A Superior / Musical Bass: Does it exist? If so, is it free? How do we recognize it? How do we achieve it?

[Tim Link]

Tim
I am impressed with your detective work !
I recall you measured at the mouth of horns and there was no dip so it's getting into the space
If there is no bass in adjacent spaces then i suspect its a wall or window acting as a membrane absorber .. it would need to be a large area to have such a comprehensive effect
That is the thing you cant accurately calculate in a design as the wall systems are not all precisely the same
Of course I am making assumptions based on a small amount of knowledge so ... ???
Would it be too crazy to measure on the other side of wall or window to see if there is a 85 Hz peak

Phil

So I got busy today and tried some things out. I disassembled one of my speakers so I could get the 18" woofer out of it and installed it in the prototype bass cabinet, which I fortunately still have. I've been trying to get rid of it thinking I'd never have any use for it. It's pretty much the same as the others.

On the prototype, I drilled a hole right above the throat so I could drop the mic in and measure there, right in front of the driver. I measured on the other side of the driver in the back chamber too. Looking at the chart, the blue line is the back chamber response and the orange is the throat. It's pretty clear what's happening. The mouth termination is too small for the length horn so an inverse polarity back wave travels back to the throat and arrives just in time to be out of phase with the driver at that frequency. The driver is definitely moving as can be seen from the back chamber pressure. It's just got no impedance match on the front side so it's highly inefficient at that frequency - about 89 Hz with this horn. Down at 60Hz the opposite is happening. The back wave from the mouth is increasing pressure in coordination with the driver moving forward, increasing its impedance match and actually reducing its motion while simultaneously increasing output from the horn, so the back chamber level goes down. With this single cabinet operating by itself and not in a corner it shows the problems very clearly. Stacking the woofers together in the corner helps them out quite a bit but not enough to fix the 85 hz issue. It actually makes the Q higher so the notch is narrower and deeper. I think this is because vertically the horn mouths are big enough, but horizontally they are not.

Earlier I said there was no dip inside the horn mouth but that's not really true. There is a dip but everything is louder in there so I can clearly hear the missing frequency. I didn't realize how much louder other frequencies sounded until I stuck my head in there at those frequencies. On measuring, the dip is not as severe just inside the mouth as it is at the throat so some interesting things are happening along the length of the horn, but overall the total sound energy being produce at that frequency is much lower.

Edit - I removed a screen so I could put the microphone at various distances along the inside of the horn. There's a huge peak at 79 Hz inside the horn. It's not a huge peak in the room, but a filler between the null at 85 Hz and the smaller null at 75 Hz.


Edit: The following paragraphs are conjecture that may not be correct.

So that's why I couldn't find 85 Hz anywhere in the room. It isn't a room cancellation issue - it's an impedance matching issue actually reducing the sound power output from the horn. It goes to show that air displacement is not always proportional to sound power output. The air has to get squeezed, not just sloshed around. Drivers move much slower than the speed of sound so the air has to get trapped at least a little to make any sound at all.

I've always kidded that direct radiator woofers are air sloshers, because at 1 or 2 percent efficiency that's mostly what they are doing. They do manage to get enough squeezing in to make great bass if they are big enough and/or have enough power and excursion capacity. With my horns, at 85 Hz the joke is on me! I'm sloshing air like it's nobody's business!

I tried some remedies - stuffing the back chamber with poly fill, stuffing the throat with poly fill, putting a cushion in the mouth. Poly fill did very little. The cushion in the mouth actually broadened the cutout zone. Not quite as deep but much broader. As usual, damping with absorption means reduced efficiency. One possible solution is to continue the horn flare gradually so that the mouth is much bigger. That means a much bigger horn. Another possibility is to put a V shaped midrange line array horn in front of the bass horn mouths. That would ease the pressure change and reduce the cancellation. Once again the speaker gets bigger.

Finally, I could just build faceplates to go in the mouths and put the drivers up there. Then I'd have direct radiators.

 

[pjwd]

So I got busy today and tried some things out. I disassembled one of my speakers so I could get the 18" woofer out of it and installed it in the prototype bass cabinet, which I fortunately still have. I've been trying to get rid of it thinking I'd never have any use for it. It's pretty much the same as the others.

On the prototype, I drilled a hole right above the throat so I could drop the mic in and measure there, right in front of the driver. I measured on the other side of the driver in the back chamber too. Looking at the chart, the blue line is the back chamber response and the orange is the throat. It's pretty clear what's happening. The mouth termination is too small for the length horn so an inverse polarity back wave travels back to the throat and arrives just in time to be out of phase with the driver at that frequency. The driver is definitely moving as can be seen from the back chamber pressure. It's just got no impedance match on the front side so it's highly inefficient at that frequency - about 89 Hz with this horn. Down at 60Hz the opposite is happening. The back wave from the mouth is increasing pressure in coordination with the driver moving forward, increasing its impedance match and actually reducing its motion while simultaneously increasing output from the horn, so the back chamber level goes down. With this single cabinet operating by itself and not in a corner it shows the problems very clearly. Stacking the woofers together in the corner helps them out quite a bit but not enough to fix the 85 hz issue. It actually makes the Q higher so the notch is narrower and deeper. I think this is because vertically the horn mouths are big enough, but horizontally they are not.

