When I first got involved with speaker design, I had some basic questions such as:
- What voltage I should give a speaker?
- What is the best voltage for a speaker?
- Can I give a speaker too much voltage?
These were just some of the newbie questions I had. I found it difficult to grasp the concept of speaker voltage and how it worked with so many other terms floating around, such as power rating and impedance.
As a general rule, to calculate speaker voltage, you need to know the power rating (watts) and impedance (ohms) of a speaker. From these values, the voltage can be calculated if needed from the formula, Voltage = Square Root (Power x Impedance)
- V = √PI
where:
- V = Voltage
- P= Power
- I = Impedance (Nominal)
In the audio industry, you may see some variations in this formula where different speaker manufacturers and designers use different values for the impedance, which will give a different result.
In the interest of simplicity, and to keep to the Audio Engineering Society‘s latest standard (AES2-2012), I use the nominal impedance of the speaker when calculating voltage.
I want to dig deeper into speaker voltage to help explain what it is and consider if it is even relevant, covering the following:
- What is speaker voltage?
- What voltage should I give a speaker?
- How much voltage can you give a speaker?
- What does increasing the speaker voltage do?
- Are speakers AC or DC?
- How do manufacturers measure voltage?
What Is Speaker Voltage?
To understand speaker voltage, we need to understand what a speaker is and how it works in its most basic terms.
A speaker is a basic transducer. It takes an audio signal (which is an AC (alternating current) electrical signal) and converts it to sound waves.
In a typical loudspeaker design based on a moving coil, we have a voice coil (which is a coil of copper wire) sitting in a magnetic field.
Our input audio signal is connected to the copper coil.
As the input audio signal (AC voltage electrical signal) passes through the coil, it creates a varying voltage across the voice coil which interacts with the magnetic field and oscillates.
This oscillating voice coil is connected to our diaphragm or cone, which moves air to produce sound.
Since our input is an alternating current, passing through a system that has impedance, we have a voltage associated with this audio signal.
From Ohm’s law, the relationship between AC current, impedance and voltage is given as:
- V = IZ or I=V/Z
where:
- V = Voltage
- I = Current
- Z = Impedance
Do We Need To Consider Voltage At All?
So, given that our input to a speaker is an AC signal, why do we rate speakers in terms of power and not in terms of current and voltage? Should we even care about input voltage?
First, if we consider current, we have to appreciate that specifying the input current of a signal is not a practical approach.
Every loudspeaker and speaker system will have a varying impedance (resistance to AC current), therefore using current is not a good way to specify a speaker input, as it, too, will vary. It is just not practical to specify speaker input in terms of current.
If we consider the input in terms of voltage, using ohm’s law, our voltage is still described in terms of current as there is a dependent relationship between voltage and current:
- V=IZ
where:
- V = Voltage
- I = Current
- Z = Impedance
Although voltage is an excellent measure of an audio signal level and gives us an audio signal strength, there is a better and more practical way to specify our speaker input and that is in terms of power.
Because our input signal is an AC signal, it will alternate from positive to negative values, therefore if we describe it in terms of voltage or current, our input will also be negative.
Using power removes the need for negative values, as power is always positive.
Power also has a relationship with current and voltage, given by the following formula.
- P= IV
where:
- P = Power
- I = Current
- V = Voltage
What Voltage Should I Give A Speaker?
So given that we specify speaker input in terms of power and not voltage, “what voltage should I give a speaker” is still a commonly asked question but is this question relevant?
You could say that there is little point in worrying about the voltage.
Let’s say you have a speaker and want to know what voltage to apply to the speaker.
To do this, you need to look at the power rating and impedance specification of the speaker.
For example, let’s say you have a 20W speaker with an impedance of 8ohms:
Using the following formula, you can calculate the voltage:
- sqrt(PR) = V = 12.64Volts
where:
- P = Power
- R = Impedance (Resistance)
The power and resistance specifications are typically nominal.
In summary, you should focus on the power and impedance of a speaker, as the voltage can be calculated as a result of power and impedance if really needed.
How Much Voltage Can You Give A Speaker?
At the end of the day, it is not necessary to be concerned with voltage as the power and impedance are far more relevant.
The power rating (ideally in RMS watts) and impedance in ohms are the specifications to focus on.
The actual functioning limit for a speaker is its power rating.
If a speaker is driven to dissipate more power than this power rating specification, then it is possible that the voice coil will overheat and possibly melt. The result is total speaker failure.
It is essential that you connect your speaker to a suitable amplifier whose wattage is suitable for your speaker.
When you adjust the volume control on your amplifier, you are in effect, varying the voltage as an audio signal is just an AC varying voltage.
Your amplifier system will scale and manage the volume levels as long as you ensure that you have a suitable amplifier.
A suitable amplifier means that:
- The speaker impedance is in the range of the amp.
- The amplifier power rating is suitable for the speaker.
What Does Increasing The Speaker Voltage Do?
Increasing a speaker voltage results in a louder speaker volume.
The volume control on your amplifier is actually a voltage control. When you turn up your volume, you are in effect, turning up the voltage.
Are Speakers AC or DC?
Speakers are AC devices, as the input audio signal is an AC signal.
Final Thoughts
Speaker voltage is the measure of an audio signal level and gives us an audio signal strength. However, there is a better way to specify our speakers, and that is in terms of power.
Power has a relationship with current and voltage, given by the following formula: P= IV.
Using power removes the need for negative values, as power is always positive.
So given that we specify speaker input in terms of power and not voltage, “what voltage should I give a speaker” is not a priority. Focus on the power and impedance of a speaker, as the voltage can be calculated as a result of power and impedance if needed.