Hi Folks,
Loose panel? Loose brace? Cone slapping the baffle? Something else entirely? Help!
I recently roughed together a front loaded horn with a rear reflex chamber for a 10 inch Faital woofer I had. This is the second iteration of a design.
Intended use was bass guitar low end (up to 500hz), squeezing as much spl as possible from the woofer I have on hand.
Giving it a test run this afternoon, I was very impressed with the volume, however the longer (and louder) I played, the more I became aware of a horrible raspy/rattling sound.
The sound is much more prevalent at certain frequencies (I initially noticed it at low D on a 5 string bass - ~73hz on the second harmonic where most of its energy is).
I unplugged the bass and hooked up a signal generator to do some more investigating. The noise is occurs most easily at around 115hz, but needs to be fairly loud for it to occur. Once it does, the louder the output, the louder the rattle.
Thinking that maybe the surround of the woofer was slapping against the baffle, I thought I would check the excursion plot in Hornresp. I suspect that this may be part of the answer to my mystery of what has gone wrong, but the plot shows the lowest excursion at around 73hz and 115hz. The diaphragm pressure plot shows peaks at these frequencies too.
Here's a cutaway of the construction, just before I screwed on the final side to complete the box. I used an expanding polyurethane glue and all panels were screwed.
So my question to the brainstrust is what do I do to fix this? I have considered removing the screws from one side and cutting it off with the table saw to expose the inner workings again. I should be able to do it neatly enough that ii can screw another side on without too much hassle.
The raspy rattle seems to sound loudest out of the reflex port, but is also louder on the left hand side than it is on the right hand side. Troubleshooting precisely where the noise is coming from is difficult, because to make it happen, I have to feed the woofer a fair amount of voltage, annoying my wife considerably
If I cut it open, what am I looking for? Is it more likely to be a loose brace than a loose panel? Every panel has two braces on each side, with the braces on different sides offset from each other
I initially thought that the surround or cone of the woofer may be slapping the baffle, but the excursion plot suggests otherwise, and the woofer would need to move well past xlim to get there.
Help would be very much appreciated
Hornresp below
Frequency response for the two iterations of the design below. Plenty of ripple in the response, which is not a problem for this application
Loose panel? Loose brace? Cone slapping the baffle? Something else entirely? Help!
I recently roughed together a front loaded horn with a rear reflex chamber for a 10 inch Faital woofer I had. This is the second iteration of a design.
Intended use was bass guitar low end (up to 500hz), squeezing as much spl as possible from the woofer I have on hand.
Giving it a test run this afternoon, I was very impressed with the volume, however the longer (and louder) I played, the more I became aware of a horrible raspy/rattling sound.
The sound is much more prevalent at certain frequencies (I initially noticed it at low D on a 5 string bass - ~73hz on the second harmonic where most of its energy is).
I unplugged the bass and hooked up a signal generator to do some more investigating. The noise is occurs most easily at around 115hz, but needs to be fairly loud for it to occur. Once it does, the louder the output, the louder the rattle.
Thinking that maybe the surround of the woofer was slapping against the baffle, I thought I would check the excursion plot in Hornresp. I suspect that this may be part of the answer to my mystery of what has gone wrong, but the plot shows the lowest excursion at around 73hz and 115hz. The diaphragm pressure plot shows peaks at these frequencies too.
Here's a cutaway of the construction, just before I screwed on the final side to complete the box. I used an expanding polyurethane glue and all panels were screwed.
So my question to the brainstrust is what do I do to fix this? I have considered removing the screws from one side and cutting it off with the table saw to expose the inner workings again. I should be able to do it neatly enough that ii can screw another side on without too much hassle.
The raspy rattle seems to sound loudest out of the reflex port, but is also louder on the left hand side than it is on the right hand side. Troubleshooting precisely where the noise is coming from is difficult, because to make it happen, I have to feed the woofer a fair amount of voltage, annoying my wife considerably
If I cut it open, what am I looking for? Is it more likely to be a loose brace than a loose panel? Every panel has two braces on each side, with the braces on different sides offset from each other
I initially thought that the surround or cone of the woofer may be slapping the baffle, but the excursion plot suggests otherwise, and the woofer would need to move well past xlim to get there.
Help would be very much appreciated
Hornresp below
Frequency response for the two iterations of the design below. Plenty of ripple in the response, which is not a problem for this application
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Missing from the first post - up to about 25 volts input, there isn't a hint of the vibrating noise. Go a few volts louder and it appears like magic.
Ok, the results of some more tinkering - the output from the horn mouth doesn't appear to have the raspy sound.
The cone is relatively light for the asymmetrical high compression ratio at higher power.
The "raspy sound" may be harmonics generated from cone flex at the high horn loading impedance/excursion minima, high pressure maxima frequencies. Higher odd order harmonics would probably sound more "raspy" than even order.
Look at the RTA while playing tones, if that is the case you should see multiples of the offending frequencies.
