{"id":2726,"date":"2015-08-14T12:24:40","date_gmt":"2015-08-14T10:24:40","guid":{"rendered":"http:\/\/www.tonmeister.ca\/wordpress\/?p=2726"},"modified":"2018-05-29T07:57:46","modified_gmt":"2018-05-29T05:57:46","slug":"bo-tech-the-heat-of-the-moment","status":"publish","type":"post","link":"https:\/\/www.tonmeister.ca\/wordpress\/2015\/08\/14\/bo-tech-the-heat-of-the-moment\/","title":{"rendered":"B&O Tech: The Heat of the Moment"},"content":{"rendered":"

#39 in\u00a0a series of articles<\/a>\u00a0about the technology behind\u00a0Bang & Olufsen<\/a>\u00a0loudspeakers<\/strong><\/p>\n

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“If you saw a heat wave, would you wave back?”<\/p>\n

– Steven Wright<\/p>\n

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Part 1: The Myth(?)<\/h2>\n

Once upon a time, back in the “olden days” (as my kids call them), before I worked at Bang & Olufsen, I did a Master’s degree at McGill University as a tonmeister (hence the domain name…). In practice, this means that I was trained to be someone who knew as much about things like room acoustics, microphone techniques, recording practices and electronics as I did about music. (Of course, if I knew nothing about music, then this equation falls apart…) In practice, what it meant was that I spent a LOT of time in recording studios (literally more time than I remember) doing recordings or just listening to them.<\/p>\n

Now, let’s think about the design of a typical “good” recording studio control room. Usually, you have a mixing console in the middle of the room, and at least one pair of small studio monitor loudspeakers sitting on the meter bridge. Also, up on the front wall, there’s a large pair of “soffit mounted” loudspeakers built into it. Figure 1, below, shows an example of this.<\/p>\n

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\"Figure<\/a>
Figure 1. A typical recording studio. Notice that there are a pair of small loudspeakers (usually called “nearfield monitors”) sitting above the mixing console, and a pair of large “soffit-mounted” loudspeakers on the walls.<\/figcaption><\/figure>\n

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One story I heard back in the “olden days” was that, if you’re building a recording studio, you should always keep the amplifiers for your big soffit-mounted loudspeakers away from (or at least above) the loudspeakers themselves. You should never NEVER put the amps directly under the loudspeakers. The story went that the reason for this was that\u00a0hot air rising<\/a> from the amplifiers would waft past the tweeters, modulating (changing) the speed of sound of the air between the tweeters and the listening position. And, since this modulation was random, then this heat would sometimes cause the right channel to arrive at the listening position earlier than the left, sometimes later. The result (according to the story) would be a constant shifting of the phantom image, moving left and right instead of staying exactly in the middle where it should be. The analogy\/metaphor\/explanation was that the image location would “shimmer” in the same way that there appears to be water on hot asphalt in the distance on a summer day, or the way you can see heat rising from a hot surface like a barbecue.<\/p>\n

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Fast-forward about 25 years, and I’m sitting in a meeting at B&O, discussing mechanical design options for high-end loudspeakers. Someone in the room says “one possibility is to put the radiators for the amplifiers on the face of the loudspeaker, just under the midranges and\u00a0tweeters” and I say “you can’t do that – you’ll screw up the phantom imaging”. Heads turn to look at me, the expressions on the faces obviously say “what on earth<\/em> are you talking about?” So, I tell the story that I heard\u00a0back in the olden days…<\/p>\n

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Quick sidebar<\/strong>: if you don’t already know, almost all Bang & Olufsen loudspeakers are active loudspeakers, which means that the amplifiers and power supply are inside the loudspeaker enclosure. Amplifiers and power supplies are never 100% efficient. In addition to this, because we use DSP to control the loudspeaker, we can push our loudspeaker drivers to their limits without melting their voice coils – this\u00a0also results in\u00a0a lot of heat.\u00a0All of this means that our loudspeakers need a lot of thought into their thermal design – basically, we need to make sure that the heat can get out, which is why we need radiators like the one shown in the photo on the right (which shows the bottom of the back of a BeoLab 20<\/a>. The vertical stripes are a metal radiator attached to the electronics inside.). We now\u00a0return you to your regularly scheduled\u00a0story…<\/em><\/p>\n

Some of the engineers in the room say “that’s crazy” – and others (the ones I like) say “let’s find out…” So, using the modern tools for finding-things-out, we started\u00a0by typing things like “loudspeaker amplifier heat phantom image” into Google. This resulted in no information – or at least no useful information. So this meant\u00a0that we had to do an experiment.<\/p>\n

