Turned out I had just about enough birch ply to throw a prototype together. Unfortunately, I don't cut particularly straight lines with a handsaw, and my circular saw is no better - just faster. It'd be nice to have a tablesaw, but I don't make enough speakers to really warrant buying one.
Anyway, the glue is drying at the mo. I'll add some sealant tomorrow, and then should be making sound early next week.
Chris
Anyway, the glue is drying at the mo. I'll add some sealant tomorrow, and then should be making sound early next week.
Chris
Haha no worries. Wood glue and acrylic caulk is your friend. At least mine is. 😂
No cone, be it loudspeaker or passive radiator, is ever going to sound any good at unfeasibly large excursions; full stop. If power density is your only goal, sound quality will suffer - whether the trade-off is worthwhile is in your hands.
Don't forget that rapid transient decay is probably more important - and certainly harder to achieve - than transient attack, and is most important to high quality LF reproduction. Think about that when a signal stops and there is 0.6kg of passive of radiator flapping around at 40mm total excursion...
(If your target audience is a room of stoned techno heads, please ignore all the above).
Have a look at track saws. They are cheaper and smaller than table saws and super accurate. I have a tablesaw and a Makita track saw, and end up using the track saw more often. They are safer and have better dust control than the table saw.
It's not that bad. A lot of bass by nature does not have fast decay. If it does, it is higher frequency, therefore the cone is moving less already, and finally, a lot of sounds have no real reference in your brain. If it sounds differently, you have no idea, it's just some bass.
Of course there are extreme cases, and this is not denial of hifi and accuracy, but it is not THAT bad. Also try to sim decay and Gd of a closed system and BR system. At the same SPL at 40Hz, the difference can be as low as negligible. Only true solution is servodrive.
MrK,
If it's of serious interest to you, I'll see about capturing some waveforms of my sealed 18"s and this 12" PR, so we can see the difference between the two. Is there a particular test signal you'd like me to use?
NB - I'd EQ each cabinet to have a very similar frequency response, so that we're only looking at the difference in reproduction between those two systems.
FWIW, in the PA world, output density is very important, and this is an exercise in seeing how far it can be taken. If it sounds bad or doesn't work for me, it'll never be heard by an audience.
Chris
I can do that if you like, but a step response has a lot of HF components which will be filtered by the subwoofer crossover.
Attached is a quick experiment in Audacity. I generated a 1Hz square wave (top waveform) and then applied a 100Hz 24dB/oct low-pass and a 34Hz 24dB/oct high-pass, resulting in the bottom waveform.
The bottom waveform, then, is the signal that would come out of the amplifier once those filters are in place. Are you sure a step response is the correct test signal?
Chris
Attached is a quick experiment in Audacity. I generated a 1Hz square wave (top waveform) and then applied a 100Hz 24dB/oct low-pass and a 34Hz 24dB/oct high-pass, resulting in the bottom waveform.
The bottom waveform, then, is the signal that would come out of the amplifier once those filters are in place. Are you sure a step response is the correct test signal?
Chris
Attachments
I second you as I also have Makita track saw. Does crazy accurate cuts, easy to get cut's exactly the same repeatedly, accuracy depending solely on how you measured. Easily sub millimeter, so that cant even feel the difference with finger if two pieces held side by side. Handy to store and take out to use. Big sheets of ply are no problem, put some xps foam on the ground for quick flat surface, ply on top and start rippin'. Very good blade as well, very clean cuts even after years of use. Good dust control, can cut speaker panels in your livingroom if you have good vacuum with it.
I 'third' a track saw. It's my shop MVP..
I too use my table saw much less, usually only for small multiple pieces, or bevel cuts beyond 45 degrees.
On big subs, even on MEH horns with their compound beveled miters, cuts are accurate enough that glue is all it takes to seal them up
Three sawhorses underneath a 4x8 ft sheet of whatever cheap but sturdy ply, with a sheet of sacrificial 1" XPS (like tmuikku mentioned) on top of the ply , makes for a great cutting bed.
If buying a track saw, highly recommend looking for packages that include tracks. Makita bundles can be found with a 55" track.
