There are lots of things to consider when designing a sound system from scratch. One that may seem confusing at first, but can be simplified once you understand a few electrical and audio concepts, is selecting the right amplifier to go with your speakers. It’s about matching power (kicked out by the amp) with performance (created by the speaker) and here are a few terms to know in order to do it right.
Speaker and Power Concepts
Amperes are the unit for measuring electrical current. It’s shortened to amps (don’t get that confused with an amplifier).
Watts are the measure of power. In speakers, watts is equal to amps times volts. Speaker manufacturers also rate speakers for how powerful they are in watts.
Voltage is a measure of potential electricity. When you turn an amplifier up you’re increasing the voltage coming out of it.
Ohms is a measure of resistance. It relates to Ohm’s Law which describes the relationship between voltage, current, and electrical resistance.
Speaker impedance is measured in Ohms. It tells you how much resistance the speaker presents to the amplifier. It’s not constant because it changes with different sound frequencies. A 6 Ohms speaker is almost never resisting at exactly 6 Ohms.
Sensitivity in speakers is complex. To keep it simple, the more sensitive a speaker is the less power it needs to draw from an amp. This blog on wattage and sensitivity goes more in depth.
Figuring Out Amp Power Needs
Speaker manufacturers list a loudspeaker’s power rating–sometimes called power handling or power capacity–on its specification sheet in watts. You need this information to create a sound system that both sounds good and is reliable in the intended application–whether that’s for a medical ultrasound device or a football stadium.
A good rule of thumb is that you want an amp that can supply double the power a speaker is rated for during its normal usage. This leaves enough “head room” for the amp to supply the extra power needed when the speaker is at its peak performance level. During normal operations, you want an amp working at about 60% to 70% capacity.
One thing to consider is whether the speakers in your audio system will be wired in parallel (which is common) or in series (which is rare). Wiring speakers in parallel means connecting the positive terminal of the first speaker to the positive terminal of the second speaker, the negative to the negative, and so on for any additional speakers. Wiring in parallel decreases the impedance and ups the amount of power the system can draw from the amp.
An amp connected to a low impedance speaker (4 Ohms) puts out more current than if it’s connected to a high impedance speaker (8 or 16 Ohms). To calculate the total impedance of multiple speakers is relatively simple–if each of the speakers in your system has the same impedance rating. The equation is impedance divided by the number of speakers. So 4 speakers with 8 Ohms impedance, wired in parallel, will have a total impedance of 2 Ohms (8 divided by 4) as a system.
Selecting the Right Amp
Amps come in mono (with one output), stereo (two outputs), and multichannel (more than two outputs). A mono amp may be right for something like a PA system. Stereo amps are common in hi-fi systems or headphones - where you’re feeding a right and left speaker. Multi-channel amps are used in surround sound systems and applications where you have different kinds of speakers in different locations working in tandem.
Manufacturing a speaker is a mix of science and art, and a lot of the performance ratings are approximations because of this. That’s one reason getting amps and speakers from the same manufacturer might make sense–they’ll have been engineered to work together based on the same testing standards.
The first amps invented operated by supplying continuous power to a speaker. This is very inefficient. However, thanks to huge advancements in electronics, today a Class D amp paired with digital signal processing (DSP) is best for most sound system needs. Class D amps operate like a digital switch (with the components turning on and off extremely fast). This makes them much more efficient, they produce less heat, are smaller, lighter, and last longer. The DSP can electronically alter the signal being fed into a speaker so that sound comes out the way it is intended to be heard.
DSP can make up for deficiencies in the listening environment, time delays between the speaker’s locations and the listener’s location, and inadequacies of the respective speakers’ performance characteristics.