Bad null at 40 Hz

Regalma · « **on:** 14 Sep 2009, 07:57 pm »

This weekend I hooked up my RTA and a pink noise generator. I found that I have a deep dip right at 40 Hz. It is 15 to 20 dB down. And it is quite V shaped.

After moving the mic around and doing some calculations I concluded that I am getting a reflection off the back wall The wall is about 7 feet behind my listening position. That is about 1/4 of a 40 Hz wavelength. In theory that means that I would have a cancellation of a 40 Hz signal right where I am sitting. As I move the mic forward and back the frequency of the dip changes accordingly. That is inline with the idea that it is caused by reflections off the back wall. I have put a pair of GIK Monster Bass traps on the wall behind me with very little effect. I have moved them around and tried the corners and different angles with little effect.

My setup consists of a pair of VMPS RM-30s w/o the built in subwoofers. They have good output down to 35 Hz and a stereo pair of TBI subwoofers sitting under them. I can't do much about the direction of the VMPSs but I have tried rotating the TBIs. They happen to be very directional. That hasn't done much. Running the VMPSs or the TBIs alone still show the same dip.

I could move my chair forward. If I move forward several feet the dip largely disappears. But that puts me too close to the speakers. Or maybe move the chair so far back that the cancellation occurs at a high enough frequency that the bass traps will have more effect. That is not a very good setup for my room though.

I guess I could tear out the back wall so that my living room opens into the garage. But I suspect that would really hurt the resale value of the house, not to mention it leading my friends to finally be convinced I am indeed crazy.

Does anyone have any suggestions?

srlaudio · « **Reply #1 on:** 14 Sep 2009, 08:08 pm »

Hello. I have a great suggestion. To defeat your null with trapping (ie bass traps) your room would be eaten up by cavernous amounts of absorption. An alternative approach is to break up the standing wave with diffusion of the proper design and size in a fraction of what it would take with absorption. Here is a link to exactly the same problem we faced here in Nashville at Hilltop Studio......

http://www.srlaudio.com/hilltop_studio.pdf

I know this will precipitate a firestorm of response, but I say the proof is in the pudding. John, the owner of Hilltop, has welcomed anyone to call him about what we did there, and I stand by the results of a properly designed, large scale diffuser.

Allen Rumbaugh
SRL Acoustics

doug s. · « **Reply #2 on:** 15 Sep 2009, 05:27 am »

try moving your subs. perhaps closer to the listening position - even nearfield...

doug s.

JLM · « **Reply #3 on:** 15 Sep 2009, 09:52 am »

Thanks for the inquiry.

Curious, what listening position is "too close"? I have two positions here in the mancave. One is Cardas nearfield (68 inch equilateral triangle). Imaging pops into focus like crazy but the effect approaches headphones (not everyone's cup of tea). The other is straight back here at my desk (about 10 feet from the speakers).

A third option would be to use equalization. I started with a fully Cardas compliant rectangular room/setup, then added absorption before applying EQ. With nearfield setup and my large single driver speakers (sans whizzer) the room interaction was minimal to start with. Results will vary, but in this situation EQ has made the most improvement (much more than just for room interaction) and the absorption the least.

It's also be good to know room dimensions.

MaxCast · « **Reply #4 on:** 15 Sep 2009, 10:58 am »

Where to you crossover your subs and RM30's?
Do you have any other treatments in the room?
What is the size of your room?

Glenn Kuras · « **Reply #5 on:** 15 Sep 2009, 12:57 pm »

Quote from: srlaudio on 14 Sep 2009, 08:08 pm

Hello. I have a great suggestion. To defeat your null with trapping (ie bass traps) your room would be eaten up by cavernous amounts of absorption. An alternative approach is to break up the standing wave with diffusion of the proper design and size in a fraction of what it would take with absorption. Here is a link to exactly the same problem we faced here in Nashville at Hilltop Studio......

http://www.srlaudio.com/hilltop_studio.pdf

I know this will precipitate a firestorm of response, but I say the proof is in the pudding. John, the owner of Hilltop, has welcomed anyone to call him about what we did there, and I stand by the results of a properly designed, large scale diffuser.

