Solving Baffle Step Acoustically

[davidallancole]

Hi everybody,

What are some ways that can be used to solve baffle step correction acoustically, or basically any way except a line level or speaker level circuit using R, L or C? I understand using a bipole configuration will do it and I think I heard that making a really wide front baffle and keeping the box against the wall will also do it. Is this right? How wide would the baffle have to be for this to work? Is there a maximum distance from the wall it can be?

 

[SY]

A second driver can work wonders. But eq is so easy, why avoid it?

For all your other questions, just scale to wavelength.

 

[a_tewinkel]

It depends on what you call "solving". Is that compensating/equalizing or really trying to minimize the baffle step? Also, you might want to solve the diffraction problems.
A wide baffle with smooth edges against the wall might solve that, or better, in-wall speakers. The sound stage though, will probably be a bit "flat" because the rear wall reflection is absent. However, I have heard that the human mind compensates for this when listening to such speakers for a prolonged period, and the image is restored after about 20 minutes of listening.
A downside of this kind of design is the strong excitation of almost all room modes. Depending on your room, this can be a good thing or a bad thing.

 

[davidallancole]

Just looking for all options available.

Maybe this http://boozhoundlabs.com/fx120-alpha10/ is a worthwile approach to solving/compensating for it.

 

[a_tewinkel]

Dipoles are also a good approach, since they do not have a baffle step. They radiate constantly in a figure of 8 pattern and have less edge diffraction because of the sound pressure nulls towards the sides. However, they should be placed at least 1 m away from the wall. Have you read the website of Siegfried Linkwitz ?

 

[davidallancole]

Thats a whole other can of worms I am playing with.

 

[Caferacer]

"Maybe this http://boozhoundlabs.com/fx120-alpha10/ is a worthwile approach to solving/compensating for it."

Or use two of the same midrange drive. And just like boozehound did to fill in his open back rolloff, cut in the second driver at whatever frequency is necessary to make up the baffle step.

 

[Hezz]

Probably the simplest way would be to use a wide enough baffle in a three way design that the baffle step would be low enough to put at the lower crossover frequency. Maybe 300-500 Hz. Then use a woofer that is 1-2 db more efficient than the other drivers. If speakers will be placed fairly close to the wall no baffle step correction is advised. Out in the room 1-2 db is about right. For outdoor speakers you will probably want up to 4-5 db unless they are hanging stadium type speakers. Then you would want a full 6db baffle step.

 

[davidallancole]

Cool guys (or gals).

Thanks for the comments.

So basically you can solve it by (including electrical methods):

1. RL circuit at the speaker level.
2. RC circuit at the line level.
3. Infinite baffle or wide baffle against wall.
4. With a 2 way or 3 way design, use a woofer with 1-2dB increased SPL rating crossed over to the mids/highs at the baffle step frequency.
5. With a fullrange design, use a woofer with the same SPL rating or the same fullrange driver crossed over at the baffle step frequency.

Look right?

 

[planet10]

http://www.t-linespeakers.org/tech/bafflestep/intro-bds.html

dave

 

[audiokinesis]

In the nearfield, the baffle step is audibly significant.

But at distances where the power response dominates, it is not.

So, one way to avoid the baffle step is to listen from farther back. Would that qualify as an acoustic solution?

Bipoles were mentioned as a way to acoustically address the baffle step. With proper geometry, a bipole can also mitigate the floor-bounce dip.

Duke

 

[MJK]

In the nearfield, the baffle step is audibly significant.

But at distances where the power response dominates, it is not.

So, one way to avoid the baffle step is to listen from farther back. Would that qualify as an acoustic solution?

Bipoles were mentioned as a way to acoustically address the baffle step. With proper geometry, a bipole can also mitigate the floor-bounce dip.

Duke

Duke,

You have made a couple of statements that really got me thinking and I want to better understand what you have said.

First, your distance and power response comment. How far away do you need to be for this to happen and what properties of the speaker system would produce this effect? Maybe my room is too small but if possible I would lie to try and see if this works.

Second, your comments on bipoles and mitigating the floor bounce caught my interest. If I design and construct the perfect bipole (two drivers in phase, one on the front baffle and one on the rear baffle) then the baffle step will be totally corrected. This means that for all practical purposes the speaker is operating in an infinite baffle. But the driver is still elevated relative to the floor which would produce floor bounce nulls at several frequencies. Is there some positioning of bipole drivers that both cancels the baffle step, or most of it, and mitigates the floor bounce null? How much does a bipole mitigate the floor bounce nulls?

After I had typed the paragraph above, I opened up my MathCad bipole and ported box worksheets to test the floor bounce behavior of each configuration. You are correct and the bipole exhibits less floor bounce then the monopole ported box, something that I had not considered. The bipole I modeled has some depth so that the baffle step was not totally corrected, it was improved significantly and the room reinforcement would probably provide the remaining correction. So I guess that the offset between the front and rear drivers must help to smooth the floor bounce response, very interesting. When designing a bipole, was the depth of the speaker enclosure an important variable? Did you consider the rear wall reflection from the driver on the rear baffle, it would seem that room placement needs to be managed.

