Mottlefeeder

I'm using D'Addario Chromes and I can recommend them. Also, although it may be a minority interest, I'm using a 5-string acoustic, an Ibanez from their 'Exotic woods' series.

My string adaptor just sits on the end of the existing nut. The nut end face is angled slightly, so the string tension pulls the adaptor towards the back of the neck. Also worth checking is the 'Overlord of Music' nut and bridge set, sometimes sold individually. That nut will take standard strings, but it clamps them by bending them through 90 degrees, which some strings will not take. I don't know if it is a drop-in replacement for the existing nut assembly. David

I'm having mixed results, probably due to low internet connection speeds - there is no fibre I can get to, and since the exchange is at the end of our street, we are not likely to be upgraded until everyone else has.been. I started with a Behringer UCA222 with an analogue mixer, and then moved on to a Behringer 302USB. Both are fine if all you want is to record onto Audacity, and play back backing tracks, but both had latency that was too great for JamKazam, even with the generic ASIO driver ASIO4ALL. I'm currently running a Steinberg UR22C which gives you the option of an HPF on each track going in, and EQ or other built-in FX on either the inward track or the outward track. It works well with Jamulus, but I'm still not flying on JamKazam. One-to-one seems to work, but when I join someone else's session I get no audio. David

Nice woody tone. The bass is big compared with a 5-string Yamaha RBX, but it's light, and comfortable whether I am standing or sitting. It's a 34 inch scale, and the neck feels good to me, and B string works well - there aren't many 5-string acoustics around, and I preferred this to the Boulder Creek at nearly double the price. David

EWB205WNE - 'exotic woods' 5-string with flatwounds. David

Thank you both, and contributors to the previous thread. In summary, I have a viable design based on the maths and the experience of others and no defined route, other than trial and error, to making the cab smaller by using a smaller volume port. I only want to build this once, so that sounds like a good place to stop. David

My earlier thoughts were to use the two 4 ohm speakers, each fed from a 20 W amplifier, with the option to connect them in series to connect to a 75 W @ 8 ohm / 150 W @ 4 ohm amplifier when I needed more volume. I've since found a 50+50 W @ 4 ohms module so I'm going up from 75 W total input to 100 W total input. We all seem to agree that the 30-60 Hz octave is unlikely to be a problem, so I am interested in your thoughts on porting at 64 Hz - do I need to allow for 100W at that frequency, and if not, how do you decide how many watts to design for? With my current layout of components, I can accommodate a 200mm long port. David

I get the impression that we are running on similar but slightly different lines of thought, so I'm not getting the answers I was hoping for. Having said that, I'm grateful for the explanations you have provided. They have filled in a lot of gaps for me. With my bass, the fundamental is 12 dB down, and since there will also be a 60 Hz 18 dB/octave HPF in the pre-amp, I agree that the fundamental is not an issue in my design. The speakers are rated at 80 watts each, and in a cab of the appropriate volume (11.4 l), are capable of taking 100 W at any frequency above 57 Hz. The excursion plots show that with the HPF in circuit, the speakers are below Xmax at all frequencies at 100 W. The proposed cab is tuned to 64 Hz, which seems to give the best compromise of size, smooth roll-of and bass extension, but a port capable of handling 100 W at 64 Hz makes the cab bigger than I want, so reducing the port power-handling capacity is a compromise I'm investigating. As I posted earlier, the spectral analysis of my lowest bass notes shows equal volumes at 60 Hz and 120 Hz, so I was hoping someone could comment on whether it was reasonable to assume that a low 100 W note could be regarded as for example 45 W at 60 Hz, 45 W at 120 Hz, and 10 W for everything else. That would give me the 'science' I need to reduce the size of the port and still be confident that it would do what I wanted. Am I oversimplifying a complex problem? Also, I was under the impression that if I reduced the port power handling capacity too much I would get chuffing, but you suggest that compression would be the more likely outcome. What does port compression do to the response of the cab? David

Unfortunately it's my nature to analyse things to death, and I still cut wood to the wrong size ... Taking your second paragraph first, I've taken on board that the harmonic mix will change as I move around the fretboard, but I chose the low C because it is the note most likely to overwhelm a port tuned to its harmonic. With regard to what the amp will deliver, I'm assuming that at some point I will turn up the amp and it will start to clip - 5 inch speakers are not as efficient as 10 inch speakers, so this is inevitable. Turning down slightly from that volume, the amp will be delivering its maximum voltage to the speaker, and the signal will initially have the lowest two harmonics predominating as each note decays. Using that scenario, I'm questioning whether I can derate the power handling of the port, but you appear to be saying that a port overload for the initial fraction of the note can be ignored. So, if the aim is to avoid chuffing, is chuffing a continuous overload of the port, or just overloading during the initial transient? David

