´I have remade the ports for my DIY subwoofer with a 15TBW100 driver. The ports were previously undersized (two round 103mm ports, 164mm² port area), the tuning frequency was about 32.5 Hz with a net internal volume of 100 liters (~3.5 cu ft). The undersized ports had a 14mm radius flare on both ends.
The new rectangular ports have a total 296mm² area and a 55mm flare radius on both ends, but they sit directly against the bottom and walls, so only 2 of 4 sides of the ports are flared. The tuning frequency increased to 38.5 Hz, and the net internal volume is now 81L, due to the larger ports.
I've made frequency response measurements of the subwoofer at different power levels before and after the changes using UMIK-1 (6db gain) and REW, and put together some port compression data in a spreadsheet.
All measurements are ground plane 2m. Velocity in m/s and compression in db.
The spreadsheet shows input voltage, WinISD simulated port velocity and measured compression compared to the 5.66V sweep for Fb - 2.5 Hz, Fb, and Fb + 2.5 Hz. It also shows compression at 130 Hz (low impedance) in order to get some indication of possible power compression, depending on sweep level.
Some things I found interesting:
Port compression mostly affects frequencies below Fb. The undersized ports had only 0.7 db port compression at 77,9V and 35Hz, despite a (very high) simulated port velocity of 36,3 m/s.
Power compression at 130hz was considerable, although later in the sweep (more time for the voice coilt to heat up), it means that power compression likley affects the high power sweeps. For instance, there is more compression at 41Hz, 112V after the mod, than at 35Hz, 77,9V before, despite lower port velocity (28,3 vs 36,3 m/s).
The new rectangular ports have a total 296mm² area and a 55mm flare radius on both ends, but they sit directly against the bottom and walls, so only 2 of 4 sides of the ports are flared. The tuning frequency increased to 38.5 Hz, and the net internal volume is now 81L, due to the larger ports.
I've made frequency response measurements of the subwoofer at different power levels before and after the changes using UMIK-1 (6db gain) and REW, and put together some port compression data in a spreadsheet.
All measurements are ground plane 2m. Velocity in m/s and compression in db.
The spreadsheet shows input voltage, WinISD simulated port velocity and measured compression compared to the 5.66V sweep for Fb - 2.5 Hz, Fb, and Fb + 2.5 Hz. It also shows compression at 130 Hz (low impedance) in order to get some indication of possible power compression, depending on sweep level.
Some things I found interesting:
Port compression mostly affects frequencies below Fb. The undersized ports had only 0.7 db port compression at 77,9V and 35Hz, despite a (very high) simulated port velocity of 36,3 m/s.
Power compression at 130hz was considerable, although later in the sweep (more time for the voice coilt to heat up), it means that power compression likley affects the high power sweeps. For instance, there is more compression at 41Hz, 112V after the mod, than at 35Hz, 77,9V before, despite lower port velocity (28,3 vs 36,3 m/s).
Of course you would find more compression with more voltage at or very close to tuning. This is a region dominated by the impedance minima, where you are dumping some real power into the coil. Driver movement is near stall, and the coil is rapidly heating.
This has nothing to do with the larger ports.
This has nothing to do with the larger ports.
“Power compression at 130hz was considerable, although later in the sweep (more time for the voice coilt to heat up), it means that power compression likley affects the high power sweeps. For instance, there is more compression at 41Hz, 112V after the mod, than at 35Hz, 77,9V before, despite lower port velocity (28,3 vs 36,3 m/s)”
What i meant by this is exactly what you are saying, higher compression despite larger ports means that thermal compression significantly affects the results at the high power sweeps.
I may not have been clear enough.
What i meant by this is exactly what you are saying, higher compression despite larger ports means that thermal compression significantly affects the results at the high power sweeps.
I may not have been clear enough.
I have some difficulty to replicate your speaker and port specifications.
the two ports are 103 mm long, correct?
the port cross section surface is 164 mm2? that seems waaay to small if I understand it correctly.
