Listening environment testing

Sound waves take up physical space and require physical distance to develop. Anything the sound “touches” can positively or adversely affect what you are listening to. For example, the length, height and width of a room can correlate to how particular sound waves will develop. Materials within the room, from carpet to work surfaces, can affect sound waves as well.

Even professional listening environments can have issues with audio accuracy. Large rooms, including sound stages or broadcast production studios, tend to have reverb (echo) issues. Smaller rooms aren't big enough for reverb problems, but they are notorious for natural resonance issues. Equipment noise can also affect what you hear and, as a result, how your mix will sound.

Fortunately, software-based sound analysis tools such as real-time analyzers (RTAs) can let you “see” what you hear through various tests, so you can compensate for your surroundings. The primary purpose of these tests is to help create a “flat” room, a space in which the listener is hearing the “real” sound that the rest of the world will hear.

A “room mode” test is generally one of the first tests to run, because you need to identify how the environment is altering the sound. Every room, particularly a small room, has its own natural resonance. A result of the shape of the room, this resonance (or room mode) can cause a buildup of particular audio frequencies.

Each audio frequency within the spectrum has a wave of a certain size. For example, the 100Hz wave is about 34in long, so you can correlate that particular frequency to how many cycles you'll get in a particular space. It's simple math, really. All right, it might not be simple with so many frequencies, but it is predictable (and your RTA will do the math for you).

You want to hear what's been recorded, unaffected by the environment in which you are listening to it. Without proper analysis, you won't know which frequency or frequencies “get stuck” in the room. As a result, you might tend to reach for the EQ to compensate for those frequencies. But when that content, which was mixed to your particular room mode, is played elsewhere, you have a hole in your audio.

An RTA will show a peak of any particular frequencies, so you can resolve any room mode issues, but the quality of your tools can affect the quality of your measurement. Garbage in, garbage out — the old industry adage — applies to sound analysis as well. You have a choice of software programs designed for real-time sound system measurement and analysis.

You will also need to invest in a measurement microphone to capture the audio coming from the speakers in the room you are testing. Some mics deliver a certain sound. When it comes to measuring listening environments, however, you want a measurement mic with a frequency response that is flat from 20Hz to 20,000Hz, the full spectrum of audio frequencies that can be heard by the human ear, which will provide an accurate representation of the room. There are a number of measurement mics on the market. They are not inexpensive but are necessary for accurate results.

The measurement mic needs to be plugged into a preamp and then an A/D converter so you can feed the signal into your laptop via USB. Essentially, you place a measurement mic in the location of the room from which you will be listening, play pink noise (for frequency response and phase tests) through the room's speakers, and analyze the results through the RTA software. Pink noise is the preferred sound to analyze because it includes the full 20Hz to 20,000Hz spectrum, with all frequencies at the same volume.

We used the Rational Acoustics Smaart software to measure the frequency response of a small audio mixing suite. (See Figure 1.) A dual-source measurement called transfer function (seen in the lower part of Figure 1) allows us to compare pink noise as electrical energy to the pink noise once it has become acoustical energy in the room. Along with the spectrograph in the upper part of the image, the test clearly shows an issue around 250Hz, as well as a smaller issue at around 500Hz, which is an octave above 250Hz (and is to be expected). It is likely that in dealing with the 250Hz room mode, the 500Hz problem area will also be resolved.

Once we are aware of the exact issues in the room, we can begin to assess how to deal with them. If you have the luxury of changing the physical structure of the space, you can change the room size or alter the angles of the walls. For most facilities, however, audio issues need to be controlled through absorption or diffusion. Absorption material removes sound energy from a room, while diffusion reduces echoes and reflections but keeps the sound in the space.

Available in a variety of designs, diffusers often have blocks or baffles in their designs. A bookshelf also serves as an excellent diffuser; the different sizes and densities of the books on the shelves serve to radiate sound energy in several directions.

Depending on the audio issues, absorption materials can be applied to specific areas of a room, not necessarily every square inch of wall space. Plus, with a little more applied math, we can select certain densities of materials that can absorb particular frequencies. Foam panels and ceiling tiles are some of the most popular absorption materials purchased to improve room sound. You can also install carpets and tapestries for further sound absorption, as well as fabric-covered office cubicle panels that are designed to reduce noise.

For the audio suite we tested, we used absorption materials instead of relying on an equalizer to compensate for these anomalies. Owens Corning 703, 2in-thick semirigid fiberglass boards, or similar materials, have particularly good absorption coefficients down to the 250Hz range. After some trial and remeasuring, we found acceptable acoustic and aesthetic locations for the panels, and we achieved more desirable results. (See Figure 2.)

Mark Miller is a sales and design specialist with Advanced Broadcast Solutions.

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A few more tests

Here are a handful of other tests that can improve your listening environment:

  • Room tuning — With room mode issues resolved, another wave of pink noise through the measurement mic can show peaks and valleys in what should be a perfect signal.
  • Speaker placement — Proper subwoofer placement is essential, particularly in 5.1 surround-sound mixing environments.
  • Time alignment — To compensate for imperfections in speaker placement, this test makes sure audio signals from each speaker arrive at the listener location at the same time.
  • Noise rating — How much noise from outside sources (such as people talking in the hall) is affecting your listening environment?
  • Reverb times — The measurement of a room's natural reverb is important so you can replicate it, such as in a sound booth ADR session after shooting on a sound stage.