Following on from my thread about whether WinISD was giving me bad advice, I'm now trying to work out whether I need a port sized for full power, or something less that full power. I downloaded a spectrum analyser and played a low C through it (lowest available fretted note). The analyser shows that the signal at ~30 Hz is about 12 dB below the signal at ~60 Hz, which is the same level as the signal at ~100-120Hz. For a port tuned to 60 Hz, my question is, if the voltage applied to the speaker is mainly two frequencies, and one transducer is reproducing those frequencies, the power must be split between those frequencies, so is it reasonable to design the port to take a maximum of half the amplifier power at 60 Hz, on the basis that the rest of the power will be at other frequencies? David

Assuming that the noise limiter was installed because of complaints by the neighbours, you aren't doing the pub any favours by playing louder than they want. David

If they are really old, then they will run at 5 volts. However the recent crop of fast chargers can run at higher voltages depending on what they are connected to. Check the charger labels - they should tell you. David

Just a quick update, and thanks to all of you who responded. I wasn't sure whether to go make a rectangular port or buy in a round one, so (amongst other things) I measured the internal diameter of the port pipe I would use, and it is 63 mm, not the 68 mm default in WinISD. This means that my port will be shorter than originally thought. Also, in calculating how much room I had in the box, I forgot that the port comes through the box wall, which means there is less of it in the box. The combination of those two means that I have room to use a straight port, and it finishes more than one diameter from the back wall, which seems to tick all the boxes. One of the things I have learned came from this thread - http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/156355-winisd-slot-ports.html - in summary, WinISD gives the option of modelling a port (a) finishing flush with a baffle, or (b) finishing hanging in free space, but a shelf port is neither of those. The 5th poster reported that a port finishing by being boxed in by the enclosure base and two sides, needs to be about considerably shorter than WinISD calculates it. He misremembered it as 40%, and later corrected that to 20%. So, if any air movement restriction in the vicinity of the port causes the port to act as it it was longer, this suggests that if you have to put a bend in your port, then you need to have at least a diameter's length of port after the bend to avoid the pipe before the bend affecting the air flow. Thankfully not something I need to worry about this time. David

And replied. David

And I'm back in the room. My basses are all 5-string, and vary from two-pup designs to a bridge Piezo acoustic bass, so I'm not planning to do any further analysis on output vs frequency. I'm in the right ball park so I'll stop there. So, firming up the design, with an 18dB/octave HPF set to 60 Hz, and limiting the input power to 20 W per speaker, and using a vent with a 2:1 cross section, I get an 80 mm x 40 mm vent 160 mm long giving me a maximum air flow of 15 m/s at 65 Hz. So far, so good. For an overall box depth of 305 mm, with an amp on the back, and a grill on the front, I have an internal depth of 220 mm for a port of 160 mm. Is the port end too close to the back wall? David

Thanks for that - I'll be away for the next few days, so I may not respond to further comments. David

Just to make sure I understand - the output from the pickup is 6dB down at 30-40 Hz because there isn't as much signal to pick up, so to model that into a port calculation, if I have say 10 watts at 60 Hz, putting me on the limit of port air-speeds, I should check that at 30 Hz, the port can cope with 4 watts without reaching that air-speed limit? Talking of compromises, and refining my design slightly, I was aiming for a similar sound and volume to my 8 ohm Eminence basslite S2010 in a small cab, taking about 10 watts from one channel of a stereo amp. This two speaker box will do it, with two 4 ohm drivers taking 20 watts each - i.e. half the size of box , but taking 4 times the power to reach the same volume. The next step up in volume would be to add a second S2010 and run one per channel, taking 20 watts in total. But the increase in output is only 3dB, and I am now carting a mini stack around. The alternative next step is to use my bigger battery-powered amp - 80 watts into 8 ohms / 150 watts into 4 ohms. At this point, the single S2010 will deliver 114 dB but the twin speaker box, with the drivers in series, will only deliver 108dB, so it is 6dB quieter. Beyond that, a pair of S2010s will deliver 116 dB, but I would also be carrying a bigger battery (27 kg!), so it would no longer be a portable rig. On that basis, there is no point in designing the twin speaker box to take more than 20 watts per speaker - if I want it louder, there are better ways to do it. David

If the port calculations are based on area times length, that implies that the volume of air is the important bit? Yes, a sharp 90 degree corner will cause turbulence, but the prefabricated pipes have radiused corners to reduce that. David

And I'm back in the room... My existing 10 inch speaker box has a thin slot port, but very low air speeds, which may be why I have never heard any problems due to turbulence. Moving away from a thin slot in this design, to something like 1:4 or 1:3, and big enough to bring the airflow down below 17 m/s, gives me a port that is longer than any of the box dimensions, so I'm now looking at a port turning a corner inside the cab. One option is a drainpipe with a 92.5 degree angle, and the other is a rectangular extractor fan duct with a 90 degree bend. Should I be aiming for a 'middle of port' length as per WinISD, or a port volume as per WinISD? And are there any rules of thumb for compensating for whatever problems are caused by bends in ports? Thanks David

Thanks for your observations and effort on my behalf. I'm on holiday at the moment and have no access to WinISD but I will be checking it out when I get back. David