Helmholtz Resonance

description19 papers

group73 followers

lightbulbAbout this topic

Helmholtz resonance is a phenomenon in acoustics where a cavity, such as a bottle or a closed container, resonates at a specific frequency due to the interaction between the air mass inside the cavity and the surrounding air. This resonance occurs when the volume of the cavity and the neck dimensions create a natural frequency of oscillation.

lightbulbAbout this topic

Determinación experimental de la carga específica del electrón.

by Daiver Juarez

In this experience, we used a PASCO scientific Model SE-9638 e/m apparatus, to estimate of measurement of electron specific charge in two methods. First we induced constant voltage and variable current, and we induced constant current and... more

descriptionView Paper arrow_downwardDownload

Internal pressure in a low-rise building with existing envelope openings and sudden breaching

by Aly Mousaad Aly and

This paper presents a boundary-layer wind tunnel (BLWT) study on the effect of variable dominant openings on steady and transient responses of wind-induced internal pressure in a low-rise building. The paper presents a parametric study... more

conservative in current codes (Sharma and Richards 2003, 2005). The present study focuses on the characterization of internal pressure due to sudden door or window breaching, effects of volume corrections, varying dominant opening areas and their location with respect to the incoming (upstream) wind direction. It also compares aerodynamic data obtained from experiments carrie¢ out at small-scale (boundary-layer wind tunnel) and at large-scale Wall of Wind (WoW). The WoW is a 6-fan large-scale testing facility primarily used to study wind and wind-driven rair effects on low-rise structures (Chowdhury et al. 2009, Bitsuamlak et al. 2009, Aly et al. 2012). Recently, a new 12-fan version of the WoW is constructed (Aly et al. 2011). For comparisor purposes, the WoW results presented in Tecle et al. (2012) are provided in the current study. The study also examines the peak internal pressure loading and compares with the wind load provisions of the ASCE 7-2010.

VV LINLw | AO Ulibv MMALOLYY Mi Gilby 42 LO Uli oVMitiviliv GQItnGA VE UWS UV ITTIIIICe Ad ieee =) te iO Uw characteristic length of the air slug, P, is the ambient pressure of air, P.(t) is the external pressure driving the inertial force, C,; and C,. are internal and external pressure coefficients respectively, V, is the effective volume of the cavity, L is the characteristic geometric length scale; @ is eave height wind speed, and subscripts m and f represent model and large-scale, respectively. For correct internal volume scaling and the appropriate measurement of the internal pressure fluctuations, the nominal volume obtained through length scaling needs to be magnified as given by Eq. (5). This could be done by providing an additional volume chamber underneath the wind tunnel turntable. In the present study, the model was prepared at a length scale of 1:9 and the test was conducted at a velocity scale of 1:4. Thus, the volume needed to be amplified by a factor of 16 as shown in Fig. 1(c). An airtight volume chamber box was attached to the base of the model building underneath the wind tunnel turntable, following the recommendation of Sharma et al. (2010). Sudden door or window opening test setup: The mechanism implemented to create door/window sudden failure during the wind tunnel testing utilized a digital servo motor system shown in Fig. 4. In order to obtain consistent aerodynamic data, it was necessary to operate the door without interfering with the building’s pressure taps. The best suited option for this application was the use of a remote controlled device. A radio control system normally used for model-aircraft was used as part of the electromechanical system and a radio transmitter was used to send out a signal of instructions (open/close the door), which were collected and interpreted by a radio receiver. The

CU IO BP - 7 ers ne all of the wind AoA examined, the internal rms values are lower than the external rms values measured at the periphery of the respective dominant opening. This is believed to be due to the uniform nominal background leakage and the presence of the vents, which caused damping and hence reduction of the intensity of the internal pressure fluctuations. The trend of the peak C,; and Cre, however, illustrates good correlation between the internal pressure and the external pressure at the dominant opening. For example, the rms of the internal and external pressure coefficients for the two biggest dominant openings (i.e., D}=7.5% and W=9%), as shown in Fig. 6, did not indicate enough internal pressure excitation. In addition, spectral analysis of the internal pressure time history did not show any significant peak due to Helmholtz resonance (see Fig. 7).

