What am I doing wrong?

I've a commercial box with an 8" bass/mid and a 10" passive. The passive died of foam-rot and having tried several replacement surrounds can not get tuning correct.
So.. opted to try a reflex port..
Driver parameters..Fs = 24.2 Hz, Vas = 2.95 cu ft, Qts = 0.28, Qes = 0.32, Qms = 2.30, Mms = 29.9 gm, Re = 3.52 ohms, SPL = 87 dB, Le = 0.32 mH.
and both Qspeakers (linux) and an online calc give the following..

Butterworth B4 Ported
Vas = 2.95 ft3
fs = 24.2 Hz
Qts = 0.28
D = 17 cm
Dimensions
Vb = 1.33 ft3 = 37.66 lts
=LxWxH
f3 = 34.99 Hz
fb = 28 Hz
Dv = 2 in = 5 cm (port diameter)
Lv = 6.59 in = 16.16 cm (port length)

which match closely the commercial box volume and so I tried the port recommendations.

However, no matter whether I try a ported enclosure as shown, or a sealed enclosure, or any of the failed passive I measure only a huge peak at approx 80 Hz.
The original passive had a very compliant foam surround which perhaps no amount of weight/loading on the replacement rubber stiffer surrounds will match but I thought the reflex port recommend would make some change.
Tomorrow I may try a very large and long port and see if that can modify the peak and provide the 2 lower tuning peaks that I am told to find.

Any and all guidance appreciated :)

(The box and box volume are immutable due to WAF so a port tuning is desired)
 
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It is not clear whether these T/S parameters are factory data or self measured, and what was the drive voltage.
The procedure for a succesful cabinet loading design is very simple, the measurements have got to be realistic, otherwise gigo effect is inevitable, leading to false conclusions.

You mentioned peaking at 80 Hz. Again, what type of peaking, SPL or Impedance, and how exactly did you get that result. It could be a room mode. To make sure bass response is as predicted as per simulation, you measure its spl near field.

And you need to make sure your measurement equipment is not the source of any anomalies, spl peaking of any kind, etc.
 
The tuning is ether too low or box loss too great. It ends up looking like a large sealed box in this case. With a Qts of about .40 is where the tuning frequency and driver Fs are the same considering a QB3 alignment. Extended shelf alignment puts the tuning lower with a larger box and usually improves transient response compared to QB3.

The problem with some alignments on drivers with higher Qts is placing the Fb tuning frequency lower than driver Fs. This increases distortion and worsens transient response forcing the driver to work lower than its own Fs.

Drivers with higher than .40 Qts don't sound that great in ported enclosures IMO. The bandwidth efficiency ratio Fs/Qts is a bit lower than 80 - 90 on these drivers. The BL is usually insufficient to allow for tuning with practically higher Fb and reflects itself in lower system efficiency past its own Fs. The box damping also has alot to do with the efficiency and can hurt port performance if damped to the point of sufficiently suppressing standing wave modes as a result of poor box dimension ratios. I would avoid drivers with a Qts outside of the .30 - .40 range for ported applications, especially when the driver has to play into the upper mids and requires alot of cabinet dampening to clean up box resonances.
 
Yes - we couldn't count the number of people that have been let down by 'rotting' foam surrounds.
I have an interesting project suggestion for you.
Get yourself a suitable pair of 4 ohm 10" 'car audio' subwoofers with very low fs and create what I call >
"an actively assisted port".
This requires using a very high value inductor, around 10 mH that has very high DCR. Also known as
a 'very lossy coil'. With a multimeter that can measure inductance, you can create this by 'easily' winding
light gauge enameled copper wire onto a cheap ferrite-rod. Neatness is not important, and aim for 4 or 5 ohms DCR.
Phase-orientation wiring of your new 10" sub is critical, but easily obvious when you get it right.
With a little experimentation, you might be amazed at what this method can achieve :)
 
How are you doing your sweeps? I'm in Kapiti depending on where you live. I have a DATSv3 if you want a second opinion
Hi Dave, I've spotted several of your posts and you are only a hop skip and jump away (here in Judgeford..not far from the golf course)
I have a few more mods to try and if they still don't return any useful results then perhaps we could overlap for some further diagnostics ?
I have a pair of 38 year old (from new) Shahinian Arcs...so no slouches in performance but due to the ott factory return and shipping hit for repair I am investigating all other options first.

Cheers,
bob
 
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The tuning is ether too low or box loss too great. It ends up looking like a large sealed box in this case. With a Qts of about .40 is where the tuning frequency and driver Fs are the same considering a QB3 alignment. Extended shelf alignment puts the tuning lower with a larger box and usually improves transient response compared to QB3.

The problem with some alignments on drivers with higher Qts is placing the Fb tuning frequency lower than driver Fs. This increases distortion and worsens transient response forcing the driver to work lower than its own Fs.

Drivers with higher than .40 Qts don't sound that great in ported enclosures IMO. The bandwidth efficiency ratio Fs/Qts is a bit lower than 80 - 90 on these drivers. The BL is usually insufficient to allow for tuning with practically higher Fb and reflects itself in lower system efficiency past its own Fs. The box damping also has alot to do with the efficiency and can hurt port performance if damped to the point of sufficiently suppressing standing wave modes as a result of poor box dimension ratios. I would avoid drivers with a Qts outside of the .30 - .40 range for ported applications, especially when the driver has to play into the upper mids and requires alot of cabinet dampening to clean up box resonances.
Hi, a question from curious: how do you evaluate this stuff in practice, outside or with some particular setup inside?

