Alternative Bass EQ Method

[bellnm]

I have been using my Behringer DSP1124P and REL Strata 3 to great effect, but I have found that in my 2 channel setup it has some disadvantages which affect the overall frequency response and transparency of the system.

In order to surpress my room peaks (caused by the main speakers), I have to install their Reflex Port bungs. This rolls off the bass output early, and still does not completely remove the troublesome room peaks. I then use the EQ'ed sub to selectively fill the bass back in, making it flatter, and of course it goes lower. The downside is that the main speakers sound thinner with the bungs in, and the whole arrangement is less transparent.


I then had the following idea - how about if I took the bungs out of my main's, and simply relied on their own bass output, which is just about sufficient and of course of very high quality.

Then simply use the EQ'ed sub output in "anti-phase" to cancel out the troublesome room modes - almost using the sub and EQ system in reverse.

I got the idea after reading about Hermoltz Resonators used to absorb specific peaks - this would be like an electronic programmable multi frequency resonator wouldn't it ?

I know that the DSP1124P may be hard to program in this way as I would need to try to generate narrow pass band's as opposed to narrow stop bands which is what the unit is designed for.

The advantage that I perceive is that the system becomes almost passive, and should sound no worse than the slight bloating that already exists at the troublesome frequencies ?

Anyone have any thoughts ?

Cheers

Nick

[brucek]

I then had the following idea - how about if I took the bungs out of my main's, and simply relied on their own bass output, which is just about sufficient and of course of very high quality.

Well that's the rub isn't it? We add the subwoofer because the mains usually don't extend with authority to < 20Hz.

Where the subwoofer is producing frequencies that the mains are also producing is indeed the way we use the BFD EQing already, albeit with a crossover engaged. Any like frequencies are EQ'd to reduce dips and peaks by offering an opposing signal from the sub (as you suggest).

Beyond that, I guess we could use your theory for higher frequencies by removing the crossovers for both sub and mains, but the experiment would only reach to the response limit of the sub, which would only be ~120Hz.....

brucek

[Chrisbee]

I can't agree that speaker bass is of high quality compared with a quality subwoofer.

Relieving a speaker of its bass load actually increases transparency and greatly improves detail, imagary and dynamics. So much so that it ruthlessly exposes inferior equipment. It can also expose colouration that was masked by the muddy bass. The increased dynamic range from unloading the speakers and amps can be an embarassment. No more "easy listening" to musak while you're browsing.

What you miss when you remove the "warmth" from a speaker is all the masking garbage the speaker is putting out at lower frequencies. The speakers do sound thinner at first but the new sense of reality lifts the speaker sound quality remarkably.

Having an active filter in one's system allows very small adjustements to level and roll off point. It takes very little effort to realise dramatic improvements in SQ. No more guesswork. You just keep dialling until you can hear the best roll off point for your particular speaker/sub combination in your room.

If you are struggling with room problems then the ability to move the subwoofer (compared with the relatively static speakers) can be a real bonus. The flexibility of the equalisable subwoofer makes it a powerful tool against room modes compared with speakers. Which must remain at the far points of the equilateral listening triangle.

Using a sub in antiphase sounds promising if you can manage the trick without ruining your overall response curve. I would imagine the sub needs to be close to your speakers or phase issues become even more of an unknown than usual.

I have had greater success pulling a sub well out from the corners than in them.

A large sub can actually lift the bass from a speaker close beside it. By coupling the speaker to the nearest room boundaries or by increased baffle effect.

[bellnm]

Chrisbee, I agree with you regarding the quality of bass output from the main speakers, and as always I have a number of conflicting issues in my listening room which lead me to this idea as a possible compromise.

I listen to mainly 2 channel output through highish end Audio Research and Meridian gear within a dedicated smallish room with ProAc Response 1SC speakers.

After much moving of speakers and the listening position, (plus room treatment) I am very happy with the sound, apart from bass humps at 46Hz (+18dB) and 86Hz (+6dB). Fitting the speaker bungs reduces both peaks to +6dB, with an associated loss of deeper bass output.

I do not want to put any EQ in my main speaker path, but I have had great success (bungs in) with the EQ'ed sub operating to augment the lower bass.


HOWEVER !!!!

I know that the bungs are affecting more than just the low end of my main speaker output, and I wanted to let them run free, and absorb the "humps" in an alternative way (ideally a passive way).

All of that said, I have just built a DIY pre-amp which is stunning, and I was considering bi-amping the ProAc's with a DIY active crossover within the pre-amp.

It may be that it would be worth making the active crossover a 3 way affair, and hand over the bass to a pair of REL Strata 3 sub's - co-located with my ProAc's.

I could then equalise the whole of the bass output without affecting the delicate mid range and treble output. I may need to place the sub's 1 foot forward of the mains to account for the 1mS delay inside the Behringer DSP1124P though.

What do you think ?


Cheers

Nick

[Chrisbee]

What is your room size?

Have you fully investigated all possible speaker and listening positions to tame your bass peaks?

Rotating the listening triangle by 90 degrees to fire across the room (rather than along the greater length) offers another avenue of exploration.

Given the wavelengths at these frequencies only careful testing will reveal whether moving the subwoofers relative to the speakers offers any advantages. Your measurement microphone (or RS meter) wants to be at your ear height exactly at the listening position. A few inches in any direction makes an enormous difference in my own room.

You might consider a far more powerful subwoofer (or pair of) with far greater headroom and much lower distortion. A sub producing massive distortion is as likely to kick off as many room modes as your speakers. The new small SVS (SB12+) is getting serious admiration for a ~14" cube for a very modest price. It has some useful controls which might save some soldering effort on your part. Being sealed, the new small sub is aimed more at the stereo music crowd rather more than their other offerings. The REL really isn't up to the standard of your other kit IMO.