similar internal pressure variation regardless of the opening size (3.75% vs 9%). The distance between the upstream wall corner and the dominant opening has a significant impact on the r.m.s. values. For shorter distance between the windward wall corner and window opening, the r.m.s. fluctuation is considerably higher than the case when this distance is larger. For example, the r.m.s. for the 9% window (W2) at 30° wind AoA is 0.36; however, at 150° AoA the r.m.s. value is 0.1. A similar trend was observed for 20° and 160° AoAs. Fig. 8(c) shows the internal pressure coefficients due to window openings reaching a peak value at 70-75° wind AoA, whereas in the case of door openings, the maximum peak value occurs at 100° wind AoA. For wind AoA above 90°, the location of the window opening is far from the left corner (where upstream flow occurs) as compared to that of the door opening, which is located at the center of the wall. Consequently, the magnitude of the internal pressure due to both window openings is lower than that due to the door openings regardless of its smaller size. It can be

Fig. 6 Relationship of (a) rms and (b) maximum external pressure at dominant opening periphery vs internal pressure concluded that the dominant openings located outside of the center region of the wall exhibit larger internal pressure for an obtuse wind AoA. For example, for wind AoA between 0° and 70°, the window W; with 4% opening generates larger internal pressure than that due to doors D; (7.5%) and D> (5%). On the other hand, for wind AoA between 100° and 180°, the door openings D, and D, generate higher internal pressure compared to windows W; and W>. This attests to the significance of the dominant opening location.

Fig. 8 Living room internal pressure (mean, rms, max and min) distribution due to various dominant Oneningec Fig. 7 Internal pressure spectra for sudden opening, (there is no significant Helmholtz resonance)

Fig.9 Comparison of internal pressure coefficient inside attic for an opened and closed vent (a) mean and (b) maximum This indicates that vent operation could be useful not only for ventilation optimization, but also for prevention of wind driven rain intrusion, and regulation of the internal pressure.

Fig. 10 Comparison of internal pressure inside living room for an opened and closed vent: (a) mean and (b) maximum

Fig. 11 Internal pressure coefficient time history for sudden breach of 7.5% door (a) and 9% window (b), and the corresponding external pressure coefficient on the periphery of 7.5% door (c), and 9% window (d) The experimental study on the transient response of wind induced internal pressure to sudden breaching was carried out with the 7.5% (Test 7a) and 9% (Test 8a) dominant door opening located at the center and window opening located off-center with volume correction, respectively. Additional tests were also performed (i.e., Test 8b) without internal volume correction to examine the sensitivity of the transient response to the internal volume correction. All these tests were conducted for 45°, 75° and 90° wind AoA.

Fig. 12 Uniformity of internal pressure distribution at various wind AoA (45°, 75° and 90°)

Amanuel S. Tecle, Girma T. Bitsuamlak and Aly Mousaad ALY

Table 1 Dominant openings and background leakage distribution in small-scale dimensions ee Se Table 2 describes the various test scenarios performed to investigate internal and external pressures. Test cases | to 6 investigate the effects of the dominant opening sizes, while test cases 7 and 8 focused on transient internal pressure response during sudden openings of a door and a window, respectively. For test cases 7 and 8, the ceiling partition was removed so that the whole building acted as a single room (i.e., attic and living rooms were combined). Test cases 8a and 8b represent the same opening with and without volume correction, respectively, to assess the necessity of internal volume correction for transient response wind tunnel studies.

Table 2 Summary of test cases for low-rise building with gable and hip roof

Table 3 Response time comparisons As described in Table 3 the response time for Test 7a (7.5% door opening, located at the center of the windward wall) was 0.09s for the three wind AoAs considered. Test 8a (9% window opening located close to the right side of the windward wall) was 0.08s at 45° and 90° wind AoA while 0.07s for 75° wind AoA. This reveals that the response time of transient internal pressure overshooting is comparatively faster for a larger dominant opening as the opening area (A) governs the resonance frequency (Eq. (3)) for a given internal volume (V). Comparing the response time with and without internal volume correction, Test 8b for the building with no volume correction exhibited 4 times faster response than that of Test 8a (i.e., 0.03s vs 0.08s) irrespective of their similar opening size. This underlines the necessity for volume correction for transient response experiments in a BLWT with velocity ratios other than unity. The distribution of the peak internal pressure after sudden breach is shown in Fig. 12 for tests 7a and 8a. The data points starting from the time the door or window opened were taken by computing the peak values. The peak internal pressure is fairly uniform across the building cavity for all tests undertaken.

