Measured monopole and dipole room responses

[john k...]

Originally Posted by markus76

Art


There really is no transient response at low frequencies in acoustically small rooms. What one hears is the room - the modal decay and the magnitude response aberrations. This makes bass sound “tubby” or “slow”, it's not the source. Of course there's always the chance that a sub didn't work as intended but this should have been obvious from free field measurements.

I would certainly argue that point. Room or otherwise, if the source passes an impulse then what is measured or heard at the observation point is that impulse convolved with the transfer function of the room between the source and observation point. It is certainly a transient response by definition. That modal decay is the transient. for any pair of source and listening positions there is a unique transfer function which is (or can be) defined by either the impulse or frequency response observed at the observation point. You may be interested at looking at my full study on room transient response and woofer types.

[markus76]

Agreed but this is not how the wording "transient response" is used by most enthusiasts. I argued using the more "colloquial" definition and I'm sure Art did too.
Sorry if my comment wasn't clear enough. Let me rephrase. It's not the source that makes bass “tubby” or “slow”, it's how the source interacts with the room. Better?

[weltersys]

Originally Posted by markus76

Agreed but this is not how the wording "transient response" is used by most enthusiasts. I used the more "colloquial" definition and I'm sure Art did too.

As I wrote in response to Earl in post #108:

“Transient response describes the behavior of a system following a sudden change in its input.
Transient response is not limited to frequencies "above 200 Hz".

My tests of the Resopump sub were outdoors, the sloppy transient response (or impulse response) it had would have only been made worse by room modes.

I agree that in acoustically small rooms, what one primarily hears is the room's contribution to modal decay and magnitude response aberrations, but starting with a sloppy speaker makes the LF sound even worse than it has to be.

Art

[john k...]

Originally Posted by markus76

Agreed but this is not how the wording "transient response" is used by most enthusiasts. I argued using the more "colloquial" definition and I'm sure Art did too.
Sorry if my comment wasn't clear enough. Let me rephrase. It's not the source that makes bass “tubby” or “slow”, it's how the source interacts with the room. Better?

Sure!

I found it interesting in my sims that if I included only the axial modes the response was very good and remained minimum phase.

[soundaatma]

Originally Posted by weltersys

As I wrote in response to Earl in post #108:

“Transient response describes the behavior of a system following a sudden change in its input.
Transient response is not limited to frequencies "above 200 Hz".

My tests of the Resopump sub were outdoors, the sloppy transient response (or impulse response) it had would have only been made worse by room modes.

I agree that in acoustically small rooms, what one primarily hears is the room's contribution to modal decay and magnitude response aberrations, but starting with a sloppy speaker makes the LF sound even worse than it has to be.

Art

Agreed that in acoustically small rooms, room contribution will smear the time window, giving a poor transient bass reproduction. But this will not hold true for acoustically large/leaky room or even in near field listening, where the inherent transient response of the bass unit will reveal itself.

From subjective perceptions of many, dipoles sound the closest to musical instruments like kick drums. From all the previous comments about equalised woofers with different loading schemes , it seems that they still do not lead to the same percepetion of transient bass quality. So it seems that none of the modelling programs or even some RTA measurements are able to capture this difference of perception of transient bass , accurately. Although the graphs by the OP do show this difference...however I am not sure of the size of the acoustic environment used for those graphs.

Once again it seems to me that the stored energy of a cabinet (sealed or posted) and the backwave seeping out through the woofer are responsible for degrading the transient response. Whereas in a dipole the back wave just escapes into the room in an accountable manner and so there is minimal stored energy and no seepage through the woofer to smear the transient response. Going by the same logic, a cardioid with a delayed backwave will still have greater energy storage and leakage through the woofer than a pure undamped/stuffed dipole....although this will be less problematic than a sealed/ported woofer.

