Measurement of Sound Fields

I. Background Sound Level

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.

II. Measurement of White Noise

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.

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.

WHITE NOISE MEASUREMENTS

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?

III. Direct versus Reverberant Sound

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.

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?

IV. Directionality of Loudspeaker Sound

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?