descriptionView Paper arrow_downwardDownload

Surface Absorption Coefficient Measurement

by Ignacio Calderon and

Low frequency absorption is an important topic in room acoustics, yet, it's still difficult to achieve it in a satisfactory manner, in relation to the ease encountered at higher frequencies. Resonant absorbers are the most popular means... more

Figure 1: Typical construction of a Helmholtz resonator. Ref. [3] Helmholtz resonators are a particular type of resonant absorbers, which transform incident high pressure into air motion of an air plug in the opening of a perforated sheet with a sealed air cavity behind. The air plug and the air volume behave like a mass spring system, where the first one is an oscillating mass and the latter the spring. Thus, a harmonic motion is achieved. Adding a damping material (such as a porous absorber) creates losses, as the air in motion encounters the porous material, which contribute to an augmented absorption coefficient and may achieve a bigger bandwidth. Ideally, the porous absorber should be placed right behind the perforated plate. Figure 1 displays a simple construction model of a Helmholtz Resonator.

Figure 2: Isometric view of the Annex 02 classroom 3D model The measurement of the surface absorption coefficient were conducted in UNTREFs’ facilities, specifically in Annex 02 classroom. For the purpose of the present work, proportions of the test room were measured with the Laser Rangefinder BOSCH DLE 70 and a 3D model of the room in Sketch Up® was made (Figure 2).

Figure 3: Results for the reverberation time of the test room Despite the main purpose of the facility used in the present work is for lessons and lectures, it is not well treated acoustically for that purpose, having compromised speech intelligibility. Nevertheless, for some specific measurements, such as sound power level of certain machinery, it can provide reliable results and suitable compliance with international standards. 3.1. Reverberation time

Figure 4: Equivalent Absorption Area of test room Moreover, the maximum allowable absorption coefficient of the room is specified in the standard, this was proven to be valid in the range of frequencies between 200Hz-5kHz, failing between 100Hz-160Hz. However, the absorption coefficient curve revealed a smooth curve with no fluctuations of 15% between adjacent bands. Figure 4 proves this point.

Figure 5: Relative standard deviation of reverberation time as a function of frequency. In 8, €29(T) is the standard deviation, T is the measured reverberation time, N is the number of measurements, in this case twelve and f is the midband frequency. Using 8, a graph for the relative standard deviation was plotted in Figure 5.

Figure 6. Outline Globe-Source-Radiator (Dodecahedron), and Global-Subwoofer-Source For the measurement of reverberation time, an Outline Globe-Source-Radiator (Dodecahedron), and Global-Subwoofer-Source (figure 6) were used with an exponential sine sweep (ESS) as test signal.

Figure 7. Aurora plug-in with sine sweep generator In accordance with ISO 354, twelve reverberation time measurement were made, since it is the minimun neccesary to comply with the standard and have a proper spatially independent measures.

Figure 9: Absorption coefficient for test sample predicted by TMM

Figure 10: Real (solid) and complex (dashed) normalized impedances of the test sample.

For those measurements with six samples (A,B,C,D), the total sample area was 4,32m7, measurement E was 2,16m*, while measurement F was 0,72m?.

Figure 11: Sabine absorption coefficient for measurements A,B and C Firstly, the results according to ISO 354 are displayed, the frequency range of analysis was, according to the standard, 100Hz-5kHz. Figure 11 shows the results for measurements A, B and C using Sabines’ equation, while Figure 12 its Eyring equivalent.

Figure 12: Eyring absorption coefficient for measurements A,B and C

In order to validate the design equations, theoretical results were compared to measurements A, B and C in the range of frequencies between 40Hz-2kHz. Firstly, results are shown in Figure 13.

Figure 15. Comparison between different arrangements The comparison between results is shown in Figure 15.

Figure 14: Comparison between mean value and TMM. Black: TMM, Magenta: Mean(A,B,C). Finally, another graph was generated taking the mean of the three measurements compared to the theoretical prediction, shown in Figure 14.

Table 1. Reverberation chamber geometric characteristic requirements. Its sound absorption characteristics should be less than or equal to the values listed in table 2. For rooms bigger than 200m’, this values should be multiplied by (“/og9) The reverberation chamber must comply with certain geometric characteristics, listed on table 1.

