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-   -   The Basilar Membrane and Crossover Distortion (http://www.diyaudio.com/forums/multi-way/229122-basilar-membrane-crossover-distortion.html)

IanKnight 1st February 2013 01:03 PM

The Basilar Membrane and Crossover Distortion
 
For the past 15 years or so I have been researching and developing my interest in the part that the basilar membrane plays in how we interpret the sound of the loudspeaker.
In particular the effects that are produced by trying to integrate two drivers over the mid-band crossover area.

Considering that the basilar membrane can set up all manor of compressions, masking effects and also distortions within the hearing system I have not yet seen any relative articles or discussions on the subject of phase beating and it's connection with speaker crossover distortions in the critical mid range area.

Beat (acoustics) - Wikipedia, the free encyclopedia

With this in mind I have looked into how the critical bandwidth and two tones in very close proximity in frequency can cause roughness, smoothness and a phase locking beat when the two tones line up exactly in frequency. As the two tones go outside the critical bandwidth region they can react in a way that they are de-tuning each other. For example an effect that happens when the multiple strings for the same note of a piano is de-tuned so it becomes a “honky-tonk” piano.

My own crossover topology and design methods reflect my findings so far in the bid to try and decrease crossover distortions, phase anomalies, and colourations that are sometimes caused by conventional crossover methods.

I would be interested if anyone on this site has ever experienced anything to do with this subject or similar.

Thanks Ian Knight.

speakerdoctor 1st February 2013 01:39 PM

This link might help explain balisar membrane a bit better..
Basilar Membrane Motion 1: Two Tones - Animation | Auditory Neuroscience

Are your xo designs including acoustic offset compensation? That alone helps a great deal with phase alignment between offset drivers on a flat BB. There is much discussion over at PE TT about minimum phase. Worth a visit. Ooops, looks like you have!

IanKnight 1st February 2013 01:44 PM

Hi,
Thanks for the link....

Yes I am on PE .
Thanks for the tips.
Cheers Ian

AllenB 1st February 2013 02:52 PM

If two drivers are reproducing the same frequency, why would there be beating? With all else being equal, a physical separation will remain a fixed phase relationship.

IanKnight 1st February 2013 05:49 PM

Hi Allen,

That's an interesting point about the fixed phase relationship.
The beating and reinforcement of certain frequencies will occur according to the crossover frequency points are imposed on each driver and what slopes are used. The closeness of the crossover points will affect the phase relationship of the two drivers. The gap between the two knee frequencies of the crossovers can be quite wide before the drivers start to reinforce each other in the crossover region.
Usually the 1st order crossovers will give the most audible lock on but a combination of low order slopes can work well if the correct implementation of the drivers is used.

Cheers Ian

speaker dave 1st February 2013 06:04 PM

Interesting links re the Basilar membrane.

I'm not seeing the connection with crossover design? Even a poor crossover can't shift frequency so I would think the occurance of beats would be the same nomatter what?

I'm an amateur piano tuner (Dad was a pro) so I am familiar with beats and find them interesting. In the absence of nonlinearity, they are more perceptual than real. That is, when two tones get quite close together we perceive more a single pitch with a varying amplitude, yet a spectrum analyzer would show two tones of constant amplitude.

An interesting phenomonon with pianos is that the overtones of a string are somewhat inharmonic (not exact multiples) This can lead to to notes beating even when the fundamentals are at an exact ratio.

Regards,
David S.

IanKnight 1st February 2013 06:16 PM

Hi Dave,
Thanks for your message.
It is playing the piano that started me off on this journey. I have never been satisfied with the sound of speakers right from an early age!.
Its good to find someone like your self who works with the beating phenomenon.

If the two drivers are used in a certain way and they are crossed over at certain frequency points then this introduces phase shifts that will alter the group delay between the drivers at these frequencies.
I first did experiments over 12 years ago of using a conventional 1st order Butterworth crossover at a middle frequency of around 3khz in a 5.25" 2 way design. I was not happy with the crusty/fizzy midband despite the drivers being smooth in response in that area and capable of reproducing the frequencies well.
I decided to start pulling the crossover frequencies apart, the fizzyness subsided and so did the midrange detail and presence. But what I tried next was to pull apart just a bit farther in frequency and the midband started to come alive and had solidity and focus. I started to pull away slightly and it all locked on. I went too far apart and it did what would be expected.....there was a gaping hole in the middle.
It led me to investigate further how this could possibly be working.

Cheers Ian
( I have more info on my website )

wolf_teeth 1st February 2013 07:46 PM

Quote:

Originally Posted by IanKnight (Post 3351798)
I first did experiments over 12 years ago of using a conventional 1st order Butterworth crossover at a middle frequency of around 3khz in a 5.25" 2 way design. I was not happy with the crusty/fizzy midband despite the drivers being smooth in response in that area and capable of reproducing the frequencies well.
I decided to start pulling the crossover frequencies apart, the fizzyness subsided and so did the midrange detail and presence. But what I tried next was to pull apart just a bit farther in frequency and the midband started to come alive and had solidity and focus. I started to pull away slightly and it all locked on. I went too far apart and it did what would be expected.....there was a gaping hole in the middle.
It led me to investigate further how this could possibly be working.

The reason the spread is needed is it produces a peak in the summation if 1st orders are used at the same frequency. BW1 do not sum flat without spread. Peak if not far enough, hole if too far, there is a sweet spot.

I don't know that the experiment listed has anything to do with beat frequencies at all. Still- great discussion thread!

Later,
Wolf

tvrgeek 1st February 2013 07:54 PM

Quote:

Originally Posted by IanKnight (Post 3351798)
Hi Dave,
Thanks for your message.
It is playing the piano that started me off on this journey. I have never been satisfied with the sound of speakers right from an early age!.
Its ..... vestigate further how this could possibly be working.

Cheers Ian
( I have more info on my website )

Are you sure you are not just realizing that no one I am aware of has ever recorded a piano realistically, let alone reproduced it in any speaker? I have only heard one reproduced instrument that almost, again almost, could convince me it was live. It was of an upright bass, half-track master tape, etc. Recorded music can be fantastic, but it sounds nothing like live. I am not sure it ever can.

IanKnight 1st February 2013 08:05 PM

Quote:

Originally Posted by speaker dave (Post 3351783)
Interesting links re the Basilar membrane.

I'm not seeing the connection with crossover design? Even a poor crossover can't shift frequency so I would think the occurance of beats would be the same nomatter what?

I'm an amateur piano tuner (Dad was a pro) so I am familiar with beats and find them interesting. In the absence of nonlinearity, they are more perceptual than real. That is, when two tones get quite close together we perceive more a single pitch with a varying amplitude, yet a spectrum analyzer would show two tones of constant amplitude.

An interesting phenomonon with pianos is that the overtones of a string are somewhat inharmonic (not exact multiples) This can lead to to notes beating even when the fundamentals are at an exact ratio.

Regards,
David S.

PS..Dave,

I forgot to mention that I find it fascinating that measurement doesn't always correlate with weat we perceive or hear!.
This I think has a bearing on why the manufacturers( B&W) that I have taken my designs to have a real job understanding what I am trying to explain to them. They always seem to rely on measurements first then tweak until the measurement is correct.
Cheers Ian


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