Setting Up and Making Acoustic Response Measurements with APx

Created on 2009-09-16 19:23:00

Setting Up and Making Acoustic Response Measurements with APx

Although we’re focusing on the APx Series, the basic hardware setup and positioning applies to the 2700 Series as well.

Selecting a Room
Before setting anything up, you’ll need to choose a suitable room. With the APx’s continuous sweep stimulus signal, you can get accurate measurement results over most of the audio range without needing an anechoic chamber. That’s because the software is able to distinguish and eliminate reflections from the measurements. In a room with 8 foot ceilings, the furthest you can get from a reflecting surface is 4 feet.

With a distance of one meter between the mic and the speaker, and given that sound travels 1116.4 feet (340.29 meters) per second at sea level, the first reflection will arrive about 5 ms after the direct sound. This means that results over approximately 200 Hz will be reliable, and results below about 200 Hz will be uncertain (they may be OK, or may be tainted by reflections).

The APx software will recalculate the results as you adjust the time window (see below). If you can find a larger room, or can measure outside with a tall stand, you’ll be able to extend this range of reliable results lower.

Measuring acoustic response in the atrium at AP.

Positioning the Speaker
The DUT (device under test) for this example will be a three-way loudspeaker. Mount the loudspeaker on a stand, and place it at least four feet away from the walls, but not right in the center of the room. Now, raise it up until the center of the tweeter is four feet off the floor. Use the tweeter as your reference point, because high frequencies are very directional and much more sensitive to position than low frequencies. The speaker should mount directly on the stand, or sit on a platform attached to the stand. If the speaker is powered, connect the APx channel one generator output directly to the speaker input. If it is unpowered, you’ll need a power amplifier of sufficient wattage between the APx and the speaker.

Speaker and measurement microphone positions.

Positioning the Microphone
It’s time now to set up the measurement microphone. The mic used here is part of Audio Precision’s MMK2 kit. It’s a pre-polarized, ½” microphone with a free-field response capsule. That’s a good choice for this application.

The measurement microphone is set into a mic clip on a microphone stand or tripod. Raise the mic stand until the center of the mic is four feet off the floor. Four feet doesn’t have to be the exact height—what’s important is that the mic and speaker are at the same level and right on axis with each other. Now, adjust the spacing between the mic and the speaker until they are exactly one meter part. One meter is a standard distance used in many speaker measurements, but other distances like two meters are used as well. If the distance is too far, the signal to noise ratio will be poor, although this problem can be greatly minimized by using the synchronous averaging function in the APx software. Too close, and the three drivers in the loudspeaker won’t blend together.

The microphone needs a power supply (also part of the MMK2 kit) to operate its electronics. Connect a BNC cable from the microphone to the power supply, and another from the power supply to the analyzer channel one unbalanced input.

Calibrating the Microphone
Microphone calibration comes next. Add the Acoustic Response measurement to the APx measurement navigator if it isn’t already there. If you have a calibrator, turn it on and put it over the end of the microphone. In the APx software, open the Mic Calibration box and click Set 1 to capture and set the dBSPL1 reference level. If you don’t have a calibrator, you can enter the calibration figures from the certificate supplied with the microphone capsule.

Use a 1 kHz sine wave to set the APx generator level as desired. Many measurements specify a level corresponding to 1 Watt at the amplifier output. For frequency response measurements, the level isn’t too critical as long as the amplifier isn’t clipping and the speaker isn’t overloaded.

Taking the Measurements
Now that the equipment is set up, hit the Start button in the software to initiate a continuous sweep: a few seconds later, APx will display graphs for 14 key acoustical measurements.

Excluding the Reflections
The acoustic response measurement includes two views to aid in identifying and eliminating reflections from the results. These are the energy time curve (ETC) and the impulse response (IR). Both show the sound pressure level at the microphone as a function of time. As can be seen, the initial arrival of direct sound appears as a strong pulse on the graph. The sound level then decays until the first reflection arrives, which is marked by a sudden increase in energy. After the first reflection, there are additional reflections that diminish in level.

Automatically Recalculating the Measurements
By moving the time window cursor in either the ETC or the IR view to a position just before the first reflection, any reflections will be excluded from the measurements. Each measurement (frequency response, phase, distortion, etc.) will be automatically recalculated based on the new time window—without needing to rerun the test.

Note that although eliminating reflections will give the most accurate and repeatable results, there are times when their inclusion may be desirable to in order to better represent the real world conditions where the speaker will actually be used.

ETC, IR, and frequency response.

ETC, IR, and frequency response with the first reflection gated out. Note the difference in the frequency response curve.

As you can see, it’s very easy to set up and make comprehensive acoustic response measurements with APx analyzers. We’ve given you general guidelines to get you started. You may need to alter some of the parameters depending on your specific application.