hi all!,
i have a question about reflex duct shape.
is better rectangular or triangular ?
help me to exam the two options
thank you
i have a question about reflex duct shape.
is better rectangular or triangular ?
help me to exam the two options
thank you
I assume you mean "port" as is commonly used around here... I'm no expert, but my guess is that neither of the two are necessarily "better" than one another. One thing you do have to keep in mind is to not make the opening too long/thin, otherwise you get into trouble with "fluid" properties of air and the inside surface of the port.
Also, it is usually better to round the edges of the opening, and this may be easier with a rectangular port. Circular ports are the easiest to round-over if you have a router.
Do you have a design where a triangular port is used? It would be interesting to see ;-)
Also, it is usually better to round the edges of the opening, and this may be easier with a rectangular port. Circular ports are the easiest to round-over if you have a router.
Do you have a design where a triangular port is used? It would be interesting to see ;-)
Rectangular is better than triangular but both are not optimal...
Optimal port shape: See the picture.
b🙂
Attachments
Bjorno: that is a very interesting attachment you have shown. Do you have a link to the article it came from? Or could you explain further how to design such a port? Thanks and best regards Moray James.
I've made some rectangular ports. I have a router and roundover bits, but I haven't put a roundover on any of the ports I have made. I wasn't sure if it would change the tuning of the port. If anyone had thoughts on that I'd be interested to know.
It is a french master thesis for a company heliaetus 😉
A french presentation
http://www-cast3m.cea.fr/html/ClubCast3m/club2007/Club07_GPellerin.pdf
The original website is here but not work:
http://old.lam.jussieu.fr/src/membres/pellerin/pub
The AES article is more interesting because you can calculate the profile. I made an excel spreadsheet to calculate them. Not so easy you need to use the excel solver.
A french presentation
http://www-cast3m.cea.fr/html/ClubCast3m/club2007/Club07_GPellerin.pdf
The original website is here but not work:
http://old.lam.jussieu.fr/src/membres/pellerin/pub
The AES article is more interesting because you can calculate the profile. I made an excel spreadsheet to calculate them. Not so easy you need to use the excel solver.
General flaring of the ports is good for reducing turbulance. If you start with a given length and then flare it, it will tend to make the effective length shorter (tuning higher) because a percentage of the port is having its diameter increased.
I'm talking about big end flares here. Roundover bits wil barely change the tuning, yet getting rid of the sharp edges will cut down turbulance a significant amount.
Regarding port cross section, we would like to have the maximum area with the minimum port wall surface. Best would be from a circular port. Worst would be a very thin and deep slot. I can't imagine an equilateral triangle would be much worse than a circle. If it fits the cabinet better, go for it.
I once tried a compact sub with an elliptical profile from end to end of the port, supposedly one of the best cross sections and patented by our friends at Bose. It was better but not by leaps and bounds. We ended up going with a PR.
David S.
Rounding ports with a 3/4" round bit will reduce chuffing (turbulence noise) and won't appreciably change tuning.
Edit: Just like Speaker Dave said :^).
Thank you jerome69 and all
That's the right origin for my drawing, agree the AES paper is the one to study.
Most important is that the port becomes directional and have output lower than fb in a BR box besides a comparable smaller size than a straight cylindrical port.
b 🙂
That's the right origin for my drawing, agree the AES paper is the one to study.
Most important is that the port becomes directional and have output lower than fb in a BR box besides a comparable smaller size than a straight cylindrical port.
b 🙂
Reflex Port & Related Abstracts
For those that may be interested, a listing of articles that address some of the issues of bass reflex enclosure design are provided below.
Regards,
WHG
File: AESJ050-0019.pdf
Title: Maximizing Performance from Loudspeaker Ports
Author(1): Salvatti, Alex (1)
Author(2): Button, Doug (1)
Author(3): Devantier, Alan (2)
Affiliation(1): JBL Professional, Northridge, California
Affiliation(2): Infinity Systems, Northridge, California
Publication(1): AES-P, No. 4855, Cnv. 105, Aug-1998
E-Lib.(1): (CD aes14) /pp9798/pp9809/4745.pdf
Publication(2): AES-J, Vol. 50, No. 1, pp. 19-45, Dec-2001
E-Lib.(2): (CD aes19) /jrnl2002/2002/0019.pdf
Abstract(1): There is a current trend in the marketplace for loudspeaker ports to have a more aerodynamic appearance. This may be as much for appearance as for performance reasons. However, the sharp discontinuity at the end of a traditional port does create turbulence at high drive levels as air is drawn into the port. As well, the axial cross-sectional shape of the port can have an influence on the turbulence generated on the air as it exits the port.
Abstract(2): Ports altered to provide a more aerodynamic shape to minimize turbulence for inlet and exit air streams show performance improvements in efficiency, compression, maximum output and distortion reduction and will be outlined in this paper. The ideal shapes for high velocity inlet and exit air streams are different and the best solution is one that balances both. Additionally, turbulence is actually preferred in matters of cooling the box through heat exchange via the air in the port.
