BBC-style thin-walled cabinets vs. tight bass?

[wolfgang]

Does a BBC-style thin-walled cabinet (i.e., 1/2" birch ply + bitumen) with lossy joints really provide the least midrange coloration?

By the same token, does a thicker-walled cabinet really provide tighter and more musical (i.e., less "one note") bass?

I am thinking of building a configuration that implements both cabinet designs and crossing the drivers over around 275Hz.

[tomzarbo]

Re: BBC-style thin-walled cabinets vs. tight bass?

An acoustic guitar is designed with walls that are able to vibrate... that's okay because the guitar is part of the music making process.

I think any speaker enclosure that adds to the signal the speakers are trying to reproduce is additional noise and should be avoided. I believe that speaker cabinets should be dead as can be, the less vibration or coloration of the sound, the better.

TomZ

[Jeff B.]

Re: BBC-style thin-walled cabinets vs. tight bass?

I agree with Tom. What the BBC did in the 70's with thin panels is counterintuitive. Intentionally adding a coloration in the lower midrange will not make the midrange cleaner. In fact, the less panel resonance you have the better.

[JRT]

Re: BBC-style thin-walled cabinets vs. tight bass?

Does a BBC-style thin-walled cabinet (i.e., 1/2" birch ply + bitumen) with lossy joints really provide the least midrange coloration?

By the same token, does a thicker-walled cabinet really provide tighter and more musical (i.e., less "one note") bass?

I am thinking of building a configuration that implements both cabinet designs and crossing the drivers over around 275Hz.

A rigid cabinet is better than one that is less rigid, toward reducing amplitude and increasing resonant frequency. Damping resonance in structure borne sound 9is better than undamped.

Any nontrivial contribution from the cabinet in the summed response is a source of performance degradation, but the threshold of what is trivial is very dependent on the application. A high end mastering studio's reference monitors are very different from a garage band's horn loaded woofers in thin plywood cabinets. When you read other's commentary on this and similar questions, it is important to understand the perspective of the person making the comment, as they may not be considering the broad view, may only be commenting from narrow perspective.

edit: I just read Jeff's and Tom's posts, and fully agree with their comments, and I wasn't referring to their comments in this, as their comments are looking at big picture. You may see another's who focuses on portable pro-sound reinforcement who often advocates thin plywood.

[fastbike1]

Re: BBC-style thin-walled cabinets vs. tight bass?

The BBC wasn't trying to add to the speaker's sound. Their position was that a completely rigid cabinet would still reach resonance and ring at some audible level/frequency while a lossy cabinet would push "unwanted resonances downwards in amplitude and frequency so that they are adequately buried below the music and then pushed down in pitch"
The quotation is from Alan Shaw of Harbeth.

It seems to me that the BBC / Harbeth objective is the same as the "rigid cabinet" school. They just (seem) to believe that the lossy construction is a better approach to the goal.

[JRT]

Re: BBC-style thin-walled cabinets vs. tight bass?

The BBC wasn't trying to add to the speaker's sound. Their position was that a completely rigid cabinet would still reach resonance and ring at some audible level/frequency while a lossy cabinet would push "unwanted resonances downwards in amplitude and frequency so that they are adequately buried below the music and then pushed down in pitch"
The quotation is from Alan Shaw of Harbeth.

It seems to me that the BBC / Harbeth objective is the same as the "rigid cabinet" school. They just (seem) to believe that the lossy construction is a better approach to the goal.

Rigidity and lossy damping are two separate aspects, analgous to springs and shock absorbers (dampers).

[wolfgang]

Re: BBC-style thin-walled cabinets vs. tight bass?

The BBC wasn't trying to add to the speaker's sound. Their position was that a completely rigid cabinet would still reach resonance and ring at some audible level/frequency while a lossy cabinet would push "unwanted resonances downwards in amplitude and frequency so that they are adequately buried below the music and then pushed down in pitch"
The quotation is from Alan Shaw of Harbeth.

It seems to me that the BBC / Harbeth objective is the same as the "rigid cabinet" school. They just (seem) to believe that the lossy construction is a better approach to the goal.

Right. Reading through the "Ask the designer a technical question" on the Harbeth forums is very enlightening. And there is no shortage of praise from Harbeth owners for their uncolored sound.

