Hi,
In a cardioid the rear source is delayed and at same level as the front source, but in this project, the rear is at -6dB, why?
Hifi-Zirkel.de • Thema anzeigen - CBA - das Cardioid Bass Array
(I used google translate).
Thanks and Regards,
WonderfulAudio
In a cardioid the rear source is delayed and at same level as the front source, but in this project, the rear is at -6dB, why?
Hifi-Zirkel.de • Thema anzeigen - CBA - das Cardioid Bass Array
(I used google translate).
Thanks and Regards,
WonderfulAudio
Here is the original project on which the above project is based on
Passive Unipole - Die Anleitung, Lautsprecher - HIFI-FORUM
Passive Unipole - Die Anleitung, Lautsprecher - HIFI-FORUM
The rear source was placed on the wall(half space), so a -6dB reduction in drive signal is necessary to make its acoustic output level match the front free-standing(full space) source.
The front too 'sees' the same gain, it too is within 1/4WL from the wall so even it should see the boundary gain, No?
Wonderful audio,
This article will better explain the "rule of thumb" why "The rear sound source should be 6 dB quieter than the front!" or one rear speaker can typically be used for two forward facing speakers in a cardioid array:
Do-It-Yourself Cardioid Sub
"...What happens at other frequencies? Well, the rear case is easy. All frequencies arrive in phase and opposite polarity. They will always cancel. The front wave is a little more difficult. For frequencies either lower or higher than the design frequency, there is a different phase relationship between the two arrivals.
As frequency lowers they become more in phase, having less than 180 degrees of phase shift (Stay with me here). Because they are still opposite polarity, the closer they come to each other in phase, the more they cancel. As frequency decreases the combined output in the forward direction is reduced. As frequency increases, the combined output in the forward direction also reduces, however at other angles, with less time-of-flight delay, there will be strong, full output lobes. With higher frequencies still, there will be more peaks and nulls. The result will be the comb filtering and lobes expected from multiple sources offset in space and time. I don’t know of a practical use for the array much above it’s design frequency. However the important aspect, the one we wish to utilize, is that from the design frequency down, the polar pattern holds constant, to as low a frequency as one wishes to go.
So we have a constant polar pattern, but a falling low frequency response. Let’s equalize it. A first order low frequency shelving boost filter, with the upper frequency set to the design frequency, and the lower frequency set to the lowest frequency we want to use, will give us a 6dB-per- octave low frequency boost. This will equalize the combined forward output of the array. Since the rear output is a null to begin with, the increased drive really makes little difference.
Herein lies the trade-off. The amplifier power required will increase as frequency decreases. The maximum output of the array will also decrease as frequency decreases. Two octaves below the design frequency, the output capability of the array will be 6 dB lower than the output of a single box by itself."
This article will better explain the "rule of thumb" why "The rear sound source should be 6 dB quieter than the front!" or one rear speaker can typically be used for two forward facing speakers in a cardioid array:
Do-It-Yourself Cardioid Sub
"...What happens at other frequencies? Well, the rear case is easy. All frequencies arrive in phase and opposite polarity. They will always cancel. The front wave is a little more difficult. For frequencies either lower or higher than the design frequency, there is a different phase relationship between the two arrivals.
As frequency lowers they become more in phase, having less than 180 degrees of phase shift (Stay with me here). Because they are still opposite polarity, the closer they come to each other in phase, the more they cancel. As frequency decreases the combined output in the forward direction is reduced. As frequency increases, the combined output in the forward direction also reduces, however at other angles, with less time-of-flight delay, there will be strong, full output lobes. With higher frequencies still, there will be more peaks and nulls. The result will be the comb filtering and lobes expected from multiple sources offset in space and time. I don’t know of a practical use for the array much above it’s design frequency. However the important aspect, the one we wish to utilize, is that from the design frequency down, the polar pattern holds constant, to as low a frequency as one wishes to go.
So we have a constant polar pattern, but a falling low frequency response. Let’s equalize it. A first order low frequency shelving boost filter, with the upper frequency set to the design frequency, and the lower frequency set to the lowest frequency we want to use, will give us a 6dB-per- octave low frequency boost. This will equalize the combined forward output of the array. Since the rear output is a null to begin with, the increased drive really makes little difference.
Herein lies the trade-off. The amplifier power required will increase as frequency decreases. The maximum output of the array will also decrease as frequency decreases. Two octaves below the design frequency, the output capability of the array will be 6 dB lower than the output of a single box by itself."
Yes, if the front source was operating by itself. At low frequencies the front source launches spherical wave front in all directions. The portion of the wave front launched toward the rear wall would reflect in-phase and sum with the wave front it launches toward the listener and provide roughly 6dB boost at low frequencies where WL is much larger than the distance to rear wall.
But, the concept of this setup is that the rear wall reflection doesn’t happen. The wave front launched toward the rear wall(by the front source) is cancelled by output from the wall mounted source just as the wavefront arrives at the wall. To do this, the wall mounted source needs to have its output inverter, delayed by distance to rear wall, and matched in level with the sound arriving from the front source. Since the rear source is located on the wall(ie it is co-located with its wall-reflection) the "matched level" requirement is meant when the rear source is attenuated -6dB relative to the front source.
This setup may be better thought of as an active absorber concept rather than a cardioid.
I tried something similar 10 years back. Had 2 different channels. I used a pair of similar speakers and positioned them to suppress the floor reflection and then on the back and front walls.
I could measure quite an improvement, but perceptionally the width of the soundstage and location of instruments worsened.
after all more sound energy is pumped into the room, the cancellation works only in one point or line.
Also our mind adapts to the room, we expect certain room reflections and filter them. For us the voice of someone sounds the same in a church or outside, but in the mike recording it is not recognizable.
I could measure quite an improvement, but perceptionally the width of the soundstage and location of instruments worsened.
after all more sound energy is pumped into the room, the cancellation works only in one point or line.
Also our mind adapts to the room, we expect certain room reflections and filter them. For us the voice of someone sounds the same in a church or outside, but in the mike recording it is not recognizable.
Think of it like this. The front sub is an apple, and you want to take a bite out of the back part of it, since you can't really make a flat cut (unless you have infinite big mouth) you have to settle with biting half way through and leave some for the sides.
If you bite further(same strength as front) you end up with a hyper-cardioid and since you bite more energy then you get from the front you end up couching some of it backwards. It's the stem looking lobe which will also be in opposite phase making a mess of things.
If you bite further(same strength as front) you end up with a hyper-cardioid and since you bite more energy then you get from the front you end up couching some of it backwards. It's the stem looking lobe which will also be in opposite phase making a mess of things.