Other possibilities include a warped baffle, making the coil drag when heavily loaded.
Coil drag noise (or buzzing cables) would be less harmonically related.
The "raspy sound" may be harmonics generated from cone flex at the high horn loading impedance/excursion minima, high pressure maxima frequencies. Higher odd order harmonics would probably sound more "raspy" than even order.
Look at the RTA while playing tones, if that is the case you should see multiples of the offending frequencies.
Other possibilities include a warped baffle, making the coil drag when heavily loaded.
Coil drag noise (or buzzing cables) would be less harmonically related.
Sometimes a clipping amplifier can produce a raspy crackling sound. Maybe try swapping a different amp in to eliminate the one you are using ?
Rob.
Rob.
Could be caused by something to do with the driver's tinsel leads. That would explain why you're not hearing it through the horn.
Thanks for the replies, folks. With the access cover off, I put a small dab of white paint on the cone so I could monitor its movement. Sure enough, the raspy rattle/buzz only appears frequencies where the speaker is barely moving (as in about 3mm). when unloaded the speaker will happily move nearly an inch without making any unpleasant noises. The more I play with it, the louder it becomes. To give an indication of how loud it is relative to the horn output, with the speaker playing a 100hz tone in my shed, I can move to my house 10m away and walk through 3 doors and still hear the rattling sound over the fairly loud 100hz tone.
@weltersys Unfortunately I had already started surgery on the box prior to your post, so seeing if there were harmonics involved is no impossible, however my gut feeling is that is is a rattling panel. Would the same rationale about asymmetric loading causing issues at pressure maxima also apply to loose panels?
On the first iteration of the design, I had a similar noise when playing initially, which went away after leaving the box for a few days (I assumed this let glue set)
I have to confess that I didn't wait very long at all with this box (3hrs) before feeding it several hundred watts.
@weltersys Unfortunately I had already started surgery on the box prior to your post, so seeing if there were harmonics involved is no impossible, however my gut feeling is that is is a rattling panel. Would the same rationale about asymmetric loading causing issues at pressure maxima also apply to loose panels?
On the first iteration of the design, I had a similar noise when playing initially, which went away after leaving the box for a few days (I assumed this let glue set)
I have to confess that I didn't wait very long at all with this box (3hrs) before feeding it several hundred watts.
Panels or braces rattling rather than the cone at the high pressure frequencies is certainly possible. Could be loose plys around knots, internal voids, or loose joints.
Since the raspy rattle seems to sound loudest out of the reflex port, and is also louder on the left hand side, might be the passive crossover slapping- you should be able to reach in and dampen it by hand and find if it is the problem.
Since the raspy rattle seems to sound loudest out of the reflex port, and is also louder on the left hand side, might be the passive crossover slapping- you should be able to reach in and dampen it by hand and find if it is the problem.
I did some surgery this afternoon
Most panels required a lot of persuasion to come off, a few not so much.
The rear brace wasn't screwed to the back of the chamber and had stuff all glue on it. Could this be the culprit? It would make sense about he noise coming from the chamber, but if it was this brace, it probably should have come from the cone, too.
HOWEVER... I also discovered that I had messed up my calculations, and it was entirely possible for the surround to smash the baffle when fed a lot of power. I am certain this is not the cause of my noise, but a revealing discovery, all the same.
The cone does not seem overly rigid, but then again, I didn’t check it prior to putting it in a box
I think @weltersys probably hit the nail on the head and the noise was from the asymmetric loading and high compression ratio. Whether I have caused permanent damage to the woofer is unknown. I ran it through DATS to look for any anomalies against the published data, and couldn't find anything significant, but they are small signal parameters, after all.
I have a revised design on my computer that has a much gentler 1.5:1 compression ratio (current design is around 4.5:1), and still accomplishes my bandwidth and efficiency goals. I guess I need to decide whether to cut my losses on my might be a damaged woofer, or invest in another sheet of ply to give it another try.
Any tips?
Most panels required a lot of persuasion to come off, a few not so much.
The rear brace wasn't screwed to the back of the chamber and had stuff all glue on it. Could this be the culprit? It would make sense about he noise coming from the chamber, but if it was this brace, it probably should have come from the cone, too.
HOWEVER... I also discovered that I had messed up my calculations, and it was entirely possible for the surround to smash the baffle when fed a lot of power. I am certain this is not the cause of my noise, but a revealing discovery, all the same.
The cone does not seem overly rigid, but then again, I didn’t check it prior to putting it in a box
I think @weltersys probably hit the nail on the head and the noise was from the asymmetric loading and high compression ratio. Whether I have caused permanent damage to the woofer is unknown. I ran it through DATS to look for any anomalies against the published data, and couldn't find anything significant, but they are small signal parameters, after all.
I have a revised design on my computer that has a much gentler 1.5:1 compression ratio (current design is around 4.5:1), and still accomplishes my bandwidth and efficiency goals. I guess I need to decide whether to cut my losses on my might be a damaged woofer, or invest in another sheet of ply to give it another try.
