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

[carlspeak]

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

Here is a cabinet resonance story I recently experienced. Two Human Speakers 005 woofers were purchased and installed in a pair of Fourier 8 cabinets. The cabinets are quite robust and braced front to back and side to side.

WT2 tests on the woofers produced very smooth imp./phase curves. Many woofers of other brands I've tested in the 8 to 12 inch range typically have a notch that occurs in the curves around 1 kHz which shows the woofers elect. response to cone edge issues manifesting themselves; even at the low voltage testing normal with WT2.



With the woofer fitted into the Fourier 8 cabinet with the port plugged to convert the cabinet to acoustic suspension, another WT2 test was done (see below). Here we see an impedance & phase spike around 150 hz. It's a similar kind of thing Atkinson finds on many of his speaker tests, including the Harbeth and others. The spike clearly was not present in the free air test shown above. I connected a function generator and found I could feel a resonance appear around 150 hz. and dissipate when freq. was increased.



I wondered what sonic effect the clearly evident resonance could have on the woofer's response so I ran a near field response test (see below). No crossover components were involved with this test. The response is quite impressive and devoid of a ripple around 150 hz. and with a complete lack of cone edge breakup mode spikes above 1 kHz.



I concluded as Atkinson has in many of his reviews that although cabinet resonances can show up when an impedance test is run, most are not to the detriment of the overall speaker's sound.

[fastbike1]

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

Thanks for the data Carl. Seems like (at least) one good example that we can measure things that don't always impact the final goal -> good sound.

[timw]

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

From John Atkinson of Stereophile Magazine, Measuring Loudspeakers, Part 2, Page 7:

"However, looking at the behavior of the 300 or so loudspeakers that I have measured, several common factors emerge from the auditioning that correlate with the presence of strong cabinet resonances between 100Hz and 500Hz. (Remember that other "objective" factors will also contribute to the same subjective perceptions.) The clarity in the lower midrange can be disappointing. Tenor instruments like cello or trombone lack clarity or acquire a "woody" character. The bass can sound muddy, diffuse, one-note, blurred, or lacking in power, rather than tight, articulate, and extended, as it does in real life. Music can seem to drag, in rhythmic terms. Male voices can "boom" and female voices "hoot" at some frequencies and not others, with the result that the little inflections of tone that are characteristic of real voices become diluted. Centrally placed images, particularly of voices, can smear toward the speaker positions at some frequencies."

http://www.stereophile.com/content/m...art-two-page-6 Measuring Loudspeakers, Part Two Page 6

[carlspeak]

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

From John Atkinson of Stereophile Magazine, Measuring Loudspeakers, Part 2, Page 7:

"However, looking at the behavior of the 300 or so loudspeakers that I have measured, several common factors emerge from the auditioning that correlate with the presence of strong cabinet resonances between 100Hz and 500Hz. (Remember that other "objective" factors will also contribute to the same subjective perceptions.) The clarity in the lower midrange can be disappointing. Tenor instruments like cello or trombone lack clarity or acquire a "woody" character. The bass can sound muddy, diffuse, one-note, blurred, or lacking in power, rather than tight, articulate, and extended, as it does in real life. Music can seem to drag, in rhythmic terms. Male voices can "boom" and female voices "hoot" at some frequencies and not others, with the result that the little inflections of tone that are characteristic of real voices become diluted. Centrally placed images, particularly of voices, can smear toward the speaker positions at some frequencies."

http://www.stereophile.com/content/m...art-two-page-6 Measuring Loudspeakers, Part Two Page 6

The link doesn't seem to work.
One can only wonder what those other 'objective factors' are?

[timw]

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

Here's another attempt at the link.

http://www.stereophile.com/content/m...art-two-page-6

[carlspeak]

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

Here's another attempt at the link.

http://www.stereophile.com/content/m...art-two-page-6

That one worked! Thanks.

[fpitas]

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

To further muddy the waters, most DIY efforts have extensive bracing compared to commercial offerings. That will push any resonances up in frequency.

[wolfgang]

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

The BBC approach was a pragmatic solution to what was required to push cabinet vibration just below the threshold of audibility for a modest cost and weight. It made use of the fact that cabinet resonances are less easy to perceive at lower frequencies and so placed the largest first few modes relatively low in frequency. It is not an approach intended to produce a "high performance" cabinet but one that is just good enough to do the job.

To make a significantly more inert cabinet than a "BBC cabinet" following a different approach is not particularly difficult but is very likely to involve more cost and more weight. Simply doubling the thickness of the walls is usually not a good idea because it shifts the first few resonant modes higher in frequency but usually without significantly changing the level and so they become easier to perceive. To be equally loud at higher frequencies the wall displacement needs to be smaller (e.g. equally loud tweeter vs woofer) so the increased stiffness reducing the wall displacement is not necessarily going to help.

That makes sense. So how would you improve upon the BBC design, i.e., to make it significantly more inert without shifting the resonance modes higher in frequency? Constrained layer damping? More flexible mass like vinyl?

[Jeff B.]

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

The BBC approach was a pragmatic solution to what was required to push cabinet vibration just below the threshold of audibility for a modest cost and weight. It made use of the fact that cabinet resonances are less easy to perceive at lower frequencies and so placed the largest first few modes relatively low in frequency. It is not an approach intended to produce a "high performance" cabinet but one that is just good enough to do the job.