To prove this point to myself, yesterday I sealed the port of my 84" long Peerless 12"XXLS based bass only MLTL and compared it to my 15" alpha Hframes. I had to put a high pass filter at 40Hz on the now closed TL to compare the same BW. Surprisingly, the subjective transient response of the two loading schemes, namely the Hframe and a long sealed TL was remarkably close !

Any thoughts on the transient response of long closed bass only TLs v/s OB bass.

[gedlee]

Originally Posted by markus76

Earl, did you follow the Fazenda papers in JAES?

Markus

I did find and read that paper, it was quite interesting (I had flagged it to read, but had not finished it). It does not contradict anything that I recommend. They did not test a multi-woofer magnitude response situation.

One thing I should point out in reading the comments here is that the "modal decay" depends entirely on the room and not on the subs or there positions. Even if the direct sound does overwhelm the modal contribution, the modal decay remains the same and will appear after the direct sound has passed.

Only an active system can affect the modal decay and this was clearly shown in the subject paper. Unless you can add-in a sound to cancel the decay (active absorption) it will always remain the same no matter what you do.

This is fundamental to why I have long recommended heavy room damping at LFs. Multiple subs helps the magnitude smoothness problem and the spatial smoothness, but it does not affect the modal decay. This can only be done with sound abosrption in the room - or active absorption. But multiple subs in a well damped room would achieve the ideal as highlighted in that paper.

As to your comment: “Two systems with very different impulse responses can have exactly the same magnitude response. “

This is true in theory, but requires a highly non-MP system to have any real practical effect, i.e. a significant difference in the impulse responses, especially at LFs. At frequencies below 200 Hz room acoustics are close enough to MP that the impulse responses of two systems EQ'd to be the same will also be the same.

Regarding "transient response" - this term is used in two different contexts here and they are NOT the same. All systems have a "transient response" the term comes from the differential equation solution, although it can often be insignificant in a forced vibration system. If the "damping" is high then the "transient" decays before any significant effect is noted and only the forced response is noted. Hence, according to the subject paper, the best bass is achieved when the "transient" is surpressed. When this term is used subjectively it does not have as well a defined meaning. What is a "good transient response"? - its one where there is no "transient at all". Is this what is meant subjectively? "Fast Bass" means no "transient"? Remeber that high damping causes a slower rise time, so "fast bass" would imply to me that the rise time is high and as such the system is underdamped.

Last point: dipoles and cardiods do not excite few modes, that is a falicy. All sources excit all modes to a greater and lessor degree. Dipoles and monopoles excite different modes and do so in different ways. Hence putting a dipole and a monopole at the same room location and comparing them does not prove a thing. Yes, they are different. So what? I did an AES paper some years back comparing dipoles, monopoles and cardioids - no one stood out as any better than another if each was placed optimally.

[john k...]

Originally Posted by gedlee

Markus


One thing I should point out in reading the comments here is that the "modal decay" depends entirely on the room and not on the subs or there positions. Even if the direct sound does overwhelm the modal contribution, the modal decay remains the same and will appear after the direct sound has passed.

That is absolutely true, but it is a mater of relative levels. This in not unlike the critical distance at higher frequencies where the direct sound is louder than the reverberant sound. Example, in a highly reverberant room if you converse with a person who is well beyond the critical distance it is difficult to understand the conversation because what is heard is dominated by the reverberant field and the sound is diffuse and detail is lost. Move close to that person and it becomes clear what the conversation is about. Additionally, the speaker (person talking) can speak more softly and still be heard which further reduces the strength of the reverberant field. If close enough the strength of the reverberant field may drop below audibility.

The same applies at low frequency. As the source is moved closer to the listener the source strength can be reduced while still producing the same direct SPL at the listener's ears. The result is that the room modes are excited to a lower level. They are still present and still decay at the same rate (assuming the room represents a linear system) but their contribution to audible sound is greatly reduced resulting in cleaner bass response.

This situation is easy to demonstrate by making a near field measurement with a specified SPL at the mic and then slowly moving the mic away while maintaining the same nominal SPL. As the mic moves further away from the source the source level will need to be increased to maintain the same nominal SPL and the smooth response measured in the near field will continue to degenerate to one which increasingly shows the impact of the room modes.

