What can 3-D sound mapping do for you?

-September 27, 2013

When it comes to the development of mechanical and electromechanical products, acoustic engineering presents a significant challenge. It's not simply a matter of performance - the negative impact of audible noise ranges from nuisance to health hazard. That's where sound-mapping tools come in.

Whether aimed at minimizing the audible noise generated by a dishwasher, isolating a vehicle cabin from engine or road noise or troubleshooting a speaker, effective acoustic engineering requires the ability to visualize and quantify the sound field. That means not just isolating the source of the sound but also characterizing how the sound waves propagate through the enclosure and into free space.

Figure 1: 3-D map of a sound field simplifies troubleshooting.

A range of traditional solutions exist. Engineers can use a single microphone in multiple positions to measure noise at specific frequencies. It's an economical solution but it can be time-consuming and the results depend greatly on operator expertise.

At the next level up the scale, sound intensity probes can also measure the acoustic intensity generated by sound sources. By breaking the surface area to be measured into sections and measuring them one at a time, users can build interpolated color maps of the sound emitted. The problem is that the cost and amount of time required for the measurements rise as the resolution of the measurement increases.

Array-based beamforming and acoustic holography methods speed up the process by mounting multiple microphones on a physical grid to capture data at all points simultaneously. The trade-off is increased hardware cost, plus data processing and result interpretation requires a fair amount of expertise.

Now, designers have another alternative. The SoundBrush  from LMS International not only detects sound sources but also maps propagation throughout a 3-D field.

The system consists of an acoustic sensor mounted on a hand-held probe, a digital camera, and associated software. The user waves the probe through the area around the DUT. It measures acoustic intensity while the camera tracks its position using a lighted sphere on the probe as a reference (see figure 2). Meanwhile, a gyroscope embedded in the probe monitors orientation.

The instruments send their data to a PC, where the software presents it as a 3-D data plot that can be further analyzed as 2-D slices, octave or narrow-band FFT plots, etc. To find out more about the system, we chatted with Dirk De Weer, product line manager for LMS International.

Figure 2: The probe for the SoundBrush  features a glowing sphere to simplify tracking. An internal gyroscope monitors orientation.

Kristin Lewotsky: How does acoustic engineering fit into the design challenges engineering teams face today when designing goods?
Dirk De Weer: You have requirements for reduced noise levels from legislation and a continuous pressure from consumers to reduce noise nuisance to a minimum. The best way to make equipment more silent is to add acoustic material. You make it heavier, add layers of acoustic isolation material. This would be a first step but typically engineers also try to design more lightweight equipment which makes it challenging to meet the acoustic requirements.

K.L.: It sounds difficult. Is the solution to the problem simply using the tool to isolate the noise source?
D.D.: There is a lot of expertise required to do effective acoustic design. Even if you know the source of the sound in your equipment, knowing the way that it transfers to operators or bystanders is not an easy challenge. There are transfer paths that during the design phase you could not anticipate. You can try to simulate them but you typically find out what you have only in the first prototype.

Most designs today have conflicting requirements. If you design for light weight, typically it will not be beneficial to the acoustic design of your product. In order to find a good way to investigate changes in your design for lightweight structures, you need to investigate where the sound is coming from, check one design against another, look for weak spots in your design and try to improve on those.

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