File: AESP4661.pdf
Title: Reduction of Bass-Reflex Port Nonlinearities by Optimizing the Port Geometry
Author(1): Roozen, N. B. (1)
Author(2): Vael, J. E. M. (1)
Author(3): Nieuwendijk, J. A. M. (1)
Affiliation(1): Philips Research Labs, Eindhoven, The Netherlands
Publication: AES-P, No. 4661, Cnv. 104, Apr-1998
E-Lib.: (CD aes14) /pp9798/pp9805/4551.pdf
Abstract(1): Bass-reflex ports are used to enhance the bass reproduction of loudspeakers. However, at higher output levels, the bass-reflex port nonlinearities, specifically the unsteady separation of the acoustic flow, can lead to blowing sounds and acoustic losses. Calculations and measurements are presented for a number of port geometries. The effect of rounding the port terminations and the effect of converging-diverging ports on the production of blowing sounds are investigated.
Abstract(2): It appears that the intensity of the unsteady separation of the acoustic flow, as well as the radiation efficiency of the port and the quality factor of the port resonances, altogether determine the level of blowing noise. Maximum reduction of the blowing sounds and acoustic losses is obtained by using a port contour geometry that slowly diverges from the center towards both port ends and is rounded with curvature radii that are not too large at both port terminations.
File: AESP3999.pdf
Title: The Nonlinear Behavior of Reflex Parts
Author: Backman, Juha
Affiliation: Nokia Mobile Phnes, Salo, Finland
Publication: AES-P, No. 3999, Cnv. 98, Jan-1995
E-Lib.: (CD aes13) /pp9496/pp9502/3959.pdf
Abstract: The results of an experimental study of the nonlinear behavior of reflex ports of various shapes are presented. The parameters that are compared between the ports include noise, harmonic distortion, and flow resistance. The result is that careful design yields significant improvements in the dynamic range, however, relatively small irregularities can increase the nonlinearity greatly.
File: AESP4523.pdf
Title: Loudspeaker Ports
Author: Vanderkooy, John
Affiliation: University of Waterloo, Waterloo, Ontario, Canada
Publication: AES-P, No. 4523, Cnv. 103, Aug-1997
E-Lib.: (CD aes14) /pp9798/pp9709/4413.pdf
Abstract(1): The demand for high-level low-frequency sound in a cinema and home theater audio systems, and the desire for compact subwoofers, is the motivation behind the work. Tapered ports were believed to perform better than simple ones, and this study sets out to measure and analyze such ports and assess their performance. A model of an acoustic port of varying cross-section is presented.
Abstract(2): Acoustic ports of various shapes are studied to show their behavior at a range of levels. It is found that flared ends do help, but at high levels, some losses and distortion remain. A gently-tapered port shows better behavior.
File: AESP4748.pdf
Title: Nonlinearities in Loudspeaker Ports
Author: Vanderkooy, John
Publication: AES-P, No. 4748, Cnv. 104, Apr-1998
E-Lib.: (CD aes14) /pp9798/pp9805/4638.pdf
Abstract: The increasing demand for high SPL at low frequencies has prompted the study of nonlinearities in loudspeaker ports. At high levels, ports produce jets of air, display acoustic compression, loss and distortion, generate turbulence noise, and rectify pressure fluctuations. These phenomena are measured and analyzed. A simplified nonlinear model of a port is presented. Flared ends and gentle tapering help somewhat. The low-level linear acoustic characteristics of ports of varying cross-section are analyzed.
Abstract: The velocity profiles of ports are measured, and a new theory for this and the associated nearfield pressure is presented, which contradicts the usual "piston-in-a-baffle" approach.
File: ASAJ104-1914.pdf
Title: Vortex Sound in Bass-Reflex Ports of Loudspeakers. Part I. Observation of Response to Harmonic Excitation and Remedial Measures
Author(1): N. Bert Roozen
Author(2): Marije Bockholts
Author(3): Pascal van Eck (2)
Author(4): A. Hirschberg (2)
Affiliation(1): Philips Research Labs, Eindhoven, The Netherlands
Affiliation(2): Eindhoven University of Technology, Eindhoven, The Netherlands
Publication: ASA-J, Vol. 104, No. 4, p. 1914, Oct-1998, DOI: 10.1121/1.423760
Abstract(1): At high sound pressure levels a bass-reflex port produces blowing sounds, especially in the case of small loudspeaker boxes with narrow bass-reflex ports. The blowing sounds are caused by vortex shedding of the acoustic flow at the end of the port at high flow velocities. It has been found that acoustic standing waves in the longitudinal direction of the port are excited in a pulsatile manner by the periodically generated vortices.
Abstract(2): This is demonstrated by time history measurements of the blowing sounds of a loudspeaker system with a bass-reflex port driven by a harmonic signal. Broadband turbulence sound appears to be weaker than these deterministic sounds.
Abstract(3): It has been found that, near the 1-kHz port resonance frequency, the power level of the blowing sounds can be reduced by 8 dB by using a port cross section that diverges gradually toward both port ends with a slope angle at the port ends of about 6°, and rounding the edges at both port ends.
File: ASAJ104-1919.pdf
Title: Vortex Sound in Bass-Reflex Ports of Loudspeakers. Part II. A Method to Estimate the Point of Separation
Author(1): N. Bert Roozen (1)
Author(2): Marije Bockholts
Author(3): Pascal van Eck (2)
Author(4): A. Hirschberg (2)
Affiliation(1): Philips Research Labs, Eindhoven, The Netherlands
Affiliation(2): Eindhoven University of Technology, Eindhoven, The Netherlands (2)
Publication: ASA-J, Vol. 104, No. 4, p. 1919, Oct-1998, DOI: 10.1121/1.423762
Abstract(1): In part I of this paper, the vortex shedding that may occur in a bass-reflex port of a loudspeaker system was discussed. At the Helmholtz frequency of the bass-reflex port, air is pumped in and out at rather high velocities, vortex shedding occurs at the end of the port, and blowing sounds are generated. It was explained that the key in the design of a port with a minimum of blowing sounds is the point of flow separation from the wall at which vortices are formed.