I'm curious because I get bad response in room with any bass speaker I've had so far, there is peaks that dominate perception of the whole loudspeaker system. Currently I'm having mains speakers and one sub playing and they together allow somewhat neutral response to be adjusted to about one listening spot, otherwise it's quite wild. Room mode peaks dominating the "impression", which is quite bad, it must be handled somehow to be useful. If there is such strong influence from room, so that it totally dominates, then how can you distinguish between better and worse alignment for example? Evaluating speaker and head always at same spot? moving like one foot changes the peaks and while quality stays poor the emphasis changes from one frequency to another and I think it is impossible to evaluate anything, it's random. So, how do you evaluate, what is your process? Thanks!
 
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This is all measured with Clio or WT3 using impedance sweeps. The room has practically no influence on this type of measurement. If you're measuring in room amplitude response, you do this nearfield or perform gated ground plane measurements in a larger room or outdoors with Rew or other measurement software. The room response will be difficult to exclude if the room is small. You need to use nearfield measurements including the use of ground plane measurements and merge them with the response over 300 ~ 400+ hz. Measuring at the prefered listening location will be necessary if you want to EQ the low end, otherwise bass will be unpredictable.

The predicted alignment you're aiming for can be verified with impedance sweeps.

Lower Qts drivers in unassisted alignments will usually require a higher tuning frequency above driver Fs. The bottom impedance peak will be lower in amplitude than the upper.

The opposite holds true for higher Qts drivers, which need a lower tuning Fb below driver Fs. The upper impedance prak will be lower in this case. If you continue lowering Fb, it will turn the alignment into what appears to mimick a lossy sealed box with a single diminished impedance peak.

Drivers with a Qts around .38 to .42 can be used in a QB3 alignment with Fb the same as driver Fs. There is also the box loss to consider, but that gets more complicated.

See this website for a good calculation tool - http://www.mh-audio.nl/Calculators/BRH.html
 
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Hi, thanks!
My intent was to ask how do you define which one sounds better, how do you evaluate?
For example "Drivers with higher than .40 Qts don't sound that great in ported enclosures IMO.", how did you arrive to this conclusion?

I'm just curious, as everything I've tried seems to sound nasty except when I use measurement and DSP and tune the system and room at listening spot. Trying to scope your context, or what to listen to, curious if some qualities are audible without tuning the system? or does evaluation require tuning of system? and so on. In general, how do you evaluate perceived quality between two bass alignments in practice at home, or can you do it at all?

Measured anechoic quality is easy to compare, but it doesn't seem to matter at all when the box is brought into a room. Perceived sound quality is what ever it happens to be where the head is at, in general quite nasty. To me box simulation results or anechoic measurements seem unrelated to perceived sound in room. I'm trying to learn how to evaluate, how others evaluate their systems? Anyone?
 
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@Mister Audio not sure where you plan the inductor to connect ? across the passive only ? or added to the x-over ? or paralleled/series to the input ?
Schematic appreciated !
The 10 mH inductor is in series with one terminal of the 10" sub then going straight to a speaker box input terminal.
The other 10" terminal goes strait to the other speaker box input terminal.
In other words, this is a simple 6dB per oct. filter for only the 10", and as such doesn't connect 'into' your existing crossover.
This method will likely create a 'system' impedance of around 4 ohms @ very low frequencies > but this is what gives you
your new very low bass :)
 
I'm away for a couple of weeks, but if you want me to pop around and do some impedance and frequency response sweeps, I can help.

I modeled your driver and box in WinISD. The problem I see with a 5cm only diameter port is major port noise (probably fast turning into sealed box behaviour) with more than 10 watts input:
1692763711870.png



Impedance peaks should be symmetrical with a minima ~ 28Hz - minus box losses and without any damping nor EQ:
 

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If there is everything alright with Impedance measurement gear, and the second Imp peak falls in the 80Hz region, assuming internal cabinet volume wasn't compromised/reduced, the cause of it could be that true Fs of the driver in free air is higher than assumed.
 
Results from experimenting with a larger diameter and longer port made no difference...However, when I removed the port tube (leaving only the 40mm ply hole @75mm dia) I could measure 2 peaks, albeit the lower was small compared to the upper...but progress!
So now having thrown all the calculations out and simply 'jfdi' I find that every increase in 'port' diameter (while maintaining 40mm depth) is increasing the lower peak whilst decreasing the upper peak.
I am at the limit of my hole-saws..102mm.. but things are looking very promising. Time to get the router or jigsaw out.
Will keep enlarging and see what transpires. If I can match the peaks then will do some critical listening but if all else fails will just close up and run as as a sealed box.
 
Hi Dave, I have concluded that the driver specs provided by another forum member are for a later revision and thus any calculations in any software may be moot until I measure the drivers I have.
These drivers were hand-built by Dick Shahinian and matched to the passive that rotted.
Having abandoned calculation and simply adjusting and measuring it may be that the port replacement for the passive is somewhat unusual.
As noted, I can now measure 2 peaks @ 52Hz and 94Hz and getting closer to the same value every time I enlarge the port diameter. Leaving the port depth @ 40mm until I can get a meaningful result then maybe rejig.
Open to any suggestions :)