Question on EQing a sub (1 Viewer)

[Dzung Pham]

I've been wondering if I should use a parametric equalizer for my subwoofer. Currently, I am using two SVS 20-39PCs setup right behind each of my main speakers against the wall. This configuration gives me both a fairly flat frequency response, as well as a fairly uniform response throughout the various sitting positions. If I put them both in the corner, I would definitely need to EQ to get the flat frequency response. However, does the EQ help yield a uniform response throughout the room? Or will I only get a flat response at the central sitting position?

 

[Holadem]

This configuration gives me both a fairly flat frequency response, as well as a fairly uniform response throughout the various sitting positions.

You have found the ideal position without EQ. SO why the heck would you want to change anything? So you could use an EQ?
--
Holadem

 

[Ned]

What makes you think the corner will be any less flat?

That corner placement=boomy thinking is just a myth. If anything it's usually the best placement.

The EQ is good for cutting any bad peaks that occur from room modes. If you don't have any significant peaks then it's not much use.

 

[Dzung Pham]

I don't think the corner yields a less flat response. I know it is measurably less flat (corner placement was the first thing I tried). It produces a huge bump at around 40Hz or so. Placing the subs as I have them results in a few small dips and peaks, while the corner placement has basically one big peak. I believe I could get a flatter response using corner placement and EQ but I'm wondering if that comes at the cost of a less uniform response at different sitting positions.
Holadem, I'm pretty satisfied with what I've got but an EQ would give me yet another toy to play with...

 

[jeff lam]

It sounds to me like you have the ideal response with your set-up right now. I am also planning to use the same setup due to a sliding glass door in my front corner. These will be one sub behind each of my main speakers. Hopefully the room will yeild a flat response as yours did.

 

[Kevin C Brown]

I assume that you have actually measured the response you get in both locations?

Test disc and a Radio Shack meter, or an RTA program on a PC, etc. ?

 

[Bob Christensen]

It helps to have a miked spectrum analyzer, but you can also use test tones and a SPL meter to graph the in-room response from your listening position. Basically, with the sub, you are dealing with relatively long sound waves and what you want to do is compensate for any standing waves and half/quarter wave reflections. First eliminate as many as you can using the phase adjustment (if present) on your sub. Then, "take down" any high points on the graph with your EQ. You will not have much luck trying to "prop up" the lows by increasing the levels in the "dips" via your EQ.

Another way to approach it is to dampen the reflections a bit. Use a partner and mirror. Sit in your primary listening position and have your partner walk around the walls holding the mirror flat against the wall. Mark the spots where you can see the reflection of the sub in the mirror with masking tape on the wall. Now, try to "block" the path of the sound waves emanating from the sub out to those points (try to deal with one at a time). This can be the easiest way to compensate for room gain/loading without the use of extra electronics.

 

[Dzung Pham]

My measurements were made with the Stryke test cd (among others) and the RatShack spl meter. Jeff, I actually found that the best response was achieved by painstakingly trying to find a good combination of phase settings on each subwoofer. This is an advantage of the PC model because it actually has the phase control built-in.

Bob, I've never heard of that trick with the mirror before. How/why exactly does that work? Thanks for all the comments.

 

[Wayne A. Pflughaupt]

Dzung,

I used to be you! I had my two subs set up just like you mentioned. Even with a pair of equalizers, it was extremely difficult betting smooth response, and I had problems with one of them bottoming out during bass-heavy soundtracks (the one that wasn’t in a corner).

After spending a year or so here at the Forum reading posts from people saying that both subs should be in the same corner, I finally bit the bullet and tried it.

I was amazed at the difference! It was everything I’d heard and more. I gained more than double the acoustic output and effortless extension down to 20Hz. During the depth charge scenes in U-571 the couch shakes as if I had tactile transducers! And, I don’t have to worry about bottoming out the drivers anymore.

The deal is, Dzung, if you get an equalizer for your subs there is no is no need to move them around in the room looking for that “magic spot.” That’s for people who don’t have an equalizer. As you have seen, what you end up with is response you can live with, but it comes at the expense of extension and output.

Not to mention, the more boundaries the sub can interact with, the more ragged response is. This is because there will be cancellation at specific frequencies relative to the distance the sub is from each boundary. In addition, with a pair of subs that are separated in the room, the half-way point between them acts like a boundary as well. As you can see, Dzung, what you have is acoustical chaos. Usually you end up with response so ragged it is beyond the “repair” of an equalizer.

Once you get your EQ do yourself a favor and just put both subs in a corner, if possible the one with the most uninterrupted wall length in each direction. This will excite the maximum number of room modes and realize a gain of 6-9dB, which is effectively double the acoustic output. Another benefit of corner placement is that it minimizes the effect of nulls.

As you have noted, Dzung, you’ll have a response peak or two. However, you can easily notch them out with the equalizer, and then you will have it all: The smoothest response, the lowest extension and the highest output.

In my experience, corner placement will give excellent response at all seating positions in the room except those on either wall from the corner where the subs are.

By the way, bass does not emanate in a straight line from a driver. It is omni-directional. The mirror trick is only relevant with high frequencies.

