[ NexT Paper ]
[ NexT Paper ]
| Listening Room Considerations | |
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如果各位想得到這方面的資訊可到Hales |
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Low frequency reflection between two parallel wells: Standing Waves |
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| Sound waves reflecting between two parallel wells set up resonance modes when one-half,or a whole multiple of one-haif , the wavelength of the sound wave is equal to the distance between the wells . These resonance modes are referred to as "normal modes"or "standing aves".These standing waves color the sound of the room by emphasizing certain musical notes and createa rough and unnatural sound energy distribution within the room . If a certain standing wave frequency is acousticlly isolated from itsmodal neighlbors , its effect is more likely to be audible andporblematic. Standing wave distribution is a property of the room's physicalcharacteristics and is not effected by the audio system.(Refer to Section 5 to learn how the position of speakers effects then excitatioof the standing waves.) See Fig. 6-8 | |
| Standing waves cause little perceptible coloration above approximately 300Hz .Below this frequency, how-ever, isolated or coincident standing wave modes can cause easily perceptible colorations . The level of low frequency coloration within a listening room is effected by the distribution of all standing wave modes present in that room . Evenly distrbuted modes are less problematic than modes which"lump together" at the same frequencies. | |
| There are three types of standing wave mode that exist in a typical rectangular listening room . These are, namely , axial modes, tangential modes and oblique modes. Investigstion of tangential and oblique modes are beyond the scope of this manual.However,because the axial modes are usually dominant and therestanding and optimizing the room/speaker interface. | |
 Fig. 6-8 |
Determining The Axial Modal Distribution Of Your Listening Room |
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The fequency of the fundamental (or lowest) resonant mode or axial standing wave occurring between two parallel walls can be easily calculated by the following equations: (1) Fo =1130/2L or (2) Fo =565/L Where the constant 1130 represents the speed of sound in feet per second; L is the distance between parallel walls in feet .If your room is rectangular , standing waves will occur between side walls , front and rear walls and between floor and ceiling . The factor of 2 in the denominator of equation ( 1 )indicates that the fundamental mode occurs when the distance between walls equal one-half the wave-length of sound.Standing wave modes also occur at the harmonics of the fundamental mode. That is , additional modes will occur at twice the fundamental frequency (2Fo),three times (3Fo) etc.See Fig. 7-8. |
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| To determine the fundamental (and harmonic) modal frequencies for your room ,simply plug the dimensions of your room , in feet,into equation ( 2 ). | |
| Example: Calculate fundamental standing wave
modes in the three axial directions for a room which measures 16'W x 26'L x 8'H. Between
side walls:
Fo w =565/16 =35Hz |
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You will notice that in this example, the room width is exactly twice its height.The result is that the fundamental mode between the floorand ceiling corresponds to the first harmonic of the fundamental mode between the sid walls. Fo h = 2Fow = 70Hz |
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| This room would experience a noticeable coloration resulting from 2 axial standing wave modes at70Hz,140Hz,210Hz.. | |
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The worst possible model distridution occurs when the room dimensions are equal in
three directions: |
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| It follows , from the above example then , that the room which gives the least problematic standing wave distribution would be a roomwhose three characteristic dimensions are unique and are not related by integer multiples. | |
| See Fig. 9-11 for axial standing wave modes for three typical room shapes. | |
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This spreadsheet calculates the first fifteen acoustical axial standuing wave modal frequencies for any room with parallel wall construction. In the table to the left, the modes are sorted by ascending frequency.Coincident modes occur when two or three axial modes occur at the same frequency,and are marked with an asterisk (*).Coincident modes also indicated by zero modal spacing.Isolated modes,a single mode whose nearest (in frequency) neighbors are more than 20Hz away ,are indicated with a"yes"in the far right "isolated?"column. In general,standing waves of frequency grester than 300Hz do nat pose a coloration problem in typical listening rooms,Problematic modal distribution below 300Hz causing audible acoustic coloration will occur when there are coincident standing waves and/or isolated single standing wave modes exist.The worst case is isolated coincident modes,indicated with a double with a double asterisk (**).A">" indicates modes above 300Hz where 20Hz separstion no longer defines isolation. |
| Fig. 9 Standing wave modal frequencies for a room 24' x 24' x 8'.Asterisk (*) indicates coincident modes and double asterisks(**)indicate isolated coincident modes.Note thst the dimensions of the room are related by whole integers.This reults in every mode being isolated and double or triple caincident! | |
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This spreadsheet calculates the first fifteen acoustical axial standuing wave modal frequencies for any room with parallel wall construction. In the table to the left, the modes are sorted by ascending frequency.Coincident modes occur when two or three axial modes occur at the same frequency,and are marked with an asterisk (*).Coincident modes also indicated by zero modal spacing.Isolated modes,a single mode whose nearest (in frequency) neighbors are more than 20Hz away ,are indicated with a"yes"in the far right "isolated?"column. In general,standing waves of frequency grester than 300Hz do nat pose a coloration problem in typical listening rooms,Problematic modal distribution below 300Hz causing audible acoustic coloration will occur when there are coincident standing waves and/or isolated single standing wave modes exist.The worst case is isolated coincident modes,indicated with a double with a double asterisk (**).A">" indicates modes above 300Hz where 20Hz separstion no longer defines isolation |
| Fig. 10 Standing wave modal frequencies for a room 24' x 16' x 8'.Asterisk (*) indicates coincident modes and double asrerisks (**)indicate isolsted coincident modes.Note that dimensions of the room are related by whole integer,but unlike Figure 9, no two dimensions are better,but still not good. | |
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his spreadsheet calculates the first fifteen acoustical axial standuing wave modal frequencies for any room with parallel wall construction. In the table to the left, the modes are sorted by ascending frequency.Coincident modes occur when two or three axial modes occur at the same frequency,and are marked with an asterisk (*).Coincident modes also indicated by zero modal spacing.Isolated modes,a single mode whose nearest (in frequency) neighbors are more than 20Hz away ,are indicated with a"yes"in the far right "isolated?"column. In general,standing waves of frequency grester than 300Hz do nat pose a coloration problem in typical listening rooms,Problematic modal distribution below 300Hz causing audible acoustic coloration will occur when there are coincident standing waves and/or isolated single standing wave modes exist.The worst case is isolated coincident modes,indicated with a double with a double asterisk (**).A">" indicates modes above 300Hz where 20Hz separstion no longer defines isolation |
| Fig.11 Standing wave modal freqencies for a room 26' x 15' x 8'. Asterisk(*) indicates coincident modes and double asterisk (**)indicate isolated coincident modes.Note that the only the width dimension changed from Fig. 10 -and only one foot ! Now there is onlyONE coincident mode at 282.5Hz and it is not isolated.What a difference a foot will make ! | |
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