This is a question that comes up often – what Amp is best suited for a pair of speakers that I own?
If you’re interested in finding the amplifier power (watt) required to achieve a target SPL (sound pressure level) use this Amplifier power output calculator instead.
Quick Overview
Active speakers that require a power source to operate, do not require an amplifier as one is already built in.
Passive speakers (those that don’t have to be powered) however, do require an amplifier. In this post we provide a calculator, guidelines on how to pick an amplifier, specifications, considerations and other precautions.
Contents
Calculator
Enter the RMS power rating of the speaker in the calculator below. It will provide the range of amplifier wattage values
Example Calculation
A 100 Watt speaker requires an amplifier with power output between 90 W and 120 W.
In general it is okay to exceed the upper wattage specification but we don’t recommend going too far below the lower wattage spec. Doing so will result in the amplifier clipping and producing harmonics that can damage the tweeter [1].
What type of Speaker do you have?
Does your speaker have to be plugged in to operate? If so then it has a built-in amplifier and in this case an external amplifier is not required. An example of this active speaker is shown in the picture below.
The speaker below (from Edifier) does not have to be powered. It is passive and needs an external amplifier.
The calculator above that provides the power rating of the amplifier. Below we discuss other considerations.
Check the Specifications
Speakers have two very important specifications: Impedance and Power rating.
What do they mean?
Impedance
Speaker impedance is typically a multiple of 2. Actual values are between 2 ohm and 16 ohm (typically). This refers to the impedance of the internal coil. It is a measure of resistance to current provided by the amplifier that drives it. The higher the value, the greater the resistance and the smaller the current draw. It’s easier for an amplifier to drive a speaker with a higher impedance as it will consume a smaller amount of current.
If a speaker impedance is 4 ohm, we recommend using an amplifier that is rated at 4 ohm or lower. Don’t use an amplifier that is rated at 8 ohm as the speaker will draw more current than the amp is normally able to provide and damage it.
The speaker shown above is made by a company called Moukey sold on Amazon. The impedance is not listed in the specifications but in response to Customer Q&A listed on the site. The impedance of the latest version of the product is 4 ohm.
Typically this number is printed on a sticker at the back.
Is Low impedance Speaker better than a High Impedance Speaker?
A speaker with low impedance will play louder for the same voltage input from the amplifier. This assumes the amplifier is able to drive the lower impedance. Let’s take a look at an example of a 2 ohm and 4 ohm speaker.
Assuming the amp is able to provide 2.83 Volt to the speaker then using the formula P = V2/R gives
P2Ω = (2.83)2/2 = 4 Watt
P4Ω = (2.83)2/4 = 2 Watt
A lower impedance speaker has a higher wattage and will therefore be louder for the same input voltage. Therefore a 2 Ω speaker will play louder than a 4 Ω speaker. A 4 Ω speaker will be louder than a 6 Ω speaker.
Power Rating
The power rating is one of the main specifications listed by the manufacturer. Units are Watt or W. It represents the amount of power that a speaker can handle. There are two power ratings: RMS and Peak.
RMS Power Rating
RMS Power Rating, also referred to as the root mean square power rating, is a measure of the continuous power handling capability of the speaker. It is expressed in watts – a unit of power.
Peak Power Rating
Peak Power Rating is the maximum power that a speaker can tolerate. A speaker is only able to handle this wattage level for a short time duration although the time period is almost never specified in the data sheet. Running a speaker at this level for an extended duration will cause the coil to overheat and result in damage so it’s recommended to never operate at this power.
What’s the difference between Speaker RMS and Peak Power Rating?
The RMS power rating indicates the Average Power that a device can handle continuously without overheating or causing damage. It is a more accurate representation of the power handling capacity compared to Peak power ratings, which only reflect the maximum power output for short bursts.
The RMS power rating helps consumers determine the appropriate power requirements for their audio systems, such as speakers or amplifiers. For example, a speaker with a higher RMS wattage can handle more power from an amplifier without damage. So, understanding the RMS power handling rating is crucial for achieving optimal performance without compromising the longevity of the equipment.
Many manufacturers will emphasize the peak power rating because it’s a larger number and looks more impressive than the average power. The peak can for instance be 300 Watt with RMS equal to only 150 Watt.
When looking at the manufacturer’s data sheet specifications, use the RMS power and not peak power number.
References
[1] Audio Clipping on Wikipedia
Related Calculators
- Subwoofer Amplifier – calculate amp requirements based on subwoofer specs
- Car Amplifier output – use the power rating and impedance to calculate the output watt delivered by the amp
- Car Amplifier Specs – use the speaker sensitivity and target SPL to derive the amp’s desired output power
- Speaker Impedance – calculate the impedance of speakers in either parallel or series configuration