Allen Rumbaugh
SRL Acoustics

Allen,
First of all I want to say I LOVE DIFFUSION!!!!!! We sell diffusion and have MANY NEW THINGS coming out. One being this bad boy.

BUT can you explain how you would diffuse 40hz???

Or why you would??

See the following
http://www.gikacoustics.com/news_060109.html

Glenn

srlaudio · « **Reply #6 on:** 15 Sep 2009, 02:22 pm »

What I know is that large diffusers do indeed affect the bass range. It will not happen with a prime 7 diffuser such as you pictured, it must be a higher prime (wider panel) to achieve this. At this studio the main complaint was a bass standing wave. I will admit there are two ways to skin this cat, a load of your bass traps would do it, but also one of our larger diffusers properly placed will do the same thing, in this case eliminating the standing wave by breaking it up rather than absorbing it. A side benefit is that the stereo field and ambience is improved with the diffusion route whereas the bass trap route would remove ambience and sound "dead" Here is a place that we achieved exactly what we are talking about.....

http://www.srlaudio.com/hilltop_studio.pdf

Glenn Kuras · « **Reply #7 on:** 15 Sep 2009, 02:34 pm »

Allen,

I would like to see some room graphs of before and after from you.

Here is a before and after room set up Frank did with bass traps.
Please post yours
http://www.gikacoustics.com/news_030709.html
I will say that if you take something like our D1 and place fiberglass behind it that would be a great solution also. But to just diffuse 40hz?

Needless to say just for fun here is a link to calculate diffusors
http://www.digitalaudiorock.com/cgi-bin/qrd.cgi
Have fun

TRADERXFAN · « **Reply #8 on:** 15 Sep 2009, 03:43 pm »

Quote from: Regalma on 14 Sep 2009, 07:57 pm

This weekend I hooked up my RTA and a pink noise generator. I found that I have a deep dip right at 40 Hz. It is 15 to 20 dB down. And it is quite V shaped.

After moving the mic around and doing some calculations I concluded that I am getting a reflection off the back wall The wall is about 7 feet behind my listening position. That is about 1/4 of a 40 Hz wavelength. In theory that means that I would have a cancellation of a 40 Hz signal right where I am sitting. As I move the mic forward and back the frequency of the dip changes accordingly. That is inline with the idea that it is caused by reflections off the back wall. I have put a pair of GIK Monster Bass traps on the wall behind me with very little effect. I have moved them around and tried the corners and different angles with little effect.

My setup consists of a pair of VMPS RM-30s w/o the built in subwoofers. They have good output down to 35 Hz and a stereo pair of TBI subwoofers sitting under them. I can't do much about the direction of the VMPSs but I have tried rotating the TBIs. They happen to be very directional. That hasn't done much. Running the VMPSs or the TBIs alone still show the same dip.

I could move my chair forward. If I move forward several feet the dip largely disappears. But that puts me too close to the speakers. Or maybe move the chair so far back that the cancellation occurs at a high enough frequency that the bass traps will have more effect. That is not a very good setup for my room though.

I guess I could tear out the back wall so that my living room opens into the garage. But I suspect that would really hurt the resale value of the house, not to mention it leading my friends to finally be convinced I am indeed crazy.

Does anyone have any suggestions?

First I would try to fix it by moving one of the subs further away, putting it further back into the room which should give you better spatial average.

If that didn't work, then I would try adding a layer of drywall, with green glue, to that back wall. The only real way to absorb frequencies in that range is structural...

doug s. · « **Reply #9 on:** 19 Sep 2009, 11:13 pm »

regalma, have you tried moving your subwoofers yet? all this talk about sound absorption/diffusion is fascinating, but to deal w/bass nulls, moving your subs seems easiest to implement...

doug s.

James Romeyn · « **Reply #10 on:** 21 Sep 2009, 02:32 am »

Hi Regalma
Sorry to see that apparently this thread is approaching the wastebin.

Read about Dr. Earl Geddes' multi sub approach. By adding one or two more subs (don't have to be costly, maybe as low as $100 ea clearance subs) you may be able to almost completely fix your problem.

