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Old 22nd Mar 2018, 4:47 pm   #21
Diabolical Artificer
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Default Re: Speaker cabinet query.

Why does something sound off? Here are the T/S parameters for 18083 or Eminence Big Cat 18/400
which was a driver made for Maplin's I believe.

05-RE OHMS 5.90 13-FS HZ 27.96

06-LE MH 1.36 14-MMS GMS 102.57

07-QM 8.89 15-CMS mm/N .3159

08-QE .280 16-RMS NS/M 2.0267

09-QT .270 17-VAS LTRS 596.18

10-XMAX MM 5.50 18-SD SCM 1159.17

11-BL TM 19.58 19-EBP 100.8

12-EFF % 4.53 20-SPL dB 98.6

Cab volume 7 cubic foot.

A.
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Old 22nd Mar 2018, 11:22 pm   #22
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Default Re: Speaker cabinet query.

The port length didn't sound right to me. I've entered the data into the latest version of WinISD, and found that a 4" diameter port should be about 8cm long, which seems more like it...

It's a bit of a disappointing response for such a big driver. -3dB is about 58Hz with the recommended port tuning of 28Hz (red curve). If you tune the port to 40Hz (green curve) it stays flatter longer (-3dB now at 44Hz) but the group delay is bad - nothing is for free, so it's a question of juggling the compromises...

Personally, I'd run it sealed, and use an equaliser to bring up the gain below 100Hz. You could halve the volume of the box and still get good results - as it is, 7 cubic feet gives you a Q of 0.56 (blue curve), which is good for transient response (but most manufacturers would aim for a Q of 0.7 as that results in a smaller box). You'd get much tighter bass in return for wasting amplifier power (but watts are cheap!). Note that at 30Hz, it's only 3dB down compared to the ported option, but there's much less group delay. At these low frequencies, group delay is more audible than frequency response per se, and group delay is what makes many ported speakers sound "soggy".

For home hi-fi, sealed operation with some well-judged EQ is a really good compromise - not necessarily viable for PA use where quality comes second to loudness, naturally. See Rod Elliot's article about ELF operation here: http://sound.whsites.net/project48.htm - I built my own version of this many years back, using just a cheap 10" JBL car driver, and it worked brilliantly in my small house. One day I might even finish it (it's just built on a prototype board at the moment).

For the input buffer, you just need 1 op-amp. I would configure it as an inverting summing amp - literally just a couple of 10k resistors from L and R going to the inverting input. Put a 10k resistor from inverting input to the output, and take the non-inverting input to ground via a 1k resistor. None of those values are critical - just use whatever you have. You can add 100K or similar resistors at the inputs (also going to the -ve input) to take the signal from the output of the power amp if you like. The output of the op-amp then goes into the Sallen and Key filter. At the output of that, connect a gain pot, then send the wiper of that off to the power amp (add a unity gain buffer if using a power amp with a low-ish input impedance - op-amps are cheap!).

As for phase, the phase between input and output will vary with frequency - it's a filter . So don't worry about that aspect. But it's useful to put a polarity flip somewhere in the chain, and the position of that switch will depend on where you place the subwoofer in the room, relative to your main speakers (the wavelength of 50Hz is about 22 foot - call that 16 degrees per foot). Sometimes this switch makes an obvious difference; sometimes it doesn't. Ideally, arrange to have it in your listening position so you can hear the differences where you're sat - in a room, there will be peaks and troughs caused by standing waves as you move about (which is why worrying about a perfectly flat response is pointless).

Finally, your method for determining the port tuning is ingenious, but you'll get more accurate results from the impedance curve. I know I keep saying this, so forgive the repetition, but all you need is an audio oscillator and a DMM or 'scope. There's no maths - we're not trying to measure the impedance. We just want to find the minimum (where the port is "fighting" the movement of the speaker). Having found that by seeing when the voltage across the speaker has got as low as it can (the output impedance of the oscillator being key to this), then measure the frequency. Simple, quick, and no mess at all
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Old 23rd Mar 2018, 8:07 am   #23
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Default Re: Speaker cabinet query.

That is about the length I have in there at present.

At the start I said it was not the best driver, I was using an older version of WINisd, ported was the only option on the bar graph. I've run it in ported mode and sealed, and can't tell the difference... tell a lie, the difference is subtle.

