Let's talk about woofer placement on the baffle and the effect on frequency response in the room

[ThatSoundsGood]

I'm in a "discussion" with someone Facebook about this. He is saying that the dips at 120Hz and 380Hz on this graph are due to the woofer placement in the speaker rather than room anomalies. I agree that the woofers being higher on the baffle can make this problem worse, but I still maintain that the room is more the cause and that this frequency response is pretty damn good. He is bringing up the "Allison Effect" which is interesting. I would love to hear this group's thoughts on this. This is a picture of the speaker and the on-axis response. Let's hear it!

 

[Sancus]

"Alllison Effect" is just an old name for Speaker-Boundary Interference Response probably derived from this paper: R. F. Allison, “The Influence of Room Boundaries on Loudspeaker Power Output,” Journal of the Audio Engineering Society, Vol. 22, No. 6, June, 1974.

As the paper states you can mitigate the problem with a *combination* of the right design and the right placement. But, in addition to requiring very specific designs that probably have other compromises, most people don't have complete freedom of placement for their speakers.

I feel like the modern version of this idea is probably speakers with cardioid bass, which reduces their interaction with boundaries.

 

[ThatSoundsGood]

"Alllison Effect" is just an old name for Speaker-Boundary Interference Response probably derived from this paper: R. F. Allison, “The Influence of Room Boundaries on Loudspeaker Power Output,” Journal of the Audio Engineering Society, Vol. 22, No. 6, June, 1974.

As the paper states you can mitigate the problem with a *combination* of the right design and the right placement. But, in addition to requiring very specific designs that probably have other compromises, most people don't have complete freedom of placement for their speakers.

I feel like the modern version of this idea is probably speakers with cardioid bass, which reduces their interaction with boundaries.

Well said. And that's similar to my point. I understand the boundary effect, but the Allison speakers are designed specifically for "against wall" or "in corner" placement. They apparently sound great, but I haven't seen any real measurements and it seems that there are other possible design compromises with this approach (looks being one of them). My point is that an anechoically flat speaker is what we generally shoot for because it is effective in more situations.

 

[LIΟN]

This is a picture of the speaker and the on-axis response. Let's hear it!

How was it measured?
That picture is gating messurement? or Just Mic height with Onaxis line means?

 

[ThatSoundsGood]

How was it measured?
That picture is gating messurement? or Just Mic height with Onaxis line means?

Not gated. It's 1 meter on axis with the tweeter, which is about 39 or 40 inches from the ground. It was measured in my garage with the door closed.

 

[LIΟN]

Not gated. It's 1 meter on axis with the tweeter, which is about 39 or 40 inches from the ground. It was measured in my garage with the door closed.

I need to studying more about the positive effects/negative changes that occur with the location and displacement of the bepple interference and woofer..
But In that graph, I think the impact of that space is going to be more than 80 percent.

 

[LIΟN]

Also, even though smoothing has already been applied from 1khz, I see compilating (the effect of early 1st reflection), so I think that's more from reflection and SBIR than the characteristics of the speaker. (Changing thoughts from 80 percent to 90 percent)

 

[sam_adams]

"Alllison Effect" is just an old name for Speaker-Boundary Interference Response

No, it is a power response.

SBIR is quarter wave cancellation caused by the room boundary.

 

[sam_adams]

 

[ThatSoundsGood]

View attachment 328675

Yes, but what is the cause of the Allison effect in this situation and how could it be mitigated?

 

[sam_adams]

Yes, but what is the cause of the Allison effect in this situation and how could it be mitigated?

Read the linked paper—several times. Ponder the information presented. You will have an epiphany.

 

[MAB]

Yes, but what is the cause of the Allison effect in this situation and how could it be mitigated?

sam_adams answered the question here:

SBIR is quarter wave cancellation caused by the room boundary.

Woofer placement will change the frequency (1/4-wavelength!).

There were even speakers (Allison) that went right up against the wall!

 

[ThatSoundsGood]

Woofer placement will change the frequency (1/4-wavelength!).

Do you mean woofer placement in the enclosure or woofer placement in relation to the boundaries? Or both? Also, if the Allison effect is the cause of the 120Hz dip then how much of a role does the room play in it?
Also, if the Allison design solves this issue, what happens when you don't put Allison speakers against the wall?

 

[Keith_W]

Interesting. For some time I have wondered why there is a dip just above 100Hz in my right speaker, with the same dip happening in the left speaker at 105Hz:

The fact that these occur at different frequencies suggests to me that it is a room / placement issue rather than a speaker issue. If you look at the placement of speakers in the room:

You will see that the right speaker is closer to the side wall than the left. So ... the RIGHT speaker is closer to the wall, yet the suckout occurs at a lower frequency, i.e. longer wavelength. I would have thought that the Allison effect means that the suckout at the right speaker should occur at a shorter wavelength?

