Elac B6 upgrade

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luckyguy

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Re: Elac B6 upgrade
« Reply #20 on: 11 Nov 2015, 01:41 am »
Thanks Danny, great response.  Ive just finished the exterior mods and they have had a remarkable effect on the clarity and apparent speed of this little speaker.  If you ever get around to the B5 crossover mods I will certainly be interested and keep up the great work.       Ron

Ultralight

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Re: Elac B6 upgrade
« Reply #21 on: 11 Nov 2015, 01:48 am »
Danny, I don't understand- the No Rez is 1 inch thick - why won't it take up air space?  I must not be understanding something.

Thanks,
UL


It would but it will also eat up some air space. I did not model the woofers to see if they needed more or less air space. The No Rez won't take up any air space.

Danny Richie

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Re: Elac B6 upgrade
« Reply #22 on: 11 Nov 2015, 02:07 am »
Danny, I don't understand- the No Rez is 1 inch thick - why won't it take up air space?  I must not be understanding something.

Thanks,
UL

The foam layer slows down the air flow in the box making the woofer act like it is a larger air space. The damping layer takes up space though. Usually it is about a wash.

mark.hayes0338

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Re: Elac B6 upgrade
« Reply #23 on: 12 Nov 2015, 01:33 am »
I'm auditioning the EMP R5Bi's and the ELAC B5's and trying to decide which I like best. The EMP's have a clearer high end, but I'm wondering if these mods would help with the high end and clean up the low end on the B5's. Makes my decision much harder!

gregfisk

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Re: Elac B6 upgrade
« Reply #24 on: 12 Nov 2015, 02:15 am »
The foam layer slows down the air flow in the box making the woofer act like it is a larger air space. The damping layer takes up space though. Usually it is about a wash.

So, since it is about a wash which way would the current flow? If the foam layer makes the air space larger than the damping layer takes up space would it not be prudent to add a couple of dowels in the box to stiffen it up? Or is it so close you would be taking up too much air space by adding some support.

From my experience with cheap book shelf type speakers the box pretty much always vibrates and is adding to the sound, and not in a good way.

Danny Richie

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Re: Elac B6 upgrade
« Reply #25 on: 12 Nov 2015, 04:03 am »
So, since it is about a wash which way would the current flow? If the foam layer makes the air space larger than the damping layer takes up space would it not be prudent to add a couple of dowels in the box to stiffen it up? Or is it so close you would be taking up too much air space by adding some support.

From my experience with cheap book shelf type speakers the box pretty much always vibrates and is adding to the sound, and not in a good way.

You could add a few small dowel rods if you like.

In this case though the No Rez goes a long way and is an easy install.

AndrewJ

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Re: Elac B6 upgrade
« Reply #26 on: 12 Nov 2015, 05:17 am »
I thought it would be pertinent to chime in on this thread given the interest!!
Danny does give praise to the overall performance of the speaker, so my design can't be all bad...... ;-)
It would seem that my designs are the target of aftermarket upgrades, so that’s either praise of the basic design being worthy of spending time upgrading, or an indictment of the design that it needs upgrading ;-)
I'll give you some of my thoughts on the mods that Danny proposes and also some warnings where necessary....
Firstly, let me say that I am required to point out that any modifications that you make will obviously void the warranty, so be careful!
Now this is not the first time that one of my designs has been the subject of a long thread of how to modify it to get better results. I will say now what I have said before, and I know that Danny doesn't deny this: The reason why it is worth even thinking about modifying it is that it begins as a tremendous value product for the money.
When I designed this product, I had to meet a certain price target. It's nice to think, as an audiophile, that price points are somewhat negotiable: $199, $209, $219, $229...they are all close enough aren't they? The actual answer is no. Price point is critical in retail sales. Miss by a little and sales can go to virtually zero as the large box retailer totally rejects the product when it is the wrong price point. This becomes a major challenge in the design process. Every part that adds cost to the bill of materials has to be very carefully evaluated as to its impact on the performance.
Let me go through some of these mods and look at the cost/performance issues and discuss why I chose the path that I did.
I note that Danny chose to lift the extreme top end of the response by bypassing the series resistor on the tweeter. That's nice, if that's the response that you want as opposed to the sound that has received such praise, but it adds cost. That option would have had to be paid for by taking it out of something else. Likewise the substitution of the sand cast resistor with a Mills resistor would have taken the cost way up, as would substituting the film cap with a Sonicap. I doubt that paying for these alternative caps and resistors by taking away from the overall design would be a good trade-off in the original. (As an aside, the added cap across the resistor does not work because of a supposed inductive component of the resistor. It simple reduces attenuation of the tweeter above a certain frequency).
 