Abstract(2): This paper presents a method for estimating the point of separation for an unsteady flow like the flow through a bass-reflex port. Assuming that the flow can be described by a potential flow up to the point where flow separation occurs, it was found that the point of separation can be estimated on the basis of measurement of the sound pressure inside the loudspeaker box and measurement of the sound pressure at a distance of 1 m from the port exit.
Abstract(3): Application of the proposed technique to a cylindrical port with rounded edges at both port ends revealed that the point of separation is determined by the particle displacement rather than by the particle velocity. It was also found that a good indicator of the onset of severe vortex shedding is the Strouhal number based on the radius of curvature of the port edges.
File: ASAJ021-
Title: The Acoustic Reactance of Small Circular Orifaces
Author(1): R.H. Bolt
Author(2): S. Labate
Author(3): K. Uno Ingard
Publication: ASA-J, Vol. 21, No. 2, Mar-1949
Abstract(1): A precise experimental study of the reactive component of the complex impedance of small circular orifices has been made for a number of orifices varying in diameter from 2 cm down to 0.357 cm, with diameter/ thickness ratios from 4 to 40, over the frequency range from 200 to 1000 c.p.s. The measurement of the impedance is performed with a precision impedance tube.
Abstract(2): A theoretical correction for the influence of the tube walls is applied by considering the orifice to act as a plane piston and taking into account the influence of all possible higher order modes of the tube in the neighborhood of the orifice.
Abstract(3): Comparison between calculated and measured values of reactance shows that, as far as this quantity is concerned, the assumption that the orifice acts like a plane piston appears valid for radii equal to or greater than 1 cm for "thin" orifices within the range of variables specified above. For orifices of radii less than 1 cm, however, a modification of the radius is necessary to make the classical theoretical equation for orifice reactance fit the measured data.
Abstract(4): The observations and analyses reported here are restricted to wave lengths much greater than the hole diameter and to the low velocity region where reactance is independent of particle velocity.
File: NASA-TM112200
Title: Exact Solutions for Sound Radiation from a Circular Duct
Author(1): Y. C. Cho (1)
Author(2): K. Uno Ingard (2)
Affiliation(1): Ames Research Center, Moffett Field, California
Affiliation(2): Massachusetts Institute of Technology, Cambridge, Massachusetts
Publication: NASA Technical Memorandum No. 112200 (Jun-1997) (Ames Research Center)
URL: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970024844_1997035016.pdf
Abstract(1): This paper presents a method of evaluation of Wiener-Hopf technique solutions for soundradiation from an unflanged circular duct with infinitely thin duct wall, including mean flows.
Abstract(2): Sound radiation from circular ducts is a classical acoustics problem. Exact solutions were previously reported: the Wiener-Hopf technique was used for radiation of propagating modes from a circular duct with negligibly thin duct wall (refs. 1-3), and the hyperboloidal wave function was defined and employed for radiation from duct with various types of termination, including a plane flange and horns (ref. 4).
Abstract(3): Exact solutions undoubtedly help one to gain physical insight into the problem and can often be used in practical designs. In this electronic computation age, another significant role of exact solutions is defined as means of cross examination of results of numerical techniques. These techniques, embraced as computational aero acoustics, are just starting to attract widespread attention as a potential tool in attacking important aero acoustic problems for which quantitative solutions are not available.
Abstract(4): Despite the elegance of the closed form solutions with the Wiener-Hopf technique, numerical presentations have been limited to mere demonstrations of its capability. As a matter of fact, no computer program is publicly available for its numerical evaluation. Numerical evaluation of the Wiener-Hopf solution is not straightforward; it requires the exercise of extreme care and, often, sophisticated mathematical tricks.
Abstract(5): This paper attempts to provide a comprehensive mathematical procedure for evaluation of Wiener-Hopf solutions. In section 2, acoustic waves will be briefly reviewed for in-duct propagation and radiation. In section 3, the Wiener-Hopf technique is applied to obtain solutions, and section 4 is devoted to the evaluation of integrals involved in the solutions.
File: AESJ050-0019.pdf
Title: Maximizing Performance from Loudspeaker Ports
Author(1): Alex Salvatti
Author(2): Allan Devantier
Author(3): Doug J. Button 19
Publication: AES-J, Vol. 50, No.1, p. 19, Jan-2002
Abstract: The low-frequency performance of a loudspeaker is significantly enhanced by the use of tapered ports, but there are numerous trade-offs involving the size of the port and the input and output tapers. Design issues include the effectiveness of heat transfer, amount of turbulence created, air velocity, smoothness of the taper, symmetry of the two tapers, effective mass in the port, and the contribution to the frequency response. Suggested design rules are based on extensive empirical studies.
File: AESP7253.pdf
Title: Improvement of One-Dimensional Loudspeaker Models
Author: Juha Backman
Affiliation: Helsinki University of Technology, Acoustics Laboratory, Espoo, 02150, Finland
Publication: AES-P, No. 7253, Cnv. 123, Oct-2007
URL: AES E-Library Improvement of One-Dimensional Loudspeaker Models
Abstract(1): Simple one-dimensional waveguide models of loudspeaker enclosures describe well enclosures with simple interior geometry, but their accuracy is limited if used with more complex internal structures. The paper compares the results from one-dimensional models to FEM models for some simplified enclosure geometries found in typical designs.
