It is easy to be baffled by the language that wireless loudspeaker makers use in order to explain the performance of their products. I will clarify the meaning of one commonly used parameter: “signal-to-noise ratio” in order to help you make an informed choice whilst purchasing a new a set of wireless loudspeakers. As soon as you have chosen a number of cordless loudspeakers, it’s time to explore a few of the specifications in more detail to help you narrow down your search to one model. One important criterion of cordless loudspeakers is the signal-to-noise ratio. To put it simply, the signal-to-noise ratio describes how much hum or hiss the loudspeakers are going to add to the music signal. This ratio is usually described in decibel or “db” for short.
A technique in order to perform a straightforward test of the noise performance of a couple of wireless speakers introduced at this hyperlink is to short circuit the transmitter audio input and then to crank up the wireless speaker to its utmost. Then listen to the speaker. The static that you hear is produced by the cordless loudspeaker itself. Be certain that the volume of each pair of wireless loudspeakers is set to the same level. Otherwise you will not be able to objectively compare the level of static between different models. The general rule is: the smaller the level of static that you hear the higher the noise performance. Whilst looking at the cordless speaker spec sheet, you want to look for a set of cordless speaker with a high signal-to-noise ratio number which indicates that the cordless speakers output a small amount of noise. Noise is created due to several reasons. One factor is that today’s cordless speakers all use elements such as transistors and resistors. These elements are going to make some amount of noise. The overall noise depends on how much noise every element creates. Nonetheless, the position of these elements is also essential. Elements that are part of the speaker built-in amplifier input stage are going to normally contribute most of the noise. One more cause of noise is the wireless music transmission itself. Usually products which employ FM style broadcast at 900 MHz are going to have a rather large amount of noise. FM transmitters are extremely prone to wireless interference which is why newer types usually employ digital audio broadcast. The signal-to-noise ratio of digital transmitters is independent from the distance of the cordless loudspeakers. It is determined by how the audio signal is sampled. Also, the quality of parts inside the transmitter will influence the signal-to-noise ratio.
Most of today’s cordless loudspeakers use power amps which are digital, also referred to as “class-d amplifiers”. Class-D amplifiers make use of a switching stage that oscillates at a frequency between 300 kHz to 1 MHz. This switching frequency is also noise that is part of the amplified signal. Yet, latest cordless loudspeakerspecifications generally only consider the noise between 20 Hz and 20 kHz. The most popular technique for measuring the signal-to-noise ratio is to set the wireless loudspeaker to a gain which allows the maximum output swing. After that a test tone is fed into the transmitter. The frequency of this tone is typically 1 kHz. The amplitude of this signal is 60 dB below the full scale signal. Then, the noise floor between 20 Hz and 20 kHz is calculated and the ratio to the full-scale signal calculated. The noise signal at other frequencies is eliminated through a bandpass filter during this measurement. Another convention in order to express the signal-to-noise ratio uses more subjective terms. These terms are “dBA” or “A weighted”. You are going to discover these terms in the majority of cordless speaker parameter sheets. This method was designed with the knowledge that human hearing perceives noise at different frequencies differently. Human hearing is most perceptive to signals around 1 kHz. Then again, signals below 50 Hz and above 13 kHz are hardly heard. Thus an A-weighting filter is going to amplify the noise floor for frequencies that are easily perceived and suppress the noise floor at frequencies that are barely perceived. Most cordless speaker will show a higher A-weighted signal-to-noise ratio than the un-weighted ratio.