What is Professional B2B Speaker Acoustic Engineering Testing?The Methods of Loudspeaker Parameters Testing and Performance Evaluation

How to Define a “Good” Sound?

Generally, we would define a “good” sound as being pleasant and appealing. Statistically, a “good” sound will have the following characteristics:
  1. A frequency ranging from 18 to 46 Hz, where the sound is lower in pitch. The human ear often perceives sounds within this range as warm and steady.
  2. At frequency 46 to 200 Hz, sound is perceived as more structured and distinct.
  3. As frequency enters 200 to 2 kHz, its density becomes higher, in which the sounds are with more clarity.
  4. Sounds that are above 2 kHz will have a sense of brightness.
  5. The overall balance and resolution of sounds is also an important index of a “good” sound.

The Methods of Loudspeaker Parameters Testing and Performance Evaluation

To understand speaker acoustics testing, we must first understand the term “acoustics”. Acoustics is the study of sound and vibration. Speaker acoustics testing, therefore, measures how speaker sounds perform under various spatial conditions. Manufacturers of sound-related products, such as speakers, often use acoustics testing to understand how product sounds will interact with the desired environment. Data collected from speaker acoustics testing helps manufacturers in designing products that meet acoustics requirements and best fit the targeted environments.

The Fundamentals of Acoustics: 8 Key Terms You Must Learn Before Diving into the World of Acoustics Testing

Sound

Sound can be divided into three categories: audio, ultrasonic, and infrasonic. Audio refers to what the human ears can hear, typically ranging from 20 Hz and 20 kHz. Ultrasonic is the high-pitch frequency that is above and beyond the range of human hearing. Infrasonic is which frequencies are below 20 Hz.

Noise

Noise is any sound that is unpleasant. A noise can be either quiet or loud, as long as it is disruptive.

Decibel (dB)

The decibel, abbreviated as dB, is a measurement unit of sound. It measures the strength (loudness) of sounds. When two sounds are compared in decibel, the larger decibel indicates that the measured sound is stronger (louder). On the contrary, the smaller decibel indicates that the measured sound is more quiet.

Frequency (Hz)

Frequency is the number of cycles of repeated events within one second. Its expression is Hertz (Hz), which one Hz equals one cycle per second. Different frequencies will express different sound pitches. A high sound frequency would be expressed as a high pitch. The audible frequency range differs across species. Typically, the audible frequency range for the human ear is between 20 Hz and 20 kHz.

A-weighting

A-weighting measures the audible frequencies and how the human ear responds to them. Though the human ear can hear frequencies between 20 Hz and 20 kHz, it is not equally sensitive to all the frequencies within this range. When two sounds are in the same loudness (dB) but in different frequencies (Hz), the sounds will be perceived as different loudness (dB) to the human ear. A-weighting can help sound device manufacturers to better understand which sound frequency is more stand out to the human ear.

Vibration

Sound is generated by vibrating structures, such as a tuning fork. The sound waves vibrating from one structure can induce another structure to start vibrating. Understanding vibration can help in reducing unwanted noise from a speaker.

Reflection

Reflection is when sound is being reflected off a surface. The amount of sound reflection depends on the material of the surface. For example, hard, non-porous surfaces reflect more sounds than soft, porous surfaces.

Reverberation

Reverberation is the persistence of sounds after a sound is produced. Reverberation happens because of the numerous reflections of one particular sound. Environmental factors are crucial when deciding whether to eliminate or enhance reverberation. For example, a conference room would want shorter reverberation time to better hear the words being said.

8 key parameters of Loudspeaker Acoustics Testing

On-axis frequency response

On-axis frequency response is the primary indicator of listeners’ preference because it refers to the sounds you hear from a loudspeaker that is set to be directly aiming towards you.

Off-axis frequency response

Off-axis frequency response is the change in sound as you move away, in an arc, from the loudspeaker directed towards you. For example, off-axis frequency response happens if the sound changes when you move 20 degrees left to a loudspeaker. Manufacturers would want their loudspeakers to have off-axis responses for a wide range of angle so that the sounding will remain the same as when it is on-axis.

Polar pattern / Pickup pattern

A polar pattern defines how much of the signal will be picked up by the microphone from different spherical directions.

Impulse response

An impulse response measures how a pulse signal, under a short amount of time, will arrive- directly and reflectively- at the measurement position.

