Speakers vibrate to create sound waves that travel outward from the speaker. A rigid enclosure can cause these back-and-forth waves to cancel out each other if their wavelengths match, degrading audio quality. To prevent this, speakers are often built with internal sound absorbing materials.
Sound Pressure Level Speaker Sound
Sound pressure is the force per unit area at which speaker sound is produced. It is determined by the distance from the source, much like how a loud noise increases in intensity when you move closer to it. This is because the distance from which you hear a particular tone changes its impact on your ears, and this relationship is a logarithmic one. Specifically, doubling the distance from the source decreases the intensity by 6 decibels.
Sound Pressure Level (SPL) is a logarithmic measurement of the instantaneous root-mean-squared sound pressure at a given location and time and with a specified reference level, usually 20 mPa. The SPL is expressed as decibels (20 times the logarithm to base 10) of the ratio of the measured sound pressure to the reference sound pressure, measured with a suitable frequency weighting (A, C or Z).
As the human hearing speaker sound range spans from 0.00002 pascals up to 200 pascals, it’s more practical to express SPL in decibels rather than in Pascal units. This also gives a useful means of expressing the relative differences between sounds that are too quiet to be heard and those that are painfully loud, as there is a large number of decibels in the range between these two extremes.
Unlike sound power, which is an intrinsic property of the source, sound pressure depends on how far away you are from the source and the characteristics of the surrounding environment (distance, reflection, absorption). Therefore, SPL measurements are made using a calibrated instrument called a sound level meter, most commonly an A-weighted one.
A-weighted data is adjusted so that it better matches the sensitivity of the human ear, and is used for most public safety and professional audio applications. You’ll also see “un-weighted” (also referred to as linear) and “flat” SPL specifications, which don’t apply any frequency weighting. These are generally more accurate for scientific or calibration purposes.
Frequency Response Speaker Sound
The range of frequencies at which a speaker sound can reproduce is known as its frequency response. This is measured in hertz (Hz), which is the number of cycles that a sound wave makes per second. The ear can perceive sounds as low as 20Hz, which represents very deep bass tones, and as high as 20,000Hz for the highest treble. A speaker with a good frequency response will be able to accurately reproduce the entire audible range.
A good frequency response curve will be smooth with minimal peaks and valleys. Look for a graph that is relatively flat through the midrange and treble without excessive boosts or dips. It’s also important to check that the graph doesn’t roll off too early at the very high end of the spectrum – a strong peak at 14,000Hz, for instance, can result in a harsh and shrill sound.
When it comes to the low end, you’ll want to be sure that the speaker doesn’t produce any excessive bass rumble. A bloated, boomy sound can ruin the overall balance of a sound system and may give it a muddy or swampy character.
If you’re shopping for a home stereo, a low frequency response is less critical than in a professional installation where the sound quality needs to be very precise and clear. Measuring a speaker’s frequency response requires specialized equipment like a calibrated microphone and a frequency generator. However, several smartphone apps can provide a simplified analysis for casual listeners.
Another important spec to look for is the speaker sound slope of the frequency response graph. It tells you how much a speaker favours bass over treble. A negative slope will make a speaker sound warm, dark, or dull while a positive slope will cause the speaker to sound bright or sharp.
A good slope should be balanced and have a fairly low standard error, which is the average deviation of the in-room target response from the frequency response curve. A lower standard error means that a speaker is closer to its target response and will therefore produce more balanced sound. Some speakers with a lower standard error, however, have a poor low-bass reproduction and may not be suitable for playback of certain genres of music.
Reflection Speaker Sound
Sound waves emitted by a speaker travel through the air and strike various surfaces. Those reflections then follow different paths to reach the listener’s ears, influencing the timbre and spatial qualities of the sound field. Properly managing these early reflections is a key to the reproduction of music and movies in home theaters and other listening environments.
A sound wave that strikes a surface and is reflected has two characteristics: its phase (the change in direction of the oncoming disturbance) and its dispersion (the variation in the distribution of the energy along its path). The first characteristic, phase, can be controlled by placing absorbent material near a speaker. The second, dispersion, can be adjusted by varying the geometry of the speaker enclosure and using different materials within it.
The first early reflections from a speaker sound typically arrive at the listener’s ears 1.334 meters later than the direct sound, due to the speed of sound. Because of this short delay, the reflected sound is not perceived as an echo, but rather as another layer of sound. However, if the speaker is positioned far enough from the wall that the earliest reflections are significant, the combined direct and reflected signal can reach the listener with more than the full strength of the original input signal.
Reflections have a number of other important effects on the sound quality of a loudspeaker. The level of the reflected signal, its delay and its spectral content can have profound consequences for the overall sound quality. Despite this, many people recommend using absorption to control early reflections, assuming that doing so will improve the performance of the loudspeaker.
Sadly, simply killing all the reflections at a particular point on an ETC does not produce a dramatic improvement in sound quality as some acousticians might lead you to believe. It is critical to understand the psychoacoustic reasons why this is so, before making any generic acoustic treatment recommendations.
Ideally, the reflected signal from a speaker sound would be as coherent as the direct signal, and it should have a narrower directional pattern at lower frequencies, which is known as a “beam” pattern. Unfortunately, even the best performing speakers cannot achieve this in most listening environments. This is why a good room requires a combination of both scattering and diffusing materials.
Absorption Speaker Sound
The quality of the sound of a speaker depends to some degree on the absorption and diffusion of sound in the surrounding room. For example, clapping hands in an empty space produces zippy echoes that fade into the distance, due to the lack of absorption and the flat reflective surfaces of walls, floor and ceiling. The addition of draperies, furniture and baroque plaster ceiling decoration changes the reverberation pattern to be less pronounced because these objects have shapes and surfaces that differ from those of the flat reflective surface.
Absorption of sound is important in a loudspeaker enclosure because it can reduce the time delay between the signal reaching the diaphragm and the corresponding pressure variations being transmitted to the outside air. This shortens the overall time of the sound wave, and results in a better reproduction of high frequencies.
A good absorbing material is speaker sound made up of a large number of small pores or cavities, filled with loose fibres. The acoustic absorption of the material increases with the number and size of the pores. Increasing the density of the porous material also improves its absorption, but only up to a certain point. This is because the morphology of the fibers and the open porosity are more important than the thickness of the material.
The earliest composite sound-absorbing materials were natural fibers like hemp and coconut shell, which had excellent absorption properties over a wide range of frequency bands. These were soon replaced by inorganic fibres such as mineral wool, polyester and synthetic fibres. These had the advantage of being fireproof and damp-proof, but suffered from a poor appearance and decay.
Wrapping It Up
The practical sound absorption speaker sound coefficient, ap, is determined according to the European standard EN ISO 11654, which defines six different absorption classes (A-E). The ap of the metal foams decreases with increasing temperature because of the change of the speed of the waves, the change in wavelength and the reduced effective porosity due to the accumulation of moisture in the structure. It is possible to achieve a good ap by varying the pore size and distribution, by adding fillers or by using a special shape.