I wish I'd seen Rod's proj 48 before building this, my sub is the biggest thing in the room.

I started out using that config EG two R's to the inverting IP thinking separate buffer's would be better, but separate buffers add problem's.

I was reading up on active filter's last night in AOE and found phase differs with frequency as you say.

I will measure the Z, I hadn't forgotten you saying to measure it, I'm still finding my way trying to assimilate and coalesce all the bits of info.

I was listening to music briefly with the sub yesterday, it really changes the whole soundstage. It's quite trippy.

I built the sub, not for a definite purpose, but for the craic. I'm now thinking why have a sub in the first place? Most subs seem to be built by lads of a certain age to put in their cars so we can all enjoy the tasty beats of techno or grime as we stroll down the high street. The other reason I guess is to reinforce the lower frequencies in a home theatre set up. the reason i mention this is that I'm still not sure what to make the cutoff frequency of the low pass filter. Rather than calculate it mathamatically subjective listing tests seems to be the solution.

Again a big thankyou Mark for all your help, Santa will be bringing you a case, well a bottle anyroad of something nice at Xmas.

Andy.
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Old 23rd Mar 2018, 1:51 pm   #24
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Default Re: Speaker cabinet query.

No problem

Building subs is a good introduction to the art of speaker building. Luckily there is a lot of latitude with a large cabinet - things get more difficult with smaller ported cabinets.

Done right, a sub shouldn't be noticeable - they say that you shouldn't know it's there until you switch it off! However, at first, you'll definitely wind it up too high and have a lot of fun in the process - been there, got the t-shirt! But the idea is not to add "punch" or whatever; it's simply to extend the frequency response of the system - "more extension", not "more level". Having got the sub to correctly integrate with the mains, it's best to use a graphic EQ to apply bass lift to the system as a whole if that's what is desired.

If you have good main speakers that play loudly enough already, then it's best to not try to filter them. Instead, let the sub take over when the mains roll away naturally. If your mains are sealed, then they will roll away at typically 12dB per octave, so that makes it easy to integrate a sub. Ported speakers have a faster roll-off, and are harder to get right. As counter-intuitive as it might sound, blocking the ports of your main speakers can make that easier (and improves the phase/group delay behaviour as they roll away).

Most people set the crossover frequency too high (so the sub overlaps with the mains, causing a hump in the bass), and they generally set the level too high as well. That usually happens because they are using music with relatively little energy below 50Hz (more common than many think). With mine, I ended up overlapping more than normal because my main speakers were positioned either side of an opening between 2 rooms, meaning that their bass output was severely attenuated. When I was eventually able to rearrange the room, I put them either side of the chimney breast and stopped using the sub. I'd like to finish it one day and put it in the front room to help the LS3/5As that we have for the TV - they do pretty well, but it'll add a bit of "excitement" to the movies

Where it really adds something is acoustic recordings - classical or otherwise. Contained in that ELF part of the spectrum is often seemingly random noise that helps our brains to reconstruct the acoustic space the recording was made in. Sometimes you get little bits of traffic noise or tube rumble that was missed because no-one checked with full-range monitors - but rather than cause a disturbance, this heightens the sense of reality. For pop/rock recordings that were multi-track assembled in a studio, you don't really get this (though perhaps you'll get a bit more weight from the kick drum), but a lot of electronica has ELF effects which are good fun to experience. Your neighbours will hate you

The more I think about it, I really reckon that of all the things I've done over the years, playing with subs really is the most fun. The trouble with audio is that a lot of people take it far too seriously. Enjoy the ride
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Old 23rd Mar 2018, 3:40 pm   #25
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Default Re: Speaker cabinet query.

I measured the impedance as suggested, but having just re-read the last paragraph, post #22 i think I've measured the wrong thing. I put a 1r resistor in series with the OP from my amp and measured the the V drop to give I whilst also taking V readings at the speaker cab IP terminals. I also scopes the cab IP terminals.

See graph attached. the resonant peak is at 60hz with a V reading of 1.5v, but the Z peak is at 65hz. this is with a 3"/8cm long 4" port. The lowest V reading is at 130hz @ 0.30v and at 15hz @ 0.32v. I will take readings with the cab sealed for comparison.

