The PM-150 Audio Power Meter FAQ

Q. How is the power meter different from a sound level meter?

A. A sound level meter measures the air pressure associated with a sound. The power meter measures the wattage flowing from amp to speaker. While related, the cabinet design and even room you are playing affect the sound level or air pressure. Think carpeted vs. concrete floors. Your amp and speaker are rated in watts, not dB SPL, so it makes sense to measure the wattage moving between them in order to monitor their performance, and that is not affected by the characteristics of the room. A sound pressure meter is a good tool box addition, though, as it can help you select a playing level that is safe for your hearing.

Q. Speakers have different power handling capability at every audio frequency. How does the power meter handle that?

A. The audio power meter makes a measurement of all the power moving from the amplifier to the speaker. There is no frequency selectivity or weighting involved, such as A-weighting. A speaker is certainly more sensitive at some frequencies than others. Speaker or cabinet resonances can cause greater cone excursion at those frequencies. The audio power meter is not intended to divine the nuances of each speaker’s response curve. However, it is able to set a broad limit to the power moving from amplifier to speaker, allowing speaker protection for musicians who treat their equipment properly.

If you are more concerned with screaming sound and consider your speakers expendable, running them near the failure limit, the protection feature of the power meter would probably not help you much. You can still use the meter to display amplifier power output, until you blow those speakers. Consider investing in a recone shop!

Q. Some engineers have said that the speaker/amplifier combination is just too complicated and nuanced for a power meter to make a meaningful measurement. Is this true?

A. Nope.

That said, the loudspeaker is a complicated beast, from a physicist’s point of view. It contains resistance, inductance, capacitance, and some other equivalents of those that arise from the spring-mass system of the speaker cone, spider, magnet, and air mass. Cabinet resonances and damping complicate matters further. Nonlinearities come into play near the limits of the cone’s travel.

However, there are two wires connected to each speaker cabinet, and in those wires flow currents and voltages which can be measured. If you can measure those, then you can compute the power flowing into the speaker cabinet, no matter what’s inside. The power meter does just that.

It is not necessary for the power meter to compute all the gory details of the operation of the speaker and enclosure in order to have a durable, usable power display at reasonable cost.

Q. I have a set of ears and can tell when I’m pushing my speakers and/or amplifier too hard. Why do I need a power meter?

A. When you are driving an automobile, you can also close your eyes and tell roughly whether you are on the road. It helps to have a visual indication, however, due to the merciless way that trees stop vehicles that have wandered off the road.

Same thing is true with speakers. A visual reference while you are playing is very informative, especially one that is independent of the instrument you are playing, the ambient environment and the loudness of the rest of the band.

Q. Isn’t a loudspeaker’s output dependant on the voltage and not the power?

A. Power represents the amount of energy being used per second. To a musician that means the amount of sound being produced. Depending on the impedance of the speaker, the voltage could be greater or less for the same power output. It is the power that is related to how much sound is being produced by the moving speaker cone. The voltage and current are used to compute the power, and that is the main number of importance.

For example, I could impress a large voltage on a speaker at a frequency of 100Khz, but the speaker would not consume much power because its frequency response does not extend so high. Here we have a large voltage, but small power.

Q. Does the power meter compress my signal in order to protect the speaker?

A. NO! When the meter is displaying the power going to your speaker, the signal is patched straight through the device. There is NO TONE OR LEVEL CHANGE in any way whatsoever to the amplifier input or output signals, whether you are playing heavy metal or “Feelings.”

The power meter looks like a couple extra inches of cable to your signals. That’s it.

Q. Can it measure clipped or distorted waveforms out of the amplifier?

A. The power meter is not sensitive to the waveform shape. It measures power by sampling the waveform present, regardless of shape, within the limits of its frequency response.

Q. How do loudspeakers fail?

A. Speakers fail for two main reasons. The first is overexcursion. When a large amplitude signal hits the speaker, it can cause the coil to push the cone so much that it moves beyond its physical limits, causing damage.

The second failure cause is too much heat dissipation. If the voice coil overheats, the wire can burn open. The speaker ceases to function because current cannot flow in the coil.

The first failure occurs due to high power transient conditions, like hitting a strong chord or unplugging your instrument with the amplifier turned up. The second failure occurs when the voice coil is steadily receiving more power than it can radiate away as heat or sound.

There are other factors that a speaker design engineer would mention, but these are the main two.

Q. If this is such a good idea, why has no one sold a power meter for musicians before?

A. Actually they have, but only a few voltage meters. There are some genuine power meter instruments usable by electronic technicians, but too expensive or fragile for stage use.

When I was starting to play guitar years ago, no one had a need for processors that could produce hundreds of different sounds. Or gold plated connectors. Or amplifier modeling. Or power dividers. Or 5.1 surround sound. Or MTV. Or the Internet. But once we had these things, they were everything from entertaining to necessary. The power meter is one of these things!

Q. How is the power meter different from a clipping indicator?

A. Many guitar players drive their amplifiers into clipping on purpose. That’s what provides that dirty sound. But the fact that the amplifier is clipping does not say much about whether the speaker is seeing a surge of power. The power meter measures power directly, regardless of what the amplifier is doing.

On solid state amplifiers, clipping can have a damaging effect on speakers. This is because many solid state amplifiers are DC coupled, meaning that they can put out unchanging voltages like a battery. Such voltages are damaging to speakers. Manufacturers of solid state amplifiers often add DC fault detection circuits to their designs to prevent DC voltages from being impressed on the speakers.

When the solid state amplifier clips, the biasing of internal stages is disturbed, causing the average output voltage to move away from zero volts, resulting in some large low frequency currents flowing through the speaker. This causes heating, overexcursion, and eventual failure. Better solid state amplifiers prevent this, but less expensive ones do not.

Tube amplifiers are coupled through transformers which cannot pass very low frequency or DC currents and voltages, so clipping is not so dangerous.

Q. Can I use the power meter on my stereo or PA?

A. The power meter’s bandwidth is optimized for guitar and bass, and is not suggested for use in PA or stereo applications.