Danny comments on the quality of the cabinet MDF and the thickness of the baffle and wrap. The cabinet is the major cost in a speaker. It would be nice, wouldn't it, to have thicker cabinet walls, internal bracing, more rigid MDF, more internal damping etc. However, all of this would greatly increase the cost, and for what? Let’s think about cabinet vibration. The major excitation source if the reaction force imparted to the magnet and hence driver frame that feeds mechanical energy into the cabinet causing it to vibrate. Now, measures to thicken up the walls, or use a higher grade of MDF, absolutely do not remove the resonances. They move them up in frequency. In fact, they can become more audible!
The grade of MDF I chose is softer and so has better internal damping, and can actually sound better than the "better" grades. In fact my early prototype cabinets were better built, with internal bracing and thicker walls. When I looked at the cost I realized that this cost would be better spent on improved cone materials, larger voice coils, larger magnets, better-than-standard xover complexity and parts etc.
 
Regarding the internal damping material, the cabinet is not stuffed full. This is deliberate. Besides costing more for a higher grade and more of a better damping material, there is a fallacy involved in the effect of damping material. In theory, a heavily stuffed cabinet changes from adiabatic expansion to isothermal and as such appears to be up to 1.4 times larger. This effective increase in cabinet size is offset by the greater resistive losses and so the performance gain is not that great. In the practical quantities that we are likely to see used, the effective increase is negligible. However, in a vented box system, putting in too much damping material will severely reduce the output of the vent and greatly reduce bass output. The choice I made of damping material was optimized between damping to some degree the standing waves while not impacting bass performance.
Danny shows a measurement of the impedance of the speaker that shows some apparent resonances around 25Hz and 60Hz. He attributes these to internal standing waves. This is erroneous. The cabinet cannot support standing waves at these low frequencies. If they exist they are more likely to be mechanically coupled. However these blips in the impedance curve do not show up in my measurements. I suggest that they are due to something in the measurement procedure. I know the repeat measurement after the modifications shows they are gone, but I wouldn’t attribute this to the added stuffing reducing internal standing waves. I would however encourage Danny to re-investigate these measurements.
 
On the subject of the trim rings, removing them will change the response measured on certain axes, but will have little effect on the frontal average response, since the effects changing at different measurement axes because they are due to diffraction, You may notice subtle changes depending on your room acoustics, and very large changes depending upon your eyesight...LOL. I designed the speaker to have the trim to improve the looks by covering the chassis and screws. A worthwhile trade I believe towards getting a successful looking visual design that will appeal even with the grille off.
 
Talking of eyesight, no wonder Danny is happy with the trims removed, as he could not see that the inductor he discusses is not actually connected at both ends to the same trace. The break in the trace is hidden under a component, but a simple test with a component bridge or multi-meter would have confirmed this is.
 
So, based on the first modification offered, you will have a speaker that looks uglier and sounds different. Your choice.
However, if you go ahead be very careful. Danny does warn about the difficulty of removing the trim. Using a flat bladed screwdriver to prize off the trim very close to the driver surround requires careful dexterity. One slip and you will tear the surround. How do I know I hear you ask? Perhaps because I’ve done it myself? At least I have lots of spare drivers at my disposal. ;-)
Also, removing components from the xover circuit board is not easy. They are all epoxied to the board. Don't expect to remove them and restore it back to original if you decide you need to.
 
Next, the level 2 mods. Be careful with this regarding the change in xover frequency. The change to the tweeter puts more power into it at the lower frequencies. Potentially as much as double the power. I am sure that Danny hasn't done long term power handling tests on multiple samples to make sure that it can still meet its specifications for max power. I know I have for the original design, but not for the modified version.
Additionally, during manufacture the tweeter is measured and tested to performance limits within its expected working frequency range. The spread in response away from the reference is holding within +/-0.75dB during production over the range from 3kHz to 10kHz, which is quite impressive for a budget speaker. Using it down to a lower frequency will take it into a range where I don't guarantee such tight tolerance, so your expected results could vary. I normally design around drivers that are selected to match the mean of production, not around a single sample.
Also, changing all the inductors to Erse 16 gauge air core will likely change the effective resistance of the inductors. I accounted for inductor resistance in my design, and in fact take this into account for the bass tuning. So changes in this will also alter the bass respone.
 
