Speaker design discussion

In an effort to get Nando's thread back to the original topic I thought I'd move the speaker discuss here. I think this is the post that started the digression (Dev):-

You change your speakers! and *always* dem replacements in your own room.

 

All speakers interact with the room though, especially in the bass. You could probably change to something which doesn't exhibit the problem you are trying to fix, but it will likely have another instead.

 

Hi simon

I'm sure you'd agree that digital correction is the poor relation of acoustic room treatment.. a second line defence more accurately.

A great sounding room which requires no room treatment sounds a LOT better in my ex. than a bad room with loads of digital treatment.

To me, Richard's there is always a speaker arguement is like 'there are always a pair of boots that will get you across exmoor'. With acoustic treatment we have the equivalent of flattening exmoor and laying a road across and driving over it.

Audiophiles as you've said before (simon) shy away from spending money on this. Either family / aesthetic concerns or a lack of understanding do this IMO.

 

Why do you think they will solve it, do they solve it in studios - no they make them dead and boring if you have been in one. Did they solve the Albert Hall or the Festival Hall - no. You seem to think they are a no loss solution, well there is no such thing, there are electronic filters and there are mechanical filter both of them are bad and should only be used as a last resort.

If you want proof just remove all the wadding from your speaker, it may honk or the cabinet may play a bit as obviously the designer didn't know how to design the bloody thing to work in the first place, but without the mechanical filter all of a sudden there is more *music*. Same with your room. A speaker is a box playing within a box, they should work in sympathy not fight each other, mechanical or electronic room correction is just fighting the problem instead of understanding and being sympathetic to the problem. Don't fight the room work with it.

 

You've always said that Richard, but in my experience (and academic understanding) it is not usually the case. In fact it isn't my experience of filters in general. I think nearly all systems in audio, be they electronic or mechanical are improved by filtering of some sort.

Actually just thinking about it, I guess your amps use feedback of some sort, and isn't that filtering? Have you built a zero-feedback , single-ended amp before, and what did you think?

 

That just encourages resonances and farks the balance of the design.
It doesn't give you more music.

If you want coloured, honky and unnatural sounding speakers then fine, remove all of the stuffing and make all of the surfaces parallel and of equal size.

 

why not open baffle? or headphones, or nothing at all, that is just me being silly as always, but richard in the recording studios it is far from dead apart from the sound engineers they , well most of them are stoned deaf,

 

I am afraid not Rob, how many speakers have you designed? Filters are used as a cure, and as with all cures it is better not to have the disease in the first place. Most designers especially those brought up in the BBC tradition of thin noisy cabinets just automatically put it in as though it is a necessity, it is not, it is a cure for problems elsewhere, just solve those problems and you don't need the cure. Same electronically as with crossover networks, they are a cure for cheap or mismatching drivers, get your driver selection and/or design and/or doping and most of the crossover is defunct, all you need is a capacitor to protect the tweeter, and if we could design good full range drivers then you don't even need that.

This is true of all filters.

OK lets address your comment on parallel and same size panels. If you think about it the only cabinet that guarantees you have unequal length reflections is a cube, because when the drive is put in the box all the reflections are differing length as the cone is *inside* the box not at the surface.

 

Enough to know that the removal of all internal damping and the adoption of a cube shape doesn't make a particularly good loudspeaker.

 

I suppose I am asking for this, but you have heard my speakers on at least three occasions and they are exact cubes internally and with no internal damping, did they sound bad

I have very happy customers who love them, Docfoster in Gloucester being one of them. Rudy a music teacher and profession pianoist in Belgium who writes for a Belgium music and hi-fi mag. A guy in Luxembourg, and finally Gromit who had a pair of Cube 1 for about a month and a pair of Cube 2 for a couple of weeks, if you don't know he is a professional clarinet player and sax and clarinet teacher, and I lend him gear for assessment as his ears are so good as it is nice to have a second opinion, they all know what music is obviously, and they all love them and don't hear any anomalies. The only thing that some people don't like is the semi omni presentation of the sound stage.

Docfoster you can catch on the forums but mostly at Subjectivist.

Gromit you all know and can catch him at the other forums, I have never seen him post here.

The Luxembourg guy posts at PFM his name is Packtec or Paktech anyway something like that.

The Belgium guy is not on any UK forums but can be reached for his opinion at [email protected]

Even old salty has had a pair on loan but can't afford them, and loads of forum members have been here to hear them and they were at the show last week and at Whittlebury.

So ask them.

That is all so far as I can't get cabinets in any quantity, my acrylic supplier Anglia plastic is still trying to make them economically.

 

Yes, sorry.

Not my cup of tea at all.

I'm sure a few people like them but they are far too characterful (in the wrong way) for me. At the show they were quickly swapped for some vintage Dynaco A25s in the TT room to universal relief of those in the room.