If I try to define a 100 liter enclosure with 32,5 Hz tuning that needs one port with about 60 cm2 and 10,3 cm length (or two with 30 cm2 each).
I did some research regarding port compression and according to my findings the relation of port radius and air displacement is the relevant one.
if you like have a look here:
https://www.diyaudio.com/community/...rbers-and-port-geometries.388264/post-7755603
This is brilliant. I have been heavily considering rebuilding my cabinets for my two Dayton Epique 7’. I used two 2” pipes which ended up being a tune of ~27hz. There’s a rap song with a stepped bass line with a bottom note of 27hz and I aim my designs to be able to reproduce this. Juggling velocity, 1st resonance, length and such with the small subs I like to employ generally ends with my fighting to get below my magic 27hz tuning and I tend to end up right at 27hz.
I have found that the lowest note tends to be attenuated to my dismay. The idea that I may be dealing with power compression as opposed to velocity compression is extremely interesting. Might it be better to use smaller diameter ports that push the stalling of the cone lower? Essentially, this should push the group delay spike down lower as well?
I have found that the lowest note tends to be attenuated to my dismay. The idea that I may be dealing with power compression as opposed to velocity compression is extremely interesting. Might it be better to use smaller diameter ports that push the stalling of the cone lower? Essentially, this should push the group delay spike down lower as well?
What levels are you listening at with the small subs? Approximate power input?
At lower volumes, our hearing becomes less sensitive to low frequencies, so that could be causing it.
https://en.wikipedia.org/wiki/Equal-loudness_contour
I made some new tests in an attempt to separate thermal and port compression.
Instead of using sine sweeps, I used short CEA-2010 bursts to minimize voice coil heating.
The subwoofer in the OP with 296 mm2 ports would compress 1 dB at 120V input and simulated port velocity of 35.6 m/s, with CEA-2010 bursts. I measured 1.6 dB compression at the same voltage and frequency when i used sweeps instead. The difference is most likely attributable to thermal compression.
It's definitely there, in copious quantities, if one's loudspeakers are up to reproducing it. 🙂 Thanks for sharing your experience.
Out of curiosity, I low-pass filtered the track with a 4th-order Butterworth filter set to 120Hz. A small segment of the 27-Hz waveform is shown below, with some kick drum-like transients shown as well. The period of the sinusoidal portions of the waveform is 0.036 seconds, corresponding to 27.8Hz.
There are also many sections in the song that include a burst of 2 or 3 sequential kick drum strikes. Below is a portion of the waveform that shows this type of signal content.
Below is a section of the song where the low-frequency tones progressively reduce in frequency over four distinct subsections. It was interesting hearing the reducing frequency, until for the last one there was not a lot of SPL being produced by my compact speaker system.
I’m having a problem reproducing the 27hz note in almost everything I build. The only time I’ve had a linear perception all the way down was in my last car with two UM12’s IB through the cargo floor with enough power to go way past excursion. For example, my home stereo has two anarchy 704’s with Dayton 8” reference pr’s tuned to just below 27hz. My suv currently has two Epique 7’s with close to twice the excursion ported low and having the aid of cabin gain.
I seem to have to add a bit of boost super low to get the perception that the last note is as strong as the upper three. I thought room modes in my townhouse were to blame, but in my Honda Pilot I have the same problem. Seems like an unlikely coincidence for both listening environments to have a cancellation right below 30hz. Until you were awesome enough to sonically analyze the recording, I thought it might be that note was slightly attenuated, or utube, or Bluetooth, or something else.