 

[Scottmoose]

Simplest (aside from listening a bit further away, given that HF levels tend to attenuate faster than LF in practice) would be to increase baffle width to ~30in, which will push step response down low enough to be countered by room-gain. Just balance the speaker to account for that, & you're in business.

Interesting re the bipoles. I hadn't considered that either. They're a great solution for nearfield listening -I prefer monopole for larger rooms / farfield though so you get maximum acoustic power at the listening position.

 

[soongsc]

How about bipole wooffer sideways?

 

[audiokinesis]

Martin, thanks for your interest in my comments - and wow thanks for running some simulations to test it out! Most people would not have gone to that length. I found the floor bounce dip mitigation through modelling. Baffle width and depth and driver height and driver diameter are the parameters that I juggle. In-room measurements confirm the modelled response. One of my commercial efforts is a bipolar.

I read a two-part JBL technical paper by John Eargle once that described how to calculate the distance in a room at which the power response dominates. It was pretty complicated as I recall, involving absorption coefficients and loudspeaker directivity among other things. Here are the links:

http://www.jblpro.com/pub/manuals/pssdm_1.pdf

http://www.jblpro.com/pub/manuals/pssdm_2.pdf

I think the easier method would be to measure pink noise while walking away from the speakers. If you get to a distance where the SPL becomes steady-state, the power response is totally dominant. I doubt you would get to that distance in a typical home room. More likely is that you get to a distance where the measured SPL is +3 db compared to the predicted anechoic SPL (which falls off at 6 dB per doubling of distance from a point source). At the distance where the SPL +3 dB, the contribution of the reverberant energy (power response) and contribution of the direct sound are equal. (It's plus +3 dB instead of +6 dB because they two contributions do not add in-phase.) Anyway, in theory at least at this distance and beyond the baffle step should be somewhat mitigated.

Soongsc, a side-firing woofer (bipolar or not) would indeed avoid the baffle step for that driver. The problem is, the baffle step usually comes into play at a much higher frequency than is practical for a side-firing woofer, so it's still there for your front-facing drivers.

Another acoustic approach to the baffle step would be to use very small-diameter drivers and a very narrow baffle such that the baffle step occurs in the crossover region between the midrange driver(s) and the tweeter.

I haven't modelled this, but mounting the drivers angled back severely (like on a 30 to 45 degree slant a la Spica TC-50 or Gradient 1.3) might have a mitigating effect on the baffle step.

Duke

 

[soongsc]

...

Soongsc, a side-firing woofer (bipolar or not) would indeed avoid the baffle step for that driver. The problem is, the baffle step usually comes into play at a much higher frequency than is practical for a side-firing woofer, so it's still there for your front-facing drivers.

...
Duke

Thanks for the answer. Wonder how this side firing bipolar configuration work with floor bounce.

 

[LAL]

Do I understand the baffle step problem correctly. ie, the the higher frequencies being shorter, reflect off the baffle and consequently increase output of those frequencies while the lower frequencies being longer, do not reflect off the baffle and consequently there is no increase in their output? If so, wouldn't an obvious solution, at least in part, be to have a non reflective baffle, perhaps one covered in felt or acoustic foam?

 

[roddyama]

Do I understand the baffle step problem correctly....

Not quite. At low frequencies the speaker on the baffle is omni-directional, essentially radiating in all directions. As the frequency increases and the wavelengths become smaller, the dimensions of the baffle compared to the wavelength of the sound forces the the speaker to radiate into a half space. But since the power output of the speaker is relatively constant more acoustic power is being radiated to the front of the speaker at the higher frequencies causing a step in the forward response, ie "baffle step".

 

[The golden mean]

One "crossover" solution was used in the German speaker building magazine Klang+Ton 1/2005. It´s about a small two-way speaker were the inductor for the bass-mid range driver (a 12dB/oct filter for it) is split into two not equal partitions and a notch filter is applied between them (to the ground) .

In this case; -which of course depends of the dimensions for the baffle, the values for "the divided coil" is suggested to be 1.2 to1.5 mH seen from the amp´s side, the notch filter (3.3 ohms+0,68 mH+6.8 uF). The "second" coil is 0,68 mH. In this case a clear baffle step response kicks in at about 700 Hz and upwards. The equalization seems to have succeeded according to the measurements.

I think this magazine is possible to download if you pay for it.
(Was a bit of a problem right now when I tried to download the new issue).

But of course all the text is in German.


http://www.lloxx.de/testberichte/heft-downloads.html

 

[Norcad]

Probably the simplest way would be to use a wide enough baffle in a three way design that the baffle step would be low enough to put at the lower crossover frequency. Maybe 300-500 Hz. Then use a woofer that is 1-2 db more efficient than the other drivers.

I have just done this with my Seas 3-way.
My enclosures are 53cm wide, and just 23cm deep. And they are standing about 20-25cm from the backwall.
Mid is CA18RNX (88,5dB) and woofer is CA26RFX (90,5dB).

I'm using active xo at 250Hz, and could compensate for BS by adjusting gain on the bassamp. But I didnt have to!

You can look at my design here:

Wide Baffle Seas 3-way