The character I hear fits perfectly with a speaker suffering from internal reflections, lack of internal wave attenuation, odd power response and a dome tweeter running on too shallow a crossover and at too low a crossover frequency.

You did ask.

I like your amplifiers though - with tone controls would be even better

 

Well Jim Kempton who was running the TT room is one who has been to my house to hear them and he loved them. He did a write up at Subjectivist which I will quote here.

I've heard the NVA system before so am familiar with the 'house' sound so to speak, and I get very much what Richard is trying to achieve. But on Friday I heard it sounding the best ever. The Cubix were driven in biamped mode by two huge stereo amps, TDS I think or was it TSS - can't remember what you said mate? Anyway they were massive! These were driven from the stepped attenuator passive preamp. The PL71/SPU was the front end - sorry guys it was a joyously 11010100101 free day

The first thing that grabbed me was the very low level of overhang, due I'm sure to the cabinets. I felt them while the bass was pounding and they moved of course BUT the important point is how quickly they release the energy. They do not rely on over-damped lossy cabinets as most wooden cabinets do hence the low overhang. This leads on to them timing incredibly well. I thought they reproduced piano as well as I've ever heard. Again I reckon this to be down to low energy storage whereby the piano note decay is free from blurring and smearing - a common problem with most other speakers IMV. At first I thought they were bass light for a largish box with 2 bass drivers but when we played some music with very deep bass they reproduced it very well with weight and attack. Again I put this down to low energy storage. It's very common for poor speakers to appear 'bassy' when there's no need for it, i.e. it's not in the signal. The obvious conclusion being that the Cubix is adding very little. They go very loud, Richard's current room is not massive but it's more than big enough to accommodate them. I would suspect they'd be fine in a larger room also. The system was perfectly able to differentiate between the drums in a kit, the size of the drum and the skin tension being really obvious. Many, many speakers fail here IMV. I took Tom Waits Blue Valentine with me which is a good test for sibilance, it's easy (or should be!) to tell if the spit and sizzle in his voice is real or artificial. This system reproduced his voice as well as I've heard ever.

I also took a mint copy of Charles Mingus Pithecanthropus Erectus (Uglymusic as Richard says - I reckon he enjoyed it really). The rhythms and melody were easy to follow - no mean feat. Stan Kenton Solo is just him and a grand piano. I heard a big sonorous piano with all the body resonance you'd expect of the real thing. Easy to hear the hammers striking and the note and chord decay was first rate.

Here comes the but!! I felt I could hear a small amount of resonance in the speaker. I'm sure it's not the cabinet for all the above reasons so I reckon it's air resonance which is an all but inevitable result of putting dynamic, acoustic suspension speakers in a cabinet. Let me qualify this by saying it's very small and not that bothersome and Richard has asked for feedback. Let me also say that I'm very attuned to this as I normally use open baffles, and although they have many problems, do not have any trapped air to resonate. I'm probably a bit hyper-sensitive to this issue.

A very good day was had. Thanks to Richard for the invite and Nadia for a terrific lunch!

JK


So has he changed his mind or was it a question of acoustics or synergy with nva amps.

 

Seeing as I am bragging you might as well have Docfoster report as well.


The Cube 1s are wonderful speakers. Straight out of Mr. Dunn's top drawer. They grew and grew over their 1st 2 months of use in my system. When new they (and the Cubix too when I heard Dr. BH's recently produced pair in their infancy) sound a little closed-in and restrained, so it's important to audition a run in pair (at least 100 hours(?)).
Not sure whether its necessary to describe their sound - they are revealing in the way that all the best NVA stuff is revealing, and in a way that few speakers are.

It gets a bit lonely over here in West Midland hifi land, so if anyone wants to hear a pair of Cube 1s, please pop in!

Ben

 

I visited his room and he was unhappy with the speakers in use.
He then visited my room and borrowed a pair of A25s - and left them in play all day as he preferred them.
Ask him.

I'm sorry but reports from a handful of fans doesn't wipe away the decades of learning and research into loudspeaker design.

 

Well how do you justify the discrepancy. I will ask him to post here.

 

He was probably being nice to you - he's a nice guy.

The simple fact is that he used alternative speakers to run his demo on the day.
He didn't have to, they were sitting in our room as spares for anyone to use.

I don't see the point of this discussions though.
I don't like your speakers and can hear the faults directly related to the design decisions. If others like them that is their business.
Quoting a handful of user reports demonstrates nothing. If i link to several dozen very positive reports on BBC LS3/5s will that change your opinion of them?
Of course it won't.

 

There is other learning and research that you obviously have no experience of and with papers written on it, you should look into Roy Allison (ex of Acoustic Research and Allison Loudspeakers designer) who did exactly the same thing with the Allison 6 , but with wooden cabs and paper drivers, but because of the wooden cabs used very light wadding and some cab damping pads, because I use 20mm acrylic I can dispense with even that.

