Figuring power for DIY speakers?

[dgunthert]

As mentioned in another thread, I'm planning on building several Modula MTs and Modula MTMs for my HT. As I start thinking about equipment, I want to make sure I spec the power requirements right without overspending. After searching both this board and Google, I couldn't find a way of determining what my amp requirements will be. Using the Modula MTM as an example, it's a tweeter rated at 100 watts RMS and two mids at 60 watts RMS each. So how do I figure/estimate the RMS of the speaker as a unit?

[Hdale85]

It doesn't really matter, buy a good quality amp with decent clean power.

[fbov]

+0.5 - it matters little, but....

My HT is based on Modula MTMs, 4 NatP's and the CC. Some things I considered...

Power ratings are more thermal than mechanical, a matter of heat dissipation, not SPL production, and tweeter ratings assume clean power and a crossover. The mid pairs are rated for 120W, but can take twice that or more for short periods of time, electrically.

Mechanically, it's all about excursion control, which depends on driving frequency and box design; size if sealed and size+tune if ported. Sealed boxes will pressurize at high excursion, acting as an air spring to limit excursion to safe levels. If sized properly, this protection is independent of frequency. Ported boxes have a hole, so they don't pressurize, but the cone interacts with the box around tuning frequency. Keep the driving frequency above tuning frequency and you can create a bulletproof design. Drive it hard at lower frequencies, and it should self-destruct quite easily, but I've never seen that failure. I must be a wimpy listener (or I valule my hearing).

That said, two of mine are large, ported towers tuned very low, my L/R speakers, and the other three are sealed boxes, not quite bulletproof, but close. I drive them with an Onkyo 707, rated at 100W RMS, 2 ch. driven, but I also have pre-outs if I ever have the desire to increase available power. I haven't had the desire, yet...

Which gets to the real question - what do you listen to, and how loud do you play it?

The MTMs are about 90dB@2.83v, 1m sensitivity, but being 4-Ohm nominal impedence, they're pulling ~2W at this input voltage. They'd be more like 87dB@1W, 1m. With 5 of them in a 2400 cu ft room, I get about 90dB@2.83v at my listening position. Given I have 100W/ch available, I should be able to hit 107dB at my listening position. Since I normally listen at 75dB, this gives me over 30dB of headroom for transients. If these SPLs sound familiar (75dB average level, 30dB headroom), it's because they're the THX specs for home theater. Shucks, I'm listening 10dB too low! (But I've retained my hearing as a result!)

So, it's a matter of:
- room size
- speaker sensitivity
- listening level + headroom
- tolerance for distortion at high levels
- listening situation (what will neighbors and living partners allow?)

In my case, I'd like to have a real 200W/ch. available, but I can't justify it based on my listening situation. 100W is plenty, and that's what Hdale85's really getting at.

Loudness is not a linear function of power. 3dB is a factor of two in SPL (A 50W amp is only 3dB softer than a 100W amp.), and 10dB is a factor of 10x more power (THX's 20dB of headroom requires 100x greater power.) 10dB also corresponds to a subjective impression of being twice as loud (or half as loud). You may be fine with a stereo pair and a 50W/ch. amp. Or you may need a half dozen 200W amps (plus subwoofer amps) for a 6.1 system in a large room. Only you know for sure...

HAve fun,
Frank

PS A pox upon this site for kicking me out mid-reply - how often should one be required to log in!!!

[Hdale85]

I never have the site kick me out, but I checked the box to keep me logged in and if you don't let it save the cookies it might be a problem?

Back to the amp topic I've run my Dynamic 4T's on 15w little chip amps and they sounded very good and got decently loud. So power is sort of one of those things people always want a lot more than they really need lol. Now if your speakers are not terribly efficient then 15w isn't going to work amazingly well, but my point is most decent amps are 75w at the very least, and many of them are well over 100 so no matter what as long as you buy a good quality amp that performs well you should be fine..... unless you're trying to get 120dB in a concert hall.

[wkhanna]

PS A pox upon this site for kicking me out mid-reply - how often should one be required to log in!!!

Frank,

Please let us know if this ever happens again.
We will report it to our webmaster & get it sorted out.

[fbov]

...I've run my Dynamic 4T's on 15w little chip amps and they sounded very good and got decently loud...

That was the point of my last paragraph, and why I gave you a +0.5... very few people know how freaking loud a 90dB sensitivity speaker is at 1W. I've built speakers that get very loud with a 10W/ch amp! But the battery life is great...

They again, perhaps he's got a large room, likes it loud, or both! I try to convey an understanding of the topic, and let the OP make an informed decision.

HAve fun,
Frank

PS WKHanna, I'll get back to you on that... right now, I'm troubleshooting web sites that keep growing in RAM... and I don't think HTGuide is one of them. Thanks for asking...