Some recent post by Mainsail and Chris got me thinking about something which might be interesting to trailer sailors – or for anyone who is trying to be frugal on power requirements.
This regards using 4 or 8 ohm speakers. As Mainsail pointed out, car stereos are intended to be used with 4 ohm loads (or even lower) and you lose 3 db going to 8 ohm . I said car stereos can safely be used with 8 ohm speakers. The interesting thing is that typical car stereos have output amplifiers which when delivering power to the speaker, also burn up or waste power in the amplifier. I ran some ballpark numbers for the wasted power for an amplifier outputting 2.5 watts into a 4 ohm and an 8 ohm speaker. In both cases, the speakers are receiving 2.5 watts so would have the same loudness.
However, the output amplifier burns up or wastes different amounts of power when delivering the 2.5 watts to the different speaker impedences. For this example, the same amplifier burns up 1.66 times more power delivering 2.5 watts to a 4 ohm load as it does delivering 2.5 watts to an 8 ohm load. The total power used by the amplifier is what is wasted plus what is delivered to the speaker and for the same 2.5 watt audio level, total power used by the amplifier is 1.41 times higher for the 4 ohm system compared to the 8 ohm system. The car stereo has other current draw independent of the amplifier /speaker so the overall gain in power efficiency by using 8 ohm speakers would be somewhat less that 1.41 but using 8 ohm speakers would still seem to be a significant way to reduced car stereo power consumption for normal “moderate” listening levels.
What is the tradeoff for using 8 ohm speakers? As Mainsail pointed out, you lose 3 db in output power level. You will also likely see amplifier distortion at 3 db lower volume. In general, you cant play the stereo as loud before it starts to sound bad.
What does 3 db mean? If you have two audio signals and you A/B between them, when you can first detect an audio level difference, the louder one will be about 1 db higher level than the other. If you did this exact thing successively 2 more times, the final audio output would be 3 db higher than the initial audio level. So 3 db is noticeably louder but its not a huge amount louder.
Seems that in an auto application, you have plenty of current available and its reasonable to trade off the efficiency for the higher output power you get with 4 ohm speakers. But in the boat, its my opinion that the better efficiency of the 8 ohm speakers might be worth the slightly lower peak power. I’ve been pleased with the performance of the $150 stereo plus the 8 ohm bookshelf speakers, somewhat of a bonus to know I’m probably also getting better efficiency (since I used the 8 ohm speakers just because I had them laying around). .
Walt,
I know you're an EE and I respect that. My only credentials are that I have been an audiophile for years and done a fair amount of recording and live sound stuff as well. I also spent a couple of winters, while at college, installing car audio systems (well before "thumpin' bass"). I know none of this makes me an EE but what you said is counter intuitive to what I've both seen, read and understood about power amplifications and 4/8 ohm loads.
Rather than argue the point I performed a very "real world" test because what you said, 8 ohm speakers will consume less battery power, has always been the opposite of what I have known to be true.
I used a Trojan deep cycle battery on a charger connected through my multimeter to the stereo set to monitor DC amps. For the test I used two different pairs of speakers a set of KEF 8 Ohm and a set of Symphony 4 ohm speakers. KEF's have always been notorious for being inefficient in terms of SPL ratings and that shows to be true and you can find more efficient 8 ohm speakers but it still does not change the outcome a whole lot. Both speakers were bookshelf type speakers I had kicking around the barn. If I had to guess I'd say the KEF's are about an 87db efficient rated speaker and the Symphony's are about a 92db efficient speaker. Regardless of efficiency the 8 ohm speakers use more power at the same exact volume setting and the meter is impartial.
The source audio, to be fair and consistent, was a white noise audio test CD that puts out a constant volume and is used to set up home or professional audio systems. Because this is a fixed frequency test CD the power output requirements remain level and constant, unlike a music CD where the volume and amp requirements are constantly changing. Along with measuring DC amps I also measured the sound pressure level with my SPL meter that I use to set up and balance my home theater system.
Below are two very simple photos.
In this first photo the Kenwood stereo is powering the set or 4 ohm rated speakers and the volume is set to 27 the power consumed at volume 27 is 1.62 amps and the SPL meter reads 103db. The SPL was hoovering between 103 & 102 (mostly 102) so to be fair I'll call it 102db not 103db..
In this second photo the Kenwood stereo is powering the set of KEF 8 ohm rated speakers and the volume is also set to 27. The power consumed at volume 27 is 2.12 amps. The SPL meter reads 94 db but to be fair it was hoovering between 95 and 94 (mostly 95) and I happened to catch it at 94. I think it is fair to call it 95db. It should be noted that this is NOT an ideal location for an SPL meter but it was in the same location for both sets of speakers. Unfortunately to catch it all with the camera it needed to be where it was..
At the same volume setting the 8 ohm speakers produced less volume output and consumed roughly 131% more current than the 4 ohm speakers did. While all speakers will be slightly different it is safe to say that 8 ohm speakers will draw more current than a comparable set of 4 ohm speakers.
The meters are 100% impartial (that's why I used an SPL meter because my ears, while fairly good, can't determine a 1db difference in output volume. All things were tested exactly equal at a steady voltage input and the same stereo volume setting and test tone CD....
As I said before, and I stand by it, car stereos on boats will consume less power using 4 ohm rated speakers and will also play a little louder.
Lets assume that your personal volume comfort volume level is 102db. With the Symphony 4 ohm speakers I used you would consume 1.62 amps to achieve 102db, as in the photo above. If you wanted to achieve the same listening level of 102db with the 8 ohm KEF speakers it would require 4.58 amps (I caught it at 101 but it was bouncing between 101 & 102 the meter is very sensitive) !!!!!!!!!!!!!!!!!!!!
Is it safe to use 8 ohm speakers with a car stereo? It probably won't hurt it any but it certainly won't save any amp hours..
P.S. Most amplifier manufacturers state rated power output based on 2, 4 or 8 ohm loads. Crown for example rates their 1000W amplifier at 500 watts per channel at a 4 ohm load and 275 watts per channel at an 8 ohm load. That means that with a 4 ohm speaker it can deliver 500 watts per channel and with an 8 ohm speaker only 275 watts per channel..
Crown Amp (LINK)
