125 is 1.25 octaves above 50Hz. Subtract the 3db that the filter is already down at it's cutoff frequency from the total attenuation needed gives us 26.5dB. 26.5dB divided by 1.25octaves gives us a slope of 21.2db/Octave

Run the numbers for a 50Hz Linkwitz Riley filter which is down -6dB at the designated cutoff frequency gives us a slope of 18.8dB/Octave.

It looks like we may be able to use a 18dB/octave filter, set at about 50Hz to put the 40Hz-125Hz response just about perfect and achieve the majority of the needed "EQ" without much EQ at all.

I'd set your highpass 40Hz 24dB/Octave Butterworth and your lowpass at 50Hz 18dB/octave Linkwitz-Riley for the sub. Run a 125Hz 24dB/Octave highpass filter on the tops. Play a 125Hz sinewave through just the subwoofer and using your smartphone with a free SPL meter app, adjust your volume until it's say 90dB. The specific number isn't important, louder than ambient noise without being too loud. Make a written note of the specific volume reading. Do not change the volume. Disable/mute the subwoofer, enable the tops, and adjust the outptut gain in the DSP for the tops to get the SPL (measured from the exact same spot as before) to be as close to the same volume as the subwoofer was. Make a written note what the specific SPL is. Then, without adjusting the volume at all, enable the subwoofer and play the subwoofer and top together. If everything is perfect, the resulting SPL will be 6dB louder than either one played on it's own. The likelihood of it being perfect is low, so don't be worried if it's less than 6dB louder than each on their own. We'll iron that out later. But!, if the SPL goes down, the polarity needs to be inverted on one of them. Inverting the polarity can be as easy as swapping the wires on the amp. But, I'd personally just invert the signal in the DSP.

What this will accomplish is an "effective crossover point" of 125Hz, even though the low pass on the sub channel is set to 50Hz and the highpass of the mid/high channel is set to 125Hz. The point where they actually crossover is 125Hz. 125Hz should work well with the 6x9's you have and also work well with the DR200's.

From here, we can adjust delay to phase match the sub and top which should get the 125Hz SPL to be as close to a full 6dB summation, EQ the tops, and do a final system EQ.

Anyway, try it out and see how you like it. It may take a little adjusting slopes and cutoff frequency to get it to sound perfect, but it really shouldn't need much. This should put you really really close to sounding great.

I forgot to have you order an inexpensive "calibrated microphone" from PartsExpress when you put your tweeter order in. But, for your needs it's not entirely imperative. The microphone in your smart phone will get you close enough for government work. There are plenty of free audio apps you can use to measure and tune the system. I use "Audio Tool" by J.J. Bunn. I forget which one Bill uses, but there's a bunch and any one with a Spectrum Analyzer will likely do you just fine.

If any of you have a different way, questions/concerns about this method, or just wanna tell me I'm wrong and/or call me a big jerk, please feel free to do so. I certainly don't know everything and I'm more than happy to learn how you would do it. :thumbsup:
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

Mishkin
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Joined: Wed May 31, 2023 11:35 am

Re: Questions about hi-passing and dialling in a T48

#8 Post by Mishkin »

Amazing !

so much to think about and work with here... hopefully i get a chance to try out what you've suggested tmrw night ... before the debut friday night!

im excited !

thanks for sharing your knowledge and approach

Keryn O'Shea
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Re: Questions about hi-passing and dialling in a T48

#9 Post by Keryn O'Shea »

Mishkin wrote: Mon Mar 04, 2024 7:55 pm C.do you mean a nudge like someone was to adjust the the DSP settings? ( this shouldnt really happen bcos its via a pc plugged into the dsp which has no manual control, so i guess itd only happen if i decided to start fiddling mid event which im going to force myself to avoid)
Oh ok, no knobs then.. should be fine
Seth wrote: Tue Mar 05, 2024 12:43 am ..call me a big jerk, please feel free to do so..
That's so tempting 😄
AT8 14", AT10 19", T30 28" LAB12, T48 36" 3015LF, THT 20" LAB15, SLA 4x3"/4x4", CArray 6x4", TLAH, TLAP, J10 FLat, J12 Flat, OT12 Flat, OT15 CD, SLAP 2x6"/4x6", WH8, DR280 Mld'd, XF210. Next J10L

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Seth
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Re: Questions about hi-passing and dialling in a T48

#10 Post by Seth »

Keryn O'Shea wrote: Thu Mar 07, 2024 12:35 am
Seth wrote: Tue Mar 05, 2024 12:43 am ..call me a big jerk, please feel free to do so..
That's so tempting 😄
:lol:
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

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