Abstract(2): Based on these results it is apparent that one-dimensional models need to be refined to take some three-dimensional aspects of the sound field in close proximity of drivers into account. Approximations matched to FEM solutions are presented for enclosure impedance as seen by the driver and for the end correction of ports, taking both edge rounding and distance to the back wall into account.
File: AESP6194.pdf
Title: Analysis and Modeling of the Bi-Directional Fluid Flow in Loudspeaker Ports
Author(1): Devantier, Allan (1)
Author(2): Rapoport, Zachary (1)
Affiliation(1): Affiliation: Harman International, Northridge, CA
Publication: AES-J, Vol. 49, No. 3, p. 117, Mar-2001
URL: AES E-Library Analysis and Modeling of the Bi-Directional Fluid Flow in Loudspeaker Ports
E-Library: (CD 117Papers) /117/6194.pdf
Abstract(1): Bass reflex ports are used in loudspeakers to enhance low frequency performance. At low sound levels the port extends the low frequency response by supplying one of the components of a Helmholtz resonator. At higher sound levels the turbulent intensity in the port increases disrupting the Helmoholtz resonance causing distortion, noise and compression. Although there has been significant work done to reduce these negative effects, no optimal solution has been found.
Abstract(2): To better understand the flow phenomena within the port, Computational Fluid Dynamics, CFD, was used to model the flow. The flow was simulated for six port profiles over a wide range of sound levels. In order to correlate the results of the CFD work to the real world, the same six ports were prototyped and subjected to several objective and subjective tests.
File: AESJ050-0019.pdf
Title: On Sound Radiation from Ported Enclosures
Author: Geddes, Earl R
Affiliation: Visteon Corporation, Dearborn, MI
Publication: AES-J, Vol. 50, No.1, p. 19, Jan-2002
URL: AES E-Library On Sound Radiation from Ported Enclosures
E-Library: (CD aes18) /jrnl2001/2001/2001117.pdf
Abstract: Assumptions made in the lumped-parameter model for ported enclosures are shown to limit the accuracy of these models. The effect of these assumptions on the real room response is discussed.
File: AESP6157.pdf
Title: Sound Source Design in the Very Low Frequency Domain
Author(1): Pellerin, Guillaume (1)
Author(2): Polack, Jean-Dominique (1)
Author(3): Morkerken, Jean-Pierre (1)
Affiliation(1): Laboratoire d’Acoustique Musicale (Paris 6, CNRS)
Publication: AES-P, No. 6157, Cnv. 116, May-2004
URL: AES E-Library Sound Source Design in the Very Low Frequency Domain
E-Library: (CD 116Papers) /116/6157.pdf
Abstract(1): Whereas the aerodynamic effects take a significant place in the behavior of sound sources in the low frequency domain and for signals containing a high specific energy, new complex fluid parameters have to be implemented to take into account possible causes of sound distortion such as the stalling phenomenon in the boundary layer around the mechanical structure.
Abstract(2): For the design of vented boxes, we show that the choice of a nozzle profile for the resonator ensure a better dynamical stability of the airflow and thus authorize extreme low cutoff frequencies in "dipole" configurations. It will be described some experimental and computed results based on phase spaces and fluid FEM about the radiating output of that kind of source under 40 Hz.
File: AESP5522.pdf
Title: Vented-box Geometry and Low Frequency Reproduction: The Aerodynamical Approach
Author(1): Morkerken, Jean-Pierre (1)
Author(2): Parzy, Benjamin (1)
Author(3): Pellerin, Guillaume; Polack, Jean-Dominique (1)
Author(4): Polack, Jean-Dominique (1)
Affiliation(1): Laboratoire d'Acoustique Musicale, Paris, France
Publication: AES-P, No. 5522, Cnv. 112, Apr-2002
URL: AES E-Library Vented-box Geometry and Low Frequency Reproduction: The Aerodynamical Approach
E-Library: (CD aes19) /pp2002/pp0205/000183.pd
Abstract(1): To-day, low frequency reproduction with loudspeakers in vented box is limited by two factors: the volume of the box, and non-linear airflow through the vent. We propose a novel approach that takes into account aerodynamical parameters, leading to original profiles and an improved functioning of the vented box. For example, under certain alignment conditions, there exists a second cut-off frequency below the first one, localised on the lower impedance hump.
Abstract(2): Using this lower cut-off frequency and an adapted vent profile makes it possible to radiate frequencies around 40 Hz with box volumes smaller than 1 liter and small drivers. A prototype will be demonstrated.
File: AESP0842.pdf
Title: The Vented Loudspeaker: A Restatement
Author: Keele, Jr., D. B. (Don)
Affiliation: Brigham Young University, Electronic Media Department, Provo, UT
Publication: AES-P, No. 842, Cnv. 42, May-1972
URL: AES E-Library The Vented Loudspeaker: A Restatement
E-Library: (CD aes8) /pp6975/pp7205/0808.pdf
Abstract(1): Additional information on the use and application of A.N. Thiele's alighments for the vented loudspeaker cabinet is presented. A rewritten alignment table which has all the frequency terms normalized to the speaker resonance frequency is included. Computer run frequency responses for all the alignments are displayed along with a new fourth order Chebyshev alignment beyond No. 9. Sensitivity functions for the cabinet output with respect to variaous system parameters (fb, fs, Qt, Vb) are derived and plotted.