This does not make sense to me. Can you help me understand why?

 

[MAB]

Do you mean woofer placement in the enclosure or woofer placement in relation to the boundaries? Or both? Also, if the Allison effect is the cause of the 120Hz dip then how much of a role does the room play in it?
Also, if the Allison design solves this issue, what happens when you don't put Allison speakers against the wall?

Read what @sam_adams posted. It is relative to the room boundary!

Seems to be raining Allison speaker references, an Allison speaker thread has just been posted to:

Anyone here own Allison Acoustics speakers?

I sold the ones I had (CD9 and Ones) before I ever got into measurements but now I want to see what their in-room response would be like. Wondering if anyone here had a pair in use and could take REW room measurements with them in their recommended locations.

www.audiosciencereview.com

These speakers go right up against the wall, so no SBIR, and generate a controlled radiation pattern that minimizes the 1/4-wavelength effect.

 

[ThatSoundsGood]

Interesting. For some time I have wondered why there is a dip just above 100Hz in my right speaker, with the same dip happening in the left speaker at 105Hz:

View attachment 328682

The fact that these occur at different frequencies suggests to me that it is a room / placement issue rather than a speaker issue. If you look at the placement of speakers in the room:

View attachment 328684

You will see that the right speaker is closer to the side wall than the left. So ... the RIGHT speaker is closer to the wall, yet the suckout occurs at a lower frequency, i.e. longer wavelength. I would have thought that the Allison effect means that the suckout at the right speaker should occur at a shorter wavelength?

This does not make sense to me. Can you help me understand why?

If I understand it correctly, the math here is that the speakers producing those specific frequencies are bouncing off of the floor and walls. They are different distances from them and that causes a phase cancellation at those frequencies. If the woofers are lower to the ground and closer to the wall then you're less likely to have that type of phase cancellation because the bounce is not within the wavelength of those lower frequencies. So, for you, the math would be the different distances from the boundaries in your room causing different frequencies to cancel. Which, to me, means that the room is playing a bigger role in the problem. But it makes some sense that having a speaker with two woofers facing in the same way as the Allison speakers could flatten the dips and peaks in the lower mid range frequencies because it would compensate for the different distances to the boundaries better. I'm not explaining it well, but it makes some sense.

 

[ThatSoundsGood]

Read what @sam_adams posted. It is relative to the room boundary!

Seems to be raining Allison speaker references, an Allison speaker thread has just been posted to:

Anyone here own Allison Acoustics speakers?

I sold the ones I had (CD9 and Ones) before I ever got into measurements but now I want to see what their in-room response would be like. Wondering if anyone here had a pair in use and could take REW room measurements with them in their recommended locations.

www.audiosciencereview.com

These speakers go right up against the wall, so no SBIR, and generate a controlled radiation pattern that minimizes the 1/4-wavelength effect.

What's the trade off with them? why don't more designers use this same approach?

 

[MAB]

Interesting. For some time I have wondered why there is a dip just above 100Hz in my right speaker, with the same dip happening in the left speaker at 105Hz:

View attachment 328682

The fact that these occur at different frequencies suggests to me that it is a room / placement issue rather than a speaker issue. If you look at the placement of speakers in the room:

View attachment 328684

You will see that the right speaker is closer to the side wall than the left. So ... the RIGHT speaker is closer to the wall, yet the suckout occurs at a lower frequency, i.e. longer wavelength. I would have thought that the Allison effect means that the suckout at the right speaker should occur at a shorter wavelength?

This does not make sense to me. Can you help me understand why?

But your left speaker looks closer to the right wall, and the right speaker is further away from the left wall! All the walls count. Each cancellation may have a different Q due to room acoustics.

 

[MAB]

What's the trade off with them? why don't more designers use this same approach?

I ask myself why true audiophiles don't soffit-mount their speakers...
I have only heard Allison twice. I thought they did bass really well in one room, and had poor boomy bass in another. I think the room dimensions are somewhat import since this really limits speaker placement options.

 

[ThatSoundsGood]

I ask myself why true audiophiles don't soffit-mount their speakers...
I have only heard Allison twice. I thought they did bass really well in one room, and had poor boomy bass in another. I think the room dimensions are somewhat import since this really limits speaker placement options.

Which is my point. It's a trade-off and a different approach. I can't find actual measurements of them anywhere.