Happy Holidays,

Wayne A. Pflughaupt

 

[Bob Christensen]

Dzung: The mirror trick works because what you are “modeling” is the reflection points for the combination of your sub’s location and your primary listening position. The spots you can see the sub in the mirror from your primary listening position are the same spots where there are direct reflections off the wall. Often spikes and lows are caused when these locations correspond to the wavelength, half-wave, or quarter-wave distances. Spikes are when the waves reflect in phase, effectively increasing room gain, and lows are caused when the waves are reflected out of phase. So when you block the route directly between the sub and the spot marked on the wall, you effectively eliminate that boundary (but you create a new one unless you use an absorber to block it). Remember that each time you move your sub, or your primary listening position, or even furniture, you are changing the boundaries, and you may need to adjust things (either physically, electronically, or acoustically) to flatten your response curve again. And when you electronically EQ any audio source, you are introducing yet another electronic filter, which is not necessarily a good thing to do. It is often easier and more effective (both in terms of cost and sound quality) to physically remedy sound problems (particularly with a sub, since you are dealing with long waves and a limited frequency band) rather than electronically remedy them.

 

[BruceD]

Bob,

Good explanation of how to use the mirror trick for mid-range and high frequencies.

It doesn't apply to bass frequencies.

BruceD

 

[Wayne A. Pflughaupt]

Thanks, Bruce! After I read this, I said to my self, “There’s an easy way to resolve this.”
Does anyone remember the old Audio magazine? They used to do comprehensive measurements on all tested equipment, including speakers. One of the measurements for speakers was off-axis response, both horizontal and vertical. For the horizontal axis, the measurements show response starts dropping at about 2kHz. The further off-axis the more the drop at 20kHz compared to 2kHz. This confirms Bruce’s (and my) contention that the mirror method is most useful at (upper) mid and higher frequencies, which are more directional.
Things are very different at the lower frequencies. On charts for the two speakers I looked at for this, from between 200 and 400Hz (depending on the speaker) and below, measured output was as strong at 90, 135 and even 180 degrees off-axis as it was at 0 and 45 degrees. The datum were consistent for measurements on both the horizontal and vertical axis.
Thus it’s entirely safe to say that measured output with any sub is very consistent at a 360-degree radius. It seems useless to try to block the output at one place on a wall when equal amounts of acoustic energy are hitting to the left and right of that location. And everywhere else, for that matter.
I’ll be happy to send a copy of some of these measurements to anyone who is interested.
Happy Holidays,
Wayne A. Pflughaupt

 

[Bob Christensen]

I am not talking about on axis vs off axis response. I am talking about eliminating (or at least reducing) standing wave and out of phase interactions at the primary listening position. Take your SPL meter and walk around the room a bit and watch the level fluctuate, even at low frequencies. These can be manipulated without electronic EQ, and sometimes fairly easily (sometimes very difficult, however).

I don’t profess to be an expert at all this, just reporting on what I have found muddling through my own experiences and experiments with my own systems.

 

[BruceD]

I believe, I may be mistaken, you would need material more than 6" thick in order to have any real effect on bass frequencies.

You know of anything that is 6" thick you want to hang on your ceiling?

BruceD

 

[Wayne A. Pflughaupt]

Bob,
You might want to check out some of Howard Ferstler’s essays at http://www.surroundfreak.com/info/howard.htm. This is from one titled “Speaker Sound – Room Sound:”

Basically, the only way to effect a standing wave is to erect a complete wall (or maybe the 6” thing Bruce mentioned). It has nothing to do with the position of the sub in the room, or “block” set between it and a wall.
The phase situation is actually aggravated by placing the sub out in the room, because of the quarter-wave length situation caused by the soundwave reflecting back from the boundary out of phase. So if the sub is out in the room, you have phase cancellation at five specific distance-related frequencies (assuming a simple square or rectangular room).
For instance, say you have a room 15x18x8 and you put the sub 7 ft. from the corner on the long dimension, and 4ft. out from the wall. So the distance between the sub and five boundaries is 7ft., 11ft., 4ft., and 11ft. again, and of course 8ft. from the ceiling. Measured response would show 3dB dips in response at 85 and 48Hz. The two 11ft. distances would “stack” to cause a 6dB null at 31Hz. The ceiling factor would add another 3dB dip at 42Hz.
This is why with out-in-the-room placement the best you can hope for is “fair” response, as Dzung described.
By comparison, corner placement puts the sub so close to three boundaries that the notches are well above 5kHz, some 7-7½ octaves above most crossover points. The three remaining boundaries (the far walls and the ceiling) would give only one significant null at 48Hz (i.e., the ceiling). The distant boundaries at 15 and 18ft. would produce nulls at 22.5Hz and well below 20Hz, effectively too low to be of consequence.
The gain advantage of corner placement is also related to the short distances between the sub and the three boundaries. This renders all sound reflecting off the walls in phase with the original signal, thus reinforcing it by up to 9dB (i.e, 3dB for each boundary).
This effectively eliminates any dips generated by the three remaining boundaries. Thus all you have to deal with is the room-induced peaks, which the equalizer can easily accomplish.
Happy Holidays,
Wayne A. Pflughaupt

 

[Dzung Pham]

Wayne, thanks for all the information. It is much appreciated and helps me understand this whole issue a bit better. I think my plan will be to eventually replace my dual SVS PCs with a single SS and equalizer. I can live with what I've got for now. I'm just waiting for pictures of the SS in piano black...