Cumulative spectral decay

The cumulative spectral decay (CSD) is a tool of analyzing impulse response. It measures the decay of frequency of a loudspeaker after an impulsive input. In other words, CSD measures the rate at which a speaker stops producing sounds right after turning off its input signal.

Step response

The step input is the time integral of the impulse response, which is a signal that rises instantaneously from zero to a fixed level. Understanding the step response of a sound system is crucial because it gives more information on the stability of the components and the overall system.

Impedance

Impedance is also known as the Thiele/Small parameters of a loudspeaker driver. It is the opposition of two acoustic flows. It is also defined as sound pressure. The difference in impedance will affect not only the speed of sound travel, but also the sound produced.

Dynamics

Dynamics is the difference between high peaks and quiet spots in the sound.

How to Evaluate Speaker Performance? 2 International Standards Used for Speaker Performance Evaluation

IEC60268

IEC60268 is categorized into several parts, however, part 3, 4 and 5 are the most relatable in evaluating speaker performance:

Part 3: The standard on analogue and digital amplifiers specifies the characteristics of amplifiers in a sound system for professional or household applications.
Part 4: The standard for evaluating microphones using measurement of impedance, sensitivity, polar patterns, dynamics, etc.
Part 5: The standard on loudspeakers, excluding loudspeakers with built-in amplifiers. The purpose of this standard is to define the characteristics of loudspeakers using the measurement parameters that have been previously mentioned, such as frequency response and impulse response.

EN50332

EN50332 is divided into two parts:

Part 1: When the standardized soundtrack is played, the speaker’s output sound pressure should be less than 100dBA. This evaluation needs to be done by a “Head and Torso Simulator (HATS)”. 
Part 2: If the speaker uses a battery as its power supply, its highest output voltage should be under 150 mV.

What to Listen to When Evaluating Speaker Performance?

Tonal uniformity

The evenness of sound and tone across the frequency spectrum, from bass to treble.

Bass extension

The sounds produced in the audio range at the 20-to-40-Hz octave.

Spatial character

The spaciousness and depth of sounds produced by a speaker in a practical environment.

Dynamic linearity

Minimal audible distortions when producing sounds across the full frequency spectrum.

Speaker Testing of Jazz Hipster

In speaker testing of Jazz Hipster, a variety of equipment were used: Frequency tester, B&K 2012 audio analyzer, Gauss meter, CLIO 12 Electrical & Acoustical tests, and Klippel measurement system. The Klippel measurement system contains 4 testing modules that are often used when testing the Jazz Hipster:

LPM

Linear Parameter Measurement identifies the Thiele/Small parameters of transducers. This measurement helps to avoid poor signal to noise ratio and malfunction on the driver or amplifier.

LSI

Large Signal Identification measures the source of linear and nonlinear distortions and their effects to radiated sounds.

DIS

3D-Distortion Measurement accesses acoustical signals, measures voice coil temperature, and detects critical distortion. It helps to adjust voltage at speaker terminals so that the transducer can be protected against thermal overload.

TRF (also known as Rub & Buzz)

Rub & buzz effects are unwanted, disruptive noises due to constructional or material defects during speaker operation. The TRF module detects and analyzes such effects.
To measure frequencies that are below the cutoff frequency (125 Hz), an anechoic chamber is used for microphone and speaker unit tests. Anechoic chamber is a room with thick sound absorption material on all surfaces. The anechoic chamber is in compliance with ISO3745. Its inner dimensions are at 250cm*250cm*205cm; Its background noise is at 11 dBA; Its noise isolation level is at NIC79.
In lower frequency range (bass), Qms/Qts above 6 means the characteristic of a sound is more powerful and clear. However, if Qms/Qts is above 15, the low frequency sound would be perceived as lacking warmth. If Qms/Qts is lower than 6, a sound would be less powerful. In a mid to high frequency range (treble), a large Γ value is most desirable, in which there will be more clarity in sound and control over dynamics.

Acoustic testing is essential to the design and manufacture of speakers because different speaker designs will produce different acoustic sounds and noise. It allows manufacturers to gain insight on how their products perform under practical conditions. From the test results, speaker manufacturers can predict user experience and adjust their product design.

 More About Jazz Hipster Technology

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