Physics 4060, Acoustics Laboratory

Using the sound level meter provided, measure the background sound level in the room on the "flat" position (no filtering) and with the A, B, and C contour filters. The meter response should be kept on "slow" and the needle watched for several seconds to determine an average position. Calculate the fraction of the sound energy detected in each of the filtered sounds compared to the flat or unfiltered measurement. For example, if you read 70 dB and 50 dBA, then the energy received through the A contour filter is 20 dB down or a factor of 10² or 100 down. So in this case the A contour admitted 1/100 of the sound energy. Since the A contour is a fairly good representation of the human hearing response for medium loudness sounds, we could say that about 99% of the energy present in the sound field is inaudible. In buildings this inaudible sound is largely the very low frequency noises of air handling units.

使用提供的声级计，测量房间内的背景声压级，在'平'的位置（无滤波）以及A、B和C轮廓滤波器下进行测量。保持计数器响应在'慢'模式，并持续观察几秒钟以确定一个平均值。计算每种滤波器检测到的声能与平或未滤波测量值的比率。例如，如果读数为70 dB和50 dBA，那么通过A轮廓滤波器接收的声能降低了20 dB或10²倍，即100倍。因此，在这种情况下，A轮廓滤波器只允许了1/100的声能通过。由于A轮廓在中等响度声音的人类听觉响应中表现较好，我们可以认为大约99%的声场中的能量是听不见的。在建筑中，这种不可听的声能主要是空气处理单元的非常低频噪声。

II. 白噪声的测量

White noise is defined as sound with equal intensity per Hz in the audible frequency range. Pink noise is sound with equal intensity per octave - so that the octave range 20-40 Hz has the same total intensity as the octave 2000-4000 Hz. This tips the energy toward the low frequency; hence the term "pink" by analogy with low-frequency boosted light. White noise will be used here for an assessment of loudspeaker sound fields.

白噪声被定义为在可听频率范围内每赫兹具有相等的强度。粉红噪声是每八度具有相等的强度——因此，20-40赫兹的八度范围的总强度与2000-4000赫兹的八度范围的总强度相同。这使得能量倾向于低频；因此，用“粉红”一词类比低频增强的光。在这里，白噪声将用于评估扬声器的声场。

Connect the white noise generator to the audio amplifier to drive one of the small enclosed loudspeakers. Increase the sound level until it is somewhat louder than conversational speech (at least 20 dB above background on the dBA scale) and measure the sound field at about one meter distance directly in front of the loudspeaker. Repeat the measurement with the flat "to 20kHz" scale and with the other contours.

将白噪声发生器连接到音频放大器，以驱动其中一个小型封闭扬声器。增加声音电平，使其略高于正常谈话水平（至少比背景噪声在dBA尺度上高20 dB），并在扬声器前方约一米处直接测量声音场。然后使用平坦的‘至20kHz’尺度和其他轮廓重复测量。

How do you interpret the excess over background? Which measurement would give you the most sensitive assessment of the difference in the room sound field produced by a local source?

你如何解释背景噪声之外的多余部分？哪种测量方法能最敏感地评估局部声源产生的房间声场差异？

III. 直达声与混响声

Connect the white noise source and turn the level up so that you get at least 90 dBA at a distance of 2 cm from the grill cloth of the speaker. Starting with the microphone of the sound level meter at this distance, make measurements on both the flat and dBA scales at successive distances which are double the previous distance from the the speaker until you reach the back of the room. Make a plot of these levels with the dB and dBA levels on the vertical axis and each doubling of distance treated as an equal increment on the horizontal scale (this amounts to a base two log plot). Also produce a plot of the data using the Excel spreadsheet. For this plot, use your lowest measured sound intensity as a reference and calculate the relative powers of the other measurements for the plot. For example, if your lowest dB reading is 70 dB, call that power 1 unit. Then a measurement of 93 dB would be 10^2.3 x 1 or 200. Produce a plot of relative sound power versus r^-2 by to assess whether you get straight-line behavior at any distance range, indicating inverse square law behavior.

将白噪声源连接起来，并将电平调高，使得在扬声器网布距离2厘米处至少得到90 dBA。从声级计的麦克风开始，在距离扬声器的每次距离是前一次距离两倍的范围内，分别在平级和dBA刻度上进行测量，直到到达房间的后方。将这些电平绘制在垂直轴上，dB和dBA电平作为垂直轴，每次距离翻倍视为水平轴上的等增量（这相当于以2为底的对数图）。同时使用Excel表格生成这些数据的图表。对于此图表，使用你测得的最低声强作为参考，并计算其他测量值的相对功率以用于图表。例如，如果你的最低dB读数是70 dB，将其称为1个单位。那么93 dB的测量值就是10^2.3 x 1或200。绘制相对声功率与r^-2的图，以评估在任何距离范围内是否得到直线行为，这表明存在平方反比定律的行为。

Comment on what distances from the speaker give the closest approximation to the inverse square law. Does the inverse square law appear to hold at any distance? Often rooms are modeled in terms of a "direct sound field" and a "reverberant sound field". The "critical distance" for PA system speaker placement is defined as the distance where you depart from the inverse square law (direct sound field) and the reverberant sound field becomes dominant. Can you estimate the critical distance in the room from your data?

评论哪些距离从扬声器处最接近逆平方定律。在任何距离上逆平方定律似乎都成立吗？通常房间是根据“直达声场”和“混响声场”来建模的。“扬声器系统的关键距离”定义为离开逆平方定律（直达声场）并使混响声场占主导地位的距离。你能从你的数据中估计出房间中的关键距离吗？

IV. 扬声器声音的方向性

A platform for the small speaker with a pivoting meter stick will be used to measure the directionality of the sound from the speaker. Connect the white noise source to the audio amplifier through the PASCO tunable filter. Set the filter dial to 2000 Hz and put the filter contour on Band Pass to get a narrow band of frequencies around 2000 Hz. Place the sound level meter at a distance of about 0.8 meters in front of the speaker (on axis) and turn the sound level up to about 90 dBA. Measure sound field profiles for high and low frequencies using the flat and dBA scales. This can be done by moving the sound level meter around in 30° increments and measuring the distance at which you get the same sound level you got at 80 cm on the axis. Repeat the procedure with the filter set for Band Pass at 100 Hz. Make a plot of the direction pattern of high and low frequencies for the dBA measurements. Make a separate one for the dB measurements and compare. Interpret your results. Does it have anything to do with diffraction?

将一个带有可旋转分贝计的平台用于测量扬声器发出的声音的方向性。将白噪声源通过PASCO可调滤波器连接到音频放大器。将滤波器旋钮调至2000 Hz，并将滤波器轮廓设置为带通，以获得围绕2000 Hz的狭窄频率带。将声音分贝计放置在扬声器前方约0.8米处（在轴线上），并将声音分贝调至约90 dBA。使用平面和dBA刻度测量高频和低频的声音场分布。这可以通过将声音分贝计以30°为增量移动，并测量在轴线上获得相同分贝水平（在80厘米处）时的距离来完成。将滤波器设置为100 Hz的带通模式，重复此过程。为dBA测量制作高频和低频方向模式的图表。制作一个单独的图表用于dB测量并进行比较。解释你的结果。这是否与衍射有关？

Measurement of Sound Fields

Physics 4060: Acoustics Laboratory

Physics 4060 Laboratory