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Designing for sound is typically an after-thought. The purpose of this paper is to help design engineers plan for the best sound system in time to complete the project. This should also help
in the form of better power management and efficient determination of the form-factor.
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Designing for sound quality in portable devices |
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Thinking about speakers at the initial stages of the
design process can yield big rewards in sound quality.
By: Sherman S. Bybee and Dr. William E. Kunz, Regal Electronics, Inc.
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There are several key points to think about before designing the sound system of your portable device.
Choosing a speaker is typically the last item covered — usually in the context of what will fit. Designing the speaker system at the initial stages of the project can allow for various possibilities and ultimately, better sound quality.
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Introduction |
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The purpose of this papaer is to help engineers design in a speaker or speaker system to best meet the acoustical requirements of a product.
Designing for sound quality should be addressed as soon as possible in the process and no later than the beginning of the enclosure design. Typically, choosing a sound component such as a speaker is an after-thought, which can cause tension and even scheduling problems. Any preplanning that can be accomplished will help make the design process go much smoother.
The following are key areas to address in designing for sound quality.
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What range of frequencies will the proposed device be generating? |
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Planning and designing for sound quality is based on the characteristics of sound needed for an application. Frequency response is one of the major characteristics used in selecting a
sound device for a particular application. In most instances a speaker will be used. For example, alarms, beepers, buzzers or voice devices all require sound with different, and in many cases, limited ranges
of frequencies.
Other applications might include sound in frequency ranges with both music and voice. For instance, handheld electronic game devices and CD/DVD players require sound generation covering a much broader range of frequencies.
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With speakers, frequency response is influenced by mechanical and electrical parameters such as frame configuration, magnet size and type, and both the shape, and material used for, the cone
diaphragm (paper, mylar, etc.).
The following are the two variables used to describe the frequency characteristics of a speaker:
- Frequency Response
as measured in decibels over a frequency range in Hz : The accuracy of reproduction of the applied signal usually referenced to the human ear range of 20 to 20,000 Hertz, or cycles per second
- Resonant Frequency (fo) as measured in Hz : The frequency, measured in free space, where the speaker’s impedance increases dramatically. At the resonant frequency, with a
constant applied voltage, the current through the voice coil is the smallest.
Typical frequency response parameters for common applications might look like this:
- Voice would require an approximate frequency response of 330 Hz to 3,300 Hz.
- Music would require an approximate frequency response of 20 Hz to 20,000 Hz.
- Specialized sound conditions, above human hearing, such as for animal training and control. This would require various frequency responses, for some animals, as low as 16 Hz and for
others, as high as 150,000 Hz.
Knowing the intended frequency range beforehand is an important first step in speaker selection.
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What is the loudness or volume required ? |
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The next major characteristic is the loudness required for the application. For example would the application be:
- Close to the ear for hand-held devices such as cell phones
- In the ear for personal mp3 players
- At arm’s length for PDAs, instruments or desk-top systems
- Also, the loudness level required should be sufficient to overcome the ambient noise of a typical operating environment.
Loudness brings a couple of other issues into play as well:
- Efficiency — because loudness directly affects battery life
- Measurement standards — that is, deciding which measurement reference will be used to determine the sound output levels.
About measurement standards:
Loudness, or volume as it is sometimes referred to, is measured in terms of dB Sound Pressure Level (SPL). However, it is important to be aware that SPL specifications are not all the same and are based on different reference standards by a speaker manufacturer to present it’s product in the best light for an application. The most recognized standard is referencing 1.0 watt (w) of RMS power driving the speaker and measuring the SPL level at one meter away from the speaker face (Referred to as 1w/1m). There are times where a speaker is designed to operate at power levels of less than a watt. In this situation, in order to measure the SPL, the 1w/1m distance reference is modified by the square of the RMS power. For example, when driving a speaker with 0.1w, the equivalent distance as referenced to one watt would be 0.316 m.
Another sound device similar to a speaker is a receiver. A receiver is used in applications where the listener is in very close proximity to the device, such as, a cell phone,
head phone or ear phone.
The receiver is a low power device, with power specified in the range of 10 to 100 milli-watts (mw) and typically has a much broader frequency range. There is a different standard for measuring the SPL of the receiver— typically measured with 1.0 mw RMS driving power and installed in a fixture called an Ear Simulator or Artificial Ear.