Abstract(2): Methods of auxiliary filter synthesis are discussed including an equalization method to compensate for incorrect speaker Qt using a single second-order peak-dip filter. a simple cabinet tuning procedure using computer printed tables is also presented. A possible new set of small box low frequency alignments requiring comparatively high amounts of auxiliary filter boost at the box resonance frequency are suggested.
For those that may be interested, a listing of articles that address some of the issues of bass reflex enclosure design are provided below.
Regards,
WHG
File: AESJ050-0019.pdf
Title: Maximizing Performance from Loudspeaker Ports
Author(1): Salvatti, Alex (1)
Author(2): Button, Doug (1)
Author(3): Devantier, Alan (2)
Affiliation(1): JBL Professional, Northridge, California
Affiliation(2): Infinity Systems, Northridge, California
Publication(1): AES-P, No. 4855, Cnv. 105, Aug-1998
E-Lib.(1): (CD aes14) /pp9798/pp9809/4745.pdf
Publication(2): AES-J, Vol. 50, No. 1, pp. 19-45, Dec-2001
E-Lib.(2): (CD aes19) /jrnl2002/2002/0019.pdf
Abstract(1): There is a current trend in the marketplace for loudspeaker ports to have a more aerodynamic appearance. This may be as much for appearance as for performance reasons. However, the sharp discontinuity at the end of a traditional port does create turbulence at high drive levels as air is drawn into the port. As well, the axial cross-sectional shape of the port can have an influence on the turbulence generated on the air as it exits the port.
Abstract(2): Ports altered to provide a more aerodynamic shape to minimize turbulence for inlet and exit air streams show performance improvements in efficiency, compression, maximum output and distortion reduction and will be outlined in this paper. The ideal shapes for high velocity inlet and exit air streams are different and the best solution is one that balances both. Additionally, turbulence is actually preferred in matters of cooling the box through heat exchange via the air in the port.
File: AESP4661.pdf
Title: Reduction of Bass-Reflex Port Nonlinearities by Optimizing the Port Geometry
Author(1): Roozen, N. B. (1)
Author(2): Vael, J. E. M. (1)
Author(3): Nieuwendijk, J. A. M. (1)
Affiliation(1): Philips Research Labs, Eindhoven, The Netherlands
Publication: AES-P, No. 4661, Cnv. 104, Apr-1998
E-Lib.: (CD aes14) /pp9798/pp9805/4551.pdf
Abstract(1): Bass-reflex ports are used to enhance the bass reproduction of loudspeakers. However, at higher output levels, the bass-reflex port nonlinearities, specifically the unsteady separation of the acoustic flow, can lead to blowing sounds and acoustic losses. Calculations and measurements are presented for a number of port geometries. The effect of rounding the port terminations and the effect of converging-diverging ports on the production of blowing sounds are investigated.
Abstract(2): It appears that the intensity of the unsteady separation of the acoustic flow, as well as the radiation efficiency of the port and the quality factor of the port resonances, altogether determine the level of blowing noise. Maximum reduction of the blowing sounds and acoustic losses is obtained by using a port contour geometry that slowly diverges from the center towards both port ends and is rounded with curvature radii that are not too large at both port terminations.
File: AESP3999.pdf
Title: The Nonlinear Behavior of Reflex Parts
Author: Backman, Juha
Affiliation: Nokia Mobile Phnes, Salo, Finland
Publication: AES-P, No. 3999, Cnv. 98, Jan-1995
E-Lib.: (CD aes13) /pp9496/pp9502/3959.pdf
Abstract: The results of an experimental study of the nonlinear behavior of reflex ports of various shapes are presented. The parameters that are compared between the ports include noise, harmonic distortion, and flow resistance. The result is that careful design yields significant improvements in the dynamic range, however, relatively small irregularities can increase the nonlinearity greatly.
File: AESP4523.pdf
Title: Loudspeaker Ports
Author: Vanderkooy, John
Affiliation: University of Waterloo, Waterloo, Ontario, Canada
Publication: AES-P, No. 4523, Cnv. 103, Aug-1997
E-Lib.: (CD aes14) /pp9798/pp9709/4413.pdf
Abstract(1): The demand for high-level low-frequency sound in a cinema and home theater audio systems, and the desire for compact subwoofers, is the motivation behind the work. Tapered ports were believed to perform better than simple ones, and this study sets out to measure and analyze such ports and assess their performance. A model of an acoustic port of varying cross-section is presented.
Abstract(2): Acoustic ports of various shapes are studied to show their behavior at a range of levels. It is found that flared ends do help, but at high levels, some losses and distortion remain. A gently-tapered port shows better behavior.
File: AESP4748.pdf
Title: Nonlinearities in Loudspeaker Ports
Author: Vanderkooy, John
Publication: AES-P, No. 4748, Cnv. 104, Apr-1998
E-Lib.: (CD aes14) /pp9798/pp9805/4638.pdf
Abstract: The increasing demand for high SPL at low frequencies has prompted the study of nonlinearities in loudspeaker ports. At high levels, ports produce jets of air, display acoustic compression, loss and distortion, generate turbulence noise, and rectify pressure fluctuations. These phenomena are measured and analyzed. A simplified nonlinear model of a port is presented. Flared ends and gentle tapering help somewhat. The low-level linear acoustic characteristics of ports of varying cross-section are analyzed.