In conclusion, for the cost of the level 2 upgrade, you will get a different sound, hopefully one you like, because there is no going back if you make the changes, and remember, no warranty.
For that much extra money on top of the purchase price, you could get a pair of the F5 speakers, or wait for my next series of speakers.... :-)
I don’t want to sound like a Debbie Downer over this. Ours is a great hobby. I started as a hobbyist. It’s fun to experiment. It’s even more fun to feel accomplishment from your experiments. Just be careful. Be sure that these mods are going to give you the sound you want before you commit to making changes.

I’ll finish with a great quote. This is taken from one of the best textbooks on Acoustics, written by Leo L. Beranek. “It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than  does anyone else’s loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in performing a person’s opinion.”

Above all, Have fun!

Andrew

Danny Richie

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Re: Elac B6 upgrade
« Reply #27 on: 12 Nov 2015, 06:36 am »
I thought it would be pertinent to chime in on this thread given the interest!!
Danny does give praise to the overall performance of the speaker, so my design can't be all bad...... ;-)
It would seem that my designs are the target of aftermarket upgrades, so that’s either praise of the basic design being worthy of spending time upgrading, or an indictment of the design that it needs upgrading ;-)

I'd say it was a platform worthy of a few upgrades. I'd take it as praise.

Keep in mind too that this is part of the fun for a lot of people.

Quote
I will say now what I have said before, and I know that Danny doesn't deny this: The reason why it is worth even thinking about modifying it is that it begins as a tremendous value product for the money.

Yep, I agree.

Quote
When I designed this product, I had to meet a certain price target. It's nice to think, as an audiophile, that price points are somewhat negotiable: $199, $209, $219, $229...they are all close enough aren't they? The actual answer is no. Price point is critical in retail sales. Miss by a little and sales can go to virtually zero as the large box retailer totally rejects the product when it is the wrong price point. This becomes a major challenge in the design process. Every part that adds cost to the bill of materials has to be very carefully evaluated as to its impact on the performance.

I hear you man. And it's tough to produce this level of quality for this price point. I did something simular for AV123 years ago when I designed a speaker that they launched at $199 a pair.

Quote
Let me go through some of these mods and look at the cost/performance issues and discuss why I chose the path that I did.
I note that Danny chose to lift the extreme top end of the response by bypassing the series resistor on the tweeter. That's nice, if that's the response that you want as opposed to the sound that has received such praise, but it adds cost. That option would have had to be paid for by taking it out of something else. Likewise the substitution of the sand cast resistor with a Mills resistor would have taken the cost way up, as would substituting the film cap with a Sonicap. I doubt that paying for these alternative caps and resistors by taking away from the overall design would be a good trade-off in the original. (As an aside, the added cap across the resistor does not work because of a supposed inductive component of the resistor. It simple reduces attenuation of the tweeter above a certain frequency).

You don't have to defend your choices. This is better quality parts than I typically see at this price point.

Keep in mind though that for a lot of these guys it's no big deal to throw a few extra dollars at something like this to see (or hear) what the results are. It's part of the fun of the hobby.
 
Quote
Danny comments on the quality of the cabinet MDF and the thickness of the baffle and wrap. The cabinet is the major cost in a speaker. It would be nice, wouldn't it, to have thicker cabinet walls, internal bracing, more rigid MDF, more internal damping etc. However, all of this would greatly increase the cost, and for what?

You don't have to defend that either. You did what you had to do to hit your price targets and did a great job of it.

Quote
Let’s think about cabinet vibration. The major excitation source if the reaction force imparted to the magnet and hence driver frame that feeds mechanical energy into the cabinet causing it to vibrate. Now, measures to thicken up the walls, or use a higher grade of MDF, absolutely do not remove the resonances. They move them up in frequency. In fact, they can become more audible!
The grade of MDF I chose is softer and so has better internal damping, and can actually sound better than the "better" grades. In fact my early prototype cabinets were better built, with internal bracing and thicker walls. When I looked at the cost I realized that this cost would be better spent on improved cone materials, larger voice coils, larger magnets, better-than-standard xover complexity and parts etc.