Additional parameters to keep in mind when designing for sound:
- Efficiency: the loudness or volume desired for a given input power, i.e., the highest SPL for a given input power level to fit the space.
- Distance to the listener: the loudness at a typically used distance for the device
- Measurement Standard: The normal standard goes out the window and usually gives way to what the marketing people hear and expect.
General guidelines for SPL specifications, depending on the requirements, may include:
- Portable multimedia devices, such as laptops, CD/DVD players or PDA’s where the distance from the sound source to the listener is about one to three feet, the speakers’ SPL would be
specified in the range of 78dB to 86 dB.
- Cell-Phones, or other devices held directly or within six inches to a listener’s ear may use both speakers and receivers.
Receivers are specified at a much higher SPL of approximately 90 to 120 dBas measured in an Ear Simulator.
- In-Ear receivers are specified the same as the cell phones above
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What is the desired sound quality? |
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Once the frequency and loudness characteristics have been determined, physical requirements can be worked on. Physical requirements, such as, size, depth and enclosure dimensions
are affected greatly by the desired acoustical and electrical characteristics. In addition, to help with the acoustics of the device, a chambering scheme or sound wave-guide may also be integrated into the enclosure design.
Sound quality can also be affected by non-linear distortion, particularly where a speaker is designed to be operated near, or even above, its specified power handling capability.
Linear distortion changes over a speaker’s frequency range and should be evaluated in the context of a specific application. The
best way to address the non-linear distortion issue is to work with a speaker manufacturer and look at specific speaker response/distortion measurements for your particular application, including the enclosure in which the speaker will be mounted.
The measurement for distortion is given as a percentage, comparing the perfect sine wave signal to the distortion generated by the acoustical output of the speaker.
As a reference, a typically specified distortion of 5% is about the maximum percentage allowable to be able to discern the original sound detail. Typical distortion is about 1% to 3% for general purpose speakers, even though a manufacturer may have specified 5%. Distortion measurements for common applications would be:
- Voice applications: may have acceptable distortion levels from 1% to 3%.
- High fidelity sound applications: require a distortion level of less than 1%.
- Alarms, beepers and buzzers: can tolerate a fairly high distortion under most applications and is typically not specified.
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What is the required impedance? |
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Specifying a speaker impedance must also take into account the amplifier circuitry requirements, the DC supply voltage, the amplifier output signal voltage (measured in volts RMS) and the
desired loudness (measured in dBSPL). For maximum transfer of energy —electrical to acoustical—the goal is to have the speaker impedance match the output impedance of the amplifier.
The amplifier impedance can be matched to the speaker impedance by specifying a particular audio IC Chip and in some cases, modifying surrounding circuitry to reach a desired impedance. The dynamic impedance of a speaker is dependent on frequency and is at its highest at the resonant frequency (Fo). The basic speaker impedance is specified as DC resistance and measured in Ohms.
The following are impedance related parameters :
- Speaker impedance: should take into account the DC supply voltage available, the signal output of the amplifier and the loudness required to make an efficient, quality device.
Generally, the lower the DC supply voltage, the lower the speaker impedance
- Speaker/Amplifier efficiency: is highest when the speaker impedance matches the amplifier output impedance
- Available speaker impedance: are typically designated by DC resistances of 4, 8, 16 and 32 ohms, but can be custom made to any impedance up to about 150 Ohms
- Amplifier impedance: is designed by choosing from a wide variety of standard output impedance IC Chips, or modified circuitry to match the required speaker impedance
The effects of mismatched impedances are seen in the areas of efficiency and sound quality. A financial impact could result by having to re-design and fabricate new circuitry.
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How and where will the speaker be mounted? |
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How and where a speaker is mounted affects several important issues, such as, frequency response, loudness and quality of the sound emanating from the enclosure.
Speaker mounting configuration brings into play a variety of variables, such as:
- Volume of air behind the speaker
- Where the speaker is mounted in an enclosure
- How the speaker is mounted, and
- Grill size and configuration.
