Helm Holtz Ports |
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White Noise
Registered User Joined: 05 January 2015 Status: Offline Points: 54 |
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Posted: 30 August 2017 at 7:51pm |
ive been considering putting a post up on speaker plans about this for a while, and I would love to hear your thoughts.
Its about helmholtz ports on bass reflex and band pass speakers. Say you have a rectangle port that has little to no end correction and is tuned at 30, and the port plays loud enough at 1w to have the box f3 at or just below tuning. When the spl increases to the middle range of SPL, so does the amount of air going through the port and it creates a vortex which in essence narrows the port making the tuning a higher note and now way more efficient. Then when pushed even harder, over excursion of the driver happens at this higher than planned tuning, meaning going louder makes the rest of the bandwidth come up in SPL to match the peak that cant go any louder so when played at the most the amp will give its all within +/-3db again but with a higher f3.. is this mechanical compression that evens it out at full tilt how I've described above ? Obviously its bad to push a driver right into over excursion, so i don't recommend it. but sometimes its fun and some drivers can take it if you have the amp headroom. What can be done to compensate for this non linear response and shift in tuning and efficiency ? Im assuming this has to happen to every cab with straight ports, yet cabs like D&B J subs don't seem to suffer it ? is this done by DSP trickery ? are they just limiting the cabs to the point before over excursion happens (explains the low wattages kinda) and using dynamic eq to calm the ports down ? interested to see what the hive mind has to say about this Cheers Luke https://www.facebook.com/WhiteNoiseSoundEnforcement/ Edited by White Noise - 30 August 2017 at 8:10pm |
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gen0me
Young Croc Joined: 20 February 2016 Location: UK Status: Offline Points: 999 |
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Helmholtz.
Narrower port on logic should have lower tunning than one with higher crossurface area. I dont understand the amp thing. Amp should give same signal not depending on excursion. Unless you mean that pushed amp sounds "thinner" than playing half power. ? Edited by gen0me - 30 August 2017 at 8:33pm |
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White Noise
Registered User Joined: 05 January 2015 Status: Offline Points: 54 |
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yes your right, the port tuning shouldnt go up if its narrower. but it defiantly gets louder above the tuning and not on it. my understanding of whats going on may be a bit skewiff, but that's why i'm posting on here..
if you can use horn resp, put in a simple reflex cab that plays flat ish and run sims at 1w then 10 w then 100w then max power. you will see what i mean by feeding it diferent powers from the amp and changing the spl vs frequency. i may just have a bad way of explaining things sorry Edited by White Noise - 30 August 2017 at 8:47pm |
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gen0me
Young Croc Joined: 20 February 2016 Location: UK Status: Offline Points: 999 |
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I have exactly the same first resonance and 2nd resonance either. 2V and 100V. Simed as Nd. S1=1220 S2 1221 Con 0.01 Classic 18" rcf 18x401. How do you sim?
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White Noise
Registered User Joined: 05 January 2015 Status: Offline Points: 54 |
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not sure if we are on about the same thing here, and if i was simming a reflex box it would be ND as you say but 0 for all s1-s5 and vented rear chamber, then vrc of 100 , ap 300 and ld of 21.83 would make a 45 hz 100l box
then if you sim that with a decent 15 it will show a flat (ish) graph at 1w... then go onto the max spl and put in diferent amounts of power and a massive peak occurs above the tuning at 10 watts if then at 1,2 or 300 or more, it starts to exceed at this peak and at max rms it levels out but with massive over excursion just above and below tuning. the port acts as an acoustic mass to the driver at tuning hence why it doesnt move much at the ports frequency you dont notice it when you measure with smart because it doesnt push it hard enough to go into a non linear respons.. and when you run just a few boxes and run them hard, you dont really notice it because of the mechanical compression at that frequency. but on a bigger pile of 6 or 8 reflex or band pass, where there is loads of headroom, this excess efficiency above tuning is really noticeable and difficult to tame with eq because its constantly changing frequency and magnitude im wondering what the pros do to overcome this other than fancy port design i hope what i'm saying makes more sense now Edited by White Noise - 30 August 2017 at 10:51pm |
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gen0me
Young Croc Joined: 20 February 2016 Location: UK Status: Offline Points: 999 |
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Max spl is dependent on efficiency. Click on tools >>efficiency. Max spl should have maximum in same place. But the voltage graph dont change shape with different Eg in input parameters(i made S1, S2, L12 =0) in my sims. For different efficiency across bandwidth different 1w in maximum spl graph is for different voltages at different frequencies.
Edited by gen0me - 30 August 2017 at 11:20pm |
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White Noise
Registered User Joined: 05 January 2015 Status: Offline Points: 54 |
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okay i get you, i just always thought that changing the input voltage in the first screen just gave you a calculation of the spl.
rather than the max spl function taking into consideration some real life things like over excursion, port vortexes and power compression.. is that not why the graphs are loads diferent. not many relflex boxes sound like the graph for just upping the input voltage, they sound like the graph looks when you press max spl |
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gen0me
Young Croc Joined: 20 February 2016 Location: UK Status: Offline Points: 999 |
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Edited by gen0me - 31 August 2017 at 12:14am |
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White Noise
Registered User Joined: 05 January 2015 Status: Offline Points: 54 |
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If anybody else has any thoughts on this id love to hear it.
i can see that i have been mistaken about the tuning going up... effectively thats impossible. But is what happens is that when a reflex gets into swing, there is an excess efficiency above tuning, because there is no acoustic mass or resistance to the driver pushing at this frequency. this means it moves loads of air with very little power. the classic one notey ness of a reflex... how can you make the response of any bandpass or reflex more linear ? i see some pro boxes with xlr's going back to the amps to monitor and controll xmax, this has to be to stop exactly what i'm saying here. how is this implemented ? im pretty sure D&B boxes dont use this, so there must be other ways too Edited by White Noise - 01 September 2017 at 4:52pm |
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gen0me
Young Croc Joined: 20 February 2016 Location: UK Status: Offline Points: 999 |
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Using different speaker. So next thought is that max spl curve has nothing to do with real speakers condition but rather dsp options. Although amps are voltage drivers driven on minimum impedance their voltage would be dropping less on tunning frequencies. If you mean speakers like ipal they seem to be last choice for bandpass from ts. |
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Crashpc
Registered User Joined: 26 February 2008 Location: Czech Republic Status: Offline Points: 463 |
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I would put the Ipal anywhere. Don´t interchange sensitivity for efficiency. If you have the amp, capable of good voltage swing, it will drive this beast and it will work well after DSP. These are just NOT mass/compliance resonance driven speakers anymore.
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Nikon and Canon people should not be married to each other. Why did you let this happen?
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White Noise
Registered User Joined: 05 January 2015 Status: Offline Points: 54 |
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This kinda answers some of what i was asking,
http://www.excelsior-audio.com/Publications/Using_Limiters_to_Help_Protect_Loudspeakers.pdf anyone had any joy doing similar to this with DEQ ? |
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