I can understand the positive effect of using the softer MDF. I attack it a different way though. The No Rez product that I talked about uses a heavy damping layer that sticks to the cabinet wall with a pressure sensitive adhesive. It really knocks out the resonances of the cabinet walls.
 
Quote
Danny shows a measurement of the impedance of the speaker that shows some apparent resonances around 25Hz and 60Hz. He attributes these to internal standing waves. This is erroneous. The cabinet cannot support standing waves at these low frequencies.


You are correct. Standing waves is not technically what it is. Those waves are too long to propagate in the box. It is more of a pressure wave. There is a resonance there though that additional damping took away.

Quote
If they exist they are more likely to be mechanically coupled. However these blips in the impedance curve do not show up in my measurements. I suggest that they are due to something in the measurement procedure. I know the repeat measurement after the modifications shows they are gone, but I wouldn’t attribute this to the added stuffing reducing internal standing waves. I would however encourage Danny to re-investigate these measurements.

Those peaks are there. I will take some more measurements to confirm.
 
Quote
On the subject of the trim rings, removing them will change the response measured on certain axes, but will have little effect on the frontal average response, since the effects changing at different measurement axes because they are due to diffraction, You may notice subtle changes depending on your room acoustics, and very large changes depending upon your eyesight...LOL. I designed the speaker to have the trim to improve the looks by covering the chassis and screws. A worthwhile trade I believe towards getting a successful looking visual design that will appeal even with the grille off.

I'm not knocking the trim rings. They do look good. But there is a subtle change in the response from them.
 
Quote
Talking of eyesight, no wonder Danny is happy with the trims removed, as he could not see that the inductor he discusses is not actually connected at both ends to the same trace. The break in the trace is hidden under a component, but a simple test with a component bridge or multi-meter would have confirmed this is.

You are correct. The break in the trace is hidden under the zip tie. And I retracted that observation from the original post.
 
Quote
So, based on the first modification offered, you will have a speaker that looks uglier and sounds different.

I think it will sound more than just different.
 
Quote
Next, the level 2 mods. Be careful with this regarding the change in xover frequency. The change to the tweeter puts more power into it at the lower frequencies. Potentially as much as double the power. I am sure that Danny hasn't done long term power handling tests on multiple samples to make sure that it can still meet its specifications for max power. I know I have for the original design, but not for the modified version.

I did carefully take into consideration the power handling of the tweeter. I really didn't let it play down much lower though. At 70db it only plays about 150Hz lower.

Quote
Additionally, during manufacture the tweeter is measured and tested to performance limits within its expected working frequency range. The spread in response away from the reference is holding within +/-0.75dB during production over the range from 3kHz to 10kHz, which is quite impressive for a budget speaker.

Yep, the consistency from one to the other was very good.

Quote
Using it down to a lower frequency will take it into a range where I don't guarantee such tight tolerance, so your expected results could vary. I normally design around drivers that are selected to match the mean of production, not around a single sample.

It is not that much lower. The crossover point is lower mainly because I pulled the woofer back a little more.

I tell what (Texas speak). You can sell me a handful of tweeters and if anyone blows a tweeter as a result of my mod then I'll replace their tweeter for free.

Quote
Also, changing all the inductors to Erse 16 gauge air core will likely change the effective resistance of the inductors. I accounted for inductor resistance in my design, and in fact take this into account for the bass tuning. So changes in this will also alter the bass respone.

That's true. The larger gauge air core inductor has .4 ohms less resistance. That is going to lift the bass output below 200Hz by about 1/10th to 1/20th of a db at its peak increase. It's not that much. And I also matched it to the sensitivity across the board. So it isn't bass heavy.
 
Quote
In conclusion, for the cost of the level 2 upgrade, you will get a different sound, hopefully one you like, because there is no going back if you make the changes, and remember, no warranty.

No worries. Sell me a handful of drivers and I'll warranty them.

Quote
For that much extra money on top of the purchase price, you could get a pair of the F5 speakers, or wait for my next series of speakers.... :-)

Or they could buy some of our higher end kits too, but upgrading stuff is fun. And most of the kits that I offer include upgrade options. I guess you can say I even upgrade my own designs.

Russell Dawkins

Re: Elac B6 upgrade
« Reply #28 on: 12 Nov 2015, 08:02 am »
So, Danny, I gather from the tone of your reply to the designer that you would change nothing in your recommendations, and that if they were your speakers you would run them with the trim rings and the grill cloth off? I think they look appalling with the trim rings off, and that appearances do count for something to most people, unless your speakers are behind scrims or screens in which case - go nuts!