Abstract: The velocity profiles of ports are measured, and a new theory for this and the associated nearfield pressure is presented, which contradicts the usual "piston-in-a-baffle" approach.
File: ASAJ104-1914.pdf
Title: Vortex Sound in Bass-Reflex Ports of Loudspeakers. Part I. Observation of Response to Harmonic Excitation and Remedial Measures
Author(1): N. Bert Roozen
Author(2): Marije Bockholts
Author(3): Pascal van Eck (2)
Author(4): A. Hirschberg (2)
Affiliation(1): Philips Research Labs, Eindhoven, The Netherlands
Affiliation(2): Eindhoven University of Technology, Eindhoven, The Netherlands
Publication: ASA-J, Vol. 104, No. 4, p. 1914, Oct-1998, DOI: 10.1121/1.423760
Abstract(1): At high sound pressure levels a bass-reflex port produces blowing sounds, especially in the case of small loudspeaker boxes with narrow bass-reflex ports. The blowing sounds are caused by vortex shedding of the acoustic flow at the end of the port at high flow velocities. It has been found that acoustic standing waves in the longitudinal direction of the port are excited in a pulsatile manner by the periodically generated vortices.
Abstract(2): This is demonstrated by time history measurements of the blowing sounds of a loudspeaker system with a bass-reflex port driven by a harmonic signal. Broadband turbulence sound appears to be weaker than these deterministic sounds.
Abstract(3): It has been found that, near the 1-kHz port resonance frequency, the power level of the blowing sounds can be reduced by 8 dB by using a port cross section that diverges gradually toward both port ends with a slope angle at the port ends of about 6°, and rounding the edges at both port ends.
File: ASAJ104-1919.pdf
Title: Vortex Sound in Bass-Reflex Ports of Loudspeakers. Part II. A Method to Estimate the Point of Separation
Author(1): N. Bert Roozen (1)
Author(2): Marije Bockholts
Author(3): Pascal van Eck (2)
Author(4): A. Hirschberg (2)
Affiliation(1): Philips Research Labs, Eindhoven, The Netherlands
Affiliation(2): Eindhoven University of Technology, Eindhoven, The Netherlands (2)
Publication: ASA-J, Vol. 104, No. 4, p. 1919, Oct-1998, DOI: 10.1121/1.423762
Abstract(1): In part I of this paper, the vortex shedding that may occur in a bass-reflex port of a loudspeaker system was discussed. At the Helmholtz frequency of the bass-reflex port, air is pumped in and out at rather high velocities, vortex shedding occurs at the end of the port, and blowing sounds are generated. It was explained that the key in the design of a port with a minimum of blowing sounds is the point of flow separation from the wall at which vortices are formed.
Abstract(2): This paper presents a method for estimating the point of separation for an unsteady flow like the flow through a bass-reflex port. Assuming that the flow can be described by a potential flow up to the point where flow separation occurs, it was found that the point of separation can be estimated on the basis of measurement of the sound pressure inside the loudspeaker box and measurement of the sound pressure at a distance of 1 m from the port exit.
Abstract(3): Application of the proposed technique to a cylindrical port with rounded edges at both port ends revealed that the point of separation is determined by the particle displacement rather than by the particle velocity. It was also found that a good indicator of the onset of severe vortex shedding is the Strouhal number based on the radius of curvature of the port edges.
File: ASAJ021-
Title: The Acoustic Reactance of Small Circular Orifaces
Author(1): R.H. Bolt
Author(2): S. Labate
Author(3): K. Uno Ingard
Publication: ASA-J, Vol. 21, No. 2, Mar-1949
Abstract(1): A precise experimental study of the reactive component of the complex impedance of small circular orifices has been made for a number of orifices varying in diameter from 2 cm down to 0.357 cm, with diameter/ thickness ratios from 4 to 40, over the frequency range from 200 to 1000 c.p.s. The measurement of the impedance is performed with a precision impedance tube.
Abstract(2): A theoretical correction for the influence of the tube walls is applied by considering the orifice to act as a plane piston and taking into account the influence of all possible higher order modes of the tube in the neighborhood of the orifice.
Abstract(3): Comparison between calculated and measured values of reactance shows that, as far as this quantity is concerned, the assumption that the orifice acts like a plane piston appears valid for radii equal to or greater than 1 cm for "thin" orifices within the range of variables specified above. For orifices of radii less than 1 cm, however, a modification of the radius is necessary to make the classical theoretical equation for orifice reactance fit the measured data.
Abstract(4): The observations and analyses reported here are restricted to wave lengths much greater than the hole diameter and to the low velocity region where reactance is independent of particle velocity.
File: NASA-TM112200
Title: Exact Solutions for Sound Radiation from a Circular Duct
Author(1): Y. C. Cho (1)
Author(2): K. Uno Ingard (2)
Affiliation(1): Ames Research Center, Moffett Field, California
Affiliation(2): Massachusetts Institute of Technology, Cambridge, Massachusetts
Publication: NASA Technical Memorandum No. 112200 (Jun-1997) (Ames Research Center)
URL: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970024844_1997035016.pdf
Abstract(1): This paper presents a method of evaluation of Wiener-Hopf technique solutions for soundradiation from an unflanged circular duct with infinitely thin duct wall, including mean flows.
