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About Mottlefeeder

  • Birthday January 31

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  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. No trips planned at the moment, so it would be a few months away, but it is an option. David
  7. I travel to Bristol/Portishead to see family from time to time - could you get there to pick up & are you in a hurry? David
  8. For sale, two Jack 10 speakers, built from BFM plans. Each is fitted with Eminence Basslite S2010 speakers (150 W RMS, 300 W program) and a bank of 6 piezo horn tweeters, switchable in pairs - 0, 2, 4 or 6, so you could have no tweeters on the lower cab and some on the upper cab. £125 each, and I can throw in the matching (now empty) 2U rack sleeve if required. Pictures of the build, the finished speakers, and the difference between the Jack 10 and the Omni 10.5 (from BFM site) bass speakers only. Collection preferred or meet-up within an hour's drive for me - however, Warrington is currently in special measures for Covid 19 so these options may be delayed by a few weeks. Courier at cost
  9. 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
  10. 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
  11. 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
  12. 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
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