Roy Allison always used cubic or triangular or square cabinets with identical dimensions, he said for the same reasons I quote, that because of the driver position and the cone being inside the cab you can guarantee you don't have standing waves, where as with odd sides you could be unlucky and end up with standing waves.

 

I'm aware of the research and designs. I like AR speakers - but there are some important differences to your designs.

- In the two way models the cone tweeters were larger than modern 1" domes and designed to cross at lower frequencies, and on shallow slopes. Modern domes are not - especially metal domes which bark and shriek if you run them too low and too shallow.

- Internal wadding. AR used internal wadding - lots of it. Both my vintage AR speakers are well stuffed. This has nothing to do with the wooden construction and is intended to attenuate internal standing waves and reflections. The dense fibrous material also alters the driver damping.

- Changing the cabinet material will not eliminate internal standing waves or reflections.. It will display different resonant behaviour as a structure and that may or may not benefit from damping applied directly to the surfaces, but that is a different issue to internal wadding.

- The vast majority of Allison designs were not cubes. I can only recall one model.

The only shared thinking your designs have with these speakers is the lack of external filter on the bass driver. In fact, the above companies also used bespoke drivers for the job.

 

I am not talking about Acoustic Research he left them to set up his own company so he could put his acoustic matching designs into production, I quote :-

Roy Allison: Bending Boundary Effects
Before other people paid attention to the phenomenon, Roy Allison noticed that loudspeaker measurements taken in conventional home living rooms typically revealed a dip in power response in the 100–300Hz range. That was in the late 1960s, when Allison was VP for engineering and manufacturing at Acoustic Research. In 1972, after designing or supervising the design of nine models at AR, he left to begin an investigation of real-room speaker behavior. Next he teamed up with former AR president Abe Hoffman and two other colleagues from that company, Sumner Bennett and Frank Callahan, who had worked in sales and quality control, respectively. The quartet founded Allison Acoustics to build loudspeakers expressly designed to perform optimally where speakers were generally placed: in the home. The first of them, the Allison Model One, appeared exactly 30 years ago, at the end of 1974. Though the firm curtailed operations about 10 years ago, versions of three original Allison designs are now available from a reincarnated Allison Acoustics, which was later re-formed under new ownership.

AND

Everybody knew about standing waves, which tended to muddy the water and make these other effects very difficult to see. I did a great deal of empirical testing of my own and racked my brain, trying to figure out how to avoid this problem—and it was indeed a problem. Reflections from room surfaces can increase or decrease the power output of a woofer. Reflected energy increases the instantaneous density of the air in front of the woofer at very low frequencies. This provides an improved impedance match, and the efficiency of the woofer is thereby increased, along with the woofer's power output. At some higher frequency that depends on the distance or distances from the room surface or surfaces, the reflected energy goes out of phase with the woofer cone motion. That decreases the instantaneous density, and the woofer efficiency decreases. That's what causes the dip.

Now if the woofer is fairly close to one room surface and distant from others, in most home listening systems, power output in the range between 100 and 300Hz will drop about 1dB below what it would be without the nearby reflecting surface. At very low frequencies, there would be a 3dB increase in power output. That means, given maximum increase and maximum decrease, there's a total variation of 4dB. With the woofer equidistant from two intersecting surfaces, the dip is 3dB; factor in the maximum rise, in this case 6dB, and you have a 9dB variation. If it's equidistant from three surfaces that intersect at right angles, the dip would be a devastating 11dB and the maximum rise 9dB—a 20dB change over the bottom octaves. If the woofer is not on the line of symmetry, which is to say the same distance from all three surfaces, the dip is less severe but can still be significant. In home listening situations, I've found this reflected impedance typically causes variations from 5 to 12dB. If a tuner or receiver exhibited variations like this, it would be rejected out of hand.

The design of cabinets that get the woofer very close to one or more adjacent room surfaces. That changes the frequency range of the dip, because the closer the woofer is to a surface or to the point where surfaces intersect, the higher in frequency the dip occurs. In the case of a three-way system, it's possible to position the woofer so the dip is above its operating range, and to place the midrange driver far enough away from an intersection for the dip to occur below its range. In effect, that eliminates the problem. This approach really isn't feasible with two-way systems, because the woofer has to handle frequencies high enough to put the destructive reflections within its range. But you can build a cabinet that has the woofer very close to one surface—the best place is on top—and then position that cabinet so distances to the other nearby room surfaces are staggered. Doing that creates mild dips that are spaced along the frequency axis. They aren't able to add in the nonlinear manner that they would if the distances between the woofer and all adjacent room surfaces were equal.

 

Very interesting, but nothing to do with cubes or internal damping.
Most of that document appears to be discussing speaker positioning within the room.
Designing loudspeakers to account for boundaries is well understood.

Have we moved onto a new topic?