To make a significantly more inert cabinet than a "BBC cabinet" following a different approach is not particularly difficult but is very likely to involve more cost and more weight. Simply doubling the thickness of the walls is usually not a good idea because it shifts the first few resonant modes higher in frequency but usually without significantly changing the level and so they become easier to perceive. To be equally loud at higher frequencies the wall displacement needs to be smaller (e.g. equally loud tweeter vs woofer) so the increased stiffness reducing the wall displacement is not necessarily going to help.

I've read the BBC papers on nearly all of these designs and I agree with your statement here. The BBC had multiple goals in these speaker designs that were very application specific. Often the goal was light weight and low cost, and in some cases they did intentionally use cabinet resonance to add weight to the vocal reproduction of small speakers. But when they did, they spent a lot of time tuning it to sound just right too. In fact, in the LS3/5a spec they specify Beech battens and said no other wood sounded right to them. This kind of tweaking is not the easiest thing to do.

[carlspeak]

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

........ they spent a lot of time tuning it to sound just right too. In fact, in the LS3/5a spec they specify Beech battens and said no other wood sounded right to them. This kind of tweaking is not the easiest thing to do.

From a prior post in this thread.... " I'm sure, they have 'tuned' to create a symbiotic 'sum of the parts' that benefits their particular designs."

[andy19191]

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

That makes sense. So how would you improve upon the BBC design, i.e., to make it significantly more inert without shifting the resonance modes higher in frequency? Constrained layer damping? More flexible mass like vinyl?

Where to place the resonances depends on what you are doing. Having the first few resonances at a low frequency is not always what you want. For example, if you have a separate woofer cabinet that is isolated from the midrange cabinet the best approach is almost certainly to go for a moderate weight but stiff cabinet that pushes the lowest resonance high enough in frequency to be above the passband of the woofer. Such a cabinet will need almost no damping (the difficult bit) and drive no resonances.

A normal wooden enclosure for the midrange is unlikely to be able to push the first few resonances above the passband of the midrange. It's small size will also mean the first few resonance are going to be higher in frequency than an equivalent large cabinet that includes the woofer. It will normally require effective damping. But the most effective way to reduce the motion of the midrange cabinet is likely to be by isolating the midrange driver itself from the midrange cabinet.

[wolfgang]

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

I agree with your statement completely regarding the low frequency cabinet.

As for the "midrange cabinet," I need to be a little more specific in my design choice. What I want to do is to build a pair of 3-way speakers that are tri-amped and actively crossed-over at line level with MiniDSPs; however, I want to use the same 6.5" woofer for both the lower and middle frequencies, crossed-over around 275Hz. (The 2nd Xover will be around 3khz).

The LF cabinet can be, as you say, stiff with higher resonance modes because it will be low-passed around 275hz. The HF cabinet will be exactly the same size as the LF cabinet in order to obtain equal Qs and group delays on both drivers, but it will handle the 275hz to 3khz range. This is the cabinet that I want to push down in resonance (ideally to well below 275hz), which is what got me interested in basing my design for that cabinet upon BBC/Harbeth in the first place. I can't remember exactly, but I think Harbeth's have their first mode somewhere just below 100Hz.

So my uncertainty is, using the driver linked above, placed in a sealed cabinet (0.61 cu ft, Q=.707), playing only midrange frequencies, is the lossy, thin-walled, lowest-possible-resonance-modes design adequate for achieving best midrange detail?

P.S. I arrived at this design mainly due to aesthics, space considerations regarding the overall width of the cabinet, a desire for near-equal power handling between the bottom-two amp channels (a lot music, on average, has it's energy evenly split right around 275hz), and a desire to keep crossovers outside of the 300hz to 3khz range.

[andy19191]

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

So my uncertainty is, using the driver linked above, placed in a sealed cabinet (0.61 cu ft, Q=.707), playing only midrange frequencies, is the lossy, thin-walled, lowest-possible-resonance-modes design adequate for achieving best midrange detail?

Using a driver designed for the job of covering the midrange would seem to me the obvious way to get the best midrange detail.

If you are going to high pass the largish midwoofer covering the midrange the size and Q of the box is going to be irrelevant. This would not be the case if you were to run both midwoofers to cover the low frequencies which would seem the obvious thing to do.

Will a "BBC cabinet" be adequate for the midrange cabinet? Probably if you get it right but the motion of the cabinet could be reduced further by using a more effective form of damping, decoupling the driver from the cabinet, etc... Having the first resonances at a low frequency is not in itself a recipe for a good cabinet.

[wolfgang]

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

Thanks for the feedback. After thinking about it for a long while and comparing various drivers to each other I've switched over to a more traditional 3-way design using a 4" mid.

If you are going to high pass the largish midwoofer covering the midrange the size and Q of the box is going to be irrelevant.

I've started to realize that and I recently read something in the loudspeaker design cookbook about Q becoming irrelevant as the driver moves out of its piston range, but I still don't fully understand what that means or how to calculate the point at which Q becomes irrelevant base on the driver's T/S parameters.

, decoupling the driver from the cabinet, etc...

Does that just mean using a gasket?

[andy19191]

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

I've started to realize that and I recently read something in the loudspeaker design cookbook about Q becoming irrelevant as the driver moves out of its piston range, but I still don't fully understand what that means or how to calculate the point at which Q becomes irrelevant base on the driver's T/S parameters.

The Q is to do with the extension of the low frequency response by using the driver/box resonance. If you high pass the midrange at 300 Hz the details of what the driver is doing below 100 Hz becomes of little importance.

Does that just mean using a gasket?

A thick gasket might work if it had the correct stiffness. Rubber grommets are probably to be preferred. Here is a bit of discussion on the topic.