Film-Tech Forum ARCHIVE: proper way to eq a subwoofer

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Posted by Antonio Marcheselli (Member # 373) on 04-23-2004, 07:19 PM:

HI everyone
Let's say that I usually do not equalize theaters so it is just curiosity.

Dolby standards says that the parametric eq should be used to have the flattest frequency responce lowering the main resonance frequency of the theater. On the example's graph (on manuals) I can see a sub that have a flat response at sides but a center resonance frequency. This is simple because I can just align the resonance frequency to the flattest part of the graph (say 20-40 and 100-150).
But in my limited experience this is not common. More frequently I saw a sw array that has no flat zone (for example rising from 20 to 60 and lowering from 60 to 200) or with two resonance frequency (say 30Hz and 80Hz).

In the first case my question is: how should I use the parametric eq since I do not have a reference in the sub response? Is the best way to apply all the eq possible to have the flattest responce?

In the second case, what resonance should I lower? In my experience lowering the lower resonance causes best result but it is just few tests made.

I appreciate all suggestions!

Bye
Antonio

Posted by Antonio Marcheselli (Member # 373) on 07-25-2004, 04:37 AM:

Hi Everyone

I raise this post again, I'm sure that someone can give me an answer...

I would like to complete my question with some example, since I really know that I'm not always clear in my question...

What I asked is how to eq a subwoofer when the graph you have on your RTA is very different from the one you saw on the Dolby Manual. Here is a shot (all the shot are not real, created using a TXT file!!) of a typical subwoofer. The resonance is on a flat zone so you have just to flatten the main resonance.

Here you found a different type instead. Similar to the first one but you do not have a flat "skirt" at sides. My question is: how much eq should I made? Since the side of the frequencies are lowering continuously, I would say "as much as possible" to have the flattest and more extended responce. But I'm not really sure.

This is a complete different situation. The sub has two resonance frequency. What should I lower? On SDDS I can use two parametric equalizer, but what about dolby's?

Thank you in advance for your help!

Bye
Antonio

Posted by John Hawkinson (Member # 1135) on 07-25-2004, 08:28 AM:

Sorry this not a helpful answer, but wow, Antonio, the question sure is clear!

--jhawk

Posted by Antonio Marcheselli (Member # 373) on 07-25-2004, 09:46 AM:

Thank you john, I just made some graph to explain better my question, I do not mean to say that you do not understand what I write, just that I do not write clear question sometime!! [Smile]

Bye
A

P.S. I just realize that the scale of the Y axis is not as I intended, to make the lowest equalization in the second picture I should apply an EQ of -40dB!!! [Eek!]

But the concept is clear anyway!

Posted by Michael Schaffer (Member # 1204) on 07-25-2004, 10:34 AM:

To be honest, I don't understand the actual question either. Everything is explained very nicely (with pictures!) in the Dolby manuals, for instance the CP65 installation manual. Basically, the "flatness" (or whatever degree thereof is doable) of the subwoofer response is achieved by the Q control, not the EQ, and the level depends on the average level of the front channel bands. If the room is more resonant in the bass region than the processor adjustments allow to take care of, you have three options
- see if you can find the problem in the room
- construct a new cinema processor
- live with it

Posted by Antonio Marcheselli (Member # 373) on 07-25-2004, 10:56 AM:

Michael

Let see if I can be more clear!

You miss the point. I'm talking about equalizing the Subwoofer using its parametric equalizers. I'm not talking about its level or if the resonance is out of the range of the processor.

Dolby's manual is very clear but, IMHO, it is not complete because the situazion explained (with pictures too!) in the manual is like my first image. IN that case you have just to flatten the main resonance frequency to the flattest part of the remaining band (skirts). Dolby manual says to flatten the main resonance to the same level of the skirts (of the subwoofer response). See image from CP650 manual

I show this case in my picture #1, where the skirts of the response speak clearly about how to flatten the resonance. This case is very clear.

But what about the other two cases? Where I have no flat skirts as reference or when I have two main resonances (and just only one parametric equalizer)?

Thank for your help and do not hesitate to ask for clarification if my questions are still not clear enough!

Bye
A

Posted by Ferdinando Innocenti (Member # 2477) on 07-25-2004, 04:35 PM:

Antonio,
the second example of yours is like the first (of Dolby): you must reach the flattest curve.
In the third, which is really a “normal” example, I think you saw that kind of graph on an RTA connected to one microphone, not to a multiplexer, or your subs are too far one from the other and the intermodulation (I hope it’s the right word in English) distortion cuts some frequencies.

You can try
1. to change the subs position, if they aren’t near
2. if there are no improvements, to change the mic position, to average parametric eq

I heard a lot of subs, one near the left ch, one near the right ch, which killed low frequencies!

Ciao
Nando

Posted by Steve Guttag (Member # 268) on 07-25-2004, 05:07 PM:

In example 1 (in Dolby's manual) one sees a good sub with a resonante frequency with the room. EQ is simple and a parametric will get rid of the spike.

The second example...doesn't show a resonate frequency so much as one with really poor response...no EQ is needed, throw the "sub" away...stage channels should have better response than that "sub."

Example 3 is a tough one. With Dolby's single parametric cut-only EQ you really can't solve the problem. If the EQ was a boost/cut like normal parametrics...then you would have no problem flatening the response. However, Dolby only designed their SUB EQs for room resonances, not for weird sub responses. If you have a tough room like that...there is nothing to stop you from adding a proper parametric EQ to the system either.

If anyone things a single cut-only parametric EQ is going to solve all problems in a sub channel then they are nuts. If you have good equipment and you have a decent room, it can be all you need. I must admit...I don't need more than that single cut EQ in the vast majority of the rooms I've done.

Steve

Posted by Antonio Marcheselli (Member # 373) on 07-25-2004, 06:59 PM:

Steve

In my limited experience I saw many cases where the sub has two different resonance frequencies. They were always array of 4 or 6 subs. I remember the second example in my old cinema with 4x4645B. I thought that the third case (two resonance) was a common case... I saw them in new screens, with array of 4 subs, places on the floor, between left and center channel... as where they stay! If I remember correctly my 4688 too has two resonance frequency in my home theater...
So my choice are two:

1. add another parametric eq
2. make some tests and see where is the best results...

Nando

Yes, is a single mic analyzer but the subs are not installed in different places, they are all togheter.

Thank to all

Bye!
A

Posted by Steve Guttag (Member # 268) on 07-25-2004, 08:42 PM:

I've used too many 4645Bs to know that they don't have the response as shown in example #2...in fact they are about ruler flat down to 20Hz.

Steve

Posted by Antonio Marcheselli (Member # 373) on 07-26-2004, 01:56 PM:

Steve,

I always felt that that array of subwoofer had something wrong.

However, that array is no more existant, that cinema is no more existant...

About the two resonance, I thought that it was a common possibility and that there is a proper way to equalize...

Thank for your help!

bye
A