Part 2: The pilot\u00a0experiment<\/h2>\n

Okay, so the question is: does heat rising in front of a tweeter have an effect on the sound that it radiates? And, if so, what is\/are the effect(s) and what kind of magnitude of effect are we talking about? The easiest way to find this out is to exaggerate the effect. So, we got a tweeter and a hot plate (which lives in the acoustic department exclusively for warming risalamande<\/a> at the yearly Julefrokost (the ubiquitous Danish “Christmas lunch” literally translated, although it can happen as early as November, it rarely starts at lunchtime, and typically extends well into the evening…) and we set them up as shown in Figure 2.<\/p>\n

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\"Figure<\/a>
Figure 2: The setup for the first experiment showing the tweeter and the hot plate on the Crane in the Cube. The BeoLab 6000 in the background was not part of the experiment.<\/figcaption><\/figure>\n

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We put a microphone in front of the tweeter at a distance far enough away that the microphone wasn’t getting heated by the hot plate (about a metre or so…), and we sent a high frequency tone (20 kHz) to the tweeter from a sine wave generator. Then we simply sent the original signal and the output of the microphone to an oscilloscope so that we could look at them in real time.<\/p>\n

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\"Figure<\/a>
Figure 3. The original signal and the output of the tweeter at the microphone location when the hot plate was turned off. (Note: Exposure time: 30 seconds)<\/figcaption><\/figure>\n

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Figure 3 shows the result when the hot plate was turned off. The original signal and the microphone signal are both clean, as you can see there.<\/p>\n

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\"Figure<\/a>
Figure 4. The original signal and the output of the tweeter at the microphone location when the hot plate was set to “medium”. (Note: Exposure time: 30 seconds)<\/figcaption><\/figure>\n

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Figure 4 shows the same thing, but the hot plate is now set to “medium”. As you can see there, the output from the tweeter is much more “blurry”. This is because it is moving on the screen over the 30 seconds of the photo exposure. Note that it is moving in both the X-axis (meaning the level was changing over time) and the Y-axis (meaning that the time of travel\u00a0from the loudspeaker to the microphone was changing over time).<\/p>\n

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\"Figure<\/a>
Figure 5. The original signal and the output of the tweeter at the microphone location when the hot plate was at maximum. (Note: Exposure time: 30 seconds)<\/figcaption><\/figure>\n

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Figure 5 shows the same thing, but the hot plate is now set to “maximum” (hot enough to boil water or to burn risalamande). As you can see there, the output from the tweeter is even\u00a0more “blurry”for the same reason. It’s also fairly easy to see in that photo that the output from the tweeter is modulating in both axes.<\/p>\n

One thing to be clear of here is that we are looking at the effect of modulating the temperature of the air<\/em> in front of the tweeter. We’re not changing the temperature of the tweeter itself (at least, not with the hot plate – the tweeter heats itself, but that’s another story) or the microphone.<\/p>\n

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Part 3: The “real”\u00a0experiment<\/h2>\n

Okay, so we had some proof that the story that I heard back in the recording studio might<\/em> be true. At the very least, we found out that it would be easy to measure the effect of heat rising in front of a tweeter. Now we had to get a little more useful data by analysing the signal from the microphone a little better. Looking at the signal on an oscilloscope is good for intuition, but it’s hard to graph the results.<\/p>\n

So, this time we changed the setup a little. We used a little studio monitor loudspeaker (with some minor alterations – we taped over the ports to ensure that we weren’t modulating the air around the loudspeaker\u00a0as a result of airflow through the port, just in case… for the lower frequencies… Since we were only playing sine tones and we’re not really interested in the absolute magnitude response of the loudspeaker, the lack of a port didn’t matter too much to us…) and we recorded the microphone output\u00a0instead of just looking at the signal on a ‘scope. (We were also careful not to move during the recordings, partly\u00a0to reduce background noise, but also to avoid creating\u00a0air currents … just in case…) The total setup (although it’s still pretty simple) is shown in Figure 6.<\/p>\n

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\"Figure<\/a>
Figure 6. A small studio monitor with sealed ports and the hotplate with a microphone. The gray panels are an\u00a0acoustically\u00a0absorptive foam\u00a0called Basotect. We use that a lot inside loudspeaker cabinets, so we have\u00a0plenty lying around…<\/figcaption><\/figure>\n

We also tried measuring the air temperature in front of the loudspeaker, but this was difficult because it was fluctuating so much and so rapidly.\u00a0So, instead, we put a power meter on the mains feed to the hot plate to get a better idea of how much constant power we were using.<\/p>\n

\"Figure<\/a>
Figure 7. The power monitor on the hotplate.<\/figcaption><\/figure>\n

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The results<\/h3>\n

Our analysis was to see how much the phase and the magnitude were changing over time with different frequencies and different settings of the hot plate temperature. This was done by analysing the recordings of the microphone signal for a 30-second period.\u00a0The results turned out to be different for different frequencies – and surprisingly large in some cases…<\/p>\n

Some details for the geeks-amongst-you:<\/p>\n