Now what do we do?
AV9000 Checklist Item Under Test: 6.2.7: The electrical-noise levels for all audio channels are 55dB below the normal operating level for all audio sources. “Noise” refers to hum, electrostatic noise, radio-frequency (RF) interference, etc.
Reasoning: For the most part, AV equipment is getting better and better with respect to noise levels. The AV9000 standard recommends systems have at least a 55dB signal-to-noise ratio (SNR) to assure that users don’t hear any hiss or noise in the sound system when there is no signal present, but it’s not uncommon to measure upwards of 70dB with today’s equipment. However, when a problem does manifest itself, it’s not enough simply to note that the system has a poor, nonconforming SNR. The team’s needs are better met by spending a few minutes to pinpoint where, exactly, the problem is. It could be a noisy program source, a faulty microphone, poor cabling practices or even a necessary algorithm in the digital signal processor (DSP). Finding the source of the noise problem is far more valuable to the team than simply noting that a problem exists.
The Story: Like you, I am a huge “Tommy Boy” fan. That movie never ceases to crack me up. Lots of people quote the “fat guy in a little coat” line or the “sunnofa…that’s gonna leave a mark” line, and they are, indeed, hilarious. But one of my personal favorite moments is often overlooked. Not only does it include the late, great Chris Farley as Tommy but it also includes the inimitable Dan Aykroyd as Ray Zalinsky. Tommy was nervous about meeting Ray to try to save his dad’s automobile-parts business, and he started to sweat. Of course, he forgot to wear deodorant. So, he rubbed a car air freshener all over his suit to cover up the smell during the taxi ride to the meeting. (This movie is a masterpiece!)
Here’s how it played out when they accidentally met in an elevator:
Ray (smelling something): Went a little heavy on the pine-tree perfume there, kid?
Tommy (naïvely, trying to make a good impression): Sir, it’s a taxicab air freshener.
Ray (as he walks away, ruining Tommy’s attempt to save the factory): Great. You’ve pinpointed it. Step two is washin’ it off.
It’s such a quick, sharp conversation, showcasing the worst first impression you could make with someone who could potentially save your company. It’s cringe humor at its finest. Naturally, this got me thinking of commissioning AV systems.
An important litmus test for proper audio levels and system configuration is measuring the system’s SNR. It assures that any electrical noise (i.e., unwanted audio) is far less than the electrical signal level (i.e., wanted audio). The AV9000 standard recommends SNRs of at least 55dB for properly balanced audio systems. With professional equipment, this is fairly easy to attain. But what happens if we don’t measure at least 55dB? Users might hear a hiss or hum in the system when no audio is present. (Think that classic guitar-amp hiss always heard in movies when someone is about to shred.) That, of course, is not acceptable for professionally installed systems. So, what should we do next?
As an example, let’s take the simple system shown in Figure 1.
If we measure an SNR of, let’s say, 35dB at the output of the amplifier (as compared to at least 55dB), how can we find out where the problem lies? It’s basic troubleshooting.
- Find the nominal electrical audio-output level of the system at its default sound pressure level (SPL) in the room. For this example, to keep the math simple, let’s say we measure 0dBu at the amplifier output.
- Find the electrical noise level of the system when there is no audio playing. For this example, let’s say the noise floor is -35dBu.
- Therefore, the SNR is 35dB.
- 0dBu – (-35dBu) = 35dB SNR.
- It’s time to find out why that is!
- Confirm that the amplifier signal has low noise by disconnecting and/or muting the output of the DSP. Therefore, only the amplifier is connected to the meter, with nothing upstream. Did the noise floor decrease?
- Confirm that the mixer has low noise by disconnecting and/or muting the inputs to the DSP. Only the output of the mixer is measured. Did the noise floor decrease?
- Unmute each input one by one. Did the noise floor decrease?
- Microphones should be muted during this troubleshooting exercise. If a microphone is picking up room noise, it doesn’t represent the electrical noise of the system.
- Disable processing blocks to confirm they’re not adding to the noise level. Note: Acoustic echo cancellation (AEC) processing might affect SNRs and total harmonic distortion (THD) measurements.
- If the noise floor of the program-audio signal from the switcher has a high noise level, select each source, one at a time, to determine if it’s the switcher or the source that has the issue.
- If the source of the noise is localized, keep in mind that it might not be the device itself. Noise might be induced into the signal due to wiring practices. You can determine this by disconnecting cables at the output or the input of the devices and seeing if there is any effect with the meter.
Only when the source of the noise is uncovered can we devise a plan to move forward. The investigation should take only minutes. Once the problem is uncovered, we can either request a return merchandise authorization (RMA) for a replacement device if it’s a nonconforming product or we might have to replace the device with a better-performing alternative. If the noise is induced into the cabling, signal paths might have to be changed or, perhaps, the audio signals might have to be balanced at their source. Once we’ve ascertained the problem, the solution is typically straightforward. The hard part is finding the offending noise source.
So, when you’re measuring SNRs to verify that an audio system is properly configured, be prepared to see the investigation all the way through with your test equipment. “Tell you what: I can get a good look at [an audio system] by sticking my head up [to each loudspeaker], but I’d rather take [my meter’s] word for it.”
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