Any tips?
Did you calculated your compression ratio? (SD / Throat Adapter area (Ap1) :::: the square hole)
Once diaphragm displacement show you minimal displacement at related frequencies you could check diaphragm pressure at the max voltage. Probably the pressure present a peak when the displacement is minimal. In a simplified way, If the diaphragm is not moving with voltage applied it means that something is holding it in that position so the force at the cone is max and equals to driver motor force the equilibrium means no motion but the cone stress will be max.
It worth to compare your driver diaphragm pressure at your Horn Design vs Bass reflex design.
To check diaphragm pressure, go to diaphragm displacement chart -> tools -> diaphragm pressure
If your Horn Design present much bigger pressure, you probably exaggerated CR leading to cone over stress and deformations.
High compression ratio applications requires reinforced cones.
Attachments
G'day @LORDSANSUI
The throat adapter has the same cross section as the horn at that point. Looking at peak diaphragm pressure as you've suggested is quite revealing. I'm about double what I am for most other designs I have in Hornresp
55hz is the helholtz frequency
And here's the pressure with just the bass reflex section
Interestingly, the revised design (hornresp record attached) doesn't show a huge reduction is pressure, despite the much lower compression ratio
The throat adapter has the same cross section as the horn at that point. Looking at peak diaphragm pressure as you've suggested is quite revealing. I'm about double what I am for most other designs I have in Hornresp
55hz is the helholtz frequency
And here's the pressure with just the bass reflex section
Interestingly, the revised design (hornresp record attached) doesn't show a huge reduction is pressure, despite the much lower compression ratio
Double pressure and more peaks in the passband.
While for throat adapter you could increase the hole, once you set the Horn S2 equals to throat adapter area you probably can't lower the pressure anymore so if this is what is causing your noise you may need a different strategy like limiting the voltage or trying to find a way to reinforce the cone, there might be some resin available.
Thanks @LORDSANSUI I had figured as much.
As for stiffening the cone…
I’m sure I will cop some flak from the people on the know here, but I wicked about 15 grams of super thin CA glue into the back of the cone from the surround to about an inch towards the centre. The before and after measurements in DATS show a very small increase in mmd and about 1db of calculated efficiency loss. The cone is considerably stiffer now.
I will still put the driver in a redesigned box as I still can’t be certain that the noise in question came from a loose panel or brace, or from the cone.
I’ll report back one the driver is in a new box. I’ll either be delighted that it works, or disappointed that the raspy sound came from the driver and I wasted another sheet of ply.
Wish me luck!
As for stiffening the cone…
I’m sure I will cop some flak from the people on the know here, but I wicked about 15 grams of super thin CA glue into the back of the cone from the surround to about an inch towards the centre. The before and after measurements in DATS show a very small increase in mmd and about 1db of calculated efficiency loss. The cone is considerably stiffer now.
I will still put the driver in a redesigned box as I still can’t be certain that the noise in question came from a loose panel or brace, or from the cone.
I’ll report back one the driver is in a new box. I’ll either be delighted that it works, or disappointed that the raspy sound came from the driver and I wasted another sheet of ply.
Wish me luck!
Good luck but considering the past, you have tons of ply and so many different designs already built that besides facing issues looking for the best sound ever it's also a fun journey 🙂
A friend and I constructed revision 3 of this box yesterday, with a much bigger throat and a better thought out throat adapter.
Measured response is very close to the simulation.
Measured response is very close to the simulation.
The combination of the reinforced cone and lower compression ratio has resulted in a cabinet that will happily take 500w+ without complaint. It’s not really a subwoofer, but there’s no subforum for bass guitar speakers. A sealed back 8 inch midrange is yet to be fitted. Even without the midrange, I’m very happy with the tone from a number of my bass amps.
Great job, with 8" midrange and if you set a titanium drive you will get a full range system at the end 😀
Selenium D220 1" is cheap and vary flat.
Do you mind to share the group delay simulation and your feeling when you play notes close to the vent tuning frequency. They are OK or do you feel delay?
Selenium D220 1" is cheap and vary flat.
Do you mind to share the group delay simulation and your feeling when you play notes close to the vent tuning frequency. They are OK or do you feel delay?
@LORDSANSUI I have a few compression drivers on hand. I hadn't considered installing on in this box, but there's room, so I may as well.
Here's the measured group delay. The simulation showed it to be closer to 60ms as the port frequency. I feel no delay to speak of, however that could be in part due to the fact that a bass guitar signal is so rich n harmonics that even when playing a note with a fundamental at the tuning frequency, there's just as much signal one and two octaves above. Not sure.
Here's the measured group delay. The simulation showed it to be closer to 60ms as the port frequency. I feel no delay to speak of, however that could be in part due to the fact that a bass guitar signal is so rich n harmonics that even when playing a note with a fundamental at the tuning frequency, there's just as much signal one and two octaves above. Not sure.