If you see one at a good price (including shipping), they make a 118" track that makes working with 4x8ft, or 5x5ft, much faster and more precise than splicing two 55"s together.
Here's an impulse response given the same treatment as above. As discussed in the impulse vs swept sine debate, the former has very little LF energy since it contains all frequencies equally. ie, when it comes to an impulse response, there's as much emphasis on 10-20kHz as there is on 0Hz-10kHz. This is why I had to increase the level by 50dB(!) to make the signals visible on-screen again.
You may spot some similarities to the step response waveform posted above.
So, the waveform at the bottom is, once again, what will come out of the amplifier when it's fed with a very-high-level impulse response. Are you sure an impulse is the correct test signal?
Chris
Attachments
Hi Chris, I like to use tone bursts for that kind of test.
Like you say, an impulse is so low in magnitude due to the linear data, it makes impulse (and step) for assessing sub transients kinda not worth much.
REW's tone burst generator, a few cycles, along with its scope work great for this.
I agree with general the idea bass is best made though higher Sd, rather than higher excursion. But I don't think high excursion is necessarily sub-optimal.
In looking at busrt tests of a few sub drivers, in both sealed and ported boxes, my take is a strong motor allows higher excursion to work fine...in either box.
The strong motor provides the necessary damping it seems.
What is interesting and unknow to me, is the question of ported time dampening vs passive radiator time dampening. I do wonder how the mass of a PR can be brought to settle without a motor. Simply dunno much about PR's....
Hi Mark,
Thanks for your insight. I was hoping to help MrK (and any other impulse/step-response enthusiasts) reach their own conclusion about tone bursts. I agree they're probably the best way of assessing a subwoofer's time-domain performance: anything else requires too much bandwidth, so you only really end up measuring the crossover.
In general, I'd always rather use larger cones moving less. Increased excursion generally means increased distortion (motor effects, suspension nonlinearities, etc etc), so reducing excursion should result in better sound.
In this case, I can squeeze 120dB@40Hz performance out of a cabinet half the size of my current cabinets which achieve that. ie, if I was to take the same total cabinet volume, I'd either be running the new subs with more headroom, or be able to hit 126dB@40Hz.
As you'll know, +6dB of output capability in the same space is a Big Deal in the PA world.
When it comes to time-domain performance of PRs vs ported boxes:
Intuitively, I see your point: PRs have a large moving mass compared to the air in a port.
What's omitted is this: PRs also have more damping.
I have another 12" in a ported box tuned to 40Hz. Different driver and chamber volume, though. Once this 12" PR box is making noise, I'll compare the two.
Chris
Thanks for your insight. I was hoping to help MrK (and any other impulse/step-response enthusiasts) reach their own conclusion about tone bursts. I agree they're probably the best way of assessing a subwoofer's time-domain performance: anything else requires too much bandwidth, so you only really end up measuring the crossover.
In general, I'd always rather use larger cones moving less. Increased excursion generally means increased distortion (motor effects, suspension nonlinearities, etc etc), so reducing excursion should result in better sound.
In this case, I can squeeze 120dB@40Hz performance out of a cabinet half the size of my current cabinets which achieve that. ie, if I was to take the same total cabinet volume, I'd either be running the new subs with more headroom, or be able to hit 126dB@40Hz.
As you'll know, +6dB of output capability in the same space is a Big Deal in the PA world.
When it comes to time-domain performance of PRs vs ported boxes:
Intuitively, I see your point: PRs have a large moving mass compared to the air in a port.
What's omitted is this: PRs also have more damping.
I have another 12" in a ported box tuned to 40Hz. Different driver and chamber volume, though. Once this 12" PR box is making noise, I'll compare the two.
Chris
The PR moving mass is the same as the air in a port of equal effective diameter (Sd) tuned to the same Fb.
STV used two sine burst signals at Fb, one with short fade-in and fade out cycles and one with hard start and stop. You might compared the port (BR) response to a PR using a similar test.
https://www.diyaudio.com/community/...ers-out-of-fashion.412687/page-8#post-7684685
That said, you will likely be comparing a port with far less moving mass than the PR.