Table 2. Room equivalent sound absorption area requirements. In the case of a plane absorber, like the one considered in this work, the test specimen should have an area between 10m*-12m?. If room volume exceeds 200m°, this value should be scaled by ("/oq9)- The width to length ratio should be between 0,7 and 1 and it should be mounted accordingly to the standards specifications (A, E, etc.) and at least 0,75m from any edge of the room (ideally in a non- parallel manner to boundaries).

Table 4. Description of the arrangements of the samples

descriptionView Paper arrow_downwardDownload

Passive Separation Control By Acoustic Resonance Yang and Spedding 2013

by S. L . Yang

At transitional Reynolds numbers, the laminar boundary layer separation and possible reattachment on a smooth airfoil, or wing section, are notoriously sensitive to small variations in geometry or in the fluid environment. We report here... more

Fig. 2 Wind tunnel setup. (x, y, z) are streamwise, spanwise, and normal directions. Origin is at leading edge and mid-span

@, is displayed on a discrete colorbar whose step size matches the measurement uncertainty.

Fig. 3. Acoustic power spectra in an empty wind tunnel with flow at U = 6.7 m/s (Re = 40 k) and without flow. Note the change in axis scales The maximum absolute deflection amplitude, 7’, was measured at the free wing tip (/b = —1.0) from image sequences taken from the camera mounted on top of the wind tunnel. z/(a) was measured for Re = 40 and 60 k fo! the wing with all holes covered (the solid wing), and results are shown in Fig. 4. At Re = 40 k before and after the low- to high-lift transition, the wing tip deflection z//c ~ 0.11 %. At Re = 60 k, a difference in wing tip deflection is observed when SI-SII transition occurs. In the low-lift (ST state, z'/c ~ 0.2 %, and after transition to the high-lift (SID state, Z/c ~ 0.07 %. The difference is likely due to the increased influence of unsteady aerodynamic forces, which

Fig. 4 Wing tip deflections normalized by the chord (z’//c) at Re = 40 and 60 k for the wing with all holes closed (i.e., solid wing)

The SI-SII transition is accompanied by a forward chord- wise movement of the mean separation line. The early separation leaves sufficient time, in fractions of (c/U), so that transition to turbulence and reattachment can occur. Chordwise local disturbances could effectively promote this transition, and this concept was tested by leaving open

individual spanwise rows of holes at a given x/c location, while closing all others. For a row with open holes, 15 holes spaced 3.2 cm apart (Ay/b = 0.12) were left open across most of the span (—0.83 < y/b < 0.83). The C,(Cp) and L/D curves for the wing with selected rows of open holes located at x, = x/c = 0.1, 0.2, 0.3, 0.4 are shown in Fig. 7 (Re = 40 k) and Fig. 8 (Re = 60 k). abrupt jump in either L or D and the curves lie close to SII over all «. At Re = 40 k, the curves once again show sharp jumps between states when x, > 0.2, and for x, > 0.3, the performance curves are much closer to, though still mea- surably improved upon, the original solid baseline wing. At Re = 60 k, the curves show the sharp jumps when x, > 0.3. individual spanwise rows of holes at a given x/c location, and L/D curves for the wing with selected rows of open For both Re cases, single rows of open holes at specific chord locations allow the envelope of C;(Cp) and (L/D) (a) to be populated with results that are intermediate between the extremes where all holes are open or all closed. As the row location moves closer to the leading edge, the per- formance curves are pulled toward the SII state, as gen- erated with all holes open. When x, = 0.1, there is no The original design of the wing included two exit ports at the right wing tip, which were meant to be used for elec- trical connections. The results shown in Figs. 5, 6, 7, and 8 were for the wing with open exit ports. Figure 9 shows the L/D curves for the wing with different rows of open holes the right wing tip, which were meant to be used for elec-

Fig. 10 Power spectra at Re = 40 k and « = 10° for the wing with open holes at x/c = 0.1 and open exit port measured at various distances, z/c, from the top surface

Fig. 11 Power spectra at Re = 40 k. a No wing; b wing with closed holes; ¢ wing with open holes (x/c = 0.1) and open exit port; d wing with open holes (x/c = 0.1) and closed exit port. For b-d, « = 10°