I made up a "music pallet" earlier, starting with Eric Satie (classical piano), a bit of Black Sabbath, Damned, Lee Perry (reggae) Dave Brubeck and finally some horrible grime. I compared this with/without the sub. Obviously it's tempting to go a bit mad with music that has heavy bass and kickdrum, but i did do subtle as well. As I said it's very trippy, by that I mean the music sounds spacy,it's like the music is inside your head. It's an odd difficult to describe feeling.

As regards the filter, it's a fourth order Bessel, fc = 100hz. after faffing around with about 10 different circuits, this one seems to do the job.

Lastly, how does "bad" group delay present itself aurally? As you say this is good fun, could muck about all day but best get some housework done.

A.
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Old 23rd Mar 2018, 5:13 pm   #26
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Default Re: Speaker cabinet query.

That looks pretty good. The peak is when the drive unit is at resonance, and the minimum at ~30Hz is where the air in the port is at resonance.

I don't know what your main speakers are, but I'd expect that 100Hz is too high. It really does need to be adjustable, but 4-gang pots are hard to find.

Group delay - the difference between a kick drum punching you or slapping you with a wet fish. If the "weight" happens some time after the "click", it sounds most unnatural. The trouble is, most folk are accustomed to slow, ported bass - but once you've lived with sealed bass done properly, you really notice it. A surprisingly good test track for this - discovered entirely accidentally, I have to add - is "Tragedy" (Bee Gees) - if your street cred can handle it
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Old 24th Mar 2018, 7:59 am   #27
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Default Re: Speaker cabinet query.

Not done a Z curve measurement before, was surprised at high it went. I'll do some more with different ports/sealed for the craic.

My main speakers are sealed, Mission 720's see pic post #5. F range 30hz to 30khz. They do have good bass response, I'll measure them later. I tried to make an adjustable filter but all the pots I have are too big - 250k. No worries, I'll build another, am getting the hang of it now.

Looking at this chart - http://www.guitarbuilding.org/wp-con...d-EQ-Chart.pdf it's only really 5 stringed bass and keyboards that go down really low and the noise etc you mentioned in post #25. I have a drum kit upstairs so could record a test track to save the ignominy of playing the Bee Gees : ) Iron man by Black Sabbath has a good kick drum on it for those wishing to do the test and keep their street cred intact.

Thanks again Mark, Andy.
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Old 24th Mar 2018, 12:15 pm   #28
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Default Re: Speaker cabinet query.

I think it's been covered on this thread (but maybe not in the language I am used to) that the very-LF contained in an impact (like a bass drum) might go 'clunk' - but might go 'boing' if the thing it excites is a lightly damped mass-spring system like a port on a cabinet. I'm not sure I've ever been able to hear this, unless standing in a reverb room with its own very high-Q room modes, which can themselves go 'boing' quite alarmingly. It will be interesting to hear what your listening test throws up.

I must dig my notes out and have a look at your small sig parameters. From memory Q_ts (total drive unit (not cabinet) Q, measured in the presence of an air load) determines whether a ported cab is going to work for a given driver - by 'work' I mean yield a Butterworth-ish alignment in a cab which isn't humongous. The 1974-ish R.Small paper in JAES presents a handy nomogram for working it all out with minimal calcs - if I can find a copy I'll try to post it.

[embarrassing tangent. I once met Small and Thiele at a conference. Screwing up my courage, I went to sit with them at a series of long tables, full of silverware and heavy table cloths that turned out to be set up on wobbly trestles. As I sat down my knee hit a trestle and this massive set of candlesticks started to wobble between them; it looked for a moment that I would achieve international recognition as the guy who set light to Small's wife or Thiele's beard.]
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Old 24th Mar 2018, 1:45 pm   #29
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Default Re: Speaker cabinet query.

A given driver will sit on a range, depending on its value of Qts:

0.2: Only suitable for porting
0.3: Best to be ported
0.4: Could be sensibly done either way
0.5: Best to be sealed
0.6 or more: Only suitable for sealed

Interestingly, most speakers used in radios have a much higher Q. But the enclosures are usually not sealed - nearer to being open-baffle in many cases.