It would (will) be interesting to see the increased excursion of the driver during the first cycle using the BR compared to the PR.
Art
Art,
Are you sure the PR mass should be equal to the air in an equivalent port?
I ran some quick numbers, and while Helmholtz maths seems to fall apart at a 1000cm^2 port area on a 28L box, Hornresp suggests a 172cm long port (ie, 172L of volume) to get a 40Hz tuning frequency. Air is about 1.23g/L, so that's 212g of air. Perhaps we're too far from Helmholtz and just looking at quarter-wave behaviour?
The PRs have Mms = 300g, and each will need another ~220g adding to get the 40Hz tuning. ie, the total PR mass will be a touch over 1kg.
If I took a pair of mics and placed them close to the driver and PR, I should be able to capture similar waveforms.
More tricky on the ported box: the port exit is right next to the driver's frame. Perhaps a fig-8 mic..?
Chris
Are you sure the PR mass should be equal to the air in an equivalent port?
I ran some quick numbers, and while Helmholtz maths seems to fall apart at a 1000cm^2 port area on a 28L box, Hornresp suggests a 172cm long port (ie, 172L of volume) to get a 40Hz tuning frequency. Air is about 1.23g/L, so that's 212g of air. Perhaps we're too far from Helmholtz and just looking at quarter-wave behaviour?
The PRs have Mms = 300g, and each will need another ~220g adding to get the 40Hz tuning. ie, the total PR mass will be a touch over 1kg.
If I took a pair of mics and placed them close to the driver and PR, I should be able to capture similar waveforms.
More tricky on the ported box: the port exit is right next to the driver's frame. Perhaps a fig-8 mic..?
Chris
I think your conjecture that Hornresp goes to quarter-wave behaviour is correct, it requires a 54.2L box before calculating a Helmholtz frequency with an AP of 1000 and Lpt of 172.
Various on-line calculators suggest much longer ports.
My statement was based on the guideline attributed to Tom Danley and Deon Bearden:
https://www.diysubwoofers.org/prd/
Example:
Driver: Vas: 2 cu.ft. Qts: 0.30 Fs: 30 Hz Diameter: 8 in.
Ported Alignment (QB3): Vb = 0.70 cu.ft. Fb = 39.4 Hz
Now, we need to select an appropriately-sized passive radiator. ALWAYS use a passive radiator that is larger in diameter than the active driver, as the displacement of the passive radiator usually has to be 1.5 to 2 times that of the driver. If it's not possible to use one large passive radiator, then you can use two or more smaller ones, and tune them by working out the effective diameter from the combined surface area of the radiators.
Note that the effective diameter of the radiator is approximately equivalent to the diameter of the passive radiator's face plus 1/3 of the surround. If unsure, use the quoted Sd for that radiator, then use the following equation to determine the effective radius:
R = (Sd/PI)^0.5
In this case, we choose to use a passive radiator that has an effective radius of 5 inches (roughly corresponding to a "12-inch" passive radiator).
"Port" Radius = 5 in.
Required Port Length = 186.1 in.
"Port" Volume = (PI*R^2)*h
= (3.14 *5^2)*186.1
= 14609 cu.in. = 8.45 cu.ft. = 0.2393 m^3
Mass = "Port" Volume * Density of Air
= 0.2393 * 1.21
= 0.289553 kg
= 290 g
The passive radiator therefore has to have a weight of 290g. To achieve this, start with a passive radiator with lower mass, then add weight to make up the difference. To measure the resonance frequency of the passive radiator, install it in a free-air baffle (e.g. the box it's going in, without the driver in place), then hold a driver, driven by a sine wave generator, as close as possible to the passive radiator, then vary the frequency. At the passive radiator's resonance frequency, you should see the greatest peak to peak excursion of the passive radiator.
Appears you use a slightly different value for the weight of air, but that shouldn't amount to much.
I've never verified the above, and it does not appear to account for the PR suspension.
What blades you use?
I read the blade is 80% of the equation
You can have a top of the line saw and if the blade is crappy your cuts are splintered or just bad.