When a speaker?s manufacturer talks about their product?s sensitivity rating, what do they mean? How can a speaker?s sensitivity rating impact the listening experience? How is it calculated? Should you trust the numbers given by the manufacturers? What should you know before choosing your speakers? Here are our answers to these questions.
Speaker sensitivity: what does it mean?
The sensitivity rating of a speaker serves to indicate the sound level delivered in proportion to the amount of power sent by the amplifier. A speaker?s sensitivity is measured in decibels and its calculation must take place in a specific setting: an anechoic chamber (acoustically non-reflective), with a microphone placed 1 meter away from the speaker and an amplifier delivering an electric tension of 2.83 Vrms.
How is a speaker?s sensitivity measured?
All major manufacturers validate the frequency response of their speakers before making them available for purchase. The operation is relatively simple and involves playing pink noise (ranging from 20 Hz to 20 kHz) through an amplifier which has been calibrated to send a tension of 2.83 Vrms to the speaker.
Why 2.83 V ?
This tension of 2.83 V did not become a standard randomly; it corresponds to one Watt of power delivered by the amplifier, while the speaker features a nominal impedance of 8 Ohms, measured at 1 kHz. Any amplifier can provide this amount of power and reach from 85 dB to over 100 dB at 1 kHz, depending on the speakers used. These values correspond to a significantly high sound level in a 30 m² room, for example.
Why measure at only 1 kHz?
At a frequency of 1000 Hz (medium range), the speaker?s nominal impedance is reached. Below this frequency, the speaker?s impedance has a tendency to drop while higher frequencies will cause impedance to increase. A drop in impedance requires the amplifier to intensify its power delivery, resulting in a higher sensitivity rating. It is therefore important to measure a speaker?s sensitivity at a frequency where its impedance is stable, thereby ensuring that the same level of quality and performance will be attained by all speakers.
Why do some manufacturers indicate a sensitivity rating for 1 W of power instead of 2.83 Vrms?
This is because different speakers, each of which feature unique characteristics, behave in different ways within an audio system: sending 2.83 Vrms to an 8 Ohm speaker or to a 4 Ohm speaker does not demand the same amount of power from the amplifier, for example. An amplifier delivers 1 W to a speaker with an 8 Ohm impedance and 2 W for 4 Ohms. Since the power is doubled, the sensitivity goes up 3 dB, thus giving the impression that a 4 Ohms speaker has a higher sensitivity rating. Manufacturers whose catalog includes 8 Ohm and 4 Ohm speakers standardize their measurements by adjusting the tension of the amplifier in order to calculate sensitivity ratings which are comparable from one model to another.
Why does sensitivity rating vary from one speaker to another?
First of all, the impedance of a speaker has a direct impact on its sensitivity. With an 8 Ohms impedance, 2.83 Vrms equals one Watt, while a 4 Ohms impedance will require 2 Watts from the amplifier. This variation corresponds to a 3 dB difference. Moreover, additional drivers can also increase the sensitivity of a speaker. Lastly, a speaker whose peak frequency is situated at 1 kHz can be marketed as a high sensitivity model, although the actual sensitivity rating is much lower when the entire bandwidth is taken into consideration.
Let?s take two examples, the Highland Audio Oran 4301 speaker, with its 84 dB/W/m sensitivity, and the Klipsch R-14M with a 90 dB/W/m sensitivity. The speakers both use 4-inch drivers and yet the model from Klipsch delivers a more intense sound (+ 6 dB, meaning almost twice the sound volume for the same amount of power sent by the amplifier). The high sensitivity of the Klipsch speaker is linked to the sound level its speaker delivers at 1 kHz (frequency used to measure the sensitivity). In practice, these two speakers do not have a 6 dB sensitivity difference over the full frequency range.
Is it better to own a pair of high sensitivity or low sensitivity speakers?
The Klipsch R-14M speaker requires less power from the amplifier to deliver the same sound level as the Highland Audio Oran 4301. This is a good thing, as it reduces the amount of power that the amplifier needs to provide to the speaker as well as the amount of heat accumulating around the drivers? mobile parts ? heat which can result in a lower thermal compression and a minimal dynamic reduction. This, however, is not the case for the Highland Audio Oran 4301 speaker, which stays very linear.
The a high-sensitivity speakers often lack linearity and peak in the medium range. This alteration of the frequency curve is not necessarily an issue and can even add an enjoyable flavor to the listening experience.
If you are planning to crank up the volume on low sensitivity speakers (88 dB and below), you should choose a very powerful amplifier (> 100 W). On the other hand, a high sensitivity speaker (90 dB and over) will function properly with an amplifier of average power (< 50 W). With very sensitive speakers (Klipsch Heresy III, 99 dB/W/m, for example) an amplifier delivering at least twenty Watts is sufficient. Very sensitive speakers are therefore a natural match for tube amplifiers.
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