Notch Filters to remove room modes

[gdan]

Ok. Now the first step.
U1 is connected as a buffer. Use a similar circuit at the input berore R1.
The + input of the opamp is the input of the circuit. The output of the buffer will be connected to the R1.
Post the circuit to see if you have modified correctly.

 

[tonescout]

Ok - thanks, do you mean add another op amp as a buffer BEFORE R1?

Does this also need a voltage supply?

 

[gdan]

Yes, before R1.
And it needs power supply.

After that you will need also another two buffers.

 

[tonescout]

ok , so without buffers the active filter circuit simply does not function?

If I fed an REW signal into the existing circuit and recorded the frequency sweep of output from it could that help determine anything useful?

 

[gdan]

It will operate, but it will have a response that will depend on the characteristics of the previous and the next circuit.
Also the two notches will need a buffer between them otherwise they will affect one the other.

 

[bentoronto]

...If I fed an REW signal into the existing circuit and recorded the frequency sweep of output from it could that help determine anything useful?

Yes, important to verify. Best if you could have as much of the final set-up intact and measure from input to final signal to driver.

B.

 

[gdan]

Here are the notch filters with the buffers.

 

[gdan]

And here is the LTspice file.

Now you need to check if the notch frequencies, the attenuation and the Q of the filters are suitable for your room.

 

[tonescout]

many thanks for this - I will let you know how I get on

 

[bentoronto]

I can't make much sense of the plots on those axes.

Maybe back to the REW and the mic with and without the notch filter - but this time without the upper-woofer playing.

 

[GM]

Have you tried moving it around?

FYI/FWIW: http://www.harman.com/sites/default/files/multsubs_0.pdf

 

[AllenB]

Yes, that is the best way (not the only method but it's practically a complete solution nonetheless).

A big peak can be indicative of a problem of distribution in the room. Equalising such a peak is not going to change that. When you find a place and setting for a second sub which works in cooperation with the first, you reduce the large peaks. After a third sub you should be closer to a few dB either way.

 

[tonescout]

yeah - I have been through this, my brother uses 3 subs in his home cinema system. Moving the sub around the room in the practical places available makes little real difference. So I either live with an uneven response or try the filters, so far experience of filters is not great either!!

 

[gdan]

Notch filters can equalize the room response usually only for a specific listening positition.
I did that for my room, and I had very good results.

If you move to another position in the room, the equalization that is needed will be different.

Using multiple subwoofers solves this problem much better.

 

[bentoronto]

yeah - I have been through this, my brother uses 3 subs in his home cinema system. Moving the sub around the room in the practical places available makes little real difference.

Don't know if you did measurements are used a golden ear. I tried 14 locations for a single sealed box (AR-1w... great speaker) and found big difference among them with the mic at my chair for all runs. Big.

B.

 

[tonescout]

well so I have got a behrigner audio interface and am testing the filter as an input output loop, I have to get more experience with REW, to know what I am really gong tbh, BUT I got this out last night. Strange no second frequency notch as per the circuit.

I will calibrate the card and make more sense of everything - the 2 plots compare the PP3 batteries connected and not connected.

Also moving the sub is not really a viable option if the room is used for other purposes (it is) see image it's 137L and made of 25mm plywood so heavy and big. It has a green grilled to the left of the cabinet in the picture.

 

[tonescout]

I mean green speaker grill sorry for typo!