The 3 subs must be distributed throughout the room, one in one corner, one above ear level, one out of phase w/ the other three.

The modes in my room are so awful that 100 out of 100 listeners would (IMO) pick a good minimonitor vs. any full range speakers or any one or two subs regardless of placement or quality. Following Earl's & Duke LeJeune's advice AudioKinesis V2.0 SWARM sub system) the modes are almost imperceptible w/ no room treatment for the bass

James Romeyn · « **Reply #11 on:** 21 Sep 2009, 02:35 am »

Oops, forgot editing verbotten.

The subs will normally be sited next to walls; if four subs are employed all can stay on the floor (one must go above ear level when three are employed).

Best wishes.

Bob in St. Louis · « **Reply #12 on:** 21 Sep 2009, 08:21 am »

Browntrout · « **Reply #13 on:** 21 Sep 2009, 05:30 pm »

I genuinely don't understand why having a null at forty hertz is a problem when listening to music. Surely the answer is to tail off below forty hertz making the null null and void as far as the music is concerned.

Bob in St. Louis · « **Reply #14 on:** 21 Sep 2009, 06:18 pm »

Depend on the genre you're listening to I suppose.

Ethan Winer · « **Reply #15 on:** 21 Sep 2009, 06:19 pm »

Quote from: Browntrout on 21 Sep 2009, 05:30 pm

I genuinely don't understand why having a null at forty hertz is a problem when listening to music.

Give this man a ceegar. Few non-HT / non-subwoofer speakers go much below 50 Hz, and most music has little energy there anyway.

--Ethan

Regalma · « **Reply #16 on:** 21 Sep 2009, 09:46 pm »

I haven't been here for a few days. Definitely see subject drift here.

I "solved" my problem by pointing my stereo subwoofers towards the rear wall and crossing over my main speakers at 80 Hz. 80 Hz because that is what I have available right now. And I moved my seat forward about 18". By putting in a high pass filter on the main speakers I got rid of most of the forward radiated 40 Hz information. By pointing the subs towards the rear wall I figure I diffused the information from them somewhat. Moving the seat forward lowered the null frequency by increasing the distance from the backwall. I had already discovered that frequencies much below 40 Hz didn't seem to be prone to nulls, at least in the area of my llistening position. The sound is great and I have plenty of energy down to the lower end of subs, which is 22 Hz. These subs are designed to sound good between 22 and about 150 Hz, so rolling off the mains doesn't seem to hurt the sound, and by having a stereo pair located directly underneath the mains I don't have a directional problem. At least not one that I can hear. The next step is to dig out my Ashley XO and see if there is an XO position better than the 80 Hz high pass I have now.

Overall I am pretty happy. I am actually reaching the point where the only thing I am not happy about is the sound stage being asymetric due to the weird room shape I have to live with.

bpape · « **Reply #17 on:** 21 Sep 2009, 10:02 pm »

Good call. Always best when you can fix a problem with positioning. One of the few things in this hobby that's free.

Bryan

James Romeyn · « **Reply #18 on:** 22 Sep 2009, 12:06 am »

I guess it depends on how close the null actually is to 40 Hz & how wide. Low (open) E on the double bass & bass guitar is 41 Hz IIRC; one octave up is (obviously) double that frequency (it's common for multiples of nulls/peaks to have similar symptoms).

woodsyi · « **Reply #19 on:** 22 Sep 2009, 02:58 am »

I own and use RealTraps, GIK traps and SRL diffusers. All of them do what they are suppose to do and well. My diffusers are of 9.5" depth which I understand is tuned to 1K Hz and up for diffusion. Most people's rooms would not be big enough to put large and deep diffusers like Allan proposes to deal with 40Hz null. This is why I use absorption for bass and front (behind speakers) and diffusion for back and sides. I do believe that deeper diffusers will diffuse lower. SRL diffusers, which came out before GIK and RealTraps, do work very well but I have to admit that I did not get them to deal with anything below 100Hz issues. I do think the size and depth of them should diffuse lower frequencies than the ones Real Traps or GIK offers.