Abstract(2): Sound radiation from circular ducts is a classical acoustics problem. Exact solutions were previously reported: the Wiener-Hopf technique was used for radiation of propagating modes from a circular duct with negligibly thin duct wall (refs. 1-3), and the hyperboloidal wave function was defined and employed for radiation from duct with various types of termination, including a plane flange and horns (ref. 4).
Abstract(3): Exact solutions undoubtedly help one to gain physical insight into the problem and can often be used in practical designs. In this electronic computation age, another significant role of exact solutions is defined as means of cross examination of results of numerical techniques. These techniques, embraced as computational aero acoustics, are just starting to attract widespread attention as a potential tool in attacking important aero acoustic problems for which quantitative solutions are not available.
Abstract(4): Despite the elegance of the closed form solutions with the Wiener-Hopf technique, numerical presentations have been limited to mere demonstrations of its capability. As a matter of fact, no computer program is publicly available for its numerical evaluation. Numerical evaluation of the Wiener-Hopf solution is not straightforward; it requires the exercise of extreme care and, often, sophisticated mathematical tricks.
Abstract(5): This paper attempts to provide a comprehensive mathematical procedure for evaluation of Wiener-Hopf solutions. In section 2, acoustic waves will be briefly reviewed for in-duct propagation and radiation. In section 3, the Wiener-Hopf technique is applied to obtain solutions, and section 4 is devoted to the evaluation of integrals involved in the solutions.
File: AESJ050-0019.pdf
Title: Maximizing Performance from Loudspeaker Ports
Author(1): Alex Salvatti
Author(2): Allan Devantier
Author(3): Doug J. Button 19
Publication: AES-J, Vol. 50, No.1, p. 19, Jan-2002
Abstract: The low-frequency performance of a loudspeaker is significantly enhanced by the use of tapered ports, but there are numerous trade-offs involving the size of the port and the input and output tapers. Design issues include the effectiveness of heat transfer, amount of turbulence created, air velocity, smoothness of the taper, symmetry of the two tapers, effective mass in the port, and the contribution to the frequency response. Suggested design rules are based on extensive empirical studies.
File: AESP7253.pdf
Title: Improvement of One-Dimensional Loudspeaker Models
Author: Juha Backman
Affiliation: Helsinki University of Technology, Acoustics Laboratory, Espoo, 02150, Finland
Publication: AES-P, No. 7253, Cnv. 123, Oct-2007
URL: AES E-Library Improvement of One-Dimensional Loudspeaker Models
Abstract(1): Simple one-dimensional waveguide models of loudspeaker enclosures describe well enclosures with simple interior geometry, but their accuracy is limited if used with more complex internal structures. The paper compares the results from one-dimensional models to FEM models for some simplified enclosure geometries found in typical designs.
Abstract(2): Based on these results it is apparent that one-dimensional models need to be refined to take some three-dimensional aspects of the sound field in close proximity of drivers into account. Approximations matched to FEM solutions are presented for enclosure impedance as seen by the driver and for the end correction of ports, taking both edge rounding and distance to the back wall into account.
File: AESP6194.pdf
Title: Analysis and Modeling of the Bi-Directional Fluid Flow in Loudspeaker Ports
Author(1): Devantier, Allan (1)
Author(2): Rapoport, Zachary (1)
Affiliation(1): Affiliation: Harman International, Northridge, CA
Publication: AES-J, Vol. 49, No. 3, p. 117, Mar-2001
URL: AES E-Library Analysis and Modeling of the Bi-Directional Fluid Flow in Loudspeaker Ports
E-Library: (CD 117Papers) /117/6194.pdf
Abstract(1): Bass reflex ports are used in loudspeakers to enhance low frequency performance. At low sound levels the port extends the low frequency response by supplying one of the components of a Helmholtz resonator. At higher sound levels the turbulent intensity in the port increases disrupting the Helmoholtz resonance causing distortion, noise and compression. Although there has been significant work done to reduce these negative effects, no optimal solution has been found.
Abstract(2): To better understand the flow phenomena within the port, Computational Fluid Dynamics, CFD, was used to model the flow. The flow was simulated for six port profiles over a wide range of sound levels. In order to correlate the results of the CFD work to the real world, the same six ports were prototyped and subjected to several objective and subjective tests.
File: AESJ050-0019.pdf
Title: On Sound Radiation from Ported Enclosures
Author: Geddes, Earl R
Affiliation: Visteon Corporation, Dearborn, MI
Publication: AES-J, Vol. 50, No.1, p. 19, Jan-2002
URL: AES E-Library On Sound Radiation from Ported Enclosures
E-Library: (CD aes18) /jrnl2001/2001/2001117.pdf
Abstract: Assumptions made in the lumped-parameter model for ported enclosures are shown to limit the accuracy of these models. The effect of these assumptions on the real room response is discussed.
File: AESP6157.pdf
Title: Sound Source Design in the Very Low Frequency Domain
Author(1): Pellerin, Guillaume (1)
Author(2): Polack, Jean-Dominique (1)
Author(3): Morkerken, Jean-Pierre (1)
Affiliation(1): Laboratoire d’Acoustique Musicale (Paris 6, CNRS)
Publication: AES-P, No. 6157, Cnv. 116, May-2004
URL: AES E-Library Sound Source Design in the Very Low Frequency Domain
E-Library: (CD 116Papers) /116/6157.pdf
Abstract(1): Whereas the aerodynamic effects take a significant place in the behavior of sound sources in the low frequency domain and for signals containing a high specific energy, new complex fluid parameters have to be implemented to take into account possible causes of sound distortion such as the stalling phenomenon in the boundary layer around the mechanical structure.