[john k...]

Originally Posted by gedlee

Markus

Last point: dipoles and cardiods do not excite few modes, that is a falicy. All sources excit all modes to a greater and lessor degree. Dipoles and monopoles excite different modes and do so in different ways. Hence putting a dipole and a monopole at the same room location and comparing them does not prove a thing. Yes, they are different. So what? I did an AES paper some years back comparing dipoles, monopoles and cardioids - no one stood out as any better than another if each was placed optimally.

I would have to disagree with the first part of this statement, but would agree that when optimally placed, good bass can be obtained with any woofer system. The problem is with a full range speaker optimal placement for the bass is not necessarily optimal for the midrange. Not everyone has the luxury of having and being able to place multiple woofers around a room.

[john k...]

Originally Posted by gedlee

Markus


Regarding "transient response" - this term is used in two different contexts here and they are NOT the same. All systems have a "transient response" the term comes from the differential equation solution, although it can often be insignificant in a forced vibration system. If the "damping" is high then the "transient" decays before any significant effect is noted and only the forced response is noted. Hence, according to the subject paper, the best bass is achieved when the "transient" is surpressed. When this term is used subjectively it does not have as well a defined meaning. What is a "good transient response"? - its one where there is no "transient at all". Is this what is meant subjectively? "Fast Bass" means no "transient"? Remeber that high damping causes a slower rise time, so "fast bass" would imply to me that the rise time is high and as such the system is underdamped.

.

Have to address this to, though I think it's more interpretation.

I consider transient response to be the time dependent output of a system when subjected to a time dependent input. If the output is a scaled function of the input then the system has perfect transient response with gain. If the output does not duplicate the input by, for example, having a tail which decays after the input is reduced to zero, then the transient is not perfect. A typical sealed box woofer does add a tail to the response. But compared to the typical resonant behavior of bass instruments it is a pretty insignificant factor in reproduction. Room modes, on the other hand, can be high Q and can decay at a rate which is of the same order or longer that bass instruments creating mushy, ill-defined bass.

[gedlee]

Originally Posted by john k...

The problem is with a full range speaker optimal placement for the bass is not necessarily optimal for the midrange. Not everyone has the luxury of having and being able to place multiple woofers around a room.

Quite true on the first sentance and a good reason to seperate the bass from the rest. As to placement and numbers its a matter of priorities. Do you want good bass or not? I have three subs, only one is visible, but even then only if you know what you are looking at.

Originally Posted by john k...

Have to address this to, though I think it's more interpretation.

I consider transient response to be the time dependent output of a system when subjected to a time dependent input. If the output is a scaled function of the input then the system has perfect transient response with gain. If the output does not duplicate the input by, for example, having a tail which decays after the input is reduced to zero, then the transient is not perfect. A typical sealed box woofer does add a tail to the response. But compared to the typical resonant behavior of bass instruments it is a pretty insignificant factor in reproduction. Room modes, on the other hand, can be high Q and can decay at a rate which is of the same order or longer that bass instruments creating mushy, ill-defined bass.

I think that this is precisely the point and I completely agree. The Q's of the room modes swamp out the Q's of the source and there are a lot more of them as well. The whole room situation swamps out any sub configuration considerations in the modal region. So why get so hung up on source type, alignments and all that other stuff. It's just blowing in the wind as far as I am concerned.

It's all the subjective handwaving explainations that I take issue with.

BTW - a closed box does not "ring" (add a tail) if its Q is < .7. And even if the Q is 1.0 the ringing is minimal.

I need to think about your other point, but I can see outright that it likely depends on the characteristics of the source material. If the source drops abruptly then so does the direct field, but the reverb field rings on. The level of the ringing depends on the source level, but not the rate of decay and hence not the duration of the sound. But yes, at some point this has to drop below audibility.