Sometimes your writing suggests to me that you presume that no one of technical discernment equal (or superior) to yours will read and ponder what you are saying. This might not, in reality, always be the case.

I, too, wondered about the notion of 22 and 55Hz standing waves inside a box of these small dimensions and wrote off your use of the term as an opportunity taken to sound technically adept, again, assuming on your part that no reader would be the wiser. In your response to this single point of Andrew's you appear to be doing this again, but twice this time, in the evoking of the terms propagation and pressure waves. You say the waves are too long to propagate in the box, but this also is false. They will propagate all right, they just will not form standing waves. And, by the way, what do you mean by "it's more of a pressure wave"? More of a pressure wave than what? Are you implying there is a difference between a "pressure wave" and a simple sound wave?

Things are complex enough without intentional technical obfuscation.

If I were looking to improve the sound of this speaker and didn't care about the warranty aspect, I would go no further than replacing the cap in series with the tweeter and maybe the resistor but would think hard about doing even that, considering the warranty aspect. I would also experiment with the effect of removing just the metal grill in front of the tweeter but leaving the trim rings. As to the addition of No Rez, I'd be concerned about upsetting what is, in all likelihood, a carefully tuned box/driver system and spoiling the lower mids and upper bass.
Harbeth also makes the design choice of flexible lightweight but lossy box sides over heavy, rigid ones for the same reason as has Andrew mentions: shifting the inevitable resonance to a less bothersome, lower frequency (with lower 'Q', often) and Harbeth is renowned for accurate tonality and low distortion levels  through the mid range, if nothing else.

OzarkTom

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Re: Elac B6 upgrade
« Reply #29 on: 12 Nov 2015, 11:49 am »
Thank you Andrew for offering such a great value in today's very expensive audio world.  :thumb:

Danny Richie

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Re: Elac B6 upgrade
« Reply #30 on: 12 Nov 2015, 03:15 pm »

Things are complex enough without intentional technical obfuscation.

I think you are mistaken in your assessment.

Quote
As to the addition of No Rez, I'd be concerned about upsetting what is, in all likelihood, a carefully tuned box/driver system and spoiling the lower mids and upper bass.

Taking away cabinet resonances isn't going to upset what is. It takes away what isn't supposed to be there.

Quote
Harbeth also makes the design choice of flexible lightweight but lossy box sides over heavy, rigid ones for the same reason as has Andrew mentions: shifting the inevitable resonance to a less bothersome, lower frequency (with lower 'Q', often) and Harbeth is renowned for accurate tonality and low distortion levels  through the mid range, if nothing else.

Allowing the cabinet to become an instrument or add bloom from cabinet resonances is not being true to the input signal. And I am not saying Andrew is deliberately using cabinet resonances as part of the design. I think his choices were made based on the price point limitations that he was working in. Harbeth does do that though. I personally find the notion of deliberately adding something that is not part of the input signal to be a move in the wrong direction. 

woodsyi

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Re: Elac B6 upgrade
« Reply #31 on: 12 Nov 2015, 03:28 pm »
Fascinating.  I love the explanations back and forth between AJ and DR.   :thumb:

S Clark

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Re: Elac B6 upgrade
« Reply #32 on: 12 Nov 2015, 03:37 pm »
Fascinating.  I love the explanations back and forth between AJ and DR.   :thumb:
Sure makes it clear that there is so much more to speaker design than just measuring a F/R curve. 

Early B.

Re: Elac B6 upgrade
« Reply #33 on: 12 Nov 2015, 04:04 pm »
Sure makes it clear that there is so much more to speaker design than just measuring a F/R curve.

Yep. It's insanely complex. Sometimes people forget that speaker design is both an art and a science. Science doesn't create a great speaker. It's the art that puts the icing on the cake. And to create a masterpiece requires a wealth of experience.

jtwrace

Re: Elac B6 upgrade
« Reply #34 on: 12 Nov 2015, 04:05 pm »
Yep. It's insanely complex. Sometimes people forget that speaker design is both an art and a science. Science doesn't create a great speaker. It's the art that puts the icing on the cake. And to create a masterpiece requires a wealth of experience.
Nope.  It's ALL science. 

nickd

Re: Elac B6 upgrade
« Reply #35 on: 12 Nov 2015, 04:17 pm »
Thank you Andrew and welcome to A.C.!
This is the best thread about speakers on A.C for some time! It's wonderful to hear two serious designers discuss thoughts and technical aspects of their choices for value and sound. I have great respect for both Andrew and Danny based on what I have heard from their designs.

