Atmos Musings and Shelf Life

A bit of an eclectic mix this time around with two topics, the first somewhat controversial, the second a useful (I hope) tip.

Elsewhere on this site, and in our June Q&A column, we recommended using the same amplifier power for the front, surround and height speakers in an Atmos setup. I don’t entirely agree, though my personal experience with Atmos is limited so far to trade demos and theatrical presentations. Most Atmos-ready AVRs will, of course, have matched power—that’s just the nature of the beasts. But if you have a pre-pro and, say, 200Wpc amps driving the front speakers, do you really need 200Wpc on the other six (for 5.1.4 Atmos) “full range” surround and height channels?

One consideration here is the sensitivity of the surround and height speakers. If they’re half as sensitive as the fronts (that is, 3dB lower in rated sensitivity) they’ll need twice the power—assuming they’re being driven to the same level. If they’re twice as sensitive (+3dB) they’ll need only half the power. The former is more likely; many (not all) smaller drivers of the sort used for the upward-firing element(s) of Atmos-enabled speakers tend to be quite low in sensitivity. I have yet to see the sensitivity of the upward facing drivers specified separately in such speakers, but if you own Atmos-enabled speakers you can check for this. When you calibrate the channel levels, simply note the difference in settings for the front and upward-facing speakers, assuming the level indicator on your AVR or pre-pro is in decibels rather than some generic number.

But will you normally drive Atmos’ surround and height speakers as hard as the three fronts? Even if you’ve calibrated them to the same level, as you should, this is unlikely with most program material. Have you ever turned off the main speakers in your home theater and listened to the contribution of the surrounds alone? On films, at least, it’s astonishing how little they contribute, compared to their subjective impact with the entire system operating. Yes, occasionally they’ll speak out more aggressively, but that’s rare.

It’s a safe bet that the same will apply to the Atmos height channels. Their contribution will be similar in level to the output of the main surround channels. One notable exception could be surround music, where the extra channels are used to put the listener in the middle of the performers, rather than merely providing ambience with the main music-making taking place in front as it would be in a live performance. But even then, if you’re considering Atmos-enabled speakers rather than discrete ceiling speakers, the small, upward firing drivers in these designs are generally bass managed very aggressively, sometimes rolling off as high as 180Hz. With so little bass to handle, this will further reduce the power needed to drive them effectively without clipping the amplifiers.

The power needed for other than the front channels will, therefore, depend on a number of factors. Do you plan on using Atmos-enabled speakers or discrete ceiling speakers? Do you have an AVR with its own amps or a pre-pro with separate amplifiers? And do you plan to listen to lot of surround music with the musicians placed all around you (though I don’t think many sound mixers will position a discrete musician or vocalist on the ceiling, unless the latter has god-complex!)?

If you already have an Atmos-ready AVR with preamp outputs, and want to increase the power to the front channels, a 5- or 7-channel external amp might be just the ticket to drive the main speakers and surrounds. You could continue to use the AVRs internal amps to drive just the (typically, for home Atmos) four ceiling channels, whether Atmos-enabled or discrete. You could even go with a 3-channel external amp (though these are less common) for the fronts and use the AVR’s amps for both the conventional surrounds and the Atmos height speakers. If you already have a smaller external amp and a surround pre-pro rather than an AVR, you could move that amp to drive the Atmos channels and add a new, more powerful amplifier for the main speakers.

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New Use for a 2x4
When I lived in California I was five minutes from the nearest IKEA and purchased five IKEA bookcases to hold not only books but also most of my collection of CDs and videos. I disassembled them for the move, and reassembled them in my new home. That’s not an exercise I’d recommend to anyone, but it did save a bundle on new bookcases, not to mention that the nearest IKEA is now hundreds of miles away. Now they’re filled mostly with videos, with only a few shelves dedicated to CDs and books. (More bookcases will be needed to house the rest of my collection!)

The only issue with this is that video and CD cases are relatively shallow and the bookcases are 9-inches deep. This isn’t an issue at eye-level, but the deep setback makes searching the lower shelves a nuisance. My solution was to cut 2x4s to the width of the shelves. I bought ten such 2x4s; my friendly neighborhood Home Depot did the cutting. That gave me forty 22-inch wide 2x4s. So for an investment of just under $40 I can use them behind the videos, CDs, and books in a number of ways, moving the contents either 1.5-inches or 3.5-inches closer to the front of the shelf. By using two of the spacers, one flat, the other on its side, I can also move the contents almost flush to the front of the bookcase for a far better view of the titles on those low shelves.

If those measurements puzzle you, it's because modern 2x4s are actually 1.5 x 3.5-inches. I bought premium ones that cost an extra buck or so each; you can’t see them, but you’ll know they’re there and you can tell your audio friends that they sound better!!!

An additional possible benefit to this arrangement is that varying the depth on different shelves might offer some useful diffusion on the wall behind the seating position where the bookshelves are located. I haven’t yet evaluated this, however, and an AB test will be impossible. (Listen, remove contents of shelves, change spacers, reload, listen again!)

William Lee's picture

Speaker sensitivity and distance to listening position determine how much power would be required. According to THX, the reference level is 85dB with 20dB headroom(105dB) at the listening position.
If the speaker is rated at 90dB at 1 meter, it would produce 90dB at 1 meter listening position with 1 watt input.
1W 90dB, 2W 93dB, 4W 96dB, 8W 99dB, 16W 102dB, 32W 105dB, 64W 108dB
According to Dolby the amplifier should have 3dB extra headroom, therefore in this case, the power requirement is 64W.
If the listening position is at 2 meters, according to the inverse square law there would be decrease of 6dB. So at 2 meters with 1 watt input, the same speaker would produce 84dB at listening position.
1W 84dB, 2W 87dB, 4W 90dB, 8W 93dB, 16W 96dB, 32W 99dB, 64W 102dB, 128W 105dB, 256W 108dB
In this case, the power requirement is 256W.
These speaker rating are measured in an anechoic chamber. For home use, you should be able to get by without the 3dB amplifier headroom.
If you start off with inefficient speaker, you can see that you need more amplifier power to accomplish the same goal.
Quick way to find out if you need extra power is to look at the AV receiver sound level adjustment for each channel. All my front, left and right are at -11dB. My Onkyo SR806 is calibrated to produce the reference level at the listening position. Since my speakers Klipsch RF62 and RC52 are rated at 97dB and 96dB respectively, my receiver are only using a few watts per channel most of the time but would still need 128W to hit 105dB as the listening position is about 4 meters away.

Tommylee99's picture

I've used the shelf trick for many years, although I went the extra mile by ripping the 2x4s to the exact width I wanted. The only downside-a lot of dust can collect back there!