# Basic measurements question?



## Sixtusv (Jan 4, 2015)

Hello, I'm looking to build a new speaker to improve my system. In order to get a feel for what I want and/or need, I started to measure voltage at my current speakers and matched that with SPL readings. I tested at 1v, 2v, and 4v, across my speaker terminals. I also have REW with an uncalibrated ECM-8 mic. but I haven't used that yet to measure my speakers more accurately thinking I could get some quick information with the above setup.

A little backrgound on my current gear: Denon AVRs700w receiver, 5 channels playing in Expanded Stereo mode, each with (2) 4-ohm Sony S300 speakers (full range) connected in series (for an 8-ohm load). 2 Sony subwoofers running from the preamp-out through a plate-amp.

I ran both pure test tones and music, get comparable SPL ranges, with the test tones providing more steady readings. The measured SPL at 3' from any speaker (with all the other speakers on also) was about low 80's at 1v, mid 80's at 2 v, and around 90's with 4 volts (at which point the volt-meter started to complain by reading "OL". I'd say that most of my listening is done between the high 70's to low 80 db level, with high 80's once in a while.

I tried to use Ohm's law to figure out amps and watts like this: 4 V divided by 8 Ohms = .5 amps of current moving through the speakers at 90 db. Furthermore, .5 Amps times 4 volts again gives me 2 watts? That seems very little (I don't know what the speakers were originaly rated at, but could it be these are very efficient speakers making a lot of sound with little power?) or am I playing with fuzzy math?

I'm trying to apply some of this information to the drivers I plan to buy for the new build - when they are rated at 15w, 50w or 100w, all with the same 8-ohm impedance, does it mean that they can handle amplification on the order of 7 to 50 times more than my current system provides? Or does it mean that they NEED that much power to make the 90 or so DB of sensitivity they are rated at? (Clarification: I don't necessarily want it louder, but I want more accurate sound closer to what I hear from my system when listening through better headphones)

Bottom line, if I build new (better?) speakers, will I need to (or want to) upgrade the amplifier too? Or is this why some people upgrade their systems by using their receiver as a preamp, and adding active crossovers and bi-amped or tri-amped speakers? 

Thanks in advance - Six from Minneapolis.


----------



## Daniel S. (Apr 20, 2015)

The sensitivity of a speaker is usually measured at 1Watt with a distance of 1Meter. Even thogh I'm not quite sure what exactly you measured (all speakers on, speaker per channel still in series?), with your setup and 2W at 90dB SPL at 3' would be roughly 86dB at 1W and 1m (lot's of handwaving here). That would be in the normal range for hifi speakers.
If you're not listening at very loud levels *your Denon amp will be just fine*. As you have noticed already, the power necessary to achieve reasonable loudness (where you are not in danger of hearing damage and/or grumpy neighbours) in a normal sized living room can be surprisingly small.

A few thoughts on building new speakers:
Since you mentioned the sound of good headphones you seem to want very neutral sounding speakers. If you're trying to design these yourself you will probably need to learn a lot of theory. If you are not familiar with crossover design and never heard of Thiele/Small parameters you will need to do a lot of reading (maybe a shoe box worth of paper) to design good neutral sounding speakers from scratch.
A good alternative might be to get a quality speaker diy-kit and just do the woodworking and maybe the soldering.

Anyway, I don't want discourage you from building your speakers - which is a lot of fun - but rather save you the frustration of a "failed" project (been there done that).


----------



## Sixtusv (Jan 4, 2015)

Thanks Daniel! I have a tendency to babble, so I will try to stay focused.

I have been reading what seems like multiple shoeboxes worth of data, and I'm obviously still confused as to whether the wattage rating and efficiency of a given driver has any relation to the SPL I can expect at a given voltage across the driver terminals. 

For example: I have read that if a driver is rated at 100w, (like say the Audax PR170M0 6.5" Midrange) The coil can sustain that much power without damage. (That sounds fine, but how do I know if I'm anywhere close to that level when I'm listening through my receiver?) This 8-ohm driver also has a sensitivity rated at 100 db, presumably measured at 1/watt/meter. Is there a way to calculate the potential SPL of that driver at the same distance when you feed it a signal that measures 2 volts across the terminals? 

Still baffled (bad pun intended) in Minnesota.