Andrew, I am anxious to hear what some of your mid price designs do in the future. This little B6 speaker is amazing for the price, but it makes me wonder what is possible if your costs were allowed to be $250.00 (assuming a retail of +/-$1200.00)

Hoping for more discussions like this in the future. "None of us is smart as all of us" :D

Danny Richie

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Re: Elac B6 upgrade
« Reply #36 on: 12 Nov 2015, 04:24 pm »
Nope.  It's ALL science.

Care to explain to us how that ERS paper works?

mlundy57

Re: Elac B6 upgrade
« Reply #37 on: 12 Nov 2015, 05:29 pm »
Fascinating.  I love the explanations back and forth between AJ and DR.   :thumb:

+1

Everything built is a trade off, not just speakers. I have yet to buy a bicycle I was completely happy with right off the bat. Sometimes I only make minor upgrades and sometimes I would only keep the frame, replacing everything else.

I have even gone so far as buying two rear cassette clusters, breakng them apart and recombining the cogs to get the gear ratios I wanted on a particular bike. And even then that was a trade off because I sacrificed some shifting smoothness.

If it wasn't for having to build to price points and for majority appeal there would be no such thing as aftermarket or boutique industries. Also, if everything was built to it's absolute maximum potential only the richest could afford anything.

Even those of us who like to tinker in this hobby still have to make trade offs whether due to budget, space or spouse constraints  :o

Then we all have our personal limitations. I am more than happy to tinker with speaker upgrades, as long as somebody who knows what they are doing does the design work, but when it comes to amps, rolling tubes is the limit of my skills. Even then I need advice.

This has been a long winded way of saying both sides of the debate are legitimate. Both have their place and the existence of one in no way diminishes the other.

Bully for All!!!

jtwrace

Re: Elac B6 upgrade
« Reply #38 on: 12 Nov 2015, 05:39 pm »
Care to explain to us how that ERS paper works?
I'm sure you know as it's science that has been used for many years.  I've used a form of it in tape for literally 15 years. 

jtwrace

Re: Elac B6 upgrade
« Reply #39 on: 12 Nov 2015, 05:41 pm »
So, Danny, I gather from the tone of your reply to the designer that you would change nothing in your recommendations, and that if they were your speakers you would run them with the trim rings and the grill cloth off? I think they look appalling with the trim rings off, and that appearances do count for something to most people, unless your speakers are behind scrims or screens in which case - go nuts!

Sometimes your writing suggests to me that you presume that no one of technical discernment equal (or superior) to yours will read and ponder what you are saying. This might not, in reality, always be the case.

I, too, wondered about the notion of 22 and 55Hz standing waves inside a box of these small dimensions and wrote off your use of the term as an opportunity taken to sound technically adept, again, assuming on your part that no reader would be the wiser. In your response to this single point of Andrew's you appear to be doing this again, but twice this time, in the evoking of the terms propagation and pressure waves. You say the waves are too long to propagate in the box, but this also is false. They will propagate all right, they just will not form standing waves. And, by the way, what do you mean by "it's more of a pressure wave"? More of a pressure wave than what? Are you implying there is a difference between a "pressure wave" and a simple sound wave?

Things are complex enough without intentional technical obfuscation.

If I were looking to improve the sound of this speaker and didn't care about the warranty aspect, I would go no further than replacing the cap in series with the tweeter and maybe the resistor but would think hard about doing even that, considering the warranty aspect. I would also experiment with the effect of removing just the metal grill in front of the tweeter but leaving the trim rings. As to the addition of No Rez, I'd be concerned about upsetting what is, in all likelihood, a carefully tuned box/driver system and spoiling the lower mids and upper bass.
Harbeth also makes the design choice of flexible lightweight but lossy box sides over heavy, rigid ones for the same reason as has Andrew mentions: shifting the inevitable resonance to a less bothersome, lower frequency (with lower 'Q', often) and Harbeth is renowned for accurate tonality and low distortion levels  through the mid range, if nothing else.
Very well said Russell.   :thumb: