Oops, Tom. Destination Audio is still around. In fact they demoed at EHX this year. They are just reshuffling speakers/lines.

The whole problem is with dealers – their dealers are fickle, scared and absolutely not loyal. As a result the whole online transition is a tough one for them.

But YES, four brands (you forgot to mention Status Acoustics) is about 2-3 brands too many.

Tom, I thought to get +27dB (117dB on 90dB sensitive loudspeakers) you would need over 500 watts at 1 meter, 1000 watts at 2 meters, and 2000 watts at 4 meters. Now having said that and its impracticality, I hope that others will take your advice to heart and choose appropriately sensitive speakers, rather than buy an amplifier that is only 20-100 watts more powerful, if they are trying achieve TXH or rock concert level of decibels.

I cringed when I heard Tom going into the math of SPL. Actually, between perceived SPL and real SPL and electrical values, I get confused myself sometimes. I typically leave the calculations of anything audio to Gene.

As for 500, 1000, 2000 Watts, I don’t think the example was meant to be as specific as it sounded. The general principle was correct.

With my basic understanding of SPL, I calculated:

In order to achieve and increase of 6 dB SPL (which is AFAIK a doubling in acoustical power) you need twice the power, so 1W = 90 dB 1m, 2W = 96 dB 1m, and so on till you reach 114 dB at 16 watts. Add, let’s say 33% to that (just a guess) to reach 117 dB…
Not quite 500 watts…
Correct me if I’m wrong! 🙂

-Simon

Simon, I think you are almost correct except that you need to double the power to achieve a 3dB increase in SPL. The formula for how much more power you need to achieve a specified gain in dB is 10^(dB/10). Therefore 10^(3/10) = 1.99 and 10^(27/10)= 501.19

Re: Doubling of distance = 3dB drop

That’s incorrect, sound intensity decreases based the INVERSE SQUARE LAW, it’s not linear. Remember that sound is radiating energy into a volume of space.
See the “Intensity, radiation and dB” section on this webpage for more details: http://www.phys.unsw.edu.au/jw/dB.html
There’s calculator at:
http://www.numericalexample.com/content/view/18/33

Glad to see that others caught some of the errors in Tom’s statements about amplifier power needs. Indeed, sound intensity decreases according to the inverse square law, so as you move further and further away, it is much more than just a 3dB drop for every doubling of distance.

For the speaker to amplifier power issue: for every 3dB increase in output, roughly twice as much power is needed.

Also, THX requires that a speaker can output peak levels of 105dB at the listening position and sustain a minimum of 85dB at the listening position at all times. Tom said 117dB, which is incorrect. The subwoofer needs to be able to reach peaks of 115dB at the listening position, so maybe that is where Tom got that rough figure.

If you consider a 4m (roughly 13 feet) seating distance, the power needs can add up really quickly in order to reach full THX spec. A typical 90db/1W/1m rated speaker will need about 500 Watts of power to reach those 105dB peaks.

Actually for every doubling of power, the perceived loudness goes up +3dB NOT 6dB assuming not factoring in boundary gain or placing multiple sources in close proximity playing correlated audio.

Check out my article on Speaker Connections for greater detail:
http://www.audioholics.com/education/loudspeaker-basics/loudspeaker-impedance-series-parallel-connection-basics

As for sound obeying the inverse square law of -6dB for every doubling of distance, that only applies to LARGE rooms or open spaces. Because of early reflections, most home theater rooms experience only -3dB for every doubling of distance thus all of the SPL calculators out there are actually too conservative when calculating loudness at a given distance.

This is a great calculator to help understand loudness and conversions of units:
http://www.doctorproaudio.com/doctor/calculadores_en.htm

but again it assumes -6dB for every doubling of distance which isn’t correct in a home theater room.

What is interesting however is if you are running 5 identical speakers rated at 90dB @ 1 watt/meter in 5CH stereo mode sitting 1 meter away from all speakers, just 1 watt per channel would produce a whopping:

97dB assuming none of the speakers are in phase with each other

104dB assuming all of the speakers are in phase

So as you can see when you start adding more speakers in the room, the sound goes up considerably and even at 1 watt with all speakers playing the same source, it’s quite loud.

A powerful amp is important for driving low impedance loads, handing high sustained output levels, and producing the large peaks in music since music is very peaky in nature. When I get back in town we will do a dedicated podcast on this.

Thanks Gene. I tried to tell Tom all this, but he edited the parts of the podcast where I was correcting him.

LOL Dina!!! I belive it LOL ja ja …funny stuff……. oh and thaks Rob for you coment it helped alot. Gene thanks for the literature, I’ll read it at my Lunch hour. Thanks again Gene!!! Cuz I was getting sad about my plans to buy a new AMP!!! And now I feel all good about it again!!!! LOL

What the crap Tom, you dont want me to buy a new amp? ….just kidding LOL.