Boundary Compensation...

[FirebirdTN]

I posed this question on another forum, and someone suggested I might try here.

I notice a few speaker manufacturer's include a switch to compensate for boundary effects (wall mouting) of speakers.

I would like to add this feature to an existing speaker. I have tried searching, but haven't come up with much. Can anyone help?

I know NOTHING about speaker design, but do have an extensive electronics background, so modifying the speaker is not a problem.

My particular problem is I am trying to improve dialog clarity in my center channel. It is wall mounted very high; right above the TV which is "inwall" so lowering isn't an option. I don't know if the coloration I am experiencing is from the screen or the ceiling, but either way would like to try this modification instead of purchaseing new speakers. My particular speaker doesn't have this circuit (Klipsch SLX), yet the nearly identical speaker in the manufacturer's reference line does (Klipsch RVX-42).

How is bondary compensation normally achieved? With a "L-Pad" and series capacitor, or notch filter?

Thanks!

-Alan

[Dryseals]

Good question. I did some looking around also and did not find any real info. But as an educated guess and noting that the manufactures use a single switch to a multiple selector switch, I think you could safely assume that the compensation would be a filter for a specific range in the lower freqs.

I think I would use an spl meter and measure the levels with a speaker in the middle of the room. Then move it up against a wall and look for any gains. The spectrum of higher gain is what I would suspect need to be reduced.

[Depth Charge]

Do a search or baffle step compensation. Basically, in free space the speaker radiates sound in all directions at low frequencies but only in the forward direction at high frequiencies. This is because waves of longer wavelength (bass) can bend around the cabinet easier, high frequiency sounds (with their shorter wavelength) can not do this. This means that there is a 6dB loss in the bass region compaired to the higher frequiencies in the forward direction (since the bass frequiencies cover a larger area). Usually this is compensated for using a shelving filter in the form of an inductor and a resistor in paralell with eachother.

So when you take your speaker which has this compensation and place it against the wall, where sound can only travel in the forward direction, you get a comparitive boost in the bass region.

Basically when you place a speaker against a wall you want to remove the bafffle step compensation components from the circuit since there is no bass loss with the speaker in this position (since all sound has roughly the same directional pattern in this situation).

Hope this is of use to you. Still, do a search for baffle step compensation.

[rabbitz]

What you can try is make sure the centre speaker is set to small in the receiver so no lower bass is going to it and angle the speaker towards the viewing area. If it still has a problem on dialogue then a crossover make over is in order to overlap the drivers a bit more.

I doubt the Klipsch would have any BSC but if it's there, get rid of it.

Some old timers might remember the AR10PI from the 70's that had a switch to compensate for room positioning.... PI, 2PI or 4PI. Someone might remember how the circuit worked and was advertised as "normalising of the woofer output for the three basic environments is accomplished via an autotransformer and individual switching networks" .... what ever that means.

[FirebirdTN]

Thanks for the exact terminology to search.

Unfortunately, now that I dig a little deeper, I don't think adding a bondary compensation mod is going to help me at all.

I do have my center set to small, cross-over at 100Hz, and aimed at the listening "area".

The real problem is, this is a humble little living room setup, nothing like a dedicated HT room. Yesterday I did a little experiement with a test signal (pink noise), and come to find out, my particular center channel (MTM) has an EXTREMELY narrow "beam" of good bandwidth. Get out of this beam, and it sounded like all the high frequency portion of the pink noise just dissappeared. Unfortunately, the "sweet spot" in the perfect center of all the speakers is just an arbitrary one. There are no seats there. My seating is left and right of center, which in both cases just happens to be right outside this narrow angle of good response from my center.

So, I guess I will be doing a DIY project after-all, just not a bondary compensation mod.

What I am thinking of doing is modifying the speaker so the tweeter is out of the center of the mids. I have read this helps to give more horizontal dispersion from a horizontally placed MTM.

I know if will probably change the response some from my speaker, but aside from turning it vertically (which I can't), what else can I do?

-Alan

[rabbitz]

You can always angle the mid woofers about 15-20° to the sides as this is often recommended on horizontal MTM's and can give up to a 15dB off axis improvement when compared to a flat baffle MTM.

[FirebirdTN]

Thanks rabbitz for that idea! I never even thought of that!

I might try several things; I'll start a new thread since baffle step compensation really isn't going to help in my situation.

-Alan

[agrobler]

You could also point the speaker down a bit, i.e. to the same level as your ear, or turn them vertical - if width of the beam's a problem - and tilt them down...
i.e.
not: OoO

but:
O
o
O

and then point downwards to listening level...

Just may work...

[Drew Eckhardt]

Originally posted by rabbitz
What you can try is make sure the centre speaker is set to small in the receiver so no lower bass is going to it and angle the speaker towards the viewing area

If you have the speaker within a foot of the wall, ceiling, TV it's going to have a substantial bass boost above the sub-woofer cross-over even at 120Hz.