Another indicator is the ratio of Fs and Qes - usually called EBP (efficiency bandwidth product). Here is a website that calculates it for you:

https://www.diyaudioandvideo.com/Cal...tedSpeakerBox/

Note that WinISD also does this for you during the "new project" wizard.

I think either approach is valid, and is only a guideline. For example, the 4" Audax drivers I used in my mini-monitor project years ago should, in theory, be better suited to ported boxes than sealed, but when you listen to them, you soon change your mind

My argument for ported vs sealed is simple: we have enough trouble with the resonant system that we can't avoid (the mass of the diaphragm against the compliance of the suspension and air in the box), so why would we want to introduce another one? That's not to say that ported speakers can't be good - there are some excellent examples out there - but it's a very tricky thing to get right. I've got it wrong more often than right. Sticking with drive units at the low end of the Qts range is the only tip I can pass on - if you pick one that's in the "grey area", you'll have "fun" getting tight bass.

I must admit that I'm too lazy to do the sums these days - WinISD does a pretty good job of getting you perhaps 80% of the way there. I used to do it all manually - and then wrote a BBC micro program to do it for me - but WinISD will do a better job in much less time. Over the years, I've found the thing that makes the most difference is knowing the TS parameters. The manufacturer's data should be tested and validated - errors on datasheets are not unknown

Nice story, Mark. Did you get a chance for a decent natter with him?
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Old 25th Mar 2018, 1:37 am   #30
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Default Re: Speaker cabinet query.

Quote:
0.2: Only suitable for porting
0.3: Best to be ported
0.4: Could be sensibly done either way
0.5: Best to be sealed
0.6 or more: Only suitable for sealed
Yes, that's ringing some bells - I think Small defined a ratio 'alpha' as Fs/Fc (or the other way up), and using a driver with large Qts meant ending up with a huge box to get Fc low enough - or using a tiny, long port which pushes the port particle velocity so high that it whistles (or does it chuff? I can't remember ). I worked on something once which detected such chuffing by looking for harmonics of the (distributed, not lumped) port pipe resonances, excited by noise components generated by the unwanted turbulence, and not harmonic with the driving tone which was selected for just that property. It sort of worked.


Quote:
Over the years, I've found the thing that makes the most difference is knowing the TS parameters. The manufacturer's data should be tested and validated - errors on datasheets are not unknown
I used to find MLSSA very handy for that - it's a shame it never made it out of DOS, though lots of firms carried on with it very late nonetheless. I used to get students to write Matlab code to extract the ssps from two impedance curves (with a known added mass, or known added cabinet volume) sometime in the distant past. The added mass method would even work for headphone drivers if you did it in a bit of a vacuum.

Quote:
Did you get a chance for a decent natter with him?
No, far too shy In fact Mrs Small probably felt a bit sorry for me and made (wait for it) Small talk.
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Old 25th Mar 2018, 8:53 am   #31
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Had a bit more of a listen yesterday and started to get the hang of it. I made the filter cutoff at 80hz and got the sub doing what you said Mark, extending the lows. It complemented the music nicely and the trippyness was absent.

Re filters, so far I've made 2 pole and four pole flavours 2nd and 3rd order. The design's I've found seem to fall into two camps, the ESP filters use common E6 caps and 5% R's with the two R's and C's the same value. Other's like the AOE design's and ones i made up using this calculator - http://www.beis.de/Elektronik/Filter...eLPFilter.html use 1% R's and E12 caps R A/B and C A/B being different.

I presume using the latter gives a more accurate roll off but it's a pain trying to get value's like 23985 ohms and is it really necessary to have such an accurate filter for this application?

I will make a variable filter as suggested - ESP project 155 but need to get a dual gang 10k pot. 50k is the nearest I have but calculations suggest that will give me too wide a pass band. this means the sub can double as a subtle low end extension speaker and by turning the filter higher as a "party thumper"

Nice story Mark, there's a good overview of Mr Thiele and Small's work and TS parameters here - https://en.wikipedia.org/wiki/Thiele/Small_parameters. I have been using some software called REW for measuring frequency response THD etc of amplifiers but it designed for measuring loudspeakers. I'll have a play with it this week.

This subject is really fascinating, I'd recommend anyone to have a go at building their own speaker if only to better understand what's happening at the noisy end of their hifi setup.