But I've wondered what kind of bass Harbeth's produce. The LS3/5A was developed as a monitor by the BBC for, I assume, monitoring vocal reproduction. Plus anyone who has built a subwoofer knows that thicker, more rigged walls gives tighter bass.

I wonder if it's a matter of having an enclosure with a resonance that is well outside the passband of the driver. A thick-walled, reinforced sub enclosure will have a much higher resonance, but the sub is only producing low frequencies. A thin-walled lossy-joint bitumen-damped enclosure will have a very low resonance around 100Hz, well below the passband of "midrange."

So my ultimate question is whether or not there is a real benefit in using the best of both worlds. Many Harbeth owner's simply add a sub to their setup -- and I'm willing to bet if they pay $$$ for Harbeth's, that their sub is well-made, i.e., with thick, rigid walls.

[craigk]

Re: BBC-style thin-walled cabinets vs. tight bass?

Right. Reading through the "Ask the designer a technical question" on the Harbeth forums is very enlightening. And there is no shortage of praise from Harbeth owners for their uncolored sound.

But I've wondered what kind of bass Harbeth's produce. The LS3/5A was developed as a monitor by the BBC for, I assume, monitoring vocal reproduction. Plus anyone who has built a subwoofer knows that thicker, more rigged walls gives tighter bass.

I wonder if it's a matter of having an enclosure with a resonance that is well outside the passband of the driver. A thick-walled, reinforced sub enclosure will have a much higher resonance, but the sub is only producing low frequencies. A thin-walled lossy-joint bitumen-damped enclosure will have a very low resonance around 100Hz, well below the passband of "midrange."

So my ultimate question is whether or not there is a real benefit in using the best of both worlds. Many Harbeth owner's simply add a sub to their setup -- and I'm willing to bet if they pay $$$ for Harbeth's, that their sub is well-made, i.e., with thick, rigid walls.

one thing I can tell you about the Harbeth speakers after hearing them in Chicago, I was very, very disappointed. this was the same reaction the other 3 people in the group I was roaming the halls with had. I would definitely have to be able to listen o them in my own home before ever considering a pair. unless they sounded much better at the dealer I would not waste my time listening in house actually. for big named speakers they were ion the bottom 5 at the show.

[djg]

Re: BBC-style thin-walled cabinets vs. tight bass?

Jeff Bagby says his Continuum is better than the BBC monitor that inspired it.

http://salksound.com/continuum%20-%20home.htm

I believe him. I am not a tweaker of designs nor a designer. I like to go with a safe bet when I choose a build. Others like to experiment. If that's you, build some cabs and see for yourself.

[Ron_E]

Re: BBC-style thin-walled cabinets vs. tight bass?

I've been doing a lot of builds with well braced 1/2" Baltic birch. All 4 builds in my sig were built that way. Some have had thicker front baffles because of deep rebates but everything else is 1/2". The most recent build came in a very close 2nd in the Iron Driver 2015 contest. The Cherry π's got some rave reviews when I took them to Grinnell in 2013 (as in "best system I've ever heard anywhere").

Champs cross bracing


Cherry π's cross bracing (I don't have a picture of all the bracing after the sides went on)


100 lb boxes are probably better but my back can't handle more than 20 comfortably so I build them light and I think there are a few other guys who have had successful builds going that route.

Ron

[wolfgang]

Re: BBC-style thin-walled cabinets vs. tight bass?

I guess that in thinking through cabinet construction I am wanting to minimize sound that is being reflected back to the driver -- the absolute thinnest "wall" of the cabinet, if you will, though which sound waves will readily and most assuredly exit.

To begin with the idea of any solid object -- a speaker cabinet, a bell, a wine glass -- having its own resonance, and of sound passing through at that resonance as if the entire structure were paper-thin, it becomes clear that mitigating amplitude peaks rather than trying to stop sound in it's tracks is the only realistic goal.

I believe the answer that the BBC researchers arrived at was that, at a certain number of decibels (20 or so I believe) below the SPL of the sound leaving the front of the driver the panels can be allowed to resonate and the effect will be inaudible.