Earlier I said there was no dip inside the horn mouth but that's not really true. There is a dip but everything is louder in there so I can clearly hear the missing frequency. I didn't realize how much louder other frequencies sounded until I stuck my head in there at those frequencies. On measuring, the dip is not as severe just inside the mouth as it is at the throat so some interesting things are happening along the length of the horn, but overall the total sound energy being produce at that frequency is much lower.

Edit - I removed a screen so I could put the microphone at various distances along the inside of the horn. There's a huge peak at 79 Hz inside the horn. It's not a huge peak in the room, but a filler between the null at 85 Hz and the smaller null at 75 Hz.


Edit: The following paragraphs are conjecture that may not be correct.

So that's why I couldn't find 85 Hz anywhere in the room. It isn't a room cancellation issue - it's an impedance matching issue actually reducing the sound power output from the horn. It goes to show that air displacement is not always proportional to sound power output. The air has to get squeezed, not just sloshed around. Drivers move much slower than the speed of sound so the air has to get trapped at least a little to make any sound at all.

I've always kidded that direct radiator woofers are air sloshers, because at 1 or 2 percent efficiency that's mostly what they are doing. They do manage to get enough squeezing in to make great bass if they are big enough and/or have enough power and excursion capacity. With my horns, at 85 Hz the joke is on me! I'm sloshing air like it's nobody's business!

I tried some remedies - stuffing the back chamber with poly fill, stuffing the throat with poly fill, putting a cushion in the mouth. Poly fill did very little. The cushion in the mouth actually broadened the cutout zone. Not quite as deep but much broader. As usual, damping with absorption means reduced efficiency. One possible solution is to continue the horn flare gradually so that the mouth is much bigger. That means a much bigger horn. Another possibility is to put a V shaped midrange line array horn in front of the bass horn mouths. That would ease the pressure change and reduce the cancellation. Once again the speaker gets bigger.

Finally, I could just build faceplates to go in the mouths and put the drivers up there. Then I'd have direct radiators.

That's a relief in some ways as it was very strange . I dont much about horns but with transmission lines it was taper and length .. with the length ideally being 1/4 wavelength of resonant frequency .. i have built them with 3.7m long lines and the worked a treat .. can you just add test wings to your mouth to increase length

Apologies to stehno .. quite a tanget here

 

[Tim Link]

Apologies to stehno .. quite a tanget here

In the navy my nickname was Tangent Tim, but here I am going off on a tangent again.

Back to musical bass, I don't think that sharp cutout I obsessed over this weekend has a serious effect with real music, at least most of the time.

Still, there's something else going on that's not as good as I remember when the system was at my friend's house in his much larger room. He also had lath and plaster walls, which I'm told are sonically superior to typical drywall. The bass was as deep and strong and effortless as I would expect with eight 18" woofers playing in the room at once. At my new place it seems a bit constrained, I almost want to say overdamped - that said as a subjective impression not of too much clarity but of energy being stunted somehow. It's clear enough, but not majestic. It definitely lost something. Besides the room change, the amplifiers also changed. Before we were using JVC amps with hybrid digital/analog feedback. They produced some very strong, deep bass on older speakers that I hadn't heard on those speakers before. Now I'm using a Cambridge audio 6 channel receiver. It's rated at 50 watts per channel, all channels playing at once for a total of 300 watts. I would think that would be enough for a highly efficient system but I guess there's no way to know for sure if a different amp might make a difference other than to try some different amps.

There's also a possibility that we just played it louder most of the time at his place. He didn't have neighbors close enough to bother. We could crank it all day and all night.

Stehno, I'm in Eugene so not too far from where you are.

 

[Tim Link]

Have we reached a conclusion if musical bass exists? I suspect that's impossible to do given the inherent subjectivity and numerous variables of this hobby.

I know something exists for me that I think of as musical bass. How much of a consensus there is among us about what that is I don't know for sure. If the bass response is smooth and extended and lacking in excessive resonance and distortion, and capable of doing all of that as loud as the listener wants, I think most people will find it highly musically enjoyable. How important is frequency response accuracy vs time domain accuracy, vs. distortion and compression issues? Might there be times when adding distortion or uneven frequency response to the bass actually makes the music more enjoyable for most people? Are there other factors?

David Greisinger wrote about stereo bass effects extending as low as 40Hz, where the ears and brain can detect the difference between a wave traveling across the head sideways vs straight on or straight down. He had some examples to listen to on headphones and I could clearly hear the difference in phase between left and right well down into the bass. The effect was a subtle difference that to me added little meaningfully to the musical experience, as I have experienced both sensations in real acoustical spaces so they both seem realistic to me. Others might get a great deal of musical satisfaction from the effect, so following his advice of having subs on either side of you set 90 degrees out of phase to add some bass spaciousness to recordings, even those recordings that have mixed the bass to mono might be a game changer.

 

[thedudeabides]

My take on bass and what is "musical". It needs to provide a "foundation" to the music without impacting the transparency and dimensionality of the critical mid range and treble. It's all part and parcel of the "seamless" music continuum without calling attention to any particular frequency.

 

[pjwd]

lath and plaster walls, which I'm told are sonically superior to typical drywall.

As well as probably being more rigid I expect they would vary in both density and thickness resulting in a wide spread of resonant frequencies whereas drywall woul be very uniform ( esp if framing was regular)
Also if drywall sheeting is same on both side of wall and there is no fluff in the cavity you get both planes vibrating to give a significant effect
( called coincident dip in the lingo)
Using a couple of layers of the high density drywall with green glue etc helps but not as good as solid masonry in my view