Andy.
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Old 30th Mar 2018, 12:59 pm   #32
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Default Re: Speaker cabinet query.

Having done a few calcs i think i can make the variable LPF 15hz to 630hz as in Project 155 - http://sound.whsites.net/project155.htm by subbing the 10k pot for a 2k5, which i have, R2/4/5 for a 100r and C1/2/3 for 100n. It doesn't say what it's power consumption is but i have found a 14/014v 70mA tfmr int shed, which I think will do.

I have a hum problem on the filter I'm using at present, I'm having to lift the ground on the OP, however this is knocked up on a breadboard so grounding etc isn't optimal.

More later hopefully.

A.
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Old 13th Apr 2018, 7:47 am   #33
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Built the variable LPF - see pics attached. It's range is 10hz to 300hz and works surprisingly well, though there is what looks like modulation on the LF setting, however it sounds fine in use. I'm using the tape out off the back of my amplifier as the stereo source.

I used the previously mentioned proj 155 as my template and used two 2k2 R's on the input prior to the 22k R to ground. I used a 50k pot instead of the 10k and 3k3 R's/100n caps instead of 1k/10n. Oh and I used a TL082 instead of the 5532.

If I'd known what I know now, I would have built this differently using this design http://sound.whsites.net/project48.htm The cab I built is way too big really for a small room. Also I found though the sub does add to sound of music, it's quite subtle, I could do without.

Still, it's been a fun instructive project, I've learned a lot especially about using opamps.

Andy.
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Old 13th Apr 2018, 9:30 am   #34
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Default Re: Speaker cabinet query.

Well done for getting to the end of it! I forgot to mention these things earlier

https://tinyurl.com/y9xus5m5

I don't know if you can see from the picture, but the port is formed by bending the 'wall' of the cabinet around and back to itself (like rolling a cylinder) but intentionally 'missing', coming back to a radius a bit bigger than you started with, and continuing on a bit. This makes the port - it's quite an elegant, compact way of doing it.

I used to visit the company (quite a long way up Finland) to check up on industrial placement students. Sitting naked in a sauna there with their staff was one of my more unusual teaching-related experiences. I think some of the (male) students were more embarrassed than I was

Ah, here's a better picture

https://tinyurl.com/y8pt8pme
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Old 13th Apr 2018, 1:26 pm   #35
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Good job! Where did you get the PCBs from - they look pretty nice

You say you're picking up the signal from the tape output - but this level is fixed, so that means you have to adjust the level of the sub separately from the main volume control? When I did mine, I used a dedicated output on my preamp that was designed for this, but if I didn't have that option, I would have just taken the output from the speaker outputs (obviously via an attenuator network). So that's a possible refinement for the future. And there's nothing stopping you from trying the ELF approach - just time and some MDF, I guess. I found that it worked really well with the drive unit I had.

Generally, I quite like Genelec stuff. I haven't heard that sub, but it's a nice bit of design. We've got loads of their smaller active monitors and a few of the bigger ones too. Providing no-one has set the DIP switches inappropriately (it happens!), they sound (and measure) very good indeed. Thanks for the tip about never visiting the factory though
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Old 14th Apr 2018, 7:31 am   #36
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That's clever design on those Genelec's, but how the Dicken's do you get low bass out of such a small enclosure? Good design I suppose is the answer. Bose (groan) use/used a similar design in their Wave Music System - https://www.youtube.com/watch?v=8xpI...index=220&t=0s over a grand each!! Strike a light!

Sitting in a sauna with your student's in the nip; hope they didn't ask you to roll around in the snow afterwards or jump into a freezing lake : )

I made the PCB's, but they're not printed, just use a fine bit in a dremmel to cut the tracks. Yep, the OP of the VLPF goes into an amp, which you need to muck about with. It is a faff, but once you have it set, it's ok. I did it that way rather than have loose wires out of the speaker outs, which to keep neat would have meant buying stackable banana jacks or similar. Tried to make this project without spending too much cash and using what I had already.

Thank you both for your kind words and interest and especial thanks to Mark H for your advice + help. Would have struggled else and took longer.

A.
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Old 14th Apr 2018, 10:53 am   #37
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Default Re: Speaker cabinet query.

In my day we used a sand filled baffle.

Peter
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