I’ve been running 3.5mm directly to my cars dsp amplifier, btw, everything I do has dsp with miniidsp mic, so I can do a proper listening curve in an automobile or a little more flat in a townhouse living room. The still needs eq work to get the throbbing. I’m starting to wonder if it’s acoustic coupling to the air or our hearing sensitivity dropping off a cliff all of a sudden just below 30hz. Come to think of it, even in my old car with high throw 12’s that note never really came through like the rest. I’m 44 and maybe the extremities of my hearing are failing me? I can’t really hear above 15khz, wouldn’t surprise me if the other end of the spectrum is as well. But, even the tactile seems lacking. Even more odd is the rta shows the presence of sub 30hz bass rising.
I’m familiar with the work fletcher-munson did. I don’t generally listen to rap unless testing systems or in my car on the highway or at least not next to people at a traffic light. Testing and driving, rap and electronic like trance are nearly always played loudly. You probably have read it but above is a bit more detail into my experience and past/present equipment.
I refreshed myself on the specs of the Epique line and 200w rms is stated. My current 6ch amplifier will do 125w at the 2ohm load presented. I’m going to run some sine’s through today and approximate the actual tuning. Everything seems to be in check though, port velocity seems under control and the power is well below thermal limits listed.
How large are the ports and the enclosure?
The old car might have had more cabin gain at 27 Hz, the T/S parameters of the driver could be different to factory specs, affecting the alignment.
Without any obvious design problems (port size etc) and no measurements, it's quite hard to know what's causing it.
If the amplifier is operating at full volume (clipping at 0dBFS) the driver should dissipate about 20 W during the 27 Hz tone with 0.4 amplitude, assuming nominal 2 ohm impedance: 125*0.4^2 = 20
It shouldn't cause a lot thermal compression, but maybe a little bit.
The old car might have had more cabin gain at 27 Hz, the T/S parameters of the driver could be different to factory specs, affecting the alignment.
Without any obvious design problems (port size etc) and no measurements, it's quite hard to know what's causing it.
If the amplifier is operating at full volume (clipping at 0dBFS) the driver should dissipate about 20 W during the 27 Hz tone with 0.4 amplitude, assuming nominal 2 ohm impedance: 125*0.4^2 = 20
It shouldn't cause a lot thermal compression, but maybe a little bit.
1.1cf with two 2”x~25” each. Tune should be ~25hz with simulated velocity around 15m/s at 27hz full tilt. I did some silicone at the elbow seam with my fingers but it’s not perfect. I can hear a slight whistling when close to them blasting. Id be surprised if it caused enough turbulence to noticeably lower the ports output, although possible.
Whitwald's frequency analysis shows the 27Hz peak is almost 5dB lower in amplitude than the 37Hz:
At 1000Hz , a -10 dB drop in SPL sounds half as loud, at 30Hz, just a -5 dB drop in level sounds half as loud.
"A bit of boost" to give the lower note the same SPL and perceived level would require about four times the power, if your hearing happened to be the same as the average young adult 😉
Art
So, I’ve been chasing my tail on this one all along, while inflating every other songs low bass. Who would have guessed that Mr Young Jeezy’s virtuosity was such as to compose differing levels in his infamous bass line.
This is a great outcome, I was fully convinced that while my Epiques were astonishing for what they are, they just fell slightly short of my needs. I halted the aesthetics on the Epique cabinets to begin a duplicate project, just with a pair of Dayton reference 10’s. Similar in every way except size, output, and the need to install a big mono block.
I managed a little time today to roughly time align and quick retune. I also found one of the subs delay almost completely pegged and was swift to get that fixed. Now I’m torn, I have two compact subs built that highly perform, and just enough of the first Dayton 10” done to wet my whistle. I thought I outgrew the whole obnoxious car stereo thing, but, sometimes it’s fking awesome being able to go to pound town.
I managed a little time today to roughly time align and quick retune. I also found one of the subs delay almost completely pegged and was swift to get that fixed. Now I’m torn, I have two compact subs built that highly perform, and just enough of the first Dayton 10” done to wet my whistle. I thought I outgrew the whole obnoxious car stereo thing, but, sometimes it’s fking awesome being able to go to pound town.