If speaker mounting is done properly, any distortion present will be the result of the speaker or amplifier design and not the mounting process. A properly mounted speaker would be:
- Mounted flush with the enclosure surface
- Mechanically secured (even if a “snap-in”), and
- Have no sound leaks around the mounting surfaces
Another item to keep in mind in the manufacturing process is to ensure that the speaker is not subjected to excessive heat. Some speakers are mounted to PC Boards and cannot survive
solder flow or re-flow temperatures. PC mounted speakers are usually manually soldered in separately after the solder flow or re-flow is completed.
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Speaker mounting alternatives include:
- Screw mounting: physical mounting to the enclosure using screws through mounting holes in the speaker frame.
This method usually requires a separate soldering operation to attach speaker leads or a cable assembly to the speaker which are then connected to the PC board. Typically a cable assembly would be installed on the speaker by the manufacturer. Sometimes a connector is used on the PC board so that the enclosure can be separated from the PC board for servicing.
- Gluing: to either the printed circuit board or to the enclosure requires careful consideration. If the speaker is glued to the PC Board, the location should ensure that maximum
radiation of sound comes out of the enclosure. When glued to an enclosure, particularly for smaller speakers, the mounting surface is usually designed with an injection molded cup shape tooled in, with
dimensions specific to the speaker being used. When glued to the enclosure, the mounting surface must be flat, because any separation between the speaker and enclosure will cause an air leak and efficiency
will be decreased. After the speaker is glued or snapped in the enclosure, speaker leads are then connected to the PC board as a separate step.
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Other location issues:
- Air volume: the area behind the speaker should be large enough so as not to restrict the speaker cone movement, due to back pressure, thereby affecting frequency response and
efficiency.
- Acoustical wave guide ducting or tuned ports: will enhance the low frequency response (a la the Bose Wave Radio). When the speaker can be placed in a closed ported box, perhaps
within the main enclosure, the result will be a better low end response and a higher volume or loudness (the same effect as high fidelity bass reflex enclosures).
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Other issues |
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Speaker Shape: By an overwhelming majority, round is the shape of choice for most design engineers. Designing for a round speaker is easier as they are familiar, and
plentiful. However, in situations where speaker size might be constrained in one dimension and not the other, an oval shaped speaker could offer advantages such as a better range and more loudness.
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Heat: Heat is not a friend of speakers.
That’s why mounting techniques are such an issue. While there are speakers that are certified for “high temperature” environments, they are custom made and in some cases can go through the wave soldering operation. In the majority of applications where fidelity is even moderately important, speakers are integrated into assemblies last, usually as a separate step in order to avoid problems with heat.
Size matters. Speakers generate sound by moving air and this means, for low frequency sounds in particular, the speaker cone, which is attached to the voice coil, must be large,
and/or, the excursion distance must increase. The excursion is the distance the speaker cone and voice coil moves in and out of the magnetic gap.
The magnetic field that is generated in the voice coil gap is a major factor in the excursion distances and is a factor of both the voice coil design and the magnetic material used in conjunction with the voice coil.
Speaker magnets.
Magnets used in speakers are of two general types: rare earth and ferrite. Rare earth magnets have a stronger magnetic field than a ferrite and are used to make more efficient speakers, particularly in mini and micro sized speakers. Ferrite magnets are generally used where there are no size constraints and the magnet can be made larger. Ferrite material is much less expensive than rare earth material and is used in speakers with much higher power requirements.
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Final recommendations |
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The best way to design for sound quality is to involve your speaker manufacturer early in the design process. This approach has many benefits, such as:
- Identifying, and testing, speakers that meet your specific sound quality needs.
- Testing sound alternatives using your actual PC board or enclosure prototypes.
- Help with the actual design of your PC board layout or enclosure to enhance sound quality.
- Development of a value added speaker subassembly for your specific application. Sub assemblies might be as simple as a speaker with lead wires and PB board connector already attached.
Or, could be as involved as an injected molded speaker subassembly specifically designed to snap into your device.
Good sound quality in portable devices is readily attainable — if you plan ahead.
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If you have questions about how to design for sound quality in your portable device, please:
- Phone Sherman S. Bybee or Dr. William E. Kunz
at Regal Electronics, Inc., 408/988-2288, or
- E-mail: hal@regalusa.com
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