Horn length-Mouth size |
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Bassaddict
Registered User Joined: 18 April 2008 Location: North East UK Status: Offline Points: 308 |
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Posted: 29 January 2009 at 3:15pm |
correct me if im wrong but as far as i am aware the cut off frequency of a horn is a quarter of a wavelength of the lowest frequency that the horn can play.
so what does the mouth size have to do with it?
cheers
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The only good system is a sound system.
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speaker monkey
Registered User Joined: 15 January 2008 Status: Offline Points: 281 |
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I think the mouth area has more to do with impedance matching with the air.
That's my 2p anyway. |
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doober
Young Croc Joined: 03 January 2006 Location: Cornwall UK Status: Offline Points: 1118 |
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A good rule of thumb for mouth size is, assuming a round horn, the circumference needs to equal the wavelength of the lowest frequency, if used in full space. The area can be halved for half space use, halved again for quarter space and so on.
If the mouth is too small there will be a reduction in level at the lower end, and peaks and dips above cut off. For a more detailed explaination look at Keele's paper If you are using an exponential horn you will also have a flare rate, or flare frequency (called F12 in hornresp) which is linked to the ratio of throat to mouth. This also determines how low a horn will play. There's a lot more to it than that, look at some of the horn design links on the main speakerplans site. Kieran |
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Blahblahblah
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doober
Young Croc Joined: 03 January 2006 Location: Cornwall UK Status: Offline Points: 1118 |
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A horns main purpose is to match the impedance with the air. It can be thought of as an acoustic gearbox. |
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Blahblahblah
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Bassaddict
Registered User Joined: 18 April 2008 Location: North East UK Status: Offline Points: 308 |
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cheers for the link. have actually been flicking through some of his AES papers today.
only one of the the links for the horn pages is working the rest are saying 'PAGE NOT FOUND'
any ideas?
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The only good system is a sound system.
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doober
Young Croc Joined: 03 January 2006 Location: Cornwall UK Status: Offline Points: 1118 |
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Blahblahblah
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Bassaddict
Registered User Joined: 18 April 2008 Location: North East UK Status: Offline Points: 308 |
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thanks again doober, some cracking links there
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The only good system is a sound system.
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Steve_B
Old Croc Joined: 29 September 2007 Status: Offline Points: 1587 |
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If the walls of a horn are parallel, a tube, it will resonate at frequencies where the length of the tube is an odd integer multiple of ΒΌ wavelength. If the mouth area is sufficiently large the horn will be non-resonant. Most bass horns are not ideal and exhibit the resonant, if damped, characteristics of a quarter wavelength pipe. |
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S DeXter
Young Croc Joined: 12 May 2008 Location: Moon Status: Offline Points: 1268 |
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So then what effect does wadding / foam on walls have on horn behaviour? I assume it probably has no useful effect as its never used...
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Deadbeat
Old Croc Joined: 12 March 2008 Location: Singularity Status: Offline Points: 3167 |
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Wadding and foam are pretty useless at LF...apart from helping marginally with standing waves in the front/rear chamber.
At HF, it's a different story.
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Away on extended leave.
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davey t
Young Croc Joined: 11 March 2004 Location: Bristol UK Status: Offline Points: 1428 |
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There's a formula which calculates the cutoff frequency of a exp horn. It uses
1) horn length 2) mouth area 3) throat area 4) horn exp coeff (eg 1 = hyp exp) I cant remember it offhand. but basically if you make the mouth/ throat ratio smaller you get a lower cutoff for a given horn length at the expense of a more peaky response and less gain. simple. |
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Minirig portable soundsystem movement
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speaker monkey
Registered User Joined: 15 January 2008 Status: Offline Points: 281 |
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Baranecks horn equations:
m = (2pi * fc) / c and S = St * e^(mx) Where m = Horn Constant/Coefficient fc = Cut-off freq c = speed of sound in air (normally 343m/s) S = Mouth Area St = Throat Area x = length of horn I have also seen the first equation using "4pi" instead of "2pi", this was in the Klipsh horn, possibly using 4pi because it is a W bin therefore a double horn. But I am really not sure about that and never did figure it out properly. Would be nice if someone would clear that up for me (not that I mean to hijack a thread) monkey x Edited by speaker monkey - 04 February 2009 at 11:48pm |
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