Fig. 12 Schematic of cavities interconnected by a channel. Each cavity was designed to house speakers with an adjacent channel for wiring

affect the global behavior. When holes are present, the flow state switches from SI to high-lift SII, with no other control input necessary. The mechanism is entirely passive, and a control strategy might simply involve sliding lids open and shut to modify local flow characteristics. 4 Discussion On a wing that 1s characterized by abrupt jumps 1n lift and drag coef ficient between what we have termed SI, where the flow separates at some point between mid-chord and the trailing edge, and SII, where separation close to the leading ed ge is followed by reattachment, the dynamics are very sensitive to a number of different perturbations. In a study originally aimed at acoustic forcing of the flow, it was found , quite by accident, that the presence of the holes themselves in the wing suction surface was sufficient to The effect on the wing is approximately local, so that chordwise strips can affect local sectional c; and cg. Since each local flow state is either in SI or SIL, one may think of the wing as a device whose lift and drag coefficients can be manipulated under digital control. Local states can be on or off, and asymmetries across the span will lead to rolling

the predominant frequency varied with impingement length and inflow velocity. Without the perforated plate, the shear ayer was separated with an inflection point, while with a perforated plate, the shear layer was bounded with no inflection point. It was also suggested in [4] that unified, arge-scale motion occurs through the plate perforations and induces jet flow at the downstream end of the perfo- rated plate. The long-wavelength instabilities along a per- forated plate of length L can be expressed as a Strouhal number, fZ/U, where f is a shedding or oscillation fre- quency. The reported values of fL/U are on the order of 0.5—0.6 [8]. If the chord of the wing is used as an effective plate length, the calculated fL/U ~ 8 is an order of mag- nitude larger. moments, while symmetrically (about mid-span) actuated hole opening can yield pre-determined total lift and drag from the envelope of possibilities. The performance characteristics of the perforated wing could be due to some form of cavity flow. In general, cavity flow can be categorized into three main types, as detailed in [20]: (a) fluid-dynamic, where oscillations come from the instability of the cavity shear layer and are enhanced through a feedback mechanism, (b) fluid-resonant, where oscillations are strongly coupled with resonant (standing) wave effects, and (c) fluid-elastic, where oscillations are linked to solid boundary motion. It is also possible to have combinations of different types of cavity flow.

Fig. 17 Instantaneous spanwise vorticity fields over the aft portion of the wing (beginning at x/c = 0.5) at Re = 40 k and « = 10° with 15 open holes at x/c = 0.1. The time between successive vorticity fields is 0.1 s

Fig. 19 Relationship between Helmholtz frequency, fy, and tube length (neck height), h, for constant orifice/tube radius and variable cavity volume (a), and for constant cavity volume and variable orifice/tube radius (b)

Fig. 20 ACgii for the E387 airfoil at Re = 60k among different facilities [22], and ACp. usc between open and closed holes for the E387 wing at Re = 60 k

descriptionView Paper arrow_downwardDownload

Medición del Coeficiente de Absorción de un Absorbente Resonador - Presentación

by Ignacio Calderon and

Presentación del trabajo "Surface Absorption Coefficient Measurement"

descriptionView Paper arrow_downwardDownload

Acoustic performance of sonic metacage consisting of Helmholtz's resonator columns with internal partitions

by Sanjay Kumar

2022, Applied Acoustics

The quest for a quiet environment has led to numerous efforts to shield noise at the source or develop an efficient acoustic proof system. We present a threedimensional sonic crystal acoustic metacage structure for hybrid purposes of... more

descriptionView Paper arrow_downwardDownload

The case of the mythical beast

by Roman Vinokur

This paper on amusing acoustics is written as a false Poltergeist story unraveled by Sherlock Holmes. The education goal is to explain the effect of Helmholtz resonance. The paper was published in "Quantum", Nov-Dec 1993.

“Neither God nor the devil has ever made a personal appearance in my life,” Holmes said thoughtfully. “T suspect that the marine beast has nothing to do with what you expe-

descriptionView Paper arrow_downwardDownload

Osservando i suoni: Chladni e la storia dell’acustica tra Settecento e Ottocento

by Riccardo Martinelli

1999, Annali del Dipartimento di Filosofia (Firenze). N.s.

descriptionView Paper arrow_downwardDownload

Modeling Consonance and Its Relationships with Temperament and Harmony

by luciano costa

2019, CDT-10

After revising the concepts of consonance/dissonance, a respective mathematic-computational model is described, based on Helmholtz's consonance theory and also considering the partials intensity. It is then applied to characterize five... more

descriptionView Paper

Helmholtz Resonance

Related Topics