One way to improve the accuracy of the measurement is to also measure the current flowing to the speaker. To do this, you need to use the AMP connector on the meter and connect the meter in series with the speaker by inserting the test leads between the amplifier and the speaker in place of one of the wires. You need to be careful here because most meters can't measure above 10 or 20 amps without blowing their internal fuses. A 450-watt power calculation at 30 volts should be about 15 amperes of current.
By measuring the current to the speaker at the same volume setting as the voltage reading was obtained, you can calculate the power. Just multiply the voltage by the current and you have the power regardless of what the impedance was at the time. For instance, 15 amps at 40 volts is 600 watts. See if this number tallies with your amplifier power meter.
Another test you can perform would be to confirm that the amplifier is not being choked by a lack of supply. You have an Exide battery and a couple of 4-gauge power wires, but there are always choke points that can occur when an untrained installer does the hookup. By setting your DMM to the DC voltage position and connecting across the power terminals of the amplifier, you can see if the voltage is sagging when you crank up the amp. Voltage drops to a low point of 12 volts would be reasonable with the engine running, but if it is below that, you can suspect a problem with the power supply wiring path between the battery and amplifier. Make sure the test leads are plugged in the right inputs of your DMM when you perform this test-since it was previously in the AMP jack, connecting across the power terminals will immediately blow the meter's protection circuit.
Don't expect the power to increase by double every time you parallel speakers. I think 670 watts from a 600-watt-rated amplifier is great, especially when you are running it at the limit of its load capacity. Time to add more speakers and amplifiers to get a real serious boost in output.
Q I recently purchased an Eclipse AVN5435 with a Sirius Satellite Radio receiver. I was excited about it, in part because of the recommendations in your magazine. After I activated it and turned it on, however, I noticed that the sound quality was very poor. It sounds like a highly compressed MP3 on most stations, and with 800 watts of amplified power backing it up, the lack of depth sounds especially horrible. As I understand it, Sirius puts out only a limited bandwidth and heavily encodes the signal, which is leading to the loss I hear. Is there any hope for improvement for satellite signals in the future? Will HD Radio suffer the same compression problem?Thanks,Jay W.Oakland, CA
A I have both Sirius and XM Radio receivers in vehicles, and also own shares in both companies. I really like what satellite radio has to offer and, like you, I notice the limitations. When you have a limited amount of broadcast bandwidth (not audio bandwidth) all digital broadcast formats have some hard decisions to make. For instance, in the band allocated to Sirius and XM, you can either have five stunningly high-fidelity stations, 25 really good stations or 1,000 really compressed streams. Broadcasters choose the balance that their marketing dictates. CNN, CNBC and all the talk stations need very little actual sonic quality; instead, they need intelligibility. Many Internet voice broadcasts are between 24kB to 56kB and they are easy to understand and sound good enough.
You can even compress a voice stream to death and still understand it, if you don't mind the grainy sound. That still leaves lots of bandwidth to mess with varied compression rates for music-based content. I suspect the classical listeners get the best fidelity through the least compression, since classical is very dynamic in nature (and the listeners are picky). Rock is much less dynamic and can be compressed more without being objectionable. Jazz is in the middle.
HD radio is in the same boat, and depending on how they deal with it, will be either better or worse. The key limitation is range on HD Radio, since it is broadcast from earth-based transmitters. This means there must be something to make you want to choose HD over satellite, since you can only receive HD Radio within range of a major city that happens to have a transmission footprint. Satellite can be received anywhere except in a tunnel, so I suspect that many of us will sacrifice fidelity for the sheer enjoyment of being able to listen to the same 150 stations on a drive from Virginia Beach to Long Beach.
Broadcasters believe that "content is king;" so paying megabucks for Howard Stern is just a good investment to them. The more content they can put on the airwaves, the more subscribers they should be able to attract. If someone offered "Car Audio Talk" as a satellite channel, I would sign up right away! The problem is where to stop. If broadcasters tried to compress every interest into a radio channel, it would sound like garbage.
Radio has never been considered a high-fidelity medium. AM is always bad, FM is a bit better and satellite raised the bar significantly, but if you want to have the top of the heap in fidelity, a pre-recorded format like CD and DVD audio is well above the crowd. For example, CDs are still compressed, but the amount of compression is 1:1 which is dramatically less than even the best MP3 compression. A ratio of 1:1 really means no compression, and you wind up with over 1,400kB per second of audio detail (which doesn't relate to why some CDs still suck). Apple's AIFF format is nearly at the CD level compared to the more common high-quality MP3 format, which is usually 192kB. When I launch iTunes and set my import properties to MP3, the maximum bit rate I can capture is 320kB per second, which is still a long way from CD rates (about 25 percent).
In England, almost all of the DAB (Digital Audio Broadcast) system uses a 128kB rate, which is described by listeners as "annoying." I don't know what bit rate the U.S.-based HD Radio system will use, but it is still limited by the bandwidth limitations of their allotted portion of the radio spectrum. The good thing is that providers need to please subscribers. Little-used content will be discontinued, and as new compression formats that maintain fidelity are developed, we will ultimately benefit.