Acoustic Consulting Equipment
There are a number of specialized pieces of equipment trained Acoustic Consultants should have for effective consulting. These include the following:
- Sound Pressure Level (SPL) Meter: sometimes called a decibel meter, this tool is used to determine how loud noises are, and at which frequencies. There are several types of SPLs.
- Type 1: accuracy within +/‐ 1.1 dB at 1kHz, and 2.5 dB at other frequencies – must be calibrated annually
- Type 2: accuracy within +/‐ 1.4 dB at 1kHz, and 6.0+ dB at other frequencies– must be calibrated annually
- Integrating-‐Averaging SPLs: used to get an average over a period of time. Most Type 1 SPLs have integrating-‐averaging functions.
- Tapping Machine: this is a machine that drops small, metal weights onto the floor above to create a consistent noise source used to measure how much sound penetrates a floor-‐ceiling assembly
- White Noise Source (Speaker): a loud, amplified speaker used to project sound across a broad range of frequencies so that they can be measured at 1/3rd octave bands
- Noise Impulse Generator: a clapping machine, popped balloon, cap gun, or similar noise source used to make a loud noise that then dissipates into the surrounding environment.
Note that Type 1 and 2 SPLs should be used for all dB measurements. Phones and other consumer electronics are not calibrated, and will often vary by 10 or dB. However, phones may be used to achieve a reasonable level of accuracy for RT60 (Reverberation Time) performance.
When to Hire an Acoustic Consultant
Types of Acoustic Tests: STC, IIC, RT60
Depending on your primary noise concern, acoustic consultants may perform a number of field tests to quantify performance. These most commonly include:
- STC Test: This test measures the amount of sound that a partition blocks.
When performed in the field, it is technically an ASTC (Apparent Sound Transmission Class) test, since the amount of sound is also dependent on the flanking paths around the partition. The test is performed by using a high-‐amplitude white noise source on one side of the wall (say 100 dBA) and measuring how much is received on the other side of the wall (say 60 dBA). As long is this amount is significantly higher than the ambient noise (often 30-‐40 dBA), then you can be sure that the sound being measured is what is passing through (or around) the partition.
If the ambient sound is too high, sometimes air conditioners and fans must be turned off, and refrigerators unplugged, to ensure that any sound being measured is primarily due to the noise source. For this reason, it is very difficult to perform an ASTC test on a live construction site, or other inhabited area.
The Noise Source minus the Noise Received is approximately the ASTC of the room. 100 dBA minus 60 dBA means the wall is blocking about 40 dBA. However, ASTC takes into account how well sound is blocked at each 1/3rd octave band. That’s because a wall may block high frequencies very well, but if low frequencies easily penetrate, then a human listener will still perceive the wall as “thin”, or ineffective. Therefore, a standard scale is used to equate the perceived performance of the wall to the Transmission Loss at each band. (For more information, see Appendix B)
- IIC Test: An Apparent Impact Insulation Class test measures how well a floor attenuates footfall, or structure‐borne noise.
This is done by placing a tool known as a Tapping Machine on the ground. It drops small, metal hammers on to the floor at a pre‐determined rate, distance, and pattern. When the sound is measured below, the AIIC of the floor may be calculated. If 50 dBA is measured below, that is better than 60 dBA being measured, since the floor “blocked” about 10 dBA. Similarly to ASTC testing, there are adjustments made for background (ambient) noise and reverberation in the space.
- RT60 Test: This Reverberation Test determines how long it takes for sound to decay in a space. “Noisier” spaces with more echo will have longer reverberation times.
The test is performed by generating a repeatable impulse noise, then measuring how long the sound takes to reduce by 60 dB. Often times the impulse is clapping hands or 2x4s together, but a better option is to pop a balloon or use a cap gun, which can be repeated more precisely. Further, there are impulse generators on the market that many consultants will use to create a noise source that is balanced at high and low frequencies.
Sound Duration Measurements
Lastly, sound measurements are only relevant in the context of their duration. If a consultant performs a 1‐hour test at an airport to determine how loud it is, the answer may very well be 70 dBA, 80 dBA, and 110 dBA. This depends on whether the 1-‐hour average is used, the loudest 1-‐minute, or the Peak Noise measured. The most complete information will provide a range of data, including 10-‐second and 1-‐hour averages, and peaks.
Ldn (Day-‐Night Levels) are determined by taking 1-‐hour averages during the day, and adding an extra 10 dB to the measurements after dark. This penalizes industrial or roadway noise for night-‐time sound levels, since these are often perceived as more annoying.
Similarly, L90 measurements provide the dBA level at which noise exceeded 10% of the time. This is helpful when trying to determine how often noisy durations occurred over the course of a measurement.