Abstract(2): For the design of vented boxes, we show that the choice of a nozzle profile for the resonator ensure a better dynamical stability of the airflow and thus authorize extreme low cutoff frequencies in "dipole" configurations. It will be described some experimental and computed results based on phase spaces and fluid FEM about the radiating output of that kind of source under 40 Hz.
File: AESP5522.pdf
Title: Vented-box Geometry and Low Frequency Reproduction: The Aerodynamical Approach
Author(1): Morkerken, Jean-Pierre (1)
Author(2): Parzy, Benjamin (1)
Author(3): Pellerin, Guillaume; Polack, Jean-Dominique (1)
Author(4): Polack, Jean-Dominique (1)
Affiliation(1): Laboratoire d'Acoustique Musicale, Paris, France
Publication: AES-P, No. 5522, Cnv. 112, Apr-2002
URL: AES E-Library Vented-box Geometry and Low Frequency Reproduction: The Aerodynamical Approach
E-Library: (CD aes19) /pp2002/pp0205/000183.pd
Abstract(1): To-day, low frequency reproduction with loudspeakers in vented box is limited by two factors: the volume of the box, and non-linear airflow through the vent. We propose a novel approach that takes into account aerodynamical parameters, leading to original profiles and an improved functioning of the vented box. For example, under certain alignment conditions, there exists a second cut-off frequency below the first one, localised on the lower impedance hump.
Abstract(2): Using this lower cut-off frequency and an adapted vent profile makes it possible to radiate frequencies around 40 Hz with box volumes smaller than 1 liter and small drivers. A prototype will be demonstrated.
File: AESP0842.pdf
Title: The Vented Loudspeaker: A Restatement
Author: Keele, Jr., D. B. (Don)
Affiliation: Brigham Young University, Electronic Media Department, Provo, UT
Publication: AES-P, No. 842, Cnv. 42, May-1972
URL: AES E-Library The Vented Loudspeaker: A Restatement
E-Library: (CD aes8) /pp6975/pp7205/0808.pdf
Abstract(1): Additional information on the use and application of A.N. Thiele's alighments for the vented loudspeaker cabinet is presented. A rewritten alignment table which has all the frequency terms normalized to the speaker resonance frequency is included. Computer run frequency responses for all the alignments are displayed along with a new fourth order Chebyshev alignment beyond No. 9. Sensitivity functions for the cabinet output with respect to variaous system parameters (fb, fs, Qt, Vb) are derived and plotted.
Abstract(2): Methods of auxiliary filter synthesis are discussed including an equalization method to compensate for incorrect speaker Qt using a single second-order peak-dip filter. a simple cabinet tuning procedure using computer printed tables is also presented. A possible new set of small box low frequency alignments requiring comparatively high amounts of auxiliary filter boost at the box resonance frequency are suggested.
I've built the Basement Blasters and Iron Lawbreakers with triangular ports (with port end radiuses) at the lower end of the enclosure because the internal port members also give a significant amount of bracing to both the cabinet sides and bottom, and the two sides of the port formed by the interior walls of the cabinet do not require any flaring at all inside the cabinet. I probably could have saved a couple tenths of a cubic foot by using an optimum port taper, but opted for simplicity there, plus the more effective cabinet bracing of a constant cross section throughout the length of the port.
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Does anyone have any comment on using multiple lengths? I'm looking to do a dual 3" port for my 15" midwoofs, tuning to about 40Hz in 5 cu ft. That' gives me a length of 2.5", but I was thinking of using 1 each 2" and 3" long- can it be treated as an average length, or do we need to get more indepth calculating?
I wouldn't generalize about more extreme cases, but I think for what you are talking about the average length is correct. I am sure that the two ports combine for a single tuning.
David S.
We had an a/d/s sub that was built that way. Worked fine and did simplify the parts count.
David S.
I haven't that much experience, but when simulating ducts it seems like the larger I make the port the more resonances I get. Is that true in real life? If that's the case I wonder what is worst, chuffing noise or resonances?
Another question, can anything be worse than a round port with regards to resonance? I had a 2 kHz problem in my speakers and put x-shaped corrugated paper in the duct (see link below and scroll down to the last picture) and things really got better, both measured and soundwise (or maybe it was just placebo).
Modding Troels Vifa PL14/XT25 - Techtalk Speaker Building, Audio, Video, and Electronics Customer Discussion Forum From Parts-Express.com
Another question, can anything be worse than a round port with regards to resonance? I had a 2 kHz problem in my speakers and put x-shaped corrugated paper in the duct (see link below and scroll down to the last picture) and things really got better, both measured and soundwise (or maybe it was just placebo).
Modding Troels Vifa PL14/XT25 - Techtalk Speaker Building, Audio, Video, and Electronics Customer Discussion Forum From Parts-Express.com
Long ports of any shape may have resonances that may be objectionable, often a sharp dip, followed by a peak.
It does not look to me like the 2.5K reduction you saw has anything to do with the port, it has to do with the crossover choices you made.
You could determine what the port resonances actually are by measuring the response with the port open, port closed (stuff it with a rag, or potato, whatever) and the X shape.
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