This is the full quote from Alan Shaw that fastbike mentioned above:

What underpins the BBC's thin-wall cabinet philosophy (and I was surprised to read that exact word in one of Harwood's papers recently) is the observation that a perfectly cast bell will ring on for many seconds. Conversely, a bell with a hairline crack will sound leaden and hardly ring at all. It's the same with cabinets: if the panels are all rigidly glued together then at some critical frequency or other a note or notes in the music will trigger the cabinet's natural structural resonance. In such a rigid structure, there is nothing that can be done to suppress the ringing - and each time that note reappears, it tops up the ringing which then becomes a permanent drone underneath the music.

Conversely, in a thin-wall cabinet, the lossy joints (i.e. removable baffle/back and the generally 9-12mm thin panels used throughout the box) each act as an acoustic hairline crack. They inhibit the build-up of resonance. Simple as that really!

Now, let's not kid ourself that it is possible to kill cabinet resonance stone dead. It isn't. Not with any approach to cabinet design because the sound pressure inside the cabinet is huge. What the thin-wall approach does is to move unwanted resonances downwards in amplitude and frequency so that they are adequately buried below the music and then pushed down in pitch. Note that I said adequately. Providing that the resonance, be it from the cone, cabinet or even recording - whatever the source - is x dBs below the fundamental, the BBC proved that it was completely inaudible. Once inaudible to trained listeners on all types of music/speech, that is the end of the matter. Inaudible to the trained listener is as good as the solution needs to be. It is neither necessary nor cost effective (nor good engineering) to continue pushing for a degree of theoretical excellence that nobody can appreciate but everyone must pay for. That pragmatism keeps our speaker affordable - and sounding natural.

What we seem to be lacking in the industry today is the good old fashioned common sense that was abundant when serious researchers with zero commercial interest (i.e. the BBC) had their hands on the tiller. Thank goodness that they thoroughly documented their efforts for posterity since physics, acoustics and our hearing are the same now as fifty years ago. Now it seems we are all conditioned by marketeers to chase theoretical perfection which is far, far beyond what our ears can reliably resolve.

So to my understanding, if we can accept panel resonances producing sound in the room at SPLs which are inaudible across all frequencies, then we can avoid a situation in which a very rigid panel is pushing most resonances way down below the inaudible SPL threshold but ringing like a perfectly cast bell at its own resonance and harmonic multiples of it.

To put it differently, I believe what the BBC tried to do was the create a flat frequency response, if you will, for the panels themselves at or below -20dB SPL relative to the sound leaving the front of the cone, instead of trying to "block" sound but ultimate allowing certain resonances to pass through unimpeded because of the structure's rigidity. This is where the lossy joints and decreased rigidity of thinner panels plays a role.

[fpitas]

Re: BBC-style thin-walled cabinets vs. tight bass?

Whatever the pros and cons of thin, well-damped cabinets, I think it's unrealistic to expect thin walled cabinet construction to have a substantial effect on sound reflected back to the driver. That purpose is better served by wall lining and/or stuffing.

[Jeff B.]

Re: BBC-style thin-walled cabinets vs. tight bass?

The BBC wasn't trying to add to the speaker's sound. Their position was that a completely rigid cabinet would still reach resonance and ring at some audible level/frequency while a lossy cabinet would push "unwanted resonances downwards in amplitude and frequency so that they are adequately buried below the music and then pushed down in pitch"
The quotation is from Alan Shaw of Harbeth.

It seems to me that the BBC / Harbeth objective is the same as the "rigid cabinet" school. They just (seem) to believe that the lossy construction is a better approach to the goal.

That statement doesn't line up with Harwood's papers, nor does it line up with measurements of Alan Shaw's enclosures. See the current issue of Stereophile for an example. I will say this though, their crossover appears to be well implemented.

[Bob Brines]

Re: BBC-style thin-walled cabinets vs. tight bass?

I have been using 12mm Russian birch plywood excusively for the last couple of years. I usually double the baffle, or at least the area around the driver so there is plenty of depth for the driver rabbet. I use longitudinal stringers to stiffen the panels. Note that 12mm birch core plywood is stiffer than 3/4" MDF that meny DIY'ers use.

Bob

[carlspeak]

Re: BBC-style thin-walled cabinets vs. tight bass?

Exploiting cabinet resonance vs a cement monkey coffin debate rages on. Audio Note in the UK and Devore here in the States and probably others are offering high end speakers that, I'm sure, they have 'tuned' to create a symbiotic 'sum of the parts' that benefits their particular designs.

I may like Coke. You may prefer Pepsi.....