----------



## Daniel S. (Apr 20, 2015)

Decibel (dB) is a logarithmic unit to compare two levels. Since the sensitivity of that driver was measured with 2.83V, at 2V there would be a 3dB decrease: 20*log(2V/2.83V)=-3dB
The link below should provide valuable information for audio engineering as it also explains the difference between power level and pressure levels. There are also some charts for comparison, including permissible exposure guidelines - which will give you an idea how much SPL is sane.
http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm

A comment on the Audax speaker you chose as an example:
This is a speaker designed for sound reenforcement. If your living room is not unusually large (you almost have to yell at people on the other end of the room to be intelligible) or you want prolonged high SPL (see link above), PA speakers might not be a good choice for a home theater.

I don't mean to be patronising but if you want to design your own speakers, I think it would be sound advice (pardon the pun) to get a textbook. I found "Loudspeaker Design Cookbook" by Vance Dickason to be very helpful to get a basic understanding.


----------



## Sixtusv (Jan 4, 2015)

Daniel S. said:


> Decibel (dB) is a logarithmic unit to compare two levels. Since the sensitivity of that driver was measured with 2.83V, at 2V there would be a 3dB decrease: 20*log(2V/2.83V)=-3dB The link below should provide valuable information for audio engineering as it also explains the difference between power level and pressure levels. http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm .


Great stuff and good advice. Thank you for taking the time to respond and put your feelings into it. I like it when people care enough to say "hey!" I'll have to look at these driver specs a bit more closely... I did not get the sense they would be unsuitable for home theater by just reading the specs. . They may be a bit of overkill for my small space, yes. (Would they not work as well at lower volumes?). That is part of my original question: do the more efficient and more powerful drivers demand full power to sound good?

I have an earlier edition of Dickasons book, and just picked up a copy of one of David Weems's book (more readable for me). Yes I have a lot to learn but I'm looking forward to the mistakes even more than the successes. I guess I like the process more than the result. I could just buy better speakers, but what would I learn from that?

Well, that last question is rhetorical. I did just get a few different drivers from Madisound and Parts Express. I'll just have to start playing and see if the specs match both my measurements and my ears!

cheers! Six-Mpls.


----------



## Daniel S. (Apr 20, 2015)

I'm not sure if these Audax 6.5" drivers are unsuitable for home theater, but they wouldn't be my first choice either. The resonance frequency is to high and Xmax to low to use them in a 2 way speaker. The diameter is pretty high for a dedicated (hifi) midrange driver, so lobing starts at around 2kHz.
The high efficiency is certainly nice (less thermal compression at higher volume), but I'm not sure if you will get to power levels where it would be that much of an issue.
How well they will work at lower levels I can't say for sure. My overall experience with PA speakers is limited and I have no personal experience with that particular driver (or driver family). I had a few chances to listen to PA systems at lower volume and the sound wasn't that great, but I'm not bold enough to claim that they have to sound bad. There are however inherent limitations like the problem with off axis response due to the larger speaker size for the given frequency range.

Since you already have some textbooks and your first batch of drivers, I wish you good luck with your project, and have fun.

-Daniel

PS: When you've build some quality speakers there is still that one horrible (and often overlooked) part of the reproduction chain left: room acoustics


----------



## mwmkravchenko (Jul 11, 2009)

@Sixtusv

A loudspeaker itself has no watts rating. It does not produce watts. It consumes watts.

I'm pretty sure you understand this already.

Testing with tones and trying to to the wattage calculations is a bit dicy. It can have application if you are careful. Most woofer and midrange efficiency rating is specified at 1000 hertz. It is also at or around this level that you have an impedance level that is the nominal rating for the driver. Like 4 or 8 ohms.
So if you play a sine wave at 1000 hertz and do a voltage measurement you will be able to use the drivers rated impedance to do a reverse calculation of the wattage into it. To be successful you will need an SPL meter and a decent volt meter that is capable of True RMS measurements. The cheap meters usually poop out in accuracy a little above 200 hertz. Or if you have an old analog meter (a needle that moves left or right) you are doing just fine as they are usually very wideband measurement capable.

Last but not least.

Most listening is done at less than 3 watts of power input. The only place where you need more power on peaks in in the low end.


----------



## Sixtusv (Jan 4, 2015)

Hi Mark, 
Thank you for all the detail! It helps to know more as I start to pull ideas together for a speaker design.


----------



## mwmkravchenko (Jul 11, 2009)

Happy to share!

I